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Voltage regulation

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Experiment and Simulation of Dynamic Voltage Regulation with Multiple Distributed Energy Resources Yan Xu, Member, IEEE, Fangxing Li, Senior Member, IEEE, John D. Kueck, Senior Member, IEEE, D. Tom Rizy, Senior Member, IEEE Abstract - Distributed energy (DE) resources are power sources located near load centers and equipped with power electronics converters to interface with the grid, therefore it is feasible for DE to provide reactive power (along with active power) locally for dynamic voltage regulation. In this paper, a synchronous condenser and a microturbine with an inverter interface are implemented in parallel in a distribution system to regulate the local voltage. Developed voltage control schemes for the inverter and the synchronous condenser are presented. Experimental results show that both the inverter and the synchronous condenser can regulate the local voltage instantaneously although the dynamic response of the inverter is much faster than the synchronous condenser. In a system with multiple DEs performing local voltage regulation, the interaction between the DEs is studied. The simulation results show the relationship between the voltages in the system and the reactive power required for the voltage regulation. Also, integrated voltage regulation (multiple DEs performing voltage regulation) can increase the voltage regulation capability of DEs and reduce the capital and operating costs. I. INTRODUCTION There are a wide range of ancillary services in the distribution system that can be supplied by distributed energy (DE) resources [1], among which voltage regulation has drawn much interest because of the reactive power shortage and transport problems in power systems. Previously, others have reported that the power electronics interface between the DE and the utility can provide several reactive power services [2]-[3]. The installation of DE and the provision of voltage regulation from DE can have a beneficial impact on transmission stability and distribution system capacity by supplying reactive power at the distribution level [4]. The total installed DE capacity for installations smaller than 5 MW in the U.S. is 195,251 MW, among which the reactive-power-capable DE is estimated at 10% of the total installations [5]. Therefore, there are a large amount of DE resources potentially available for voltage regulation. II. SYSTEM CONFIGURATION A parallel-connected (to the load) DE with power electronics inverter interface is shown in Fig. 1. The interface, including the inverter, the DC side capacitor vdc, and the coupling inductor Lc, is referred to as the compensator because voltage regulation from DE is the main topic of this paper. The compensator is connected in parallel with the utility at the point of common coupling (PCC) vt. By generating or consuming reactive power, the vs Ls Rs is vt (PCC) il Load vt is ic il vdc ic Controller switching signals Lc DE vdc vc Fig. 1. Parallel connection of a DE with power electronics converter interface. Zs vsystem isc vsystem isc Controller M v excitation Excitation Load Fig. 2. System configuration of a synchronous condenser. compensator regulates the PCC voltage vt. The compensator current ic only contains the reactive power component. The system configuration of a parallel-connected synchronous condenser is shown in Fig. 2. The synchronous condenser generates reactive power during over-excitation, and consumes reactive power during under-excitation. By controlling the excitation voltage vexcitation, the synchronous condenser regulates the system voltage vsystem by changing the reactive power it provides or consumes. The synchronous condenser current isc only contains the reactive power component. Two system configurations of the compensator and the synchronous condenser are studied in this paper, which are shown in Figs. 3 and 4. In Fig. 3, the two devices are connected on two different circuits from a 2.4 kV substation. The synchronous condenser is connected to the 480 V Panel A with some other loads through transformer 1, and the compensator is connected to the 480 V Panel B with some loads through transformer 2. In Fig. 4, both the synchronous condenser and the compensator are connected to Panel A. The electrical distance between the compensator and the synchronous condenser in Fig. 4 is less than the case 2.4 kV Substation 2.4 kV/480 V transformer 1 2.4 kV/480 V transformer 2 Panel A Panel B Load 1 Synchronous Condenser Load 2 Inverter Fig. 3. Parallel connection of the two DE on different panels. 2.4 kV Substation 2.4 kV/480 V transformer 1 Panel A 2.4 kV/480 V transformer 2 Panel B Load 1 Inverter Load 2 Synchronous Condenser Fig. 4. Parallel connection of the two DE on the same panel. in Fig. 3, and therefore different voltage regulation methods are used. III. INSTANTANEOUS POWER THEORY An instantaneous nonactive (i.e., reactive) power theory [6] is adopted in this paper for the real-time calculation and control of DE voltage regulation. In the theory, definitions of instantaneous active current, instantaneous reactive current, average active power, average reactive power, apparent active power, and apparent reactive power are defined. Only the definitions that are related to the calculation and control in this paper are presented in this section. The full description of the theory is presented in [6]. In all the following equations, the lower case t indicates time. All the definitions are functions of time. For a threephase voltage vector v(t) and current vector i(t) (vectors for voltage and current will be denoted in bold), v(t) = [va (t), vb (t), vc (t)]T (1) i (t ) = [ia (t ), ib (t), ic (t )]T (2) The instantaneous power p(t) and the average power P(t) over the averaging interval [t-Tc, t] are defined by (3) and (4): 3 p(t) = vT (t)i(t) = vk (t)ik (t) (3) k =1 P(t) = 1 t p( )d (4) Tc t Tc The averaging interval Tc can be chosen arbitrarily from zero to infinity, and for different Tc values, the resulting active current and reactive current will have different characteristics. In a periodic system with period T, Tc is normally chosen as integral multiples of T/2 to eliminate current harmonics. The instantaneous active current ia(t) and instantaneous reactive current in(t) are defined by, respectively, ia (t ) = P(t) V 2 p (t ) v p (t ) (5) in (t) = i(t) ia (t) (6) The voltage vp(t) is the reference voltage, which is chosen based on the characteristics of the system and the desired compensation results. Vp(t) is the rms value of the reference voltage vp(t), i.e., . Vp (t) = 1 Tc t t Tc vTp ( )v p ( )d (7) The rms values of the voltage v(t) and the current i(t) are V (t) = 1 t vT ( )v( )d (8) Tc t Tc I (t) = 1 t iT ( )i( )d (9) Tc t Tc In this theory, all the definitions are instantaneous values; therefore, they are suitable for real-time control and provide advantages for the design of control schemes, which will be discussed in the next section. IV. CONTROL METHODS Most of the voltage fluctuation in the power system is because of a shortage or surplus of reactive power. Capacitor banks are widely used in power systems for voltage regulation. However, capacitor banks are switched on or off, which are not a continuous variable real-time source of reactive power. Moreover, the reactive power from capacitor banks decreases as the system voltage vt RMS Vt PI vc* 1 Vt* Fig. 5. Control diagram for compensator voltage regulation. vsystem Vsystem RMS PI vexcitation* 85 Vsystem * Fig. 6. Control diagram for synchronous condenser voltage regulation. decreases (by voltage squared) when reactive power is most needed. Therefore, capacitor banks cannot provide sufficient reactive power when it is most needed. DE with a power electronics converter can provide continuous real-time voltage regulation service. A voltage regulation method is developed based on the system configuration in Fig. 1. The control diagram is shown in Fig. 5. The PCC voltage is measured and the rms value is calculated. The rms value is compared to the voltage reference Vt*, and the error is fed to a proportional-integral (PI) controller. The reference compensator output voltage vc* is the reference to generate pulse-width modulation (PWM) signals to drive the inverter. The output voltage of the compensator is controlled so that it is in phase with the PCC voltage and the magnitude of the compensator output voltage is controlled so that the PCC voltage is regulated at a given level Vt*. The control scheme is shown in (10). t v*c (t) = vt (t)[1+ KP1(Vt*(t) Vt (t)) + KI1 (Vt*(t) Vt (t))dt] 0 (10) In a three-phase system, a voltage or a current is balanced if the magnitudes of the three phases are equal, and the phase-angles between consecutive phases are also equal. Most of the voltage unbalance in power systems is because of the magnitude inequalities, and only this case is considered in this paper. In this control method, the rms value of voltage in each phase is controlled independently; therefore the unbalance in the PCC voltage can be corrected by the compensator. The voltage regulation method of the synchronous condenser is designed based on the configuration in Fig. 2 and the control diagram is shown in Fig. 6. For the synchronous condenser which is used in the experiments, the synchronous condenser does not generate or consume any reactive power when the excitation voltage is 85 V. Controlling the excitation voltage to be higher or lower than 85 V generates or consumes reactive power, and therefore the system voltage vs is regulated. The control scheme is shown in (11). 1 vt RMS PI Vreference vinverter_reference X iinverter RMS Current limit X >= PI vexcitation_reference X PI Fig. 7. Integrated control of the compensator and the synchronous condenser. v* excitation (t ) = t KP2 (Vs*ystem (t) Vsystem (t)) + K I 2 (Vs*ystem (t) Vsystem (t))dt + 85 0 (11) When the compensator and the synchronous condenser are connected to different panels as shown in Fig. 3, the electrical distance of the two devices are large so that the controls of the two devices are independent, as shown in Figs. 5 and 6. However, when they are connected to the same panel as shown in Fig. 4, they are close to each other and control the same panel voltage so that they must be controlled together. The integrated control diagram is shown in Fig. 7 for this case. The compensator response time is much shorter than the synchronous condenser, therefore when the two devices are operating together to control a voltage, the compensator is responsible for the fast and small changes in the voltage while the synchronous condenser is responsible for the long- term changes. The reference voltage for the synchronous condenser (vexcitation_reference in Fig. 7) is only changed if the required compensator current exceeds the pre-set current limit. V. VOLTAGE REGULATION The parallel voltage regulation system is implemented in a DE system on the local distribution system at Oak Ridge National Laboratory. The simplified system diagram is shown in Fig. 3. The synchronous condenser and the compensator are operated simultaneously to regulate Panel A voltage (referred to as system voltage in Fig. 8) and Panel B voltage (referred to as PCC voltage in Fig. 9). In the synchronous condenser voltage regulation, the system voltage was 478.2 V before the regulation. The reference voltage is set to be 480 V (the red line in Fig. 8a), and the regulated voltage is shown as the blue waveform in Fig. 8a, which tracks the reference voltage with the error within ±0.1 V. The three-phase synchronous condenser current waveforms are shown in Fig. 8b together with the phase a voltage to show the phase angle between the voltage and the current. The phase a current is about 90º leading the phase a voltage, which shows that the synchronous condenser is generating reactive power. The spikes in the current waveforms are because of the electromagnetic interference (EMI) in the current measurement. The dynamic response of the synchronous condenser is shown in Figs. 8c and 8d. The reference voltage is increased from 480 V to 482 V (the red waveform in Fig. 8c), and decreased from 483 V to 478 V (the red waveform in Fig. 8d), respectively. The corresponding system voltage waveforms are the blue waveforms in Figs. 8c and 8d, respectively. The synchronous condenser can regulate the system voltage to a reference level at steady state and dynamically by controlling the amount of reactive power generated (to raise voltage) or consumed (to lower voltage) by the synchronous condenser. (a) rms line-to-line voltage (b) Synchronous condenser current (a) rms line-to-neutral voltage (b) Compensator current (c) Voltage increase control (d) Voltage decrease control Fig. 8. Experimental results of synchronous condenser voltage control. (a) rms line-to-line voltage (b) Compensator current (c) Voltage increase control (d) Voltage decrease control Fig. 9. Experimental results of compensator voltage control. In the compensator voltage regulation, the PCC voltage was 473.7 V before regulation. The reference voltage is set at 475.6 V (the red line in Fig. 9a) instead of the rated value 480 V, which is because of the current limit of the inverter which permits a smaller range of voltage regulation. The regulated voltage is shown as the blue waveform in Fig. 9a. The three-phase compensator current waveforms are shown in Fig. 9b together with the phase a voltage to show the phase angle between the voltage and the current. The phase a current is about 90º leading the phase a voltage, which shows that the compensator is generating reactive power. The ripple in the compensator output current is partly filtered by the coupling inductor (Lc in Fig. 1), so that the current waveforms are fundamental sinusoids with some harmonics. Larger coupling inductors could reduce the ripple to a lower level, but this would require higher DC link voltage on the inverter. The dynamic response of the compensator is shown in Figs. 9c and 9d. The reference voltage is increased from 474.4 V to 475.5 V (the red waveform in Fig. 9c), and decreased from 475.3 V to 474.4 V (the red waveform in Fig. 9d), respectively. The (c) Synchronous condenser current Fig. 10. Simulation results of parallel operation on the same panel. corresponding PCC voltage waveforms are the blue waveforms in Figs. 9c and 9d, respectively. The compensator can regulate the system voltage to a reference level at steady state and dynamically by controlling the amount of reactive power generated or consumed by the compensator. The parallel operation of the synchronous condenser and the inverter on the same panel (Fig. 4) is simulated, and the results are shown in Fig. 10. The reference of the rms lineto-neutral voltage is set at 275 V from t = 1 s to t = 1.5 s, and then at 277 V from t = 1.5 s to t = 2.5 s, as shown by the red waveform in Fig. 10a. The blue waveform is the Panel A voltage. At steady state, it follows the reference voltage, and at transient state, the dynamic response is less than 0.2 s. The compensator current limit is set at 90 A according to the current rating of the inverter. If the compensator current is below 90 A, the reference voltage for the synchronous condenser remains the same value, and the compensator responds to all the changes in the voltage. If the required compensator current is above 90 A, as shown in Fig. 10b because of the sudden voltage reference change at t = 1.5 s, a short period of current over 90 A is allowed for the compensator so that the system can have a faster dynamic response. After this short current overshoot, the compensator current is controlled below 90 A. At the same time, the synchronous condenser increases the reactive power output which is required by the system to increase the panel A voltage according to the increased reference voltage. There are two advantages to controlling the synchronous condenser and the compensator on the same voltage panel. 1) The two devices share the reactive power requirement for voltage regulation; therefore the voltage regulation capability is improved. 2) The devices of different characteristics are operated in parallel together, and these characteristics are combined together to achieve faster system dynamic response, and to reduce the capital and operating costs at the same time. vs vs Zs Zs ZA VA Panel A Load 1 ZB ZA VB Panel B VA Panel A Load 2 Load 1 Qc1 Compensator ZB VB Panel B Load 2 (a) A radial system (b) A radial system with voltage regulation Fig. 11. Simulation results of parallel operation on the same panel. (a) Panel B voltage VB with respect to ZAB, normalized to load impedance Fig. 12. Coefficient of correlation rAB vs. impedance ZAB, normalized to load impedance. TABLE I. IMPEDANCES IN FIG. 11 Impedance ZS ZA ZB Value ( ) 0.05+j0.1 j0.049 j0.049 VI. INTERACTION OF MULTIPLE DES The electrical distance (impedance) between the two DEs in Fig. 3 is large so that they are almost independent of each other, while the distance (impedance) between the two DEs in Fig. 4 is nearly zero. Thus, the control schemes are different correspondingly for the two situations. However, this still leaves unresolved what happens in regards to control when the two DEs are connected to the electrical system with an electrical impedance somewhere in between the above two extreme cases. In Fig. 11, there is a load on each of Panels A and B, and there is a sudden load increase to test the DEs’ dynamic response. Also assume there is a compensator, either SC or inverter, connected to Panel A. The impedances between the inverter and the SC are ZA and ZB, which are changed to test the interaction of the DEs with different electrical distance between each other. In Fig. 11 a-b, the loads on Panels A and B are L1-0=L2-0 = 60+j40 kVA, and after the sudden load change on Panel B, the loads are L1-1 = 60+j40 kVA, L2-1 = 100+j70 kVA. The other impedances are shown in Table I. (b) Reactive power for voltage regulation on Panel A with respect to ZAB, normalized to load impedance. Fig. 13. Simulation results of parallel operation on the same panel. A. Coefficient of Correlation between Two Voltages A coefficient of correlation r is defined to describe the relationship of voltage changes in a system. As shown in Fig. 11a, there is a load change (Load 2) on Panel B, and the corresponding voltage changes at Panel A and Panel B are VA and VB, respectively. The coefficient of correlation for the two panel voltages, VA to VB, referenced to Panel A is defined as: rAB = VA ×100% VB (12) The coefficient rAB is related to the electrical distance, i.e., the impedance of VA to VB. An extreme case is that rAB is 1 if VA to VB move together which means the impedance between Panel A and Panel B, ZAB=ZA+ZB, is zero. Fig. 12 shows the coefficient of correlation of VA to VB rAB vs. the impedance between Panel A and Panel B. The curve shows that rAB decreases when the impedance (electrical distance) between VA and VB increases. The two voltages are still closely related (75%) as the impedance ZAB is doubled or about 10% of the load impedance. More rigorous investigation of rAB w.r.t. electrical distance in a more complicated system is necessary in the future. B. Reactive Power and Voltage Regulation As shown in Fig. 11b, a reactive power compensator is installed on Panel A to regulate the Panel A voltage, VA. The compensator can be any dynamic reactive power source, such as a synchronous condenser, or a DE with an inverter interface that can provide reactive power. The load change on Panel B is the same as in the previous subsection, and the Panel A voltage is regulated by the compensator at the rated value, which is line-to-neutral voltage 277 V. In Fig. 13a, the lower curve is the Panel B voltage (in p.u.) without any reactive power compensation, as the impedance changes from zero to 1.5 p.u. The upper curve in Fig. 13a is the Panel B voltage, VB with a compensator on Panel A to regulate the Panel A voltage to the rated value. VB is more regulated by the compensator on Panel A when the impedance ZAB is smaller, or the electrical distance between Panels A and B are closer. Therefore, it is verified that the most effective way to regulate a voltage is to provide reactive power locally with a short electrical distance. Accordingly, the reactive power (in p.u.) required from the compensator to regulate VA is shown in Fig. 13b. Because of the electrical distance increases, the impact on Panel A voltage from the load change decreases, and to maintain the same voltage level, a smaller amount of reactive power is required. It should be noted that the amount of reactive power needed for the voltage regulation is independent of the type of the reactive power sources. It is only related to the change of load and the pre-set voltage schedule, since the post-disturbance (of load change) power flow shall give a unique solution. The reason is that the regulated bus with a given voltage schedule is a PV bus, and the Q amount will be uniquely determined in general. However, the choice of reactive power compensator will affect the transition from post-disturbance state to the final, new steady state. The detailed instantaneous voltage curves, very similar to Figs 8-9, are not shown here for simplicity. Further research in the coordination of multiple DEs to regulate voltage should be studied to obtain the optimal combination to achieve the best transient performance. VII. CONCLUSIONS Based on the analysis of the simulation and experimental results, the following conclusions can be drawn. 1) DE can perform real-time local voltage regulation by providing a certain amount of reactive power to the system. 2) Local voltage regulation is a more effective voltage regulation method with increased capacity and less system losses. 3) Power electronic converters have fast dynamic response, which can effectively compensate voltage sags and other sudden changes in the system. 4) Integrated voltage regulation (multiple DEs performing voltage regulation) can increase the voltage regulation capability of multiple DEs and reduce the capital and operating costs. 5) Local voltage regulation and the required reactive power are dependent on the electrical distance between the disturbance and the controlled voltage (the impedance between the reactive power source and the controlled voltage). Further research in simulation and experiment will be necessary to investigate the correlation of two voltage changes with respect to electrical distance in a more complicated system. Also, it needs to be studied to identify an optimal combination of multiple DEs to regulate voltage in a decentralized approach with minimum communication to achieve the best transient performance in voltage regulation. VIII. REFERENCES [1] J. B. Campbell, T. J. King, B. Ozpineci, D. T. Rizy, L. M. Tolbert, Y. Xu, X. Yu, 2005, “Ancillary services provided from DER,” Oak Ridge National Laboratory Report, ORNL/TM-2005/263. [2] B. Meyer, Y. Bamberger, I. Bel, 2006, “Electricite de France and integration of distributed energy resources,” IEEE Power Engineering Society General Meeting. [3] B. Kroposki, C. Pink, R. DeBlasio, H. Thomas, M. Simoes, P. K. Sen, 2006, “Benefits of power electronic interfaces for distributed energy systems,” IEEE Power Engineering Society General Meeting. [4] R.T. Guttromson, 2002, “Modeling distributed energy resource dynamics on the transmission system,” IEEE Transactions on Power Systems. [5] F. Li, J. Kueck, T. Rizy, T. King, 2006, “A preliminary analysis of the economics of using distributed energy as a source of reactive power supply,” Oak Ridge National Laboratory First Quarterly Report for Fiscal Year 2006. [6] Y. Xu, L. M. Tolbert, F. Z. Peng, J. N. Chiasson, J. Chen, “Compensation-based Non-active Power Definition,” IEEE Power Electronics Letters, vol. 1, no. 2, June 2003, pp. 45-50. IX. BIOGRAPHIES Yan Xu received her Ph.D. in electrical engineering at The University of Tennessee in May 2006. She is working as a post doctoral research member at Oak Ridge National Laboratory in the distributed energy test center. She received the B.S. degree from Shanghai Jiaotong University, China, in 1995 and the M.S. degree from North China Electric Power University, Beijing, China, in 2002. Her research interests include power quality and reactive power compensation. Fangxing (Fran) Li (M’01, SM’05) received the Ph.D. degree from Virginia Tech in 2001. He has been an Assistant Professor at The University of Tennessee (UT), Knoxville, TN, USA, since August 2005. Prior to joining UT, he worked at ABB, Raleigh, NC, as a senior and then a principal R&D engineer for 4 and a half years. His current interests include energy market, reactive power, and distributed energy resources. Dr. Li was the recipient of the 2006 UT Eta Kappa Nu Outstanding Teacher Award, voted by undergraduate ECE students. He also received the recipient of UT Chancellor’s Professional Development Award in April 2007. John D. Kueck (M’1975, SM’2000) earned a BS in Physics from Purdue University and an MS in Electrical Engineering - Power Systems, from Ohio State University. Over the first 20 years of his career, Mr. Kueck worked in the design and operation of fossil fuel and nuclear generating stations. From 1992 to present, Mr. Kueck has been a researcher at the Oak Ridge National Laboratory. His major interest is the local supply of reactive power from distributed energy resources as a reliability service. D. Tom Rizy (SM’87) is a senior research power systems engineer at Oak Ridge National Laboratory (ORNL) in the Engineering Science and Technology Division. His current interest and activities focus on new controls for distributed energy resources (DER) and applications for synchro-phasor measurements. He is a cofounder of the Distribution Energy Communications and Control Laboratory (DECC) at ORNL for testing dynamic voltage regulation controls using DER. He has over 29 years experience in power systems R&D and received his MSEE and BSEE from Virginia Tech and the University of Virginia, respectively. His is a corecipient of the IEEE Prize Paper Award (1990) for “Adaptive Relaying Concepts for Improved Performance” and a chapter author of the book on the Athens Automation and Control Experiment, a large-scale distribution automation project conducted in the 80s by DOE, TVA and EPRI.

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