Robust Control Strategy of Dynamic Voltage Restorer for Compensating Voltage Sags in Distribution System

Robust Control Strategy of Dynamic Voltage Restorer for Compensating Voltage Sags in Distribution System

D. N. Katole

Asstt. Prof., Dept. of Electrical Engg. Priyadarshani J.L.C.E., R.T.M. N.U, Nagpur,

Maharashtra, India

Dr. M. B. Daigavane Principal,GHRaisoni.IETW,Hingna, Nagpur.Maharashtra,

India

AbstractOut of all power quality problems Voltage sag is the major one which results in a failure or a mis-operation of end use equipment and the effective device known as dynamic voltage restorer is recommended to mitigate voltage sags. This paper concentrates on effective control strategies for dynamic voltage restorer and analysis is based on the compensation of voltage sags with phase jump. The aim therefore, is to recommend effective control method that can improve voltage sag. In this paper a method of determining the exact amount of voltage injection required to systematically correct voltage sag with phase jump is described. This paper presents the Dynamic Voltage Restorer (DVR) with ESS based PI Controller method to compensate balanced voltage sag. Simulation results show that this proposed method can compensate balanced voltage sag effectively.

KeywordsPower quality, voltage sag, Custom power Devices, DVR, Energy Storage System, pulse width modulation.

1. INTRODUCTION

   In most cases voltage sags which occurs frequently are considered less critical as compared to interruptions. Voltage sags are characterized by momentary decrease in rms voltage magnitude lasting between half a cycle and several seconds [1]. Two important voltage sag parameters are magnitude and time duration. However, the sag magnitude is not constant, due to the induction motor load [2]. Fig. 1 shows 50% voltage sag for 300ms. Mostly occurring short circuit faults have become one of the most important problem in voltage sag facing industrial customers. Disturbances due to short circuit of line, changes in load and starting of inductive type load has effect on voltage waveform causing problems related with the operation of electrical sensitive devices. A DVR can eliminate most sags and minimize the risk of load tripping at very deep sags.

   The controller of DVR requires fast response also it has to take care of large variation in type of sags to be compensated and changes in type of connected load hence the controller is not straight-forward. Industrial and sensitive load needs constant magnitude of voltage sine wave, constant frequency and symmetrical voltage with a constant rms value to continue the production. This increasing interest to improve overall efficiency and eliminate variations in the industry have resulted more complex instruments that are sensitive to voltage disturbances [3]. Dynamic Voltage Restorer (DVR) device used in distribution side injects a voltage in series with

   the system voltage provides the most cost effective solution to mitigate voltage sags by improving

   Fig. 1 Voltage Sag

   power quality level that is required by customer [4]. When a fault happens in a distribution network, sudden voltage sag will appear on adjacent loads. DVR installed on a sensitive load, restores the line voltage to its nominal value in few milliseconds. Sags are often nonsymmetrical and accompanied by a phase jump. Control strategies for DVRs have been addressed in [3] and [7]. In [5] the problems with phase jump have been reported but no control methods have been proposed.

2. DYNAMIC VOLTAGE RESTORER

   The concept of custom power is to use power electronic or static controllers in the medium voltage distribution system aiming to supply reliable and high quality power to sensitive users [4]. DVR as a custom power device could be the effective means to overcome some of the major power quality problems such as voltage sag by way of injecting active and/or reactive power into the system [5].

   Fig. 2 shows the DVR connected in series with the distribution system used for voltage correction. Typically, DVR consists of energy storage system, force commutated VSI, filters and series transformer.

   When the supply voltage Vs changes, DVR injects dynamically controlled voltage Vi in series with the supply voltage through a series transformer to correct the sag in such a way that the desired load voltage magnitude can be maintained. However, not only voltage injection but also active and/or reactive power injection is needed. The DVR itself is capable of generating the reactive power, however, but the injected active power must come from the energy storage system of the DVR. When the injected voltage is in phase with the supply voltage, the desired voltage correction

   V Vi

   VL 0

   Where: VL is the load voltage (reference phasor), is the

   POWER SYSTEM

   S

   R L

   FILTER

   VSC

   ESS DVR

   Fig. 2 Dynamic voltage restorer

   CRITICAL LOAD

   IL

   power (or torque) angle, IL is the line current and VS is the supply voltage. A disturbance or fault in the system may reduce the supply voltage magnitude VS to a new value VS- new. The supply voltage can be maintained by the injection of Vi.

   The equation for the compensated system is:

   VS-new=VL+ILZ – Vi (2)

   Where: Vi is the injected voltage is the phase angle of Vi.

   The rating of the ESS is:

   Si = 3Vi IL (3)

   Where the above current represents the complex conjugate.

   can be achieved with a minimum voltage injection but it may require a considerable amount of active power injection into the system [5]. When the injected voltage leads the supply voltage, however, the same correction can be made with a lower value of active power injection [5]. This is possible at an expense of higher voltage injection. Such an operation

   Vs_new

   new

   old

   Vs_old

   Vi

   IL.Z

   requires careful determination of injected voltage magnitude and angle.

   In general, the active and reactive power flows are controlled by the angle between the voltage that is injected in series with the line and the line current as shown in Fig. 3. For example, if the voltage is in phase with the current, only active power is changing with the line. Otherwise, if the voltage is in quadrature with the current, nothing more than reactive power, will change with the line, also minimum active power injection will be required if the power factor of supply is unity [6].

   Without energy storage system, the DVR can only inject voltages in quadrature with the load current and hence a larger voltage injection is required to mitigate the voltage sag. In addition, reactive power compensation is only effective for small voltage sag. Energy storage system gives the flexibility to inject voltage at any phase angle and compensate for deeper voltage sags, voltage sags with phase jumps and longer duration voltage sag.

   As From fig 2, the equation governing the system without series compensation is:

   VL

   IL

   Fig. 4 phasor of injected voltage

   Fig. 4 shows the vector diagram of the compensated system. The direction of the vector (ILZ) depends on the power factor of the load (in this case a lagging power factor) and the impedance of the line. For distribution feeders, the ratio of reactance to resistance is less than for transmission lines, which implies that the impedance angle will be less than 90 degree.

   Fig. 4 illustrates that the injection of an arbitrary voltage Vi can maintain the load voltage constant when the supply voltage dips. However, the injected complex power depends on the amplitude and phase angle of Vi. Visual observation of Fig. 4 suggests that Si will be minimum wen = . This is shown in Fig. 5

   VS=VL+ILZ (1)

   Consume P and Q Prov ide P consume Q

   Vs_new

   ,

   Vi

   IL.Z

   VQ

   VPQ

   VL

   IL

   consume P Prov ide Q

   Vp IL

   Prov ide P and Q

   Fig. 5 phasor representation

   The effectiveness of the series compensation can be obtained by the determination of injected power as a function of system parameters. Therefore, minimization of injected kVA from the ESS should be used as the criteria.

   Solving for Vi in (2) and substituting into (3) yields:

   Fig. 3 Active reactive power flow

   VS-new=3. (VL+ILZ – VS-new).IL (4)

   Vs-new, the new source voltage is simply a function of the transient voltage dip (Vd) in the line from the original source voltage (VS):

   POWER SYSTEM

   Series

   Fault

   OTHER LOAD

   Vs-new =

   (1- V

   d). VS

   (5)

   Transformer

   CRITICAL LOAD

   Note that Vd is a phasor quantity since both VS and VS-new are phasor quantities. Substitution of (5) and (1) into (4) yields after simplification:

   Si = 3[Vd I.VS.ej(+) ] (6) Where: is the phase angle of Vd

   Filter

   VSC

   DC Link

   	Firin Pulse
   PWM Generator

   	Firin Pulse
   PWM Generator

   g s

   Energy Storage System (ESS)

   The voltage deviation (VD) of the system which is referred to as voltage regulation in the utility industry is found from:

   VD =VS-VL/VL (7)

   Since VL is the reference vector, the absolute value marks can be removed from it. Solving for the absolute value of the supply voltage and substituting into (6) yields the amount of KVA injected by ESS of DVR:

   Fig. 6 System and DVR model layout

   SAG DETECTOR

   PLL

   Voltage Regulator

   Si = 3[Vd. I.VL.(1+VD)] (8)

   Equation (8) is the calculation of the injected KVA of the ESS of DVR as a function of the system parameters.

   Vs ABC

   DQO

   0.90

   –

   + SD

   COMPERATOR

3. MODELING OF DVR

   – VT

   +

   +

   The system under consideration consists of simple radial system with a source, a bus, and two parallel loads as shown in Fig 6. When a three phase fault occurs in the system, the

   1.0 Kp

   Vdq_ref

   TORQUE

   critical load experiences balanced voltage sag. Fig.6 also LIMITER

   shows the basic model layout of DVR with ESS. ESS can be used to protect sensitive production equipments from shutdowns caused by voltage sags. These are usually DC storage systems such as UPS, batteries, superconducting

   SAG CORRECTOR

   Fig. 7(a) Sag detector and sag corrector

   magnet energy storage (SMES), storage capacitors or even flywheels driving DC generators [8]. The output of these devices can be supplied to the system through an inverter on a momentary basis by a fast acting electronic switch. Enough energy is fed to the system to compensate for the energy that would be lost by the voltage sag or interruption. A delta/open winding is used in series transformer since it prevents the third harmonic and zero sequence currents from entering into the

   –

   VDC Measured

   VDC Measured

   +

   VDC

   Reference

   PI

   Controller 1

   system and also maximizes the use of the dc-link compared to y/open winding. A delta-connected LC filter bank is used to smooth the injected voltage [9]. This series compensation method is capable of restoring the load voltage to its rated value for a much larger load than the parallel compensation method [10].

4. CONTROL SYSTEM

   The outer control system consists of a sag detector, sag corrector and energy control system as shown in Fig. 7(a) and 7(b). As shown in figure instantaneous supply voltage is transformed in synchronous reference frame.In sag detector the actual system voltage is compared with the threshold value which is 0.90 p.u.

   Fig. 7(b) Energy control system

   1. Sag detector

      The sag detector detects the voltage sag and activates the control system for sag correction. The output of it is a pulse with duration equal to the duration of voltage sag. The inputs are the voltages measured on the supply side. The measured voltages are converted to d-q space vector in p.u. in synchronously rotating reference frame. The magnitude of the space vector is compared to a reference value (1.0pu). The detector can give accurate result only for balanced voltage sags.

   2. Sag corrector

      In sag corrector, the controller input is an error signal obtained from the reference voltage and the p.u. rms value of the terminal voltage measured. Such error is processed by a PI

      controller and the output is the angle , which is provided to the PWM signal generator. The PWM generator then generates the pulse signals to the IGBT gates of voltage source converter.

   3. PWM Technique

      The inverter is the core component of the DVR, and its control will directly affect the performance of the DVR. In the proposed DVR, a discrete PWM scheme will be used. The inverter used in this study is a six-pulse inverter. The carrier waveform is a triangular wave with high frequency (3000 Hz). The modulating index will vary according to the input error signal. The basic idea of PWM is to compare a sinusoidal control signal of normal 60 Hz frequency with a modulating (or carrier) triangular pulses of higher frequency. When the control signal is greater than the carrier signal, three switches of the six are turned on, and their counter switches are turned off. As the control signal is the error signal, therefore, the output of the inverter will represent the required compensation voltage.

   4. Energy Control System

   A proportional integral feedback control is used for controlling DC bus voltage. If the sag detector detects sag SD goes low. The DC bus voltage controllers during sag improve the response time.

5. SIMULATION RESULTS

   The ESS performance is analyzed by creating a three phase fault at the location shown in Fig. 6. The fault results in a balanced voltage sag of 60% on the supply side of the series transformer. Distance of the fault from the bus decides the depth of the voltage sag. The first simulation contains no DVR and second simulation contains DVR. Using the facilities available in MATLAB [7], the DVR is simulated to be in operation only for the duration of the fault, as it is expected to be the case in a practical situation.

   Extensive simulation is done by considering super capacitor and battery as ESS. The results shown in fig. 8 indicates that the variable SD is one until the fault is detected and then goes to logic zero when the supply voltage is out of tolerance. The output Vdqs is per unit magnitude of supply side voltage space vector, and Vdql is the per unit magnitude of load side compensated voltage space vector. Note that the space vector voltage magnitude changes much more quickly since it is based on instantaneous quantities, and not averaged over a cycle as the RMS voltages are. Therefore, a RMS voltage reference based sag detector will react more slowly, and hurt the response of the system. The space vector based d- q voltage is used to provide faster, more accurate detection of the voltage sag in the system described in this paper.

   When SD goes low as shown in Fig 8, the sag corrector is activated. The DC bus controllers are bypassed when SD is one. The ESS was in standby mode until SD goes low. Fig.9 shows the per-unit phase to phase RMS voltages on the supply side and critical load side. Figure 10 and 11 shows the voltage injected by DVR in abc & dqo system.

   Figures 1 show the pu line voltage of phase A on the supply side and critical load side respectively. The ESS responded within 2 cycles to keep the critical load voltage within the 10% tolerance (i.e. the sag is corrected to 0.90 per unit, not 1.0 per unit). The sag correction can be done for 0%

   tolerance but it will result in over voltages at the end of the sag, with the potential to cause insulation damage. Figure 13 and 14 shows the active power on load side with and without DVR. Figure 15 shows the state of charge of the battery. In this case, the critical load voltage was regulated below 10% of the nominal.

   Fig. 8 Sag detector response, balanced voltage sag, compensated critical load voltage in pu in dqo system

   Fig.9 Balanced Voltage sag & compensated load voltage in pu in abc system

   Fig. 10 Voltage injected by DVR in abc system

   Fig. 11 Voltage injected by DVR in dqo system

   Fig. 12 Pu line voltage of phase A on the supply side and critical load side

   Fig. 13 Active power on load side without DVR

   Fig. 14 Active power on load side with DVR

   Fig 15. State of charge of Battery

6. CONCLUSION

In this paper, a simple, fast, and cost effective Dynamic Voltage Restorer (DVR) is proposed for mitigating the problem of voltage sags in industrial distribution systems, with a large portion of its load consisting of induction motors. Calculation of the compensating voltage is done with reference to voltage only, since induction motors are not sensitive to changes in phase angle. A controller based on feed foreword technique is used which utilizes the error signal (difference between the reference voltage and actual measured voltage) to trigger the switches of an inverter using a Pulse Width Modulation (PWM) scheme. A new PWM-based control scheme has been implemented to control the electronic valves in the two level VSC used in the DVR. The proposed DVR utilizes energy drawn from the ESS during abnormal condition and stored in capacitors, and which is converted to an adjustable three phase ac voltage suitable for mitigation of voltage sags. An energy control system that regulates the DC bus voltage charges the ESS has been proposed. The advantages of a d-q based sag detector over rms voltage have been shown.

The simulation shows that capacity of power compensation and voltage regulation depends mainly on the rating of ESS of DVR.

TABLE 1 provides a list of system data used in this paper.

TABLE I SYSTEM DATA

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