Compensating of voltage flicker and disturbance using Dynamic Voltage Restorer based on matrix converter

Abstract: Rapid voltage variations in general and voltage flicker in particular caused by arc furnaces or welding plant can be harmful for sensitive loads. In this paper a dynamic voltage restorer (DVR) is used for compensating the voltage flicker and severe disturbances. This topology is based on a 3-three phase to a single phase matrix converter (MC) where the ac to ac conversion is provided without any dc energy storage units.Therefore, unlike the conventional DVR the faster response andthe reduced size of DVR are prepared. This structure also can be used to restore the voltage sags and swells to protect sensitive loads. To control the DVR, various compensation algorithms have been applied on this matrix based compensator.Its performance under different conditions is examined through simulation studies using PSCAD/EMTDC.Keywords: matrix converter, flicker, DVR, voltage disturbance, sensitive load.

1. INTRODUCTION

Voltage disturbances like voltage sag, voltage swell, flicker and harmonics can cause significant problems for sensitive loads. The problem of power quality has received an increasing interest during recent years. The concept of power quality includes the quality of the source voltage regarding voltage sags, swells,flicker and harmonics [1]. Voltage sag is defined as dropping of the voltage below 90% of the nominal value. Voltage sag is often occurs as a result of network faults, and lasts until fault is cleared. Typical time duration of this phenomenon is a half cycle to 1 minute [2]. Also Voltage swell is defined as an increase in voltagemagnitude from 1.1 to 1.8 per unit during half cycle to 1 minute. Unloading large inductive loads is one cause of the voltage swell [3]. Voltage flicker is the voltage fluctuation in the frequency range from about 5 to 15 Hz emanated from large industrial loads, such as electric arc furnaces or welding plant [4]. Dynamic voltage restorer (DVR) is a promising custom power device which can be used to alleviate these problems. This series connected compensator can inject three phase or single phase voltages with suitable amplitude, frequency and phase to isolate sensitive load from grid side voltage disturbances

[5]. Conventional DVRs are based on a dc link that usually consists of more than one stage of power electronic converter. In this structure, the input ac power is initially converted to dc by means of a rectifier. Then the dc voltage is converted to an ac voltage with controllable phase and magnitude using an inverter. Different control strategies have been proposed for DVRs. In [6] some strategies are analyzed for different types of voltage disturbances. The used energy storageelements in conventional systems result in a higher cost,bigger size and difficulties in controlling the stored energy on the onset of faults [7]. The matrix converter (MC) has attracted considerable attention in academy and industry especially for applications in motor drives, power supplies, and wind energy conversion systems [8]. The MC is a switch-mode, direct AC to AC power converter which uses a set of controlled bidirectional switches. The matrix converter eliminates the need forbulky electrolytic capacitor, so there is the possibility for a more compact design. In [9] a DVR based on a matrix converter is presented. A flywheel is utilized to store energy for compensating voltage sag and short period voltage interruptions, but the storage unit has increased the total cost of the system. In [10] a zero-energy sag corrector (ZESC) structure has been introduced, without using a capacitor but it is not able to compensate the voltage swell and flicker. To compensate the voltage flicker, in [1, 11] a STATCOM is used. In [12] also a DVR is used for voltage flicker mitigation. However, in all these structures there exists an energy storage unit with its attendant disadvantages. To eliminate the dc link a three-phases to a single phase topology is proposed in [14] which can compensate the voltage unbalance and disturbances excluding the voltage flicker. In the present paper attempts have been made to provide full compensation using the same topology. To do so the necessary simulations are accomplished using PSCAD software.

Hojjat Daghani, Reza Ghazi, Hadi GhasemabadiFerdowsi university of mashhad, mashhad, iran

hojjat.daghani@gmail.com; rghazi@um.ac.ir; ghasemabadi.hadi@gmail.com

S DVR LV V V 1 p.u.

( ) ( ) ( )

( ) ( ) ( )

( ) ( ) ( )

L a S a DVR a

L b S b DVR b

L c S c DVR c

V t V t V t

V t V t V t

V t V t V t

2. DVR CONFIGURATION Several power circuit topologies are available for

DVR based on voltage source inverter (VSI) using IGBT switches. The output of the DVR is connected in series with distribution feeder using injection transformer as shown in Fig.1 In this structure for full voltage compensation the following constraint must be satisfied for all grid voltage disturbances:

(1)

The energy storage system may be a capacitor, a battery or a flywheel. The size of the storage unit is defined by the voltage dips magnitude and its duration [13].

Fig. 1: Basic topology of a typical DVR

In this paper, to eliminate the troublesome dc link the same topology of Ref. [14] is employed. This topology is based on a three phase to a single phase MC as shown in Fig.2 The MCs inputs are directly connected to the grid on source side and their output connected to the grid on load side through three single phase injection transformers. This structure is depicted in Fig.3. Bidirectional switches are implemented by back to back IGBT and diode switch sells. By eliminating the energy storage unit, DC capacitor and minimizing the size of filters in the DVR configuration, it is possible to pack all the modules into an small size equipment [15].

Considering Fig.3 the voltage equations of load would be as follow:

(2)

In (2), L subscripts represent load, S used for source, and a, b, c subscripts is used to show the first, second and third phases, respectively.

Fig.2: Configuration of three-phase to single-phase AC/AC matrix

converter whit 6 switches

Fig. 3: Structure of used DVR

This MC can produce 64 different switching modes. These switching modes are reduced to 7 modes considering the constraints as shown in Table I. These modes subscribed 7 different voltage levels can be obtained from this converter. We can observe that the instantaneous maximum output voltage amplitude is confined to 3 times of maximum input voltage amplitude [14].

TABLE I: Permissible switching modes for purposed structure

Mode ON switch ov oi

1 S11 & S22 va-vb 1ii

2 S21 & S32 vb-vc 2ii

3 S12 & S31 vc-va 3ii

4 S12 & S21 vb-va 2ii

5 S22 & S31 vc-vb 3ii

6 S11 & S32 va-vc 1ii

7 (S11 & S12 ) or (S21 & S22 ) or (S31 & S32 )

0 0

, ,mod ( )1, ..., 6( ) ( ) ( );

i o desired o e iiE t v t v t

11 22

21 32

1 2 3 4 5 6 1

1 2 3 4 5 6 2

&

&

{min(| |,| |,| |,| |,| |, | |) | |}

{min(| |,| |,| |,| |,| |, | |) | |}

.

THEN S ON S ON

THEN S ON S ON

IF E E E E E E E

IF E E E E E E E

1 2 3 4 5 6

.

{min(| |,| |,| |,| |,| |, | |) | |}k

THEN The Switchs in Mode k will be ON

IF E E E E E E E

2

1

1

1 2 cos( )

sin( ) sin( )tan ( )

cos( ) cos( )

V V V

V ( )DVR

DVR

postfault postfaultSLoad

k k

k

k

k

x

1V ( )

1

DVRk

x k

3. GENERATING REFERENCE VOLTAGE AND FIRING PULSES

Different strategies can be used to control the DVR. Three basic strategies are used to achieve this goal that analyzed in [6]: the pre-fault compensation, in-phase compensation, and the energy optimized compensation strategies. In this paper the pre-fault and in-phase methods are used. The pre-fault method introduced as pre-flicker method in [12]. This strategy is mostly used for loads that are sensitive to phase jump such as thyristor controlled drives. In other word, both magnitude and phase of the load voltage are compensated in this method [6]. The control mechanism of these converters is based on the minimum difference between the desired output voltage and the input voltage for each modes of converter:

(3)

In this strategy, the output voltage in different permissible modes of MC is compared with the desired output, and the error is calculated for each operatingmode. The minimum absolute value of error is recognized and the best mode is chosen. So according to the mode chosen, the switch turn on and turn off signals are sent to MCs through driver circuits [7]. In other words we have:

Such control method will reduce the number of switching actions so leading to lower losses and higher efficiency. Figure 4 shows this kind of compensation in per-unit plane.

Fig.4: Pre-fault compensation strategy

1 0ILoad

prefault is the load current before disturbance

that is considered as the reference vector and

1VLoad

prefault is the load voltage before fault.The

disturbance might change the source voltage magnitude from 1 to k with δ degree phase jump. Therefore

V ( )S

postfaultk is the source voltage after

disturbance. DVR should inject a voltage (V )DVR

x

with appropriate magnitude and phase to compensate this fault. Also the load voltage and current after compensation are the same as their values before

disturbance which means V Vpostfault prefault

Load Load and

I Ipostfault prefault

Load Load . By these definitions, the DVR injected

voltage can be calculated as follows [6]:

(4)

(5)

(6)

This control strategy is very useful for loads that are sensitive to phase jump. But for loads that are not sensitive to phase jump the in-phase method can be used. In the present control method, the DVR only compensates the load voltage magnitude. Figure 5 shows this kind of compensation in per-unit plane. In this method:

(7)

(8)

(9)

Using this method for loads that are not sensitive to phase jump will increase the efficiency and the correctoperating range of the proposed DVR as this strategy utilizes the minimum of injection voltage magnitude for compensating. For the proposed structure, the energy optimized compensation method does not provide superior results than two presented methods since this topology does not have any energy storage unit.

Fig.5: In-phase compensation strategy

Flicker Mitigation with DVR based on Matrix Converter

0.000 0.050 0.100 0.150 0.200 ... ... ...

-400

-200

0

200

400

VS_a

-400

400 VDVR_a

-400 -300 -200 -100

0 100 200 300 400

VL_a

4. SIMULATION RESULTS

In this section, simulation results are rendered to verifythe performance of the DVR configuration. The systemparameters are given in Table II. The system is simulatedusing PSCAD/EMTDC software. The voltage variation ±25% p.u with frequency of 8.3 Hz added to the three phase voltages of network to simulate the voltage flicker. As shown in Fig. 6, the proposed method can welldetermine the reference voltages of DVR and compensatethe voltage flicker. Also this DVR configuration cancompensate other type of voltage disturbances, such assags, swells and severe voltage distortions as shown inFigs.7 &8, respectively.

As shown in Fig.7, the input voltage has not any voltage disturbances between 0 to 0.02 seconds, so DVR does not generate any voltage and the load voltage is equal to nominal voltage during this time interval. Between 0.02 to 0.06 seconds, the input voltage magnitude decreases to 0.7 p.u, and DVR injects required voltage to the grid. Therefore the magnitude of the load voltage remains in 1 p.u. Between 0.08 to 0.12 seconds, the magnitude of source voltage increases to 1.5 p.u and DVR restores the voltage to 1 p.u. In Fig.8 the inputvoltage is severely distorted and the DVR can compensate such disturbances. In addition in all cases the balanced condition is provided.

TABLE II: System parametersSource Load Injection Transformer Filter

220 ( )SV rms v

50i Hf z30 LR

17LX

0.01tR

0.023tX

0.2a (turn ratio)

1 mhfL

30 μffC

3 fR

Fig.6: Simulation results before and after the voltage flicker: (a) grid voltage; (b) DVR injected voltage; (c) load voltage;

a

b

c

Fig.7: Simulation results for voltage sag and swell: (a) input voltage; (b) DVR injected voltage; (c) load voltage;

Fig.8: Simulation results for for severe distortion of input voltage: (a) input voltage; (b) DVR injected voltage; (c) load voltage;

Harmonics Voltage Compensating in Unbalanced Condition with DVR based on Matrix Converter

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 ... ... ...

-500

500 VS_a VS_b VS_c

-500

500 VDVR_a VDVR_b VDVR_c

-500

500 VL_a VL_b VL_c

Voltage Sag and Swell Compensating with DVR based on Matrix Converter

0.000 0.020 0.040 0.060 0.080 0.100 0.120 0.140 ... ... ...

-500

500 VS_a VS_b VS_c

-500

500 VDVR_a VDVR_b VDVR_c

-500

500 VL_a VL_b VL_c

a

b

c

a

b

c

5. CONCLUSION

In this paper a DVR employing a 3-phase to single phase matrix converter (MC) without energy storage is studied for voltage flicker mitigation and compensating severe disturbances in unbalanced conditions. Two control strategies for this structure are presented. The pre-fault method to compensate loads which are sensitive to phase jump. The in-phase method to compensate loads that are sensitive to magnitude rather than phase jump. The method is more efficient and increases the correctoperating range of the proposed DVR. In this paper, itwas shown that this structure can well compensate any voltage fluctuations including flicker resulting from special loads. Simulation results are provided to verify theperformance of the proposed DVR.

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