MODELING AND SIMULATION OF A DYNAMIC VOLTAGE RESTORER FOR VOLTAGE SAG MITIGATION AND HARMONICS ELIMINATION.
Power quality has become an important issue in recent years due to the sensitivity of new equipment to its variations. Custom power devices are used to overcome power quality problems. Dynamic voltage restorer (DVR) is one of the most efficient and effective custom power devices that can mitigate voltage sags swells and harmonics coming from supply side. In this paper a dynamic voltage restorer is modeled based on hysteresis voltage control method for voltage sags/swells mitigation and harmonics elimination. Effect of rated power of injection transformer connected in series with distribution feeder on the capability of DVR to compensate harmonics is also investigated. The system is modeled in MATLAB/Simulink environment and simulations are carried out to verify improvement in system operation.
Key Words: Power quality Voltage sag Voltage Swell Harmonics Dynamic Voltage Restorer (DVR)
Power quality has received increasingly more attention by researchers due to its impacts on utilities as well as both industrial and commercial electrical consumers . Power quality problem is defined as Any power problem manifested in voltage current or frequency deviation that results in the failure or misoperation of customer equipment " . Among various power quality problems voltage sag is the most frequent one. Voltage sag is a decrease in root mean square (rms) voltage magnitude in the range of 0.1 to
0.9 per unit (p.u) at fundamental frequency with the duration of half cycle to one minute . It is often caused by balanced or unbalanced faults in the distribution systems or by large induction motors startup . Voltage swell on the other hand is an increase in rms voltage magnitude typically between 1.1 to 1.8 per unit at fundamental frequency and duration of half cycle to one minute . Turning off a heavy load or fault conditions particularly for ungrounded or floating delta systems could cause swells. Although effects of a voltage swell is often more destructive than sag it is not as important as voltage dip. This is because voltage swells are less common in comparison with sags in distribution systems.
Custom power devices are based on power electronic technology which is much faster than conventional electromechanical based protection devices. These devices mitigate voltage sags/swells originated from supply side and improve power quality of customers especially critical customers at distribution level . There are different types of custom power devices such as Dynamic Voltage Restorer (DVR) Distribution Static Synchronous Compensators (DSTATCOM) Static Var Compensator (SVC) and Uninterruptible Power Supplies (UPS). Each of them has its own advantages and disadvantages. Among all of them DVR is considered to be the most efficient and effective one for voltage sag/swell mitigation. DVR is a series compensator which injects voltage to the point of common coupling (PPC) to maintain the voltage of sensitive load at nominal voltage. DVR can
also have some other features like harmonics elimination and power factor correction .
In this paper DVR and its operation principle are introduced. Then the control strategy and phase-locked loop (PLL) algorithm is described. In section 4 a DVR is modelled based on a hysteresis voltage control method. The
DVR performance under different power quality problems such as voltage sags voltage swells and harmonics distortion is verified and the effect of rated power of injection transformer connected in series with distribution feeder on capability of DVR to compensate harmonics is investigated. The simulation tool is the MATLAB/Simulink Power System Blockset.
2. DYNAMIC VOLTAGE RESTORER
DVR is a power electronic converter based series compensator that can protect critical loads from almost all supply side disturbances other than voltage interruptions. DVR is generally installed between supply and sensitive load feeder figure 1 .
The main components of DVR are: Voltage Source Converter (VSC) injection transformer passive filter energy storage unit DC link and control system .
The voltage source converter is the main block for DVR. VSC consists of an energy storage unit and switches that can generate a sinusoidal voltage at required frequency magnitude and phase angle. Because of the fast switching ease of control and low conduction losses of Insulated Gate Bipolar Transistors (IGBT) two-level converters employing IGBTs are suited for DVR applications.
Injection transformer links the DVR to the distribution network. It connected in series with the distribution feeder and introduces the compensating voltages generated by the VSC into the distribution system. It also isolates the load from DVR. One of the drawbacks of using injection transformer is the increase in loss plus it has a nonlinear behavior and can be a limiting factor regarding the bandwidth of the DVR system.
In order to block the high frequency harmonics generated by VSC a passive is used. The filter usually connected to the VSC output. In this case an extra inductor is employed to each phase. The passive filter can also be connected in parallel with the primary winding of the injection transformer. This connection is not recommended when the converter switched at high frequency in normal operation because the transformer winding would be under stress by the fast changes of the VSC output voltage .
Small voltage variations can be compensated by injecting reactive power to the PCC. But deep voltage sags/swells require active power in addition to reactive power in order to be compensated properly. A form of energy storage unit such as battery supercapacitor Super Magnetic Energy Storage (SMES) or flywheel is used to provide this real power. Energy storage device can be connected either directly or via a chopper to the DC capacitor of the VSC figure 2.
DVR has three modes of operation which are: protection mode standby mode and injection/boost mode.
In protection mode if the current on the load side exceeds its acceptable level due to fault or short circuit on the load DVR will be bypassed mechanically or electrically. In
standby mode the voltage winding of the injection transformer is short circuited through converter. In injection mode a voltage sag/swell has been detected and the DVR injects the compensating voltage through the injection transformer .
3. CONTROL STRATEGIES
The basic functions of a controller in a DVR are the detection of voltage sag/swell and harmonics distortion
events in the system computation of the compensating voltage generation of triggering pulses for converter and termination of the trigger pulses when the event has passed .
Voltage detection is important because its accuracy determines the dynamic performance of the DVR. Several voltage detection method such as rms value evaluation method peak value method and missing voltage technique have been developed for voltage compensation. In this paper the missing voltage technique is used. It defines as the difference between the desired instantaneous voltage and the actual distorted voltage .
For voltage sag/swell detection the software phase-locked loop developed in  is employed to lock the phase angle in the pre-fault value of the supply voltage and generate a sinusoidal waveform. For harmonic detection Discrete Fourier Transfer (DFT) is used to extract the fundamental and harmonic components from the distorted supply voltage. In this paper a conventional hysteresis voltage control technique is used . It is type of nonlinear voltage control based on voltage error. Voltage hysteresis control consists of a comparison between the output voltage Vo and the tolerance limits VH and VL around the reference voltage Vref. As long as output voltage is between the upper and lower limit no switching occurs but when the output voltage crosses to pass the upper or lower band the output voltage is decreased or increased respectively figure 3.
As shown in figure 4 Three-phase supply voltage fed into the harmonic detection block. In this block by subtracting the harmonic components from the distorted supply voltage its fundamental component obtained. The output signal of harmonic detection block fed into PLL to generate the desired sinusoidal waveform. Reference voltage obtained by subtracting the desired sinusoidal waveform from the distorted supply voltage. To generate the switching pulses of VSC the generated reference voltage is compared with the DVR output voltage and then hysteresis voltage control is employed.
4. SIMULATION RESULTS AND DISCUSSION
The MATLAB/Simulink model of DVR test system is shown in Fig. 5. Three different scenarios are simulated to evaluate the efficiency of control strategy. The system parameters and constant values are listed in table 1.
Figure 6 represents the results of simulation when three-phase to ground resistive fault occurs. The fault starts at 0.1s and last for five cycles of the fundamental frequency. Figure
6 illustrates how quickly the DVR respond to sudden changes to keep the load voltage at nominal value.
Figure 6 (b) and (c) shows the voltage generated by the DVR
and compensated voltage respectively.
The second scenario shows the DVR performance during voltage swell. In this case programmable voltage source is used as a supply voltage to simulate voltage swell and no fault is present. The source voltage has increased about 25% of its nominal value and voltage phase angle is shifted 20. The simulation results of the balance voltage swell are represented in figure 7. Figure 7 (b) and (c) shows the injected and load voltage respectively. The total voltage swell duration is 0.1s and it starts at 0.1 sec.
It can be observed that initially there is no voltage injection from DVR to the load because no voltage swell is sensed. As soon as the voltage swell occurs VSC injects the necessary voltage through injection transformer to the load so the voltage swell do not affect the load. The load voltage is kept almost 1 pu with the help of DVR.
TABLE 1 SYSTEM PARAMETERS
###power supply###Vph = 240 V f = 50 Hz
###240 Vph / 240 Vph
###injection transformer###r1 = r2 = 0.004 p.u
###x1 = x2 = 0.08 p.u
###AC filter###R = 1###C = 50 f
###Load###R = 15###L = 2.5 mH
###DC source###500 V
In the third scenario the DVR operation under harmonic distortion is verified. By the use of programmable voltage source the fifth and seventh harmonics are programmed to be superimposed on the fundamental signal of the source. The magnitudes of the fifth and seventh harmonics are 15% and 10% of supply phase voltage respectively. In addition to harmonic distortion a sag voltage starts at 0.1s and last for
0.1s. The distorted supply voltage is shown in figure 8 (a). The injected voltage which is produced by the DVR in order to compensate the load voltage and the load voltage which is maintained at constant level are shown in figure 8 (b) and (c) respectively. The THD of the supply voltage is 24.12% whereas the three phase average THD of the load voltage and current are 1.08 % and 0.7 % respectively. This indicates that the DVR can eliminate harmonic distortion as well.
The DVR maximum compensation ability considerably depends on rated power of injection transformer. The effect of rated power of transformer on THD of the load in third
scenario is investigated. As it can be seen from table 2
oversizing the transformer not even increase the investment cost but also increase the THD of load voltage.
TABLE 2 EFFECT OF TRANSFORMER RATED POWER ON VOLTAGE LOAD THD
###Phase A###phase B###phase C###Average
In this paper a DVR based on hysteresis voltage control has been modeled using Matlab SimPowerSystem" toolbox.
Time domain simulation of the DVR under different conditions including voltage sag caused by a three-phase to ground fault voltage swell and distorted supply voltage has been carried out to verify the ability of DVR to maint the load voltage around nominal value. Simulation results shows the DVR abilities in harmonic elimination as well as voltage mitigation. It is concluded that DVR successfully mitigated long duration voltage sags/swells and perfectly restored the load voltage almost 1 p.u. study on rated power of injection transformer shows although larger transformer has a better performance in sags/swells mitigation it increases THD of load voltage when the supply voltage distorted so there is a trade-off between rms value and THD of the load voltage.
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