Introducing a New Method for UPQC Control to Solve the Power Quality Problems

A. Kazemi, R. Rezaeipour IEEE Conference Publishing

Center of Excellence for Power Systems Automation and Operation, Iran University of Science and Technology, Tehran, Iran kazemi@iust.ac.ir, r.rezaeipour@yahoo.com

Abstract-In this paper a new method has been offered to control the UPQC as a device for power quality improvement in distribution networks. Using the time domain analysis of reference signal, we can instantaneously adjust the performance of series and parallel UPQC filters such that several problems of power quality including notch, sag, swell, flickers and existence of harmonics and load imbalancing can be solved. Simulation of the recommended method was carried out on two sample systems and the results of simulation show that the proposed method is able to provide the desirable power quality in the presence of a wide range of disturbances.

I. INTRODUCTION

Usually the power system faults, non-linear loads such as power electronic devices and dynamic response of system cause some disturbances including sag, swell, flicker, voltage notch and harmonics in a distribution system. On the other hand, the increased use of sensitive electronic circuits by industries and households together with privatization and competition in electric energy systems, posed the power quality improvement as one of the major problems in electricity industry.

Considering the high speed of response and flexibility in operation of power electronic devices, these equipments are seriously noticed to solve the power quality problems. UPQC is one of these devices, which has undeniable capability in improving the power quality status [1]-[3].

Although the UPQC, with the possibility of injecting active and reactive power in series and parallel branches, can considerably compensate the power quality events, its correct function depends on applying a suitable control method.

Different methods have been used for controlling UPQC in order to solve several problems of power quality [4] including: neural networks[5], application of optimal control [6], sliding mode control [7], wavelet transform [8], predictive control [9] and many other heuristic methods [10-12]. However, there is complexity of calculations in classic methods and slowness or low quality of response is observed in some methods of frequency domain.

Also, some of the previous methods can not solve all the problems of power quality. A new control strategy for UPQC control is suggested in this paper, which has high speed and precise response besides the capability of solving some of power quality problems.

The other parts of this paper are as follow: the structure of UPQC is studied in detail in part 2. The suggested control method is explained in part 3 and the results of simulation are presented in part 4 using two case study for considering two typical power quality events and finally the conclusion is presented in part 5.

II. STRUCTURE OF UPQC

Principally the structure of UPQC includes two active parallel and series filters. Figure 1 shows the arrangement of these filters in the network. Series active filter acts as a voltage source in series with the network and produces any form of wave given to it by the series controller using the PWM converters. On the other hand, the parallel active filter acts as a parallel current source with the network and is controlled using the parallel controllers.

Fig. 1. General Structure of a UPQC in the Network

Considering that UPQC should be able to supply the

active and reactive power, a fast energy storage, like capacitor, is used in the DC side of power electronic converter of active filters.

III. PRESENTATION OF CONTROL METHOD

The control method suggested in this section includes two parts, that is, series and parallel control parts, which are applied on active parallel and series active filters, respectively.

A. Series Control

The series control signal applied on active series filter should be in a way that we can obtain complete sinusoidal voltage in load side through applying it in series with non-sinusoidal and unbalanced source.

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For this purpose it is necessary to obtain a reference signal which the complete corrected sinusoidal voltage should be in its form. The development stages of signal for series control are shown in diagram Figure 2.

Fig.2. Series Control Diagram

Regarding to figure 2, if the voltage signals of ua, ub, and

uc, are three-phase voltages include distortion, they will only have fundamental frequency component after passing the low pass filter. Now using the equation (1), the components of their instantaneous positive sequence, those are +

au , +bu and +

cu will be calculated:

−−

−+

−−−−−−

=

+

+

+

c

b

a

c

b

a

c

b

a

uuu

0DDD0DDD0

63

uuu

15.05.05.015.05.05.01

31

uuu

ω

(1)

where D is the derivative operator.

Then, the effective value of positive sequence voltage is determined through equation (2) instantaneously.

( )

= ∫

++

tT

t

2uT1absU (2)

Now the waveform of the main components of distorted

voltage which for phase a is as equation (3) can be accessed:

( )αω +=+ tsinU2ua (3) Now we can create the waveform of the reference voltage

according to equation (4) through correcting the effective value of voltage in equation (2):

( )αω +== + tsinV2uUVv aa (4)

where V is the effective value of desirable voltage. Finally, the control signal of series active filter is obtained through comparing the reference signal obtained from equation (4) and distorted signal. B. Parallel Control

Parallel active filter control should be in a way that the reactive power of load be completely supplied by UPQC in order to let the load power factor be %100 in the network point of view. So, this principle was used in parallel control signal preparation and the reference current is calculated after computing the instantaneous active power of load.

The instantaneous power of load can be determined through equation (5):

ccbbaa iuiuiup ++= (5)

As we know the DC amount of power obtained from the

above equation, is the load active power. Therefore, we can calculate this active power at any instance using a low pass filter. Now considering that the reference current should be in a way that only the active power is observed under the voltage calculated through the equation (2), we have:

c,b,aiuU3pi i2i == (6)

where p is the DC amount of the instantaneous power of load (instantaneous active power of load).

Equation (6) offers the reference current of each phase and through comparing it with the load current, we can obtain the parallel control signal of UPQC.

IV. SIMULATION RESULTS

In this section, the simulation study was performed using SIMULINK software in MATLAB environment for two different power quality events in order to study the performance of the suggested method.

A. Existence of Harmonic & imbalance in Source Voltage & Load Current

The system being studied in this stage, is a 400 V three-phase network with harmonic content shown in table (1). This system feeds a load including two linear and non-linear loads. Linear part is a three phase balance 10 kW constant power load with the 0.8 lag power factor. But the non-linear part is a current constant load with harmonics of table (2).

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The study was performed for 0.1 second time interval and UPQC is considered to operate at t=0.05 s.

Table 1: Harmonic Components of Source Voltage

Phase shift against phase a % Harmonics 20 deg %50 3 60 deg %30 5

Table 2: Components of Non-Linear Load Currents

Phase

Harmonic Component

a [A] b [A] c [A]

0 0 20 0 1 150 80 0 3 20 45 0 5 50 0 0

Figure 3:A-C shows harmonic voltage of source,

compensating voltage of UPQC and well-corrected load voltage, respectively.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-500

-400

-300

-200

-100

0

100

200

300

400

500

Sou

rce

Vol

tage

(v)

time (s)

3-A

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

time (s)

UP

QC

Vol

tage

(v)

3-B

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-500

-400

-300

-200

-100

0

100

200

300

400

500

time (s)

Load

Vol

tage

(v)

3-C

Fig. 3. Harmonic correction of voltages; A: Source Voltage; B: UPQC Voltage; C: Corrected Voltage at load side

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

-150

-100

-50

0

50

100

150

time (s)

Sou

rce

Cur

rent

(A

)

4-A

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

-150

-100

-50

0

50

100

150

time (s)

UP

QC

Cur

rent

(A

)

4-B

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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-200

-150

-100

-50

0

50

100

150

200

time (s)

Load

Cur

rent

(A

)

4-C

Fig. 4. Harmonic and imbalancing correction in currents; A: Load current; B: UPQC Current; C: Corrected current at the source side

Also, figure 4:A-C shows that despite high level harmonic

pollution and imbalance in load, the current has a perfect sinusoidal and balanced shape from viewpoint of the network.

To show the ability of proposed method in voltage regulation, the RMS value of load voltage is illustrated in Figure 5. This figure shows that after operation of UPQC, the RMS voltage is reduced to its desirable value after passing a short transient.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

50

100

150

200

250

300

time (s)

RM

S V

olta

ge o

f Lo

ad (

V)

Fig. 5. Effective value of load voltage before and after UPQC operation

B. Existence of voltage notch

In this stage, a 6-puls thyristor rectifier which is connected to a 400 V network through Ω= 01.0R and mHL 1= is considered as a voltage notch generator. This rectifier feeds one DC motor with Ω= 1aR and mHLa 20= at its DC side.

Figures 6:A-C show the network voltages, UPQC and corrected voltage of load, respectively. As it is seen in

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

Sou

rce

Vol

tage

(v)

time (s)

6-A

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

time (s)

UP

QC

Vol

tage

(v)

6-B

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-400

-300

-200

-100

0

100

200

300

400

time (s)

Load

Vol

tage

(v)

6-C

Fig. 6. Voltage correction of notch generator load A: Source Voltage B: UPQC Voltage

C: Corrected Voltage at load side

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0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-40

-30

-20

-10

0

10

20

30

40

time (s)

Load

Cur

rent

(A

)

7-A

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-40

-30

-20

-10

0

10

20

30

40

time (s)

UP

QC

Cur

rent

(A

)

7-B

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-40

-30

-20

-10

0

10

20

30

40

time (s)

Sou

rce

Cur

rent

(A

)

7-C

Fig. 7. Harmonic & imbalancing correction in currents of notch generator load; A: Load current;

B: UPQC Current; C: Corrected current at the source side

Figure 6-C the load voltage is completely corrected after t=0.05 s which UPQC is activated.

Also Figures 7:A-C show the load, UPQC and network side corrected currents, respectively. As it is observed, the suggested control method managed to solve the problem after passing the short temporary state and correct the non-sinusoidal and unbalance current of the load.

Figure 8 shows the RMS value of load voltage before and after connection of UPQC to circuit. As it is seen there is a short transient in correction around one power cycle due to time needed for exact amount computation by RMS block calculator.

Study of the load voltage and current frequency spectrum in Figures 9:A-B indicates a wide range of harmonic pollution, which was completely removed using the suggested method according to the Figure 9-C.

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

50

100

150

200

250

time (s)

RM

S V

olta

ge o

f Lo

ad (V

)

Fig. 8. Effective value of load voltage before and after UPQC operation

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

Frequency (Hz)

Har

mon

ic P

erce

nt (%

)

9-A

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

Frequency (Hz)

Har

mon

ic P

erce

nt (%

)

9-B

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0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

120

Frequency (Hz)

Har

mon

ic P

erce

nt (%

)

Fig. 9. Frequency content spectrum; A: Source voltage; B: Load Current;

C: Corrected Load Voltage and Corrected Source Current

V. CONCLUSIOns A new method for UPQC control based on calculation of

reference signal in time domain is proposed in this paper. The suggested method is very simple and fast and acts based on the conditions of network in any time, regarding to remove any power quality problem from the distribution system. The proposed control method includes series and parallel parts which the former part corrects the load voltage to a perfect sinusoidal form with exact RMS value and the later one tries to shape the load as an active balance linear one from the network point of view. Computer simulation was performed using SIMULINK on two typical case studies. One system with harmonic distorted source and nonlinear imbalance load and the other with the voltage notch. The results show that the suggested method managed to completely remove the disturbances in all cases.

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