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Research on TCR Type SVC System and MATLAB Simulation Zheng Shicheng Electrical and Information School of AnHui University of Technology Ma’ Anshan China 243002 e-mail : [email protected]

Fang Sian Zhang Gaoyu Electrical and Information School of AnHui University of Technology Ma’ Anshan China 243002 II.

Abstract—In this paper, the principle of Static Var Compensator ˄SVC˅is analyzed in detail, and a kind of SVC system which consists of Thyristor Controlled Reactors ˄TCR˅and Fixed Capacitor (FC) is designed. Bridge rectifier and L-R circuit is used to simulate the symmetrical three-phase load. Based on instantaneous reactive power theory, the SVC system signals are detected rapidly and precisely. In order to maintain the stability of the client node voltage and satisfy the requirements of system power factor, the voltage and admittance double loop control strategy is adopted, and the well-behaved steady-state effects is obtained. The break of node voltage caused by load disturbance or any other factors is restrained, and system power factor is well improved too.Finally, the SVC model is established under the environment of MATLAB simulation.The simulation results show that the proposed system can stabillize the fluctuation of power grid voltage and raise power factor, and the merits of fast response speed and high precision are verified.

PRINCIPLE OF SVC

Figure 1 is power circuit of the system. aǃbǃc is three phase power grid, and FC is reactive power compensators and filter banks. It can smooth 5,7,1 Ă harmonics, meanwhile provide reactive power. Taking aǃb phases for example, antiparallel thyristors are triggered in form of phase control, and there is 180ephase defference between the two devices at the same branch. Supposed trigger delayed angle of thyristor is Į ,according to the principle of TCR, when Į changes between 90 e and 18 e , the relationship between fundamental component of current within reactor and Į can be indicated as (1) :

Keywords- SVC, Instantaneous Reactive Power, Power Factor, MATLAB Simulation

I.

INTRODUCTION

With the development of power electronic technology, more and more high power non-linear devices, such as electrolyzers, electric arc furnaces etc, are applied in industry. Although productivity have been improved greatly, serious harms are brought to power grid system, such as low power factor and voltage waveform distortion. The common method taken by power sector is fixed capacitor compensator which has such advantages as simple constructure and low costs etc. However, due to the constant impedance, capacitor can only offer constant reactive power,it can not adapt transient changes of load and fulfil dynamic compensation. In addition, harmonics of system are easily amplified by capacitor, so phenomena as overload, superheat and excessive loss often appear. The capacitor is easy to be expanded even exploded for a long running[3]. According to these situations, the client SVC unit is designed applying TCR +FC style in this paper.Three-phase non-conctrolled rectifier in series with L-R load is adopted to simulate the three-phase symmetrical load, and thyristor switched load is used to simulate instantaneous disturbance. The volume of this SVC system ranges -300kvar to +300kvar

c 978-1-4244-5046-6/10/$26.00 2010 IEEE

Figure 1.The power circuit of SVC

˄1˅ Here,Umab is voltage peak value between phase a and b ; XL is react- ance of reactor in series with thyristor. When Į=90 e , thyristor is on fully, and i1 reaches the highest value. When Į=180 e , thyristor is off entirely, i1 reaches the lowest value. So, changing Į can control the reactive current within reactor, namely, TCR could absorb varying reactive power from power grid [1]. As the load of power grid is time-variant, the reactive power’s variable quantity is supposed Δ Q lo a d ,changing Į timely could keep reactive power steady in system. So, to

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stabilize the voltage of power grid and obtain the desired results, the reactive power offered by TCR is only equal to the variable quantity of the load. III.

APPLICATION OF INSTANTANEOUS REACTIVE POWER THEORY

The precondition of dynamic compensation is that the needed voltage and current signals are detected out rapidly and accurately. Supposed the three-phase voltage is ua䫺 ub䫺 uc , the three-phase current is iaǃ ibǃ ic, the current in FC is isaǃisbǃisc, and the current in TCR is iTabǃiTbcǃiTca respectively. The threephase voltage used for calculation is expressed as (2) : ea = E

m

s in ω t

eb = E

m

s in (ω t-2 π /3 )

ec = E

m

s in (ω t+ 2 π /3 )

˄2˅

Here,Em is the peak value of voltage. Converting (2) to be (3):

α ǃβ

§ eα · ¨ ¸= © eβ ¹

coordinates,and showed as

§ sin ω t · 3 Em ¨ ¸ 2 © − cos ω t ¹

2 2π 2π [ua sin ωt + ub sin(ωt − ) + uc sin(ωt + )] ˄10˅ 3 3 3 In (7) (8)and (9) , IFC,ITCR and ULine are DC quantities corresponding to the fundamental components of threephase current and voltage respectively [2]. U Line =

IV.

DESIGN OF FC AND FILTER

Inductor is in series with capacitor within FC branch. To filter out the specific harmonics, the L and C should be resonated at a giving frequency. At the same time, for the fundamental wave, reactive power provided by capacitors is greater than consumed by inductors, hence, reactive power is provided to the power grid as a whole. According to the character of the load, three-phase bridge rectifier in series with L-R is used to simulate symmetrical three-phase load. The currents in system and TCR are analyzed with FFT in MATLAB , the results are shown in Figure2.From it, the main harmonics are 5 ǃ 7 ǃ 11 ǃ 13… ˄ 6k±1 ˅ respectively, single tuned filters are designed to filter 5ǃ7ǃ11 ǃ13 and 17 ,and high-pass filter is used to filter 19 and higher harmonics.

˄3˅

According to instantaneous reactive power theory, formulae could be get as follows:

§ p· 3 § sin ωt − cos ωt · § iα · ˄4˅ ¨ ¸ = Em ¨ ¸¨ ¸ 2 © − cos ωt − sin ωt ¹ © iβ ¹ ©q ¹

§ ip ¨¨ © iq

· § sin ω t ¸¸ = ¨ ¹ © − cos ω t

− cos ω t · § iα ¸¨ − sin ω t ¹ © iβ

· ˄5˅ ¸ ¹

Three-phase current ia䫺 ib䫺 ic is shown as (6) after 3/2 conversion. § iα ¨ © iβ

· ¸= ¹

§1 2¨ ¨ 3¨ ©0

−1 / 2 · § ia · ¸ ¨ ¸ ˄6˅ ¸ ¨ ib ¸ ¨ ¸ 3 / 2 − 3 / 2 ¹¸ © ic ¹



−1 / 2

By substituting (5) into (4), (7) is get as follow: 2π 2π · § sin(ωt − ) sin(ωt + ) ¸ ¨ sin ωt § ia · 3 3 § ip · ¸ ¨ ¸ ˄7˅ 2¨ = ¨ ¸ • ¨ ib ¸ ¨¨ i ¸¸ 3¨ ©q¹ 2π 2π ¸ ¨© ic ¸¹ ¨ − cosωt − cos(ωt − ) − cos(ωt + ) ¸ 3 3 ¹ © Supposed IFC=iq, it is obtained as (8).

2 2π 2π [isa cos ωt + isb cos(ωt − ) + isc cos(ωt + )] ˄8˅ 3 3 3 The current of TCR is get as (9).

I FC = −

Figure 2. The system current waveform after three-phase non-controlled rectifier in series with L-R load and analysis with FFT.

According to the volumeof system, FC should provide 300kvar reactive power in total, so each phase should provide 100kvar. In order to make the capacitors of FC have the same harmonics voltage in actual applying, formula (11) is used commonly to assign the volume of compensation [4]

Q cn =

In / n

¦

× Qc

(11)

Im / m

Here, Qc is the total volume of compensation. Q c n is the nfilter volume of compensation. The formula (12) is used to calculate the value of capacitor.

6 2π 2π ITCR = − [iTab cos ωt + iTbc cos(ωt − ) + iTca cos(ωt + )] ˄9˅ 9 3 3

The same reason for the voltage, (10) is obtained.

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C =

Q c n (1 −

ω 1U

1 ) n2

2 p

(12)

Here,Up is the virtual value of phase voltage. According to resonant frequency, the value of L could be get from formula (13).

L =

1 ( nω 1 ) 2 C

(13)

For high pass filter, R is decided by formula (14). R =

1 nω 1 L m

(14)

the dynamic response speed of system to load disturbance. When reactive power admittance changed, according to the quantity detected, TCR computes out Į to offset it. At the same time, when the active power admittance of load also changes, the PI regulator produces a given value of admittance to assure the voltage constant. The power factor of system changes due to the variation of the given value[5], and the power factor ranges 0.95 to 1 in the system. The control part also includes non-linear segment and six pulse generator. According to (1), the relationship between fundamental component of the current within reactor and Į is non-linear, so linearlization segment was brought in to make the output of PI regulator consistent with i1. The lookup table model of simulink in Matlab is adopted to fulfill the linearlization in this design. Simulation model of control section is shown in figure 4.

In engineering application, m ranges 0.5 to 1 normally. Applying the above formulae and considering the requirements of compensation volume in design, taking a phase for example, the relative parameters could be used as follows: C5a = 0.00013074F

L5a = 0.0031H

C7a = 0.0016F

L7a = 0.00012924H

C11a = 0.0066723F

L11a = 0.0001255H

C13a = 0.00047856F L13a = 0.00012528H C17a = 0.00028148F L17a = 0.00012455H C19a = 0.00022558F L19a = 0.00012442H R19a = 0.8Ω Figure 4. Control section of SVC in Matlab

V.

Six pulse generators are used to trigger TCR and its model in Matlab is shown as figure5.The trigger order is as following:

CONTROL STRATEGY OF TCR

The goal of the control system is as following: When load disturbs, the voltage of power grid could be controlled to keep an invariable value.

Uab → Uac → Ubc → Uba → Uca → Ucb → Uab →!

Make sure the power factor of the systemı0.95. The control strategy of SVC is shown as figure 3.

Figure 5. The block of TCR and six pluse generator.

 Fgure 3. The control strategy of SVC

According to the control requirements, voltage-closed loop must be lie in outer-loop to make voltage constant, and innerloop is an admittance compensation loop, it aims to increase

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In this paper, the total volume of TCR is 600kvar, so, each phase consume 200kvar reactive power. Taking aǃb phase for example, the value of L can be obtained from (15). Q =

U

2 ab

ω 1L

= 200000

So, Lab=0.00231094H.

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(15)

VI.

EFFECTIVENSS OF FILTER AND DYNAMIC SIMULATION

100

0

-100 Q / Var

The phase current waveform and the anaylsis with FFT is shown as figure 2 and figure 6. Figure2 is the result without FC, and figure 6 is the result after FC is switched into the system.

200

-200

-300

-400

-500

-600

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.35

0.4

0.45

t / s

(c) The reactive current of system 120

100

a/ deg

80

60

40

20

0

0

0.05

0.1

0.15

0.2

0.25

0.3

t / s

Figure 6. Phase current waveform and the anaylsis with FFT after FC is switched into system

(d) The changes of Į in the system

TCR is switched into the system at the time of 0.1s. At 0.2s, the load disturbance appears when Resistance-Inductance Load is switched on by thyristor at the side of DC. The dynamic response of system is shown as the following figure7.

600

400

200

800

I/A

600

0

400

-200

black:I/A

200

red:U/V

0

-400 -200

-400

-600 -600

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

t/s

(a) The waveform of system phase voltage and line current Red:voltage,black:current

0.1

0.15

0.2

0.25 t/s

0.3

0.35

0.4

0.45

0.5

(e) The phase current of TCR

The variation of bus voltage waveform of system is shown as figure 7(a), and TCR is switched into the system at the time of 0.1s. The result shows that system voltage is raised by capacitor firstly, and rapidly drops to the given value after 0.1s during 1 to 2 periods. The system current changes from leading phase to lagging phase compared with voltage phase, and the desired power factor is achieved.

400 350 300

U/ V

0.05

Figure 7.The simulation waveform of the system

450

250 200 150 100 50 0

0

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

t/s

(b) The effective value of system line voltage

0.4

0.45

At the time of 0.2s, the Resistance-Inductance Load is connected to system by thyristor. Then, the system voltage scarcely changes and the system current increases, reactive power nearly reduces to 0, and the power factor nearly turns to 1. Meanwhile, trigger angle Į raises and the consumption of inductive reactive power in TCR decreases. From figure 7, some conclusions are drawn as follows: (1) When load disturbs, the bus voltage value of system can be kept constant rapidly, namely the dynamic response speed is better.

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(2) After TCR is connected to system, the power factor is more than 0.95.

REFERENCES [1]

VII. CONCLUSIONS Based on instantaneous reactive power theory, a TCR+FC type of SVC system is designed, and the model is established in MATLAB software. Simulation results shows that each index of the system can satisfy the requirement of power sector, and the power grid harmonics are restrained effectively. The simulation also offers theoretical direction for the further development of the device which is based on DSP. ACKNOWLEDGMENT

[2] [3] [4]

[5]

R.Mohan Mathur, Rajiv k.Varma,xuzheng. thyristor-based facts controllers for electrical transmission systems[M] Beijing, China Machine Press,2004. Wang Zhaoan, Yang Jun, Liu Jinjun Harmonics elimination and reactive poler compensation[M] Beijing, China Machine Press, 2006. George J. Wakileh. Power Systems Harmonics[M] Beijing, China Machine Press,2003. Hung Lifen,Jiang Jianguo.Study of novel SVC control system based on double closed-loop PID controller journal of system simulation[j] 2007,19(8) page(s) 1803㨪1806 Mathur, R. M. Varma, R. K., Thyristor-Based FACTS Con- trollersfor Electrical Transmission Systems, Wiley-IEEE Press, February 2002.

This work is supported by the project numbered 50707003 from National Natural Science Foundation of China.

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