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Voltage Control in Networks with Distributed Generation – A Case Study B. Blaži, Member, IEEE, T. Pfajfar, Member, IEEE, and I. Papi, Senior Member, IEEE
Abstract—This paper analyzes the influence of distributed energy resources (DER) on distribution network voltage profile. A general description of the impact of DER on voltage profile is given and three advanced voltage control techniques are discussed: central voltage control with multiple voltage measurements, reactive power control with sources that enable such operation and the use of an autotransformer as a voltage regulator. The aim was to maximize and to facilitate DER integration. The influence of DER on voltage profile and the effectiveness of the investigated solutions were evaluated by means of simulation in DIgSILENT. The simulated network was an actual distribution network in Slovenia with a relatively high penetration of distributed generation. Recommendations for voltage control in networks with DER penetration are given at the end. Index Terms—Distributed energy resources, distribution networks, power quality, voltage control, voltage profile.
I. INTRODUCTION
D
ISTRIBUTION utilities have to maintain network voltage between tight limits [1], which is essential for correct operation of customer loads. Distributed energy production leads to variations of voltage profile [2], [3]. Due to bidirectional and fluctuating active power and altered reactive power flows, along with insufficient control, the system voltage may increase or decrease over the set limits. Moreover, most distribution system operators require distributed energy resources (DER) to operate with a constant power factor (or at zero reactive power) and DER usually do not provide any ancillary services to the network. In such circumstances traditional voltage regulation schemes may fail to ensure the required voltage levels. This paper presents a study case of an actual distribution network in Slovenia with a relatively high penetration of distributed generation. A general description of the impact of DER on voltage profile is outlined first [3], [4]. Next, the classic approach to voltage regulation in distribution networks is described and some methods for advanced voltage control are presented. The following advanced voltage control methods are described next: centralized voltage regulation B. Blaži is with the Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia (e-mail:
[email protected]). T. Pfajfar is with the Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia (e-mail:
[email protected]). I. Papi is with the Faculty of Electrical Engineering, University of Ljubljana, Ljubljana, Slovenia (e-mail:
[email protected]).
with multiple-point voltage measurements [5], [6], DER voltage control through reactive power exchange [7] and the use of an additional voltage regulating transformer [8]. The solutions are evaluated with simulations of the analyzed network and recommendations for efficient voltage control are given at the end. II. DER IMPACT ON VOLTAGE PROFILE Fig. 1 shows a simple circuit with DER connected to the low-voltage (LV) feeder. The feeder is connected to mediumvoltage (MV) and high-voltage (HV) levels through two transformers. The HV/MV transformer is usually an on-load tap-changing (OLTC) transformer, while the MV/LV transformers have off-circuit taps. Voltage profile of the presented network is also shown in Fig. 1. The MV level is usually regulated to a fixed value. Voltage boost of the MV/LV transformer is also shown. The two lines show the voltage profile with DER connected and disconnected. Depending on the DER production and load consumption also a voltage rise may occur in the network. The voltage rise can be evaluated with the equivalent circuit shown in Fig. 2 [3]. The circuit shows a load (consumption PL and QL), a source (production PDER and QDER) and reactive power compensation at the DER site (QC). US and UR are the sending and receiving end voltage. R and X are the resistance and reactance between the transformer busbars and DER connection point. The voltage US can be calculated as: U S U R ( R jX ) I R (1)
Fig. 1. Voltage profile of a radial distribution line with DER
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III. VOLTAGE CONTROL IN NETWORKS WITH DER
Fig. 2. Simplified equivalent circuit of DER connection to the network
Expressing IR with PR and QR one gets:
UR
US
RPR XQR U *R
j
XPR RQR U *R
(2)
If we choose the voltage at DER busbar as the reference voltage U R U R 0q then (2) can be simplified to:
RPR XQR XP RQR j R UR UR
UR
US
UR
U S 'U jG U
(3)
The voltage phasor diagram according to (3) is shown in Fig. 3.
In a typical regulation scheme the voltage is controlled with the OLTC HV/MV transformer with the automatic voltage control (AVC) relay. The ratio change is typically within ±10 % and ±15 % in steps of 0.6 - 2.5 %. OLTC control is mostly based on only one substation voltage measurement point. Such scheme is robust and performs well in most operating conditions. However, it may fail if DER sources are connected in the network as they may cause voltage rise on a particular feeder. With the AVC relay line drop compensation (LDC) can also be used. The technique is illustrated in Fig. 4. In addition to voltage US measurement at the substation, the AVC relay also receives a signal proportional the load current (Is). This current is used to estimate the voltage drop to a remote network point. The estimation is made with appropriate selection of the drop impedance R+jX. Ideally, the voltage calculated with LDC is equal to the voltage at the load connection point (UR). Therefore the OLTC actually regulates the voltage at a remote network location. A mayor practical difficulty with the line drop compensation is that loads are distributed among many feeders and are often spread along the entire length of the line. The inclusion of DER only complicates the situation further on. Due to usual complex distribution network structures it is difficult to adjust the linedrop compensation to suit all network points. HV network
OLTC transformer
US CT
RL
XL
UR Load
Line
Down
AVC relay
Up
Tap changer IS
VT
Fig. 3. Voltage phasor diagram
X Time delay +
The receiving end voltage amplitude is mostly influenced by the in-phase component U: U R | US
RPR XQR , UR
RPDER . UR
-
R Uc +
-
-
(4) Fig. 4. AVC relay with the line drop compensation
where PR=PL-PDER and QR=±QDER+QL-QC. If we assume an extreme case, i.e. no-load conditions (PL=QL=0) and DER operation with unity power factor (QDER=0) then: U R | US
Deadband
Uref
(4)
According to (4) the voltage at DER connection point can be higher than the transformer busbar voltage. Taking into account the mentioned assumptions the voltage rise depends on DER active power generation. Different strategies for a more efficient voltage control are described next.
A. Advanced voltage regulation Several approaches can be used to control voltage profile in case of DER penetration. The simplest methods include rules limiting the maximum DER share in a particular network. These rules may give indications on how much power generation can be installed on various voltage levels, or may require that the short-circuit level at the point of connection is a minimum multiple of DER rating. However, such simple measures are usually rather restrictive and represent a barrier for wider DER employment. One of the ‘traditional’ approaches is also network reinforcement, e.g. replacement of cables or lines with lower impedance ones [9]. Despite this is a relatively costly solution it may be sensible in cases of regular equipment replacements.
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One of the more advanced voltage control techniques includes OLTC voltage control with multiple voltage measurement points. This of course requires an ICT infrastructure and algorithms that enable to maintain voltage within limits at all measurement points. However, in some cases it is not possible to maintain voltage at all network points within levels. Another option is also the control of DER reactive power generation. With reactive power voltage can be controlled to some extent but on the other hand it also contributes to energy losses. One option is also the use of passive or active voltage regulators. A passive regulator is for example an autotransformer with a transformation ratio of 1 and variable taps. Such transformer enables to split the network in two sections. Active regulators are based on power semiconductors (often called custom power equipment) and enable also voltage control. All means of voltage control in a distribution system must be properly coordinated. In the next section the case study is presented. IV. CASE STUDY For evaluation of the effect of DER on network voltage profile and investigation of the described voltage regulation concepts a section of a distribution network at 20 kV voltage level was analyzed. The network includes a large share of small hydro power plants (sHPP) which may be the cause for voltage profile problems.
Fig. 5. Single line diagram of the analyzed network 1.2 1 0.8 P (p.u.)
A. Network and simulation model descriptions The single-line diagram of the analyzed network is shown in Fig. 5. The network was modeled in the DIgSILENT simulation program. Five points where voltage was observed are marked with MP1 – MP5. The 20 kV network is connected to the HV level (fault level at 110 kV was 2200 MVA) through a 20 MVA transformer equipped with an OLTC (regulation ± 12 %, step 1.33 %). The AVR regulates the voltage at 20 kV and maintains its value between 103.0 % and 104.5 % of the nominal voltage, i.e. 20 kV. The voltage reference is set to 103.75 %. The AVR was included in the simulation model. There are 13 sHPP’s connected in the area. The total installed power is around 2.5 MVA. Two of the plants are equipped with synchronous generators (nominal powers 810 kVA and 594 kVA) while others use induction generators. All generators are connected to the 20 kV level thorough 20/0.4 kV transformers. The synchronous generators were operating with a constant power factor cos = 1.00. Loads were modeled as R-X impedances that are also voltage dependent (in the voltage range 80 % - 120 % of the nominal value). Based on the loading measurements data (10minute intervals) four different daily load patterns (active power) were developed. One of the patterns is shown in Fig. 6. For all loads a power factor cos=0.95 was presumed. Substation loads were modeled as single impedances at 20 kV voltage level. Simulation results are presented next.
0.6 0.4 0.2 0
Time
Fig. 6. LV substation daily load pattern
V. SIMULATION RESULTS The aim of simulations was to analyze voltage levels in the network, to estimate the influence of DER on voltage and to evaluate the effectiveness of different voltage regulation schemes. The voltage at MV level was observed with simulations. According to the legislation in Slovenia the voltage in the LV network should not exceed the limits of +6/10 % of the nominal value 230 V. Taking into account that the
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voltage drop on a MV/LV transformer feeding loads and LV feeder can reach 5 % and that the voltage regulation step is 1.33 % we set the MV voltage limits to +5/-5 %. The goal of voltage control techniques therefore was to keep the MV within these limits, and consequently that should enable to maintain the LV within the defined range. Two scenarios are introduced next. In the first one the current network situation is analyzed, and in the second one different voltage control options are evaluated. Simulation length was set to 144 s where each second corresponds to one 10-minute interval. Simulations therefore cover one day. A. Current situation Fig. 7 shows the active and reactive power of the HV/MV transformer feeding the area when no DER are operating. At maximum loading the apparent power reaches 100 % of the transformer nominal power. Fig. 8 shows the voltage at various MV network points. As we can see the voltage at all observed points is within the defined boundaries. The transformer OLTC is operating and the AVR voltage reference is set to 103.75 %. The tap changes are also clearly visible from the voltage curves. Fig. 9 and 10 show the same situation with the exception that all sHPP-s were operating at their nominal power. The total generation reaches 1.7 MW and the maximum transformer loading decreases to 92 %. Voltage profile shows that at two substations the voltage exceeds the upper limit (MP3, MP4), which occurs in low loading conditions. Both substations are located in the area where most of the power plants are connected. This clearly indicates that DER generation increases voltage levels. On the other hand the voltage at substation at bus MP2 is relatively low which means that voltages can not be adjusted only with the adjustment of the HV/MV regulating transformer reference voltage setting. Some options for voltage regulation are discussed in the next section.
Fig. 8. Network voltage – without DER
Fig. 9. Transformer active and reactive power – with DER
Fig. 10. Network voltage – with DER
Fig. 7. Transformer active and reactive power – without DER
B. Options for voltage control Three options are described below: central voltage control, voltage control with reactive power and the use of an additional voltage regulating transformer. One of the advanced options for controlling voltage profile is also central voltage control with multiple voltage
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measurements. In contrast to the classic regulation scheme, where voltage is measured only at one point (MV busbars), in the centralized approach voltage is measured at more points in the distribution network. An appropriate algorithm must be used to define a reference voltage for the HV/MV transformer that enables to maintain network voltages within limits. Therefore the AVR reference voltage changes according to network conditions. Measurements at points MP1 – MP5 were used in the model. The results are shown in Fig. 11. As we can see the control algorithm is capable of maintaining the appropriate voltage in every loading condition. However, the highest and the lowest voltage in particular moments are close to the upper and lower limit respectively, suggesting that further increase of DER in the area could make the approach ineffective. In such cases central voltage control can be used in combination with other methods, e.g. reactive power control or voltage regulating transformers. Both are described next. The next evaluated option is the use of reactive power generation from sHPP-s for voltage control. In our case there are only two synchronous generators which enable reactive power control. Both generators were set to operate with a fixed power factor of cos = 0.90, consuming reactive power. The results are shown in Fig. 12. It can be seen that the reactive power from the two synchronous generators alone can not lower the voltage at MP4 below the upper limit. However, this method would be effective if used in combination with central voltage control. The results of the combined approach are shown in Fig. 13. In comparison to the results in Fig. 11 the node voltages are closer together suggesting there is still enough flexibility for the connection of additional generators. We have to bear in mind also that additional reactive power flow increase network and generator losses. The last evaluated option is the use of an additional voltage regulating transformer. The transformer is basically an autotransformer with voltage ratio equal to 1 that was connected to the feeder between buses MP3 and MP4 where all the generation is connected. The transformer enables the separation of the network into two areas with independent voltage control. The main transformer operates with the classic voltage regulation scheme. The voltage regulating transformer is equipped with regulating taps (+10/-10) with 1 % step. The reference voltage was set to 101 % of the nominal voltage. The results are shown in Fig. 14. The additional voltage transformer enables to keep all voltages at bus MP4 within limits. Despite that the voltage at MP3 is still too high at low loading. In this case the transformer is not capable to solve the voltage rise problem. Serious drawbacks are the needed investment and additional losses. The transformer is most effective when network separation is possible, i.e. where most of DER are located on one feeder. Simulation results show that in some circumstances the network may already be operating with voltages outside the requested limits. Such circumstances include cases of low consumption and high DER generation. The investigated
solutions for voltage control were evaluated by means of simulations. Results have shown that centralized voltage control with multiple voltage measurements is a viable approach that also does not require major infrastructure changes. With continuing DER penetration this may not be enough so that central voltage control could be upgraded with reactive power control from DER sources. The installation of a voltage regulating transformer is probably also too costly in terms of price and additional losses.
Fig. 11. Network voltage – central voltage control
Fig. 12. Network voltage – reactive power control
Fig. 13. Network voltage – central and reactive power voltage control
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Fig. 14. Network voltage – additional voltage regulating transformer
For a more detailed analysis of voltage levels in the network a statistical approach should be used taking into account load and generation profiles [10]. The results would be given as percentages of time with voltages outside the limits. VI. CONCLUSION The paper deals with the problem of voltage profile control that may arise in the case of DER connection to distribution networks. This may bring to a situation where the classic approach to voltage regulation with an OTLC transformer and measurements in one point fails to maintain the voltage within the required limits. Three different approaches for voltage control were presented in the paper: central voltage control with multiple measurements, reactive power control with sources that enable such operation and the use of an autotransformer as a voltage regulator. The aim was to maximize and to facilitate DER integration. Therefore power curtailment was not considered as an option. Simulation results suggest that each distribution network should be analyzed individually in case of DER penetration. In the first step limits have to be set for the maximum penetration that is possible without major changes to the existing voltage profile generation scheme. In next steps measures that enable further DER integration should be analyzed. Future work will comprehend extensive field measurements, calculation of voltage levels with the use of statistical methods and evaluation of losses for different voltage regulation approaches. VII. REFERENCES [1] [2] [3] [4]
EN 50160 standard, "Voltage characteristics in public distribution networks," EN standard, 2000. R.C. Dougan et al., Electrical Power Systems Quality, 2nd ed., New York: McGraw-Hill, 2002. N. Jenkins et al., Embedded generation, IEE, London, UK, 2000. R. O'Gorman and M. Redfern, 'The impact of distributed generation on voltage control in distribution systems," 18th International Conference on Electricity Distribution, Turin, 6-9 June 2005.
Thomson, M., "Automatic voltage-control relays and embedded generation - part 1", Power Engineering Journal, Vol. 14, Issue: 2, June 2000, pp. 71-76. [6] P. N. Vovos, et al., "Centralized and Distributed Voltage Control: Impact on Distributed Generation Penetration," IEEE Trans. on Power Systems, vol. 22, pp. 476-483, Feb. 2007. [7] P. M. S. Carvalho, P. F. Correia, L. Ferreira, "Distributed Reactive Power Generation Control for Voltage Rise Mitigation in Distribution Networks," IEEE Trans. on Power Systems, vol. 23, pp. 766-772, May 2008. [8] F. Kupzog, H. Brunner, W. Pruggler, T. Pfajfar, A. Lugmaier, "DG DemoNet-Concept - A new algorithm for active distribution grid operation facilitating high DG penetration", IEEE International Conference on Industrial Informatics (INDIN), pp. 1197-1202, Vienna, July 2007. [9] C.L. Masters, "Voltage rise: the big issue when connecting embedded generation to long 11 kV overhead lines," Power Engineering Journal, Vol. 16, Issue: 1, Feb. 2002, pp 5-12. [10] F. Demailly, O. Ninet, and A. Even, "Numerical Tools and Models for Monte Carlo Studies of the Influence on Embedded Generation on Voltage Limits in LV Grids," IEEE Trans. on Power Delivery, vol. 20, pp. 2343-2350, July 2005.
VIII. BIOGRAPHIES Boštjan Blaži received the B.Sc., M.Sc. and Ph.D. degrees, all in electrical engineering from the University of Ljubljana, Ljubljana, Slovenia, in 2000, 2003 and 2005, respectively. From 2000 to 2006 he worked as a researcher at the Faculty of Electrical Engineering. Currently he is as an assistant at the same faculty. Beside teaching, his work includes research in the fields of power quality, active compensators, operation of power converters and integration of distributed generation. Tomaž Pfajfar received his B.Sc. degree in electrical engineering from the University of Ljubljana, Slovenia, in 2004. Since 2004 he has been a researcher at the Faculty of Electrical Engineering in Ljubljana. In 2006 he was with Arsenal Research Distributed Generation Group in Vienna, Austria. Currently he is a junior researcher at the Faculty of Electrical Engineering in Ljubljana, Slovenia. His research interests include power quality, distributed generation and active network operation. Igor Papi (S'97-A'99-M'00) received his B.Sc., M.Sc. and Ph.D. degrees, all in electrical engineering, from the University of Ljubljana, Slovenia, in 1992, 1995 and 1998, respectively. From 1994 to 1996 he was with Siemens Power Transmission and Distribution Group in Erlangen, Germany. Currently he is an associate professor at the Faculty of Electrical Engineering in Ljubljana. In 2001 he was a visiting professor at the University of Manitoba in Winnipeg, Canada. His research interests include power quality, power system simulations, control and modeling of FACTS devices and Power Conditioners.
