Sag/Swell Compensation and Displacement Factor Improvement using IDVR in Distribution Network

Transcription

1 Voltage Sag/Swell Compensation and Displacement Factor Improvement using IDVR in Distribution Network Vinothini.R 1 Balamurugan.M 2 PG Scholar, Power Electronics and Drives, Associate Prof, Head of EEE Department, Dr.Pauls Engineeringg College, Dr.Pauls Engineering College, Villupuram, India. Villupuram, India. rvinothinieee@gmail.com balam06@gmail.com Abstract In this Paper, an Interline Dynamic Voltage Restorer (IDVR) with Displacement Factor (DF) control is proposed for Voltage Sag/Swell Compensation and DF improvement in Distribution Network. An Interline Dynamic Voltage Restorer (IDVR) is used to mitigate voltage sag/swell during abnormal condition in distribution system. It consists of several back-to-back Voltage Source Converters (VSC) with common DC-link. During normal condition an IDVR is used to improve the Displacement Factor (DF) of one of the involved feeder. DF improvement can be achieved by active and reactive power exchange (PQ sharing) between different feeders. Detailed simulation for the proposed IDVR system with DF improvement were carried out in MATLAB7/SIMULINK platform. Index Terms Voltage Sag/Swell Compensation, Harmonic Suppression, Displacement Factor Improvement, Interline Dynamic Voltage Restorer (IDVR), PQ sharing mode. I. INTRODUCTION In recent years, the most noticeable topic for Electrical Engineering is Power Quality(PQ)[1]. Some of the typical Power Quality (PQ) disturbances present in distribution system are voltage sags, voltage swells, interruptions, phase shifts, harmonics and transients. Among the disturbances voltage sag is considered as the most severe problem due to sensitive loads at distribution network[2]. Several devices such as STATCOM, tap changing transformer, UPFC and DVR are existing to mitigate voltage sag problems. Among these, dynamic voltage restorer can afford the most commercial solution to mitigate voltage sag by injecting voltage in addition to power into the system. Dynamic Voltage Restorer is a series connected power electronics based device that can quickly mitigate the voltage sag in the system and restore the load voltage to the pre-fault value. Consequently, the amount of energy storage within the DVR becomes one of the main restrictive factors in mitigating long-duration voltage sags. As a result, researchers presently pay greater attention to the DVR energy storage and its optimum use. The first DVR was installed in North 22 America in a kv system located in Anderson, South Carolina[3]. Practically, the capability of injection voltage by DVR system is 50% of nominal voltage. In distribution systems, load voltage restoration can be achieved by injecting active and/or reactive power into the distribution feeder[5]. Active power capability of the DVR is governed by the capacity of the energy storage element and the employed compensation technique. Several control techniques have been proposed for voltage sag compensation, such as presag, in- control approaches[6]. If the phase, and minimal energy required power for voltage restoration is obtained from the neighbouring feeder(s), the compensating device is technically called an Interline Dynamic Voltage Restorer (IDVR)[16]. The basic concept behind the IDVR is derived from the Interline Power Flow Controller (IPFC) proposed by Gyugyi in 1999 to exchange power between parallel transmission lines. The two converters of the IPFC shown in Fig.1 are used to control the transmitted power in each line (P1 and P2) and active power transfer between lines(p12). Fig.1 Single line diagram of an IPFC in transmission system With respect to the line current, the injected voltage has two components. The quadrature component provides reactive power compensation for the line, while the in-phase component absorbs or generates the required active power. The main difference between an IPFC, normal IDVR, and an

2 IDVR with Displacement Factor improvement system[19] are summarized in Table 1. It should be noted that the IPFC was the inspiration for proposing the IDVR for distribution networks. IPFC IDVR IDVR (DF improvement) Function It is used in transmission systems to control the power flow of parallel It is used in distribution system for voltage sag/swell restoration. It is used in distribution systems for voltage sag/swell restoration and improvement of displacement power factor during normal conditions. Operation Employed is normal operation Employed in abnormal conditions It can be employed in normal as well as abnormal conditions In-phase Voltage injection Active control power When the feeder is switched to power control mode, the in-phase voltage component represents the active power to be pumped/ absorbed by that feeder to/from the DC-link (Active power control When the feeder is switched to power control mode or DF improvement mode, the in-phase voltage component represents the active power to be pumped/ absorbed by the feeder to/from the DC-link(Active power control) Quadrature voltage injection Line reactive impedance control When the feeder is switched to power control mode, the quadrature voltage component is used to keep the load voltage magnitude of that feeder constant (Load voltage control) When the feeder is switched to power control mode or DF improvement mode, the quadrature voltage component is used to keep the load voltage magnitude of that feeder constant (Load voltage control) Table-1 Salient Features of the IPFC, normal IDVR and IDVR with Displacement Factor improvement During normal operating conditions (i.e., all feeders are healthy), the DVRs are typically bypassed via bypass switches, or they can be alternatively used for load sharing purposes. Instead of bypassing the IDVR in normal operation, this project proposes a new operational mode, namely PQ sharing mode[19], to improve the DF of one of the involved feeders by sharing active and reactive power among different system feeders through the common DC-link. In its general form, the IPFC employs a number of inverters with a common DC-link to provide series compensation for a selected line of the transmission system. In a similar way, the IDVR system is formed by using several DVRs protecting sensitive loads in different distribution lines to share a common DC-link energy storage[12]. This project also presents an extensive analysis to develop suitable control schemes for voltage sag compensation, harmonic suppression and Displacement Factor (DF) improvement. Fig.2 represents the proposed topology for Voltage Sag/Swell Compensation and Displacement Factor (DF) improvement using IDVR in distribution network. For normal voltage levels, achieving active power exchange P ex between the feeders (from sourcing feeder to receiving feeder), requires controlled voltage injection in each feeder by the corresponding converter (see Fig.2). II. PROPOSED IDVR TOPOLOGY Fig.2 Proposed topology for NPC inverter based IDVR. 23

3 This injected voltage should not perturb the load voltage magnitude of both feeders; therefore, both converters are operating under PC mode. A. Sourcing Feeder The converter in the sourcing feeder is responsible for feeding energy into the dc link via injecting a controlled voltage (magnitude and phase) through the series coupled transformer allowing for power exchange. In this paper, in order to emulate the effect of voltage injection on the feeder DF, the injected voltage is emulated using a voltage drop across a series virtual impedance, as shown in Fig.3(a). The resistive component of this virtual impedance absorbs active power (P ex ) from the source, while the function of the capacitive reactance component is to maintain a constant load voltage magnitude. After voltage injection, the supply s active power increases while its reactive power decreases due to the virtual injected capacitive reactance, hence, the sourcing feeder DF eventually increases. B. Receiving Feeder The converter in the receiving feeder is responsible for absorbing the transmitted power from the sourcing feeder via voltage injection; hence, the power controller has a power command of Pex. The injected voltage in this case is equivalent to injecting a virtual negative resistance r2 in series with an inductive reactance x2, as shown in Fig.4(a). Fig.4 Receiving feeder: (a) per-phase circuit with virtual impedance injection and (b) phasor diagram. III. PROPOSED IDVR CONTROLLER Fig.3 Sourcing feeder: (a) per-phase equivalent circuit with virtual impedance injection and (b) phasor diagram. Assuming a three-phase balanced load is connected to the feeder; the per-phase equivalent circuit of the feeder with series virtual impedance injection is shown in Fig.3(a),while corresponding phasor diagram shown in Fig.3(b). Fig.5 shows the proposed IDVR controller (two feeders are involved, namely, feeder x and feeder y), which is able to manage the power transfer through the dc-link in normal as well as abnormal operating conditions. As a general controller, voltage sag/swell and DF improvement problems are merged into one control circuit. Referring to Fig.5, each converter may be switched to one of four possible modes (off mode, VC mode, PC mode, or PQ mode). The PCC voltages are continuously monitored by a logic unit that is responsible for choosing the appropriate mode of operation for each converter based on the voltage levels. Fig.5. Proposed IDVR Controller system. The main cases are summarized in Table-2 and in the following subsections (where the hyphenated The following section will show how different condition describes the state of one of the feeders to the modes of operation are handled individually in the left of the hyphen and the other feeder to the right of the controller. A set of scenarios can be envisioned for the hyphen). For all other cases, the converters will be 24

4 ISSN: Volume 13 Issue 2 MARCH switched to the off position. Cases CONVERTER - x CONVERTER - y VC PC PQ Of f VC PC PQ Off Vx is normal & sag at Vy Vy is normal & sag ay Vx Normal condition, PQ mode enabled Normal condition, PQ modedisabled Table-2. Proposed IDVR Controller Combinations A. Normal Normal (PQ Sharing Mode is Disabled) In this case, the logic unit selects the OFF positions (see Fig.5) for both converters. B. Normal Normal (PQ Sharing Mode is Enabled) In this case, the logic unit selects the PQ positions (see Fig.5) for both converters after verifying all constraints that accompany this mode. Based on the DFs of the loads connected to the involved feeders (DF Lx and DF Ly ), the direction of active power flow will be defined. The feeder with a lower load DF will be the sourcing feeder with a positive active power reference, and the other feeder will be the receiving feeder with a negative active power reference. C. Normal Voltage Sag If one feeder exhibits voltage sag, the logic unit has to switch its series converter to VC position (see Fig.5) to regulate the load voltage, and the required power for restoration will be absorbed from the dc link[22]. The converter of the healthy feeder will be switched to its PC position (seee Fig.5) to replenish the dc-link voltage[22]. The needed power to restore the dc link voltage will be the output of the dc voltage controller. This power is used to estimate the corresponding converter reference voltage. D. Normal Voltage Swelll If one feeder exhibits voltage swell, the logic unit switches its series converter to the VC position (see Fig.5) to regulate the load voltage. Additional power is then fed to the dc link[19]. The converter of the healthy feeder will be switched to its PC position (see Fig.5), to avoid increasing the dc-link voltage. The amount of power, which should be absorbed by the healthy feeder, will be the output of the dc voltage controller. IV. SIMULATION RESULTS A detailed simulation has been carried out for a simple IDVR system consisting of two lines of 400V. The IDVR is modeled and simulated using the MATLAB 7/Simulink (Sim Power System) platform. The MATLAB model of an VSC based IDVR connected system is shown in Fig.7. In this system feeder-1 is considered as healthy feeder and feeder-2 is faulty feeder. Fig.6 Single Line Diagram of VSC based IDVR 25

5 ISSN: Volume 13 Issue 2 MARCH Fig.7 Simulink model of VSC based IDVR Fig.8 DVR-Voltage Source Converter The sub-system model of Voltage Source Converter with LC-filter and series injecting transformer is shown in Fig.8 and Fig.9 represents the source voltage (400V) and current (12A) waveform for the VSC based IDVR system. Fig.10 Feeder-1 Output Voltage & Current Waveform Whenever there is any voltage sag occurs in feeder, the by-pass switch is turned ON. In feeder-2 the sag occurs from ( )sec because of connecting the heavy load. Fig.11 Feeder-2 Output Voltage & Current Waveform Fig.9 Source Voltage & Current Waveform A. Voltage Sag Compensation The output voltage & current waveform for feeder-1 & feeder-2 is shown in Fig.5.8 & Fig.5.9. Here feeder-1 is considered as the healthy feeder, hence this feeder is used to replenish the DC-link storage capacitor. The heavy load is connected from ( )sec in feeder- 2. In feeder-2 the voltage dip is from 320V to 260V, so the VSC based DVR need to mitigate the voltage sag by injecting 60V by switching ON the bypass switch from ( )sec. Fig.12 represents the DVR injecting voltage waveform. 26 Fig.12 Injecting Voltage waveform

6 ISSN: Volume 13 Issue 2 MARCH B. Displacement Factor Measurement Fig.13 represents the Displacement Factor (DF) measurement for feeder-1 & feeder-2the feeder-2 displacement factor decreases and hence During sag voltage during normal condition it maintains the unity displacement factor. Fig.13 Feeder-1 & Feeder-2 Displacement Factor Measurement C. FFT Analysis The FFT analysis for feeder-1 & feeder-2 is represented in Fig.14 & Fig.15. Table-4 represents the simulation results of the proposed IDVR system. Fig.14 FFT Analysis For Feeder-1 Voltage Injection Time Displacement Factor THD 4.08% Table-3 Simulation Results Table-3 shows the simulation VSC based IDVR. This paper addresses the problem of voltage sag and Displacement Factor(DF) control. For a two level inverter, the THD value is 4.08% in feeder-1 and 4.32% in feeder-2. Also the Displacement Factor of the system is maintained at unity during Normal operation of IDVR. CONCLUSION ( ) sec Normal Voltage 1 Sag Voltage % In this paper, the simulation of 3-level Voltage Source Converter (VSC) based IDVR for 3ɸ, 400V, 50Hz distribution system has been developed by using Matlab7/ Simulink platform. During abnormal condition, the proposed IDVR is used to mitigate the voltage sag due to heavy load in distribution network. Also, a new operational mode for the IDVR to improve the Displacement Factor (DF) of different feeders under normal operation is proposed.. In this mode, the DF of one of the feeders is improved via active and reactive power exchange (PQ sharing) between feeders through the common dc link. Under PQ sharing mode, the injected voltage in any feeder does not affect its load voltage/current magnitude. Based on the simulation results obtained the IDVR compensates the voltage sag effectively. Displacement Factor (DF) is also maintained at unity during normal operation of IDVR. PARAMETERS Input Voltage Output Voltage Output Current Fig.15 FFT Analysis For Feeder-2 FEEDER-1 (Healthy) 400V 326 V Normal Voltage 6mA Injected Voltage - FEEDER-2 (Faulty) 400V Sag Voltage Normal Voltage Sag Voltage 60 V 320 V 260 V 20 A 45 A 27 REFERENCES [1] Arindam Ghosh Power Quality Enhancement Using Custom Power Devices, Kluwer Academic Publishers, 2002 [2] M. Vilathgamuwa, A. A. D. Ranjith Perera, and S. S. Choi, Performance improvement of the dynamic voltage restorer with closed-loop load voltage and current-mode control, IEEE Trans. Power Electron., vol. 17, no. 5, pp , Sep [3] D. M. Vilathgamuwa, A. A. D. R. Perera, and S. S. Choi, Voltage sag compensation with energy optimized dynamic voltage restorer, IEEE Trans. Power Del., vol. 18, no. 3, pp , Jul [4] P.C Loh, D.M Vilathgamuwa, S.K Tang, H.L Long, Multilevel Dynamic Voltage Restorer, International Conference on Power System Technology, vol.2, pp , November [5] J. G. Nielsen, M. Newman, H. Nielsen, and F. Blaabjerg, Control and testing of a dynamic voltage restorer (DVR) at medium voltage level, IEEE Trans. Power Electron., vol. 19, no. 3, pp , May [6] C. Fitzer, M. Barnes, and P. Green, Voltage sag detection technique for a dynamic voltage restorer, IEEE Trans. Ind. Appl., vol. 40, no. 1, pp , Jan./Feb [7] M. J. Newman, D. G. Holmes, J. G. Nielsen, and F. Blaabjerg, A dynamic voltage restorer (DVR) with selective harmonic compensation at medium voltage level, IEEE Trans. Ind. Appl.,

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