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Voltage control in low voltage networks by

Photovoltaic Inverters PVNET.dk

Case-study Bornholm

Adrian Constantin, Radu Dan Lazar and Dr. Sren Bkhj Kjr

Danfoss Solar Inverters A/S, December 2012

This report can also be downloaded athttp://www.danfoss.com/solarand atwww.PVNET.dk

Research supported partly by Energinet.dk, under grant number ForskEL 10698

Danfoss can accept no responsibility for possible errors in catalogues, brochures and other printedmaterial. Danfoss reserves the right to alter its products without notice. This also applies to productsalready on order provided that such alterations can be made without subsequential changes beingnecessary in specifications already agreed. All trademarks in this material are property of the respectivecompanies. Danfoss and the Danfoss logotype are trademarks of Danfoss A/S. This work is licensedunder aCreative Commons Attribution-NoDerivs 3.0 Unported License
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Summary

This report provides an analysis on the main voltage regulation techniques that can be

applied in the low voltage (LV) network with standard photovoltaic (PV) inverter technology. The

main purpose of the research is to verify if reactive power can be used in LV networks to increasethe hosting capacity, by controlling the voltage and hereby increase the hosting capacity of the LV

networks.

The hosting capacity of a LV network is defined by the amount of PV power which can be

installed in the network before certain limits are reached. The evaluated limits in this report are:

overloading of LV cables, overloading of MV/LV transformers and overvoltage at the outermost

distribution box. Two types of Volt-VAR control schemes are presented: the power factor

depending on active power output of the PV inverters - PF(P) and reactive power depending on

the terminal voltage of the PV inverter - Q(U). A generic LV grid model with 71 users and a 100 kVA

feeding transformer is used in the simulations, being considered as representative for the

Bornholm network. All users are equipped with PV systems, each of equal power size and PV panelorientation. Furthermore, a simple MV network is implemented in order to observe the voltage

variations in the 10 kV network as well. The energy consumption for each of the 71 users is based

on time-series of generic consumption. The PV generation is based on synthesized hourly

irradiance by the PVsyst software, taking both clear sky and covered sky into consideration.

The results indicate that without voltage control the overvoltage phenomena starts for a PV

capacity of 1.5 kW per residence (total 107 kW). By applying a standard PF(P) control scheme, the

overvoltage condition is avoided up to a PV penetration level of 1.8 kW per residence (total 124

kW) and still keeps the amount of exchanged reactive power low, PF > 0.95. By applying Q(U)

control, the overvoltage condition is mitigated up to 2.0 kW (total 144 kW) penetration and still

keeping the power factor above 0.90. The overvoltage issue is not solved by upgrading the feedingtransformer, on the contrary increasing the size of the transformer has a slight negative effect. The

total hosting capacity for the LV networks on Bornholm is estimated to 50 60 MW, which can

cover 1520% of the yearly energy consumption.

Overall, it has been found that applying standard voltage control techniques in the LV

networks helps to increase the PV penetration by approximately 30% from 1.5 kW to 2.0 kW per

residence. For higher PV penetration levels, additional solutions must be applied: more complex

voltage control schemes, increased self-consumption, storage solutions or active power

curtailment. Both PF(P) and Q(U) control types are improving the maximum voltage profile at the

outermost distribution box. Based on these results, if overvoltage is observed at the end-customer

site the following order of actions are suggested:

Apply voltage control for the inverters, e.g. PF(P), Q(U) or more complex schemes

Lower the tap position in the LV/MV transformer and apply Q(U) round the clock

Increase self-consumption at peak production hours

Lower active output power of inverters (only in emergency cases for short periods)

Upgrade cables, upgrading the transformer does not help

Install storage, i.e. battery systems


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Table of Contents

1 INTRODUCTION 7

1.1 Problem statement 7

2 METHODS 10

2.1 Definition of hosting capacity for the electrical network 10

2.2 Definition of PV penetration for the electrical network 13

2.3 Reactive power by grid voltage - Q(U) 14

2.4 Power Factor by Active Power PF(P) according to VDE AR N 4105 17

2.5 Generic LV network 18

2.6 Statistical Data Used for Consumption and Generation 202.6.1 Electrical energy consumption 202.6.2 PV generation 20

2.7 Simulation model 222.7.1 Dynamic inverter model description 222.7.2 Dynamic AC load modeling 23

2.8 Simulation study cases 23

3 RESULTS 26

3.1 Results of the base study case 26

3.2 Overview of comparisons between the simulated results 34

3.3 Varying the dead-band of the Q(U) controller 36

3.4 Varying the voltage sensitivity of the Q(U) controller 38

3.5 Comparison for varying the transformer power rating with and without Q(U) voltage control 42

3.6 Comparison for varying the transformer power rating with PF(P) voltage control 44

3.7 Comparison of Q(U) and PF(P) 46

3.8 Comparison of hosting capacity for the cases 48

4 CONCLUSION 50

4.1 Obtained results 50

4.2 Recommendations for voltage control in LV feeders 52


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4.3 Future Work 52

5 WORKS CITED 54

6 APPENDIX A 56

7 APPENDIX B 58

7.1 Study case 2 58

7.2 Study case 3 62

7.3 Study case 4 67

7.4 Study case 5 71

7.5 Study case 6 76

7.6 Study case 7 80

7.7 Study case 8 84

7.8 Study case 9 89

7.9 Study case 10 93





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Nomenclature

DNO Distribution Network Operator

EPIA European Photovoltaic Industry Association

HV High Voltage

LV Low Voltage

MV Medium Voltage

OLTC On load tap changer

PF Power factor (acc. To IEEE convention)

PV Photovoltaic

TSO Transmission System Operator


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1IntroductionThis study is part of the project Application of smart grid in photovoltaic power systems

PVNET.dk, ForskEL Programme 2011 partly sponsored by Energinet.dk. This work package

provides one possible and economically viable solution of overcoming technical difficulties which

may appear due to the ever increasing amount of PV generation at the LV distribution network

level. Namely, it analyses the performance of reactive power control of the PV inverters connected

to the LV network. This solution is intended to be tested in the laboratory and afterwards

deployed on PV inverters on the island of Bornholm. Two types of Volt-VAr control are under

study: reactive power by voltage - Q(U) and power factor depending on the output power - PF(P)

of the PV inverters, both which are commonly used in Germany. The advantages and

disadvantages of using reactive power control in a LV feeder are exemplified based on a generic LV

network model for the Bornholm network. This network describes a typical residential topology on

the island.

1.1 Problem statement

In Denmark only, PV installations are expected to increase at a very high pace with an

estimated amount of installed power exceeding 1000 MW in 2020 [1]. As of February 2012, there

are a total of 11 MW installed PV capacity in Denmark providing approximately 0.03% of the total

energy consumption. This figure is comparable to approximately 3% in Germany [2]. Most of these

PV installations in Denmark are being built in the 0.4 kV feeders of the distribution network.

Reference [3] estimates a total of 140 MW by the end of September 2012 comprising more than

25 000 installations.

There are several technical issues which may appear when increasing the penetration levelof renewable energy in a LV distribution network:

Transformer overloading

Cable overloading

Overvoltage phenomena

Voltage unbalance

Reverse power feed-in

Protection System failure

The first three issues are the most urgent and the DNOs are trying to address them first. This

document will focus on these. Voltage unbalance has been studied within this project in paralleland documented in reference [4]. Reverse power flow and protection system failure are not

covered by this report due to the limited influence that PV inverters can have in mitigating these

effects without corresponding communication channels or storage devices (which increase self

consumption).

The first three forementioned issues are present with different occurrence probabilities

depending on the type and layout of each LV distribution network and the PV penetration level.

Thus, from the point of view of categorizing the distribution networks upon the type of end

customers, the technical literature uses the following grid naming [5]: Urban, Sub-urban, Rural and

Farms. The current study has the objective of focusing on a sub-urban grid type due to the high

relevance in connection to the electrical network of Bornholm.


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A comprehensive analysis has been done by Dansk Energi in a recently published report [6].

As in most theoretical analyses simplifying assumptions have been used (3 phased PV systems,

hourly load/generation data, grid data only from one DNO, even distribution of PV throughout the

network). In this report a total of 1110 0.4 kV feeders are evaluated for susceptibility of violating

the +/- 10% voltage design criterium when introducing PV generation from 0 to 50% penetration.One of the results in their report is that while a PV penetration level of 13.5% in 2030 is expected,

only 0.7-0.8% of the total number of feeders (containing more than 5 consumers) will experience

overvoltage at that moment. For higher penetration levels the probability of having these types of

problems will eventually increase, therefore, the associated costs of reinforcing the network will

increase accordingly.

Several solutions have been suggested until now in order to cope with the overvoltage

phenomena at high PV penetration levels of distributed generation in LV:

1.Voltage control using reactive power generation from PV inverters

2.

Voltage control at the LV side of the LV/MV transformer by on-load tap changers3.Active power derating of the PV production in case of overvoltage conditions

4.Battery storage/Energy buffer at PV generator and MV distribution level

5.Network upgrade

6.(Seasonal) changes of the tap position of the LV/MV distribution transformer

Each solution is currently investigated by different stakeholders and their feasibility is

assessed.

Method 1, voltage control through reactive power generation from PV inverter, is one of the

easiest to implement because of the versatility of the inverter unit in providing a plethora of

voltage control techniques. A good overview of the available Volt-VAr methods is presented in [5].

These are constant power factor (PF), constant reactive power, local Q(U) and local PF(P). The

same reference provides two proposals for optimized local Volt-VAr control: local Q(U,P) and local

PF(U,P). The last two methods are claimed to increase the PV penetration levels while keeping the

reactive power flow to a minimum (thereby minimizing the reactive power exchange) and also

avoiding overvoltage situations. Regarding the local Q(U) regulation algorithm a recent study has

assessed its stability when applying it to PV inverters installed in LV networks [7]. In their report

the integration of Q(U) in the standard requirements for LV connected PV inverters in Germany is

recommended. Another author analyses and verifies the efficacy of several other different types

of voltage control methods based also on the local Q(U) principle [8]. Several claimed

disadvantages of the existent methods are: need for overrating the PV inverter, increasing the

losses in the grid due to reactive current circulation, compensation in the MV network of the

generated inductive reactive currents. All the previously mentioned control methods are intended

to function autonomously. Still, in the future, by leveraging the Smart Grid functionality,

communication means will be available to the inverters as well. Therefore, the voltage control

through reactive power generation can be optimized by coordinated/scheduled control.

Method 2, is proposed by several transformer manufacturers and DNOs [9] [10]. Pilot

projects are already under development in order to assess the efficacy of the on load tap

changer type transformers[6] [10] [11]. Still, the solution is not mature enough to be accepted as

being feasible. Moreover, the high investment costs and the additional service required make this

option less attractive.


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Method 3, active power derating, has been investigated by researchers [12] [13] and DNOs

[14]. Usually, the PV plant owner cannot estimate the impact of this control scheme upon the

economical aspects of the investment. Furthermore, the PV owners which will be at the end of the

LV feeders will be the first to be affected. An interesting conclusion is that: At first glance it seems

that local or central regulation of reactive power comes first among the possible strategies. Activecurtailment would then be activated when reactive compensation is no longer sufficient to avoid

upper voltage constraints[14].

Method 4, battery storage/energy buffer at PV generator level, is promoted by several PV

inverter manufacturers with the scope of shifting the PV grid injection peaks, thus avoiding

overloading the network [15]. EPIAs report Connecting The Sun [16] describes as a valid solution

the combination between active power derating and storage which will result in peak shaving at

noon hours and feeding energy during the evening hours.The energy buffer system can be a

battery (bidirectional power flow) or a controllable AC load (unidirectional power flow e.g. heat

pumps). The main disadvantage of the storage solution is the high cost of an integrated battery +

inverter system even for small capacity batteries. The life-time of the battery is also difficult to

estimate considering the unpredictability of the full load hours to be applied. Regarding the use of

batteries at the MV distribution level, several pilot projects are being implemented by DNOs in

order to get more experience with the use and advantages of such a control method. Although

providing a very good technical solution, the storage technologies are not yet price competitive

enough for the lifetime and capacity required. Further investigations are necessary.

Method 5, comprises the standard approach of increasing the grid capacity by upgrading the

LV/MV transformer to a higher power rating or by reinforcing the LV feeders by addition of parallel

lines or replacement of old lines with higher ampacity ones.

Method 1 is the main focus in this work, the authors wishing to take advantage of theversatility of the PV inverters in providing voltage control schemes. Thus, the subject of employing

reactive power control techniques in the LV distribution networks is discussed in this analysis. The

main outcome of this work is intended to be a generic design guideline for mitigating effects of

increased PV penetration, thus increasing the network integration of solar energy in the power

system.


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2MethodsThis Chapter represents the main part of the study and contains an analysis method

description in order to draw guiding conclusions over the feasibility of the use of reactive power in

LV grids for increasing PV penetration.

2.1

Definition of hosting capacity for the electrical network

The reactive power supply from PV inverters is currently used in all types of networks in

Germany (e.g. in LV and MV [17] [18]), while many other european countries are in the process of

enforcing such requirements. On the other hand, in islanded systems it is common practice to use

droop control to maintain the grid operation. It is therefore interesting to find out to what extent

the reactive power control can be used in low voltage networks in terms of voltage control. This

analysis provides study case answers to questions such as:

What is the benefit of using the PV inverters in reactive power mode in LV networks?How much can we increase the level of PV penetration in the distribution network if

reactive power support methods are used in the PV inverters?

What are the limiting factors when increasing the level of PV penetration (transformer

overloading, cable overloading or overvoltage situations) ?

What is the influence of different grid specific parameters to the overall results (MV grid

impedance, transformer size, cable length)?

Reference [6] discusses in great detail the overvoltage limitations that can be encountered

when increasing the PV generation in the LV network. Due to the high number of studied feeders

it provides a high confidence level of the results. The voltage allocation ranges are clearly specifiedfor each component in the 10 and 0.4 kV network in such a manner that the +/- 10% voltage range

criterium holds according to the EN50160 standard [19]. The analysis from [6] is based on

measuring the relative difference between the secondary side of the transformer and the

outermost distribution box in LV network. As a guiding rule, the relative voltage is allowed to vary

to maximum -5% in case of a load and maximum +2.5% in case of PV generation [6]. In the future

with more distributed generation in the network this might change to -2.5% to +5% voltage

variation.

A relatively different analysis approach is taken in this report in comparison with reference

[6]. An absolute voltage range allocation is performed based on Figure 1. While this case is

considered to be representative for the current analysis, other different designs may also be foundin real situations. The figure contains a one line diagram of a distribution system and the

corresponding voltage allocation ranges. It contains a 60/10 kV transformer with on load tap

changer controlling the voltage on the secondary side. A MV cable distribution system is shown

connecting the 60/10 kV transformer to a 10/0.4 kV transformer. The latter contains an off load

tap changer adjusting the voltage on the secondary side. A LV distribution network follows

towards the farthest located distribution box. A load and a PV system are depicted as being

connected to the network. It can be seen that at each location in this network certain minimum

and maximum voltage limits apply. The narrowest allocated range is at the secondary side of the

60/10 kV transformer where the voltage is allowed to vary within a 2% interval. The widest band is


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allocated to the outermost distribution box for which the +/-10% voltage limits are applied. This

figure represents one of the main requirements for the voltage ranges in this report.

According to [6] the voltage ranges fromTable 1 are reserved from the full +/-10% interval.

Based on the above relative values of Table 1, the absolute voltage ranges from Figure 1 have

been deduced.

Voltage range allocated for: Size [%] Size applied

inFigure 1

[%]

Tap changer at 60/10 kV transformer 2 2

Voltage fall in the 10 kV network 5 5

Non-optimal tap position of the 10/0.4

transformer

2.5 -

Voltage drop of the 10/0.4 kV transformer 1.5 1.5

Unsymmetrical loading 1 1

Voltage increase in the LV installation (after the

outermost distribution box)

0.5 -

Voltage increase in the LV lines - 2.5

Voltage increase in the MV lines - 2.5

Voltage increase in the 10/0.4 transformer - 0.5

Tap position 10/0.4 transformer (maximum load) - 2.5

Tap position 10/0.4 transformer (maximum PV) - 2.5

Total 12.5 20

Table 1: Voltage reserves allocated in the 10 and 0.4 kV network [6]


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Figure 1: Allowed voltage ranges for different points in the 10 and 0.4 kV distribution network


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2.2

Definition of PV penetration for the electrical network

The PV penetration levels are defined in this study in accordance to the definition used in

[6]. Therefore, under the present legislative framework it is expected that an economical

investment in a residential solar plant will result in an installed capacity of 5 kVA. Having all theconsumers in a residential feeder installing a 5 kVA/kWp solar system would therefore be

equivalent with 100% PV penetration in the respective network feeder [6]. This method to

estimate the amount of PV in the network has the advantage of creating a uniform distribution

across the entire feeder of PV power. Practical cases may differ from this situation. The PV

penetration level is expressed in percent and in combination with the number of consumers

and the maximum rated power of one PV inverter Sr (5 kVA) it determines the total

installed PV power in the respective feeder as in Equation 1.

Equation 1

InTable 2 are shown the chosen values for which calculations have been done in Section3.

PV penetration level

[%]

Power Rating of

inverter [kVA]

Total PV power for a LV

network with 71 customers

[kVA]

0 0.0 0

10 0.5 36

20 1.0 71

30 1.5 107

40 2.0 142

50 2.5 178

60 3.0 213

Table 2: Selected PV penetration levels in the simulation

Regarding the orientation of the PV panels in the PV plant it is assumed that all systems are

oriented south with a 45 degrees vertical inclination, seeFigure 2.This case is considered to be the

worst case scenario since in the summer months the peak production will be higher for this

orientation than in any other situation (any orientation and any season).Figure 2 also shows that

by scattering the PV systems in orientation and inclination the PV penetration can be further

increased with more than 10% without negative impacts from overloading.


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Figure 2: Typical output power from a 1 kWp PV system in clear sky conditions. The green curve shows the

power from aggregated systems pointing all south. The red curve shows the corresponding power for 10 systems

which are scattered from East to West with 30 and 45 inclination.

2.3 Reactive power by grid voltage - Q(U)

The main purpose when applying a Q(U) control algorithm is to use the reactive power of the

inverter in such a way that in case of overvoltage conditions the control will decrease to a certain

degree the mains voltage and in case of undervoltage situations the control will tend to increase

the voltage towards a prescribed value. This control needs to take also into account the standard

tap changer position of the MV/LV transformer.

The Q(U) control is normally implemented as in Figure 3. The voltage at the inverter bus

terminals can be used as an input value to the controller. This voltage is computed as the averaged

RMS value of the three phases and expressed in per unit system like suggested in IEC 61850-90-7[20]. A reactive power versus voltage dependency is defined using a piecewise linear curve as

drawn with red and green in Figure 3. The red curve contains additionally a dead band

characteristic, commonly used in medium voltage applications. A low pass filter is added to the

controller in order to increase stability by making the controller slower (e.g. the inverter will not

interact with faster automatic voltage regulators).


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Figure 3: PV inverter Q(U) control algorithm

The slope m of the Q(U) characteristic represents the sensitivity of the reactive power

controller versus voltage changes. In this report the term voltage sensitivity is used

interchangeably with the term slope m of the Q(U). It is defined as inEquation 2.In the case of a

multi segment curve as it is the case of the red curve inFigure 3,one voltage sensitivity value is

defined for each interval. Dead-band may also be programmed in the reference curve. Thus,

different voltage sensitivity values can be defined for under/over excited operation modes,therefore the voltage sensitivity will be calculated using a different references inEquation 2.

maxmin

minmax

UU

QQ

U

Qm

Equation 2

If analyzing a Q(U) curve there are several defining parameters (denoted by their name) and

intervals (marked by a capital letter) as shown inFigure 4.

Q*[pu]

Uinv[pu]

Umin Umax

Qmax

Qmin

A B C

Udmin UdmaxUref

Figure 4: Generic Q(U) curve and defining parameters. The curve could also have another shape, with more points.

IEEE power factor convention, see Section2.4.

The operation intervals defined inFigure 4 are:

Interval A: overexcited (capacitive) operation of the PV inverter; the main objective is

to increase the mains voltage.

Interval B: Dead-band in which the controller is not injecting reactive power for a

predefined voltage range.

Interval C: underexcited (inductive) operation of the PV inverter; the objective is to

decrease the mains voltage.


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The relevant parameters shown in the figure above are:

: Minimum voltage value for which the controller should apply maximum

capacitive reactive power at the inverters terminals .

: Minimum voltage of dead-band interval. For lower voltages overexcited(capacitive) inverter operation is chosen while for higher values the PV inverter does

not inject any reactive power.

: Maximum voltage of dead-band interval. For lower values the PV inverter

does not inject reactive power while for higher values the underexcited (inductive)

inverter operation mode is chosen.

: Reference voltage for dead-band selection. This parameter is chosen according

to the selected output voltage of the LV/MV transformer at the secondary side (LV

side) and in accordance with the voltage tap-setting. This parameter has no other

purpose but to correctly determine suitable values for and .

: Maximum voltage value for which the controller should apply minimum(inductive) reactive power at the inverters output terminals .

: Minimum reactive power generated by inverter. This parameter refers to the

underexcited reactive power operation of the inverter. The value can reach up to the

maximum underexcited reactive power capability of the inverter.

: Maximum reactive power generated by inverter. This parameter refers to the

overexcited reactive power operation of the inverter. The value can reach up to the

maximum overexcited reactive power capability of the inverter.

Additionally to the fore-mentioned parameters, the controllers low pass filter is defined by

a time constant. The German authority BDEW recommends in the MV networks the use of a

settling time between 10 and 40 seconds for Q(U) [17].

The parameter is usually chosen depending on the applicable under-voltage limits of

the inverters. For EN50160 [19] standard the -10% limit is used in case of long-term voltage

variations with averaging periods for 10 minutes. Therefore, if the fore-mentioned standards

apply:

The same design criterion is employed for defining the parameter: a value of maximum

+10% in voltage will correspond to:

Parameters and are defining the width of the voltage dead band in which the

Q(U) control should not generate any reactive power. This region should restrict the inverters

injecting unnecessary reactive power while small variations in voltage are present around the

nominal prescribed value at the LV side of the distribution transformer. A too broad dead band

will also have negative effects since inverters closer to transformer station will not participate at

all in regulating the voltage while inverters at the remote ends will provide maximum reactive

power.

Parameter may be chosen based on the rated PCC voltage and the position of the tap-

changer on the LV/MV transformer.


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2.4

Power Factor by Active Power PF(P) according to VDE AR N 4105

In this report only one typical PF(P) curve has been analyzed, identical with the one defined

in VDE AR N 4105 and shown in Figure 5. The IEEE power factor convention has been chosen

throughout this report, as shown in Figure 6. Throughout the report lagging power factor andinductive reactive power are describing the same phenomena. It can be observed inFigure 5 that

the PV inverters are required to inject reactive power (inductive) starting at 50% power generation

and at 100% the power factor reaches 0.9 (lagging) for units with rated power above 13.8 kVA and

0.95 (lagging) for units below this level. For the Danish standard TF 3.2.1 the same requirement is

stated but with a rated power limit of 11 kVA for the 0.95/0.9 power factor. One property of this

type of control is that the inverters will inject reactive power independently of the location in the

feeder in comparison with Q(U) algorithm in which the farthest inverter would inject always more

reactive power than the ones closer to the transformer. Thus, overall better control of the voltage

is assumed, since all inverters in the network are taking part. The disadvantage is that the

inverters might inject reactive power into the network even though it may not be required (noovervoltage situation).

Figure 5: Power factor by active power control as defined in VDE AR N 4105

Figure 6: IEEE power factor convention


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2.5

Generic LV network

One representative LV network has been selected for the Bornholm distribution network for

the project PVNet.DK [21] and it is depicted inFigure 7.It contains two LV feeders supplied by one

MV/LV 100 kVA Transformer with parameters shown in Appendix A,a number of 71 consumersand the corresponding cable interconnections (also detailed in Appendix A). The two feeders

contain 52 and 19 consumers respectively. This LV network supplies energy to a residential (sub-

urban) area on the island of Bornholm. The generic LV network described in this Section is

assumed as representative for a residential Danish LV distribution network in the island of

Bornholm and corresponding to a suburban network (shown inFigure 7).

Figure 7: Generic LV network analyzed in this study


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For an average consumption case and no PV generation, the voltage profile in this network

would look as inFigure 8.

Figure 8: Voltage profile in the generic LV network for average consumption levels and no PV generation


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2.6

Statistical Data Used for Consumption and Generation

In order to obtain realistic results, several sets of historical data have been used. Statistical

and historical energy consumption and generation data are presented in this section.

2.6.1

Electrical energy consumption

The electrical energy consumption in the residential sector in Denmark has been obtained

at an hourly rate for one typical year as can be seen inFigure 9.A total of 8760 sample points are

stored. The plotted values are normalized to the maximum hourly recorded consumption value

through the entire year. This data will be later normalized to the average yearly consumption level

of one Danish residence which in 2009 was 3.44 MWh [22].

Figure 9: Electrical energy consumption over one year for a typical Danish residence.

2.6.2

PV generation

For PV generation yearly profile data, synthesized values (using PVsyst) for one location in

Denmark have been used as shown inFigure 10 andFigure 11 at an hourly time sample. In order

to make use of these data a simple linear scaling to any other inverter with different power ratinghas been done. Therefore, if considering a 2 kW PV system, all the production values have been

multiplied by 2 thus obtaining the equivalent production data for a 2 kW reference system size.


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Figure 10: Yearly synthesized PV production data for Brdstrup, Denmark, of a 1 kWp PV plant

Figure 11: Power generation each hour of day for a 1 kWp PV unit for Brdstrup, Denmark, during one year

The data shown inFigure 10 andFigure 11 corresponds to a PV system oriented to south and

with an inclination of 45%. To have a better understanding of the difference between yearly

production levels and peak production for different types of PV systems,Table 3 is shown below.

In this report, in order to address the worst case scenario for PV panel orientation, all PV systems

installed are pointing south with an inclination of 45 degrees, marked with green inTable 3.


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PV Orientation [] PV inclination [] Peak hourly

produced energy

[Wh]

Total produced energy

[kWh]

-90 (west) 30 870 895

-45 (west) 30 892 1021

0 30 850 1066

45 (east) 30 864 1013

90 (east) 30 809 884

-90 (west) 45 888 847

-45 (west) 45 918 1004

0 45 940 1059

45 (east) 45 876 995

90 (east) 45 798 833

0 20 825 1043

Table 3: Peak and yearly PV production for different orientations of a 1 kWp PV system in Brdstrup,

Denmark, (synthesized data from PVsyst)

2.7

Simulation model

2.7.1

Dynamic inverter model description

The dynamic model of the inverter is a three-phase RMS model. It supports several

operating modes:

o

Normal mode: Q = 0o Local Q(U) mode

o Local PF(P) mode

o Constant PF mode

o Constant Q mode

A functional diagram of the implemented model is shown inFigure 12.The AC active power

output data is used by the PF(P), constant power factor and by the apparent power rating

limitation block P,Q limit. The control mode switch is operated by a separate reference signal.

The measured RMS voltage of the connection point is used to compute the reactive power

contribution in case of Q(U) operation mode. The AC grid block is entirely describing the

components in the AC grid and modeled within Power Factory DIgSILENT grid diagrams. The


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inverter model injects balanced, three phase currents. In steady state the inverter will act as a

constant power source.

PV power

P

Q

P,Q

limit

Pout

Qout

AC

Grid

Urms

Q

U

Control

Mode

Cos )

P

Const Q

Const

PF

Q=0

Figure 12: PV inverter RMS model

2.7.2

Dynamic AC load modeling

A simple AC load model has been developed in Power Factory to model the consumption of

a typical consumer connected to the LV network. A reference power factor has also been used.

Throughout the simulations, for simplicity, a constant power factor 0.95 inductive has been

selected.

P consumptionP

P,Q

calculation

Pout

Qout

AC

Grid

Cos()

Figure 13: AC load RMS model

2.8

Simulation study cases

To the base network of Figure 7, PV inverters have been added to each consumer and

different PV penetration levels have been chosen as inTable 2.Additionally, a very simple 10 kV

MV network has been included to model the voltage variations in the 10 kV network. The lengths

of the MV cables have been intentionally increased in order to simulate a high impedance MV

network. Also, to represent a cluster of 10/0.4 distribution networks a simple aggregated model of

the reference network of Figure 7 has been added. Details regarding the components are

documented inAppendix A.The resulting electrical network is shown inFigure 14.


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Aggregated

distribution

networks

P,Q

4

Figure 14: Generic network with PV generation at each consumer. The indicated number of inverters in the

boxes means a lumped inverter model.

Themain assumptionsfor all study cases are:

Hourly samples for both PV generation and AC loads consumption are used. This translates

as described in Section2.7.1. PV irradiation profile is the same for each connected inverter in the network. This

assumption can be considered valid if the simulation is done on hourly measurement samples.

Local loads follow a similar consumption curve, each load varying from the reference

measurement with a maximum of +/- 10%; All loads are three phase balanced.

The same control characteristic is set for each PV inverter in the network.

All PV inverters are balanced, three phased units. All PV inverters have the same power

rating as described in Section2.2.

The relevant parameters used to select different study cases are documented in Section2.3.

Additionally, two more sizes of the LV/MV transformer have been selected (160 and 200 kVA) in


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order to observe the influence of the transformer rating to the overall Q(U) and PF(P) efficacy.

Considering a 60% PV penetration in the generic LV network, this will correspond to 213 kW PV

capacity. Hence, the biggest chosen value for the connecting transformer is 200 kVA. Since in the

cases of PV penetration higher than 30% the rating of the cables may also be exceeded an analysis

of the cables loading is also of interest. The simulated study cases are shown inTable 4 for Q(U)andTable 5 for PF(P).

For Qminand Qmaxequal to 0.6 p.u. values inTable 4,this corresponds to a maximum active

power of 0.8 p.u. for the apparent power S= 1. Study cases 5 and 6 are using different maximum

intervals: for case 5 a Qminand Qmax equal to 0.48 p.u., while for case 6 a Qminand Qmax equal to

0.4 p.u. Thus, the minimum power factor is equal to 0.8 (lagging & leading) in the Q(U) cases

2,3,4,8 and 10. To make a better comparison of Q(U) against PF(P) control as defined in VDE AR-N

4105 [23] study cases 5 and 6 are performed. For these cases the equivalent minimum power

factor (when voltage is at overvoltage limit) is 0.89 and 0.92 respectively.

Studycase

no.

PVpenetration

[%]

[pu]

[pu]

[pu]

[pu]

Voltagesensitivity

m

[pu]

[pu]

Trafo

[kVA]

1 0:10:60 1 1 0 0 0 1 1 100

2 0:10:60 0.9 1.1 -0.6 0.6 6 1 1 100

3 0:10:60 0.9 1.1 -0.6 0.6 7.5 0.98 1.02 100

4 0:10:60 0.9 1.1 -0.6 0.6 10 0.96 1.04 100

5 0:10:60 0.9 1.1 -0.48 0.48 4,8 1 1 100

6 0:10:60 0.9 1.1 -0.4 0.4 4 1 1 100

7 0:10:60 1 1 0 0 0 1 1 160

8 0:10:60 0.9 1.1 -0.6 -0.6 6 1 1 160

9 0:10:60 1 1 0 0 0 1 1 200

10 0:10:60 0.9 1.1 -0.6 -0.6 6 1 1 200

Table 4: Simulated study cases and chosen parameters for Q(U) control strategy

Study

case

no.

PV

penetration

[%]

Trafo

[kVA]

Maximum

PF (lagging)

11 0:10:60 100 0.95

12 0:10:60 160 0.95

13 0:10:60 200 0.95

Table 5: Simulated study cases and chosen parameters for PF(P) control strategy


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3 ResultsEach study case provides numerous results to be analysed. Due to the high number of

parameters that can be observed for each case, only the base case (no. 1) will be shown in Section

3.1, while the complete set of results can be viewed in Appendix B. Moreover, a comparison

between the study cases is of greater interest and will be discussed in Sections3.2 and onwards.

3.1

Results of the base study case

Study case 1 represents the situation in which the PV inverters have no voltage control

algorithm implemented. It is also assumed that the low voltage side of the 60/10 kV transformer is

dispatched to a constant 0.995 per unit voltage level, as depicted in Figure 1. The 10/0.4 kV

transformer is using a constant tap position resulting in a +2.5% voltage increase at the secondary

side.Figure 15 andFigure 16 display the minimum and maximum year round voltage levels at each

busbar of feeder 1 and 2 respectively for variable PV penetration levels from 0 to 60% in steps of10% as inTable 2.

The voltage level is represented by vertical bars of different width for each PV penetration

level. On the horizontal axis, the voltage of each busbar is ordered from left to right according to

the electricaldistanceto the transformer. The first set of vertical bars (left most) corresponds to

the voltage at the primary side of the 10/0.4 kV transformer, while the second set of bars

corresponds to the secondary side.

It is obvious that at the outermost busbar (vertical bars on the right) the highest voltage

variations will be observed (bus 360 inFigure 14). The minimum voltage value for a specific busbar

is the same for all PV penetration levels since at maximum load hours (during evening) no changes

are performed by the PV inverters. Increasing the PV penetration to 60% will result in a high

maximum voltage increase of approximately 1.17 per unit in feeder 1. A smaller variation is

observed in feeder 2 (app. 1.13 p.u. voltage for 60% PV penetration at the outermost distribution

box bus 9925 in Figure 14) since the cable distances are smaller, as well as the number of

customers being serviced.

InFigure 15 andFigure 16 there are also shown the +/-10% voltage limits. It can be seen that

for PV penetration levels higher than 30% (excluded) the allowed voltage variation is exceeded in

feeder 1. These 2 feeders will therefore be declared as being challenged for a PV penetration level

higher than 30%, corresponding to 107 kW. Thus, their hosting capacity is said to be 30%.


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Figure 15: Study case 1 - Maximum/Minimum yearly voltage for each bus of feeder 1 for variable PV

penetration

Figure 16: Study case 1 - Maximum/Minimum yearly voltage for each bus of feeder 2 for variable PV

penetration


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Figure 17 displays an overview of the simulated voltage levels at different relevant points in

the network and for different penetration levels. This locus plot is shown in connection with the

allowed voltage limits in each point. If analysing the locus of the voltage for the outermost

distribution box it can be seen that the maximum PV penetration level would be just below the

30% marker, which also is the case for most of the other locations in the network.

Figure 17: Study case 1voltage locus at different points in the network and different PV penetration levels

Figure 18 shows the evolution of the transformer loading (in per cent) when an increasing

amount of PV penetration is installed in the LV network (as inTable 2). The graphic represents a

histogram plot viewed from above. Therefore, it can be observed that irrespectively of the PV

penetration, most of the hours in a year are spent by the transformer under a loading from 10 to

40%. Also, by increasing the PV penetration it is seen that the peak at 40% loading is shifting

towards the 20% loading. This could potentially increase the life-time of the transformer. The

uncoloured regions represent zero hours of operation at that specific loading level. For PV

penetration levels lower than 30% (and including) the peak transformer loading occurs during

hour 18:00 the 25th

of December.

Furthermore, due to increasing PV power capacity for PV penetrations higher than 30%,there appear a few hours per year with a very high transformer loading (at noon, mostly during

spring). For a 60% PV penetration the transformer is maximally loaded at approximately 150%.

This overloading coincides with high reverse power generation of the transformer and has a

duration of not more than 2 hours per occurrence (3 times per year) as shown inTable 6.It can be

seen that in this specific case of the Bornholm generic network, the limiting parameter for

increased PV generation is also the transformer rating. The hosting capacity of the transformer is

therefore around 40%, corresponding to 142 kW PV power, when all PV systems are pointing

south. When the PV systems are scattered, the hosting capacity could increase up to

approximately 160 kW, corresponding to a 45% penetration.


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Figure 18: Study case 1Histogram of the yearly transformer loading for variable PV penetration (legend

hours per year)

PV

penetration

[%]

Number of hours for a loading level at:

100% 110% 120% 130% 140% 150%

0 1 0 0 0 0 0

10 1 0 0 0 0 0

20 1 0 0 0 0 0

30 1 0 0 0 0 0

40 14 0 0 0 0 0

50 142 76 27 3 0 0

60 147 131 127 48 17 6

Table 6: Study case 1 - Number of overloading hours of the 10/0.4 transformer

Figure 19 displays the loading performance of the LV cable system in the network. A similar

histogram as inFigure 18 has been plotted for the loading of the cables. For each hour in the year

the maximum loading of any of the cables has been selected as the reference for the calculation.

Based on these 8760 result points, the histogram ofFigure 19 is obtained while the PV penetration

level is varied as in Table 2. The uncoloured regions represent zero hours of operation at that


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specific loading level. A PV penetration of maximum 50% is possible without overloading the LV

cables, corresponding to approximately 180 kW PV systems.

Figure 19: Study case 1Histogram of the yearly cable loading (all maximum) for variable PV penetration

(legendhours per year)

As it is expected, the reactive power of the outermost located inverter is always zero for this

study case, as shown inFigure 20.

Figure 20: Study case 1No volt-VAr controlQ(U) locus plotsfarthest inverter


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Figure 21: P(U) locus for Study case 1farthest inverter

The active power losses of the distribution transformer, the LV cable system and the total

losses for this simulation case are plotted inFigure 22.Each point represents the sum of the yearly

losses of all LV cables in the distribution network for that specific PV penetration level. As in the

case of the transformer losses, a minimum is reached for the 20% PV penetration level.

Figure 22: Active power losses for Study case 1

The transformer losses are varying slower than the cable losses when installing more PV in

the network. The minimum power loss is achieved for a PV penetration level of 20%. This


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minimum level is due to the fact that at 20% PV the power transfer in the network is minimized

both in the generation and the load hours. As it will be seen in the cases for applying Q(U) control

in the undervoltage region (practically this means the high load hours), the reactive power transfer

will increase the power losses both in the generation and consumption periods. To understand the

magnitude of potential energy savings in study case 1 for 20% PV installations one intuitivecalculation is given. Subtracting the total power losses at 20% PV from the value at 0% PV more

than 800 kWh can be saved per year. With 957 10/0.4 kV stations on Bornholm [24] with the same

rated power as here, a 20% PV penetration would save 766 MWh energy per year, which

corresponds to the average consumption of 220 households.

The active power transfer at the MV side of the distribution transformer is displayed in

Figure 23.Minimum, maximum and average values are shown. It can be observed that there are

high differences between the average and the minimum/maximum recorded values, especially in

cases of high PV penetration.

Figure 23: Active power transfer through 10/0.4 kV transformerStudy case 1


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The reactive power transfer at the MV side of the distribution transformer is displayed in

Figure 24.Since the load profile is not changing there are no visible modifications to the reactive

power consumption during a full day.

Figure 24: Reactive power transfer through 10/0.4 kV transformerStudy case 1


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3.2

Overview of comparisons between the simulated results

Starting from this base case, several comparisons have been performed on the obtained

results of all of the simulated study cases. A synthesis of the contents of this Section is shown in

Table 7.

Study

case

Deadband

evaluation

Voltagesensitivity

w&w/oQ(U)

100/160/200kVA

NoQ(U)

100/160/200kVA

Q(U)

100/160/200kVA

PF(P)

Q(U)vs.PF(P)

1 X X X x

2 X X X x

3 X

4 X

5 X

6 X

7 X

8 X

9 X

10 X

11 X x

12 X

13 X

Table 7: Comparisons performed between different study cases

Figure 25 displays a comparison of all study cases regarding the active power losses of the

distribution transformer. It can be observed that no significant changes are present. For the NO

PV case the power losses are around 2.77 MWh for a 100 kVA unit, 3.8 MWh for a 160 kVA unit

and 4.5 MWh for a 200 kVA unit. It can be seen that by increasing the PV power penetration, a

minimum of the transformer losses is achieved at the 20% PV level, irrespective of the simulated

study case. Even for a 30% PV generation level, the transformer losses are kept below the NO PV

case, while at 40% a slight increase is observed.


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3.3

Varying the dead-band of the Q(U) controller

By varying the dead band of the Q(U) controller, one may get a good impression of the

influence this dead band has upon the voltage regulation capability of the inverters. Figure 27

shows the Q(U) scatter plots for the two applied dead bands (+/-2% and +/-4%) and the case withno dead band selected for the inverter at the outermost distribution box. It can be easily observed

that the bigger the dead-band the steeper the Q(U) curve will be in the active region. Additionally,

the plot shows also that by applying a dead band, the maximum voltage (during one year) will

increase at that specific inverter terminals. Still, even for an 8% dead band the maximum voltage

will still be lower than the maximum in the case of no Volt-VAr control.

Figure 27: Q(U) locus plots for variable Q(U) dead bandfarthest inverter

A good overview of the influence of different dead band selection is seen inFigure 28.It can

be observed that in the case of using a Q(U) control scheme with no dead band, the highest

voltage regulation capability is obtained at the outermost distribution box (case (b) shown in

figure). Therefore, if comparing the case of having no Volt-VAr control (a) with the case (b) the PV

penetration level can be increased from below 30% to more than 40%. If applying a dead band (as

in cases (c) and (d)) the efficacy of the reactive power control method decreases slightly but still

providing a higher PV penetration level than in the base case (a). Note that the minimum voltage

level in case of Q(U) is increased in Figure 28, since the inverters also exchange reactive power

with the network during the night (corresponding to the locus in the left half side ofFigure 27).


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Figure 28: Comparison of different deadband settings - voltage locus for specific points in the network (with

variable PV generation)

Figure 29 displays a comparison between the transformer yearly power losses when varying

the dead-band of the Q(U) controller. As it is expected, a larger dead-band would result in lower

losses in the transformer. For the case with no dead-band and a 40% PV penetration level there is

an increase in the transformer loss of only 7 % compared to Study case 1 No Volt VAr Control.

Figure 29: LV transformer power losses for variable Q(U) dead band study cases


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The total cable losses are shown in Figure 30.As expected, by adding a dead-band to the

controller the total power losses are being decreased compared to Study case 2 Q(U) no dead-

band. At 40% PV penetration level an increase of around 35% is observed compared to Study

case 1No Volt VAr Control

Figure 30: LV Cable system total power losses for variable Q(U) dead bands

3.4

Varying the voltage sensitivity of the Q(U) controller

By varying the voltage sensitivity of the Q(U) controller a less steeper reference curve is

programmed in the inverter, resulting in a lower reactive power injection for the same variation of

input voltage. The voltage sensitivity m (as defined inEquation 2)of the compared study cases is

shown inTable 8.

Study case number Voltage sensitivity m

1 0.0

2 6.0

5 4.8

6 4.0

Table 8: Chosen voltage sensitivities for compared cases

This can be easily observed inFigure 31.The consequence is that the PV inverter will be able

to control less the terminal voltage, reaching a maximum of 2% voltage decrease when comparing

study case 1 and 6.


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Figure 31: Q(U) locus plots for variable voltage sensitivityfarthest inverter

An overview of the influence of variable voltage sensitivity settings towards the maximum

and minimum simulated voltage levels is shown inFigure 32.It is observed that by lowering the

Q(U) slope of the Q(U) characteristic the maximum observed voltage at the outermost distribution

box increases.

Figure 32: Comparison of different voltage sensitivity settings - voltage locus for specific points in the

network (with variable PV generation)


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Figure 33 describes the transformer losses for variable Q(U) voltage sensitivity. The results

are similar between each other, a decrease in total power losses being observed for decreasing

sensitivity levels and high PV generation levels. For smaller PV penetration levels there are no

significant differences, since the reactive power flow is also at minimum.

Figure 33: LV transformer power losses for variable voltage sensitivity study cases

The LV cable system power losses are shown in Figure 34. While for low PV penetrationlevels the total losses are decreased by using Q(U) control, from the 30% generation level there

are increased losses in comparison to the No Volt VAr Control case. For a 60% PV penetration

the cable losses are doubling between Study case 1 and 2. By decreasing the voltage sensitivity

(Study cases 3 and 4) the power losses in the cables are also decreased.

Figure 34: LV cable system losses for variable voltage sensitivity


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Several diagrams of the reactive power transfer for variable voltage sensitivity and different

PV penetration levels are displayed in Figure 35,Figure 36 and Figure 37.By analysing all these

figures, it is observed that a minimum of reactive power transfer (on average) is achieved for PV

penetration levels from 10 to 30%. If observing the curve for Study case 2 with PV generation

levels of 10% inFigure 35, it can be seen that it provides the minimum reactive power transferround the clock at the transformer site. If the PV generation increases, there will be an even

larger amount of inductive reactive power transfer at the MV side of the transformer during the

noon hours, and a capacitive reactive power transfer during the high load hours in the evening.

Figure 35: Reactive power at MV side of transformer for varying voltage sensitivity of Q(U) (0,10,20%PV)



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3.5 Comparison for varying the transformer power rating with and without Q(U)

voltage control

The influence of upgrading the 10/0.4 kV transformer upon the maximum/minimum voltage

limits is shown in Figure 38. Comparing base case (a) 100 kVA unit with a higher ratingtransformer it can be observed that the maximum simulated voltage at the outermost distribution

box is slightly increasing while the minimum value is slightly decreasing. The overall effect is that

although solving the overloading problem, the voltage at the feeder ends may slightly increase.

Figure 38: Comparison of normal operation (No Q control) with different LV/MV transformers - voltage locus

for specific points in the network (with variable PV generation)


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An overview of the influence of upgrading the 10/0.4 kV transformer towards the maximum

and minimum simulated voltage levels is shown inFigure 39.

Figure 39: Comparison of Q(U) with different LV/MV transformers - voltage locus for specific points in the

network (with variable PV generation)

Figure 40 displays a comparison between the transformer yearly power losses when using

Q(U) and upgrading the transformer power rating.

Figure 40: LV transformer power losses for variable Q(U) dead band study cases

200 kVA

160 kVA

100 kVA


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3.6

Comparison for varying the transformer power rating with PF(P) voltage

control

Using a power factor depending on the injected PV power level, PF(P), has the primary

objective to decrease the voltage level only at the moment the PV inverter is producing power.This has the advantage that the inverter will not produce reactive power outside the production

times and also it will be independent of the location of the inverter in respect to the distribution

transformer. One disadvantage may be that it will inject reactive power even at times when it is

not necessary e.g. voltage is well within limits. Study cases 11, 12 and 13 use inverter models with

the PF(P) reactive power control.

The PF(P) scatter plot is shown in Figure 41. With small exceptions there is no significant

difference when using each of the three types of transformers.

Figure 41: PF(P) locus plot of farthest located inverter

The transformer power losses can be observed inFigure 42.It is obvious that by upgrading

the MV/LV transformer the overall power losses will increase due to higher no-load losses. By

upgrading to a 200 kVA from 100 kVA transformer, the transformer losses will increase by 50%.


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Figure 42: Losses in the 10/0.4 transformer - Comparison for different power ratings of transformer and use of PF(P)

control

As it was expected, the power losses on the cables have no significant changes irrespective

of the PV generation level, as shown inFigure 43.

Figure 43: Losses in the LV cables - Comparison for different power ratings of transformer and use of PF(P) control

200 kVA

160 kVA

100 kVA


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3.7

Comparison of Q(U) and PF(P)

A small comparison between the efficacy of Q(U) and PF(P) control algorithms is shown in

Figure 44.In all displayed cases the transformer used was fixed to a 100 kVA rating. Plot set (a)

corresponds to Study Case 1 and represents the voltage levels in different points of the networkwithout using any reactive power control strategies. Plot set (b) corresponds to Study Case 2 and

uses a higher maximum reactive power generation level than the PF(P) plot set (d). Finally plot set

(c) corresponds to Study Case 6 which has a similar maximum reactive power generation level

compared to PF(P). Both Q(U) control cases are contributing to lift the minimum voltage at the

outermost distribution box while PF(P) has no influence.

Figure 44: Comparison of Q(U) and PF(P) - voltage locus for specific points in the network (with variable PV

generation)

Figure 45 displays the transformer power losses in the power transformer. If comparing both

Q(U) methods (Study case 2 and 6) with the PF(P) case it is observed that minimum power loss is

achieved by using PF(P). This fact is normal since both Q(U) cases are injecting reactive power also

during night (round the clock operation). Also note that by using Q(U) at night time, the

minimum voltage at the outermost distribution box is increased. Thus Q(U) could be used at nightin combination with lowering the tap-position of the MV/LV transformer. Q(U) at night could also

be used to compensate for nightly heavy duty loads, e.g. heat pumps and electrical vehicles. For a

60% PV penetration level, the PF(P) algorithm increases losses by 3%, the Q(U) study case 6 by

13% and Q(U) study case 2 by 20%.


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Figure 45: Losses in the 10/0.4 transformer - Comparison between the use of PF(P) and Q(U) control

The losses in the LV cables when using PF(P) and Q(U) control types is shown in Figure 46.

The same dependency as in the case of transformer power losses fromFigure 45 is observed. In

the case of 60% PV generation, for PF(P) control the losses increase by 8%, for Q(U) study case 6

by 60% and for Q(U) study case 2 by 95%.

Figure 46: Losses in the LV cables - Comparison between the use of PF(P) and Q(U) control


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3.8

Comparison of hosting capacity for the cases

The results from 13 study-cases are compiled inTable 9.The cases are rearranged in order of

hosting capacity. The first column shows the hosting capacity for the particular case seen from an

overvoltage and a transformer loading perspective; the second is the case-number and the third isa short description of the case. The fourth column is the maximum amount of exchanged reactive

power for the inverters at the outermost distribution box and the fifth column is the

corresponding PF. The sixth column shows the yearly energy losses in the transformer and cables

and finally the seventh column shows the maximum exchanged reactive power through the

transformer into the MV network.

Hosting capacity

[%]

O.V. / Trafo

Study

Case

Note Maximum Q

outermost

PV inverter

[p.u.]

Minimum PF

outermost PV

inverter

(lagging)

Yearly

energy

loss

[MWh]

Maximum hourly

reactive power

exchange through

transformer [kVArh]

30% 40-50% 1 Base case 0.0 1.00 5.5 -30 (ind)

30% n.a. 7 160 kVA trafo 0.0 1.00 6.6 -30 (ind)

30% n.a. 9 200 kVA trafo 0.0 1.00 7.3 -30 (ind)

35% ~40% 11 PF(P) -0.25 0.97 6.0 -53 (ind)

35% n.a. 12 160 kVA trafo

PF(P)

-0.25 0.97 7.1 -52 (ind)

35% n.a. 13 200 kVA trafo

PF(P)

-0.25 0.97 8.6 -53 (ind)

40% 30-40% 6 Q(U)

low sens.

-0.4 0.92 7.0 -66 (ind)

40% ~40% 4 Q(U)

8% dead band

-0.6 0.8 6.8 -72 (ind)

40% 30-40% 5 Q(U)

mid sens.

-0.5 0.87 7.2 -74 (ind)

40% n.a. 10 200 kVA

Q(U)

-0.6 0.8 9.5 -91 (ind)

45% 30-40% 3 Q(U)

4% dead band

-0.6 0.8 8.2 -93 (ind)

45% n.a. 8 160 kVA

Q(U)

-0.6 0.8 10 -104 (ind)

47% 30-40% 2 Q(U) -0.6 0.8 10 -98 (ind)

Table 9: Comparisons of hosting capacity for the 13 cases, in increasing order. For the loading of the

transformer: a range is given when the sum of overloading hours is higher than 86 hours per year and an

approximate value is given when the sum is below 86 hours per year.

Case number 1 is the first entry with a hosting capacity of 30% (1.5 kWp per residence)

without taking any reactive power control measures into consideration. Increasing the capacity of

the transformer (cases 7 and 9) does not increase the hosting capacity when overvoltage is the

issue. Thus, case 1 is marked green as the optimum solution for the 30% penetration.

The usage of the standard PF(P) in case 11 increases the hosting capacity to 35% (1.8 kWp

per residence). The maximum hourly exchange of reactive power is increased from 30 to 53 kVArh

and the yearly losses in the transformer and cables are increased with 500 kWh. Again, using the

PF(P) with large transformers does not increase the hosting capacity. Therefore, case 11 is marked

green as the optimum solution for the 35% penetration.


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The hosting capacity is increased further to 40% (2.0 kWp per residence) in case 6 by

applying Q(U) control with a low sensibility. This could also be achieved with the Q(U) scheme with

high sensibility and 8% dead-band, but this would increase the exchange of reactive power

through the transformer. The yearly energy loss is increased with 1.5 MWh compared with the

base case and using a large transformer does not have any impact on the hosting capacity.Consequently, case 6 is marked green as the optimum solution for the 40% penetration.

A hosting capacity of 45% (2.3 kWp per residence) requires Q(U) control with high sensitivity

and 4% dead-band as in case 3 and can be further increased a few per cent by removing the dead-

bands as in case 2. However, the transformer is being overloaded to some extent.

The hosting capacity for the 100 kVA transformer is in section3.1 found to minimum 40%

and maximum 50%, corresponding to a minimum of 140 kW PV systems, when the PV systems are

pointing south. If the PV systems are scattered, the hosting capacity is increased to 45%, which

corresponds to a total of 160 kW PV systems. In both cases, no voltage control is applied. The

PF(P) control scheme decreases the hosting capacity of the transformer to around 40% and usingthe Q(U) with low sensitivity reduces it even further to minimum 30% and maximum 40%. The

scale 30% - 50% corresponds to 120 kWp200 kWp when the systems are scattered.

A PV penetration of maximum 50% is possible without overloading the LV cables,

corresponding to 180 kW PV systems, when no voltage control is applied. Applying voltage control

will decrease the hosting capacity of the LV cables due to the additional reactive current.

According to [24], there are 957 MV/LV transformers on Bornholm with a total of 28 000

private and commercial customers. The peak demand is 55 MW and the yearly energy

consumption is 268 GWh. When each of the 28 000 customers have a PV system and by using the

results obtained in this research, the hosting capacity of the LV feeders on Bornholm is in the

range 4256 MW, when all systems are pointing south. If the PV systems would be scattered, the

range is 4864 MW. Assuming that each of the 957 transformers can host 140 160 kW PV, the

hosting capacity of the transformers is within the range 130 MW 150 MW. Summing up, the

hosting capacity of the LV networks and MV/LV transformers on the island of Bornholm is in the

range 50 60 MW, which could cover the peak demand (during noon hours) and 15-20% of the

yearly energy consumption.


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Another interesting result is the advantage of using Q(U) feature of the PV inverters even for

low PV penetration levels. For example, at 20% PV generation, the PV inverters can supply reactive

power to AC loads (even at evening hours) and thus minimizing the reactive power flow in the

network. As a consequence, the power losses in the transformer and cable system are lowered

slightly.

The results indicate that without any Volt-VAr control the overvoltage phenomena starts for

PV generation levels at 30%, corresponding to 1.5 kW PV per residence.

By applying a PF(P) control scheme as in study case 11, the overvoltage condition is avoided

and the PV penetration can be increased to 35% and still keep the amount of exchanged reactive

power low. By applying a Q(U) control scheme with a low sensitivity as in study case 6, the

overvoltage condition is avoided up to 40% PV penetration and still keeping the power factor

above 0.92. Finally, by using Q(U) with high sensitivity and dead-band, study case 3, the

penetration can be increased further to 45% and perhaps also to 50% if the dead-band is removed

as in study case 2. However, the yearly losses in the transformer and cable system are almostdoubled compared with the base case with only 30% penetration.

One more conclusion that can be drawn is that the overvoltage phenomena is not overcome

by upgrading the transformer, on the contrary increasing the size of the transformer has a slight

negative effect. Transformer upgrading should be taken into consideration only with the purpose

of mitigating the transformer overloading issues.

If reactive power control is provided round the clock then the PV inverters can help

controlling the voltage during high load hours, e.g. during charging of electrical vehicles or when

the heat pumps are turned on in order to utilize free energy from the wind -turbines. For the

case of 40% PV penetration the minimum yearly simulated voltage has improved from 0.91 p.u. to

0.93 p.u. (an increase of 2%-point). This feature may allow the PV penetration level to be

increased even further by lowering the tap position in the 10/0.4 distribution transformer

provided that sufficient PV capacity is installed in order to provide reactive power during the load

peaks.

For PV generation levels lower than and including 30% the 100 kVA transformer will not be

subjected to overloading. The peak-load of the transformer is reduced for PV penetrations up to

20%. This could potentially increase the life-time of the transformer. For 40% PV penetration

levels, a limited amount of overloading hours are observed, thus having a limited impact over the

lifetime of the transformer. The hosting capacity of the transformer is therefore around 40%,

corresponding to 142 kW PV power, when all PV systems are pointing south. When the PV systemsare scattered, the hosting capacity could increase up to approximately 160 kW, corresponding to a

45% PV penetration. For higher PV penetration levels, additional solutions must be applied:

increased self-consumption, storage solutions, transformer upgrading or active power curtailment.


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4.2

Recommendations for voltage control in LV feeders

As a guiding rule, if overvoltage is observed at the end-customer site the following order of

actions should be followed:

1.

Apply PF(P) for all inverters on the feeder2. Apply Q(U) for all inverters on the feeder

3. Lower the tap position in the LV/MV transformer and apply Q(U) round the clock

4. Increase self-consumption at peak production hours

5. Lower active output power of inverters (only in emergency cases for short periods)

6. Upgrade cables, upgrading the transformer does not help

7. Install storage

Overall, it has been found that applying reactive power control strategies in the LV networks

have a beneficial influence in respect to a limited increase of the PV penetration. Both PF(P) and

Q(U) control types are improving the maximum voltage profile at the outermost distribution box.

Only by using Q(U) round the clock, thus by providing capacitive reactive power during high AC

load hours, it is possible to provide an improvement of the minimum voltage profile.

4.3

Future Work

The work presented in this report provides a starting point for other more in depth analyses.

One example is redoing the simulations of Chapter2.8 on a large number of grid types. Such

an exhaustive approach would offer a much better indication of the actual efficacy of the reactive

power control techniques for many types of LV networks. Moreover, one reactive power control

technique may be recommended for one network type while it would have negative impact on a

different type. Another possibility may come from the need of applying different control strategiesdepending on seasonal, daily and hourly schedules.

Finding an optimum generic Q(U) curve could also be investigated. For this reason, one

suggestion is shown in Figure 47. The main objective of the curve is to provide higher voltage

sensitivities to the inverter connected closer to the transformer terminals.

U

Q

96,0

102.5,0

105,33

110,44

Figure 47: Proposed Q(U) curve for further analysis


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Several more study cases could be performed on Q(U) and PF(P) where not only one PF(P)

reference is given but several. Each PF(P) curve could then be compared with its similar

counterpart in terms of maximum reactive power contribution. Additionally, several Q(U) curves

with dead band and fixed voltage sensitivity (varying end point) could be selected and analysed.

One more study case could be performed with a different set value for the fixed tap position ofthe 10/0.4 kV transformer, e.g. neutral position, together with Q(U). Also Q(U) and PF(P) curves

which generates the same amount of reactive energy should be compared, as well as curves which

have the same limits for exchange of reactive power.

One additional future study direction is the analysis of more advanced reactive power

control algorithms such as Q(U,P) or PF(U,P) documented in [5]. An optimized control would

ensure minimum reactive power injection from the PV inverters to avoid overvoltage phenomena

and thus would minimize the power losses in the AC cables.

As another possible future analysis, the same subject can be studied with the inclusion of

new and modern types of consumers. Heat pumps and electrical vehicles (EV) are showing strongmomentum in mass deployment. The influence of these consumer types towards the PV inverter

performance can be thus analysed. A more detailed modelling of the EV units can be developed

and thus smart storage control algorithms may be included.

This report provides a starting point for additional economic analyses on the total cost of

using reactive power control methods in LV networks in order to increase the PV penetration level.

These cost analyses may include the cost of energy losses in the power network, the transformer

lifetime expense when temporarily overloading it, as well as the amount of power factor

correction capacity in the 60 kV station to cover peak generation hours. A comparison with the

cost of upgrading the network in order to accommodate more PV can also be performed. This also

includes the changes in transformer life-time as function of the PV penetration.


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5Works Cited

[1] Dansk Energi, Energinet, Dong Energy, Scenarier for solcelle udrulning i Danmark, 2012.

[2] S. F. Jensen, Solceller Nu og i fremtiden, 30 01 2012. [Online]. Available:

http://www.dansksolcelleforening.dk/FileArchive/Generel%20orientering%20fra%20Energinet

.dk.pdf. [Accessed 19 07 2012].

[3] EnergiNet, EnergiNet DK news, 29 10 2012. [Online]. Available:

http://energinet.dk/EN/El/Nyheder/Sider/Naesten-20000-familier-har-faaet-solceller-i-

2012.aspx. [Accessed 29 10 2012].

[4] R. D. Lazar and A. Constantin, Voltage Balancing in LV Residential Networks by Means of ThreePhase PV Inverters, Danfoss Solar Inverters A/S Denmark: EUPVSEC 2012, 2012.

[5] Erhan Demirok, Dezso Sera, Remus Teodorescu, Kenn H. B. Frederiksen, An optimized local

reactive power control for high penetration of distributed solar inverters in low voltage

networks, in EUPVSEC, 2011.

[6] Dansk Energi, Solceller og spaendingsvariationer i 0.4 kV net, Copenhagen, 2012.

[7] P. Esslinger, Studie Q(U), Technischen Universitt Mnchen, Mnchen, 2012.

[8] G. Kerber and R. S. H. Witzmann, Voltage Limitation by Autonomous Reactive Power Control of

Grid Connected Photovoltaic Inverters, IEEE Catalog Number: CFP09851; Conference on

Compatibility and Power Electronics, pp. 129-133, May 20 - 22, 2009.

[9] ABB, Case Study: Voltage regulation for utility grids with distributed generation, [Online].

Available:

http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/8cb3000da66fdcaf4825792000

6e4efe/$file/2ucd301118_a%20pcs100%20avr%20case%20study%20-%20english%20vers.pdf.

[Accessed 19 07 2012].

[10] C. Krner, M. Hennig, R. Schmid and K. Handt, Gaining experience with a regulated

distribution transformer in a smart grid environment, in CIRED Workshop, 2012.

[11] A. Einfalt, F. Kupzog and H. Brunner, Control Strategies for Smart Low Voltage Grids The

Project DG DemoNetSmart LV Grid, in CIRED Workshop, 2012.

[12] R. Tonkoski, . L. Lopes and . T. El-Fouly, Coordinated Active Power Curtailment of Grid

Connected PV Inverters for Overvoltage Prevention, Sustainable Energy, IEEE Transactions on,

vol. 2, no. 2, pp. 139 - 147 , April 2011.


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55

[13] L. Chia-Hung, . H. Wei-Lin, C. Chao-Shun and H. Cheng-Ting , Optimization of Photovoltaic

Penetration in Distribution Systems Considering Annual Duration Curve of Solar Irradiation,

Power Systems, IEEE Transactions on, vol. 27, no. 2, pp. 1090 - 1097 , 2012.

[14] C. Gaudin, A. Ballanti and E. Lejay, Evaluation of PV curtailment option to optimize PV

integration in Distribution Network, in CIRED Workshop, 2012.

[15] SMA Energie AG, SMA SMART HOME, 01 06 2012. [Online]. Available:

http://files.sma.de/dl/2485/SMART_HOME-KEN123012.pdf. [Accessed 19 07 2012].

[16] European Photovoltaic Industry Association, Connecting the sun - Solar Photovoltaic on the

road to large scale grid integration, Brussels, 2012.

[17] BDEW, Technical guideline: Generating plants connected to the Medium Voltage Network,2008.

[18] ENBW AG Deutschland, Vorgaben der ENBW Regional AG zur Blindleistungs-

/Spannungskennlinie Q(U) fur Erzeugungsanlagen am Mittelspannungnetz, 2011.

[19] EN, Standard EN 50160 - Voltage Characteristics in Public Distribution Systems, 2004.

[20] IEC, Draft IEC 61850-90-7 TR - Communication networks and systems for power utility

automation - Part 90-7: IEC 61850 object models for photovoltaic, storage, and other DER

inverters, 2012.

[21] S. Bkhj Kjr, R. Lazar, A. Constantin, U. Borup, H. P. Ballegaard, J. stergaard, G.-Y. Yang, C.

Stephansen, M. Rolsted Mattesen and J. Bergholdt, Application of smartgrid in photovoltaic

power systems on the island of BornholmPVNET.DK, in EUPVSEC, Hamburg, 2011.

[22] Dansk Energi, Danish Electricity Supply - Statistical Survey '09, 2010.

[23] FNN, VDE-AR-N 4105:2011-08 Power generation systems connected to the low-voltage

distribution network - Technical minimum requirements for the connection to and parallel

operation with low-voltage distribution networks,August 2011.

[24] M. R. Mattesen, G. Y. Yang, S. B. Kjr, R. D. Lazar, A. Constantin, J. stergaard and C.

Stephansen, Analysis of Thevenin Equivalent Network of a Distribution System for Solar

Integration Studies, in ISGT EU, Berlin, 2012.

[25] PVsyst , PVsyst - Photovoltaic software, PVsyst - Photovoltaic software, [Online]. Available:

http://www.pvsyst.com/en/. [Accessed 11 2012].


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6Appendix ALow voltage cable parameters:

Cable name Busbar1 Busbar2 Type Length in km

125LV_528 125LV 528 4X150 AL-M PEX 0,084

125LV_533 125LV 533 4X95 AL-M PVIKS 0,03

528_529 528 529 4X50 AL-M PEX 0,105

528_1046 528 1046 4X50 AL-M PEX 0,033

533_535 533 535 4X95 AL-M PVIKS 0,047

533_534 533 534 4X50 AL-M PVIKS 0,103

529_530 529 530 4X50 AL-M PEX 0,053

529_531 529 531 4X95 AL PVIKS 0,076

529_10150 529 10150 4X150 AL-M PEX 0,055

529_10157 529 10157 4X150 AL-M PEX 0,003

10146_10147 10146 10147 4X50 AL-M PEX 0,068

535_536 535 536 4X95 AL-M PVIKS 0,068

531_532 531 532 4X50 AL-M PEX 0,076

531_540 531 540 4X50 AL-M PEX 0,056

10150_10151 10150 10151 4X150 AL-M PEX 0,07

10147_10148 10147 10148 4X50 AL-M PEX 0,072

540_10149 540 10149 4X150 AL-M PEX 0,032

10151_10152 10151 10152 4X150 AL-M PEX 0,026

10151_10156 10151 10156 4X50

control de voltaje en baja tenson con inversores y redes fotovoltaicas.pdf

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