A Statcom Simulation Model To Improve Voltage Sag Due To Starting Of High Power Induction Motor

National Power & Energy Conference (PECon) 2004 Proceedings, Kuala Lumpur, Malaysia

148

A STATCOM Simulation Model to Improve Voltage Sag Due to Starting of High Power Induction Motor A. F . Huweg, S . M. Bashi- and N. Mariun

Abstmct--A simulation model of static synchronous compensator (STATCOM) has been constructed on Matlab/simulink software t o examine its capability for voltage sag mitigation due to starting high power induction motor. i n this paper, the main structure of simulink (STATCOM) model is described briefly. Its capability to compensate reactive power to the system when the voltage sag occurs was described. A phase control thyristor (SCR) based voltage source inverter (VSI) is employed for this appIication. The influences of the initial operation point and DC capacitance are considered. The behavior of this system during voltage sag caused by starting of motor load has been examined. Simulation result shows the fast response and the STATCOM capability for mitigate voltage sag.

sags as shown in Fig .1 may cause tripping, production disturbances and equipment damages. The concern for mitigation voltage sag has been gradually increasing due to the huge usage of sensitive electronic equipment in modem industrial.

Index Termr--voltage sag, voltage sag mitigation, static synchronous compensator, voltage source inverter.

I. INTRODUCCION

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n the past, equipment used to control indushal process was mechanical in nature, which was rather tolerant of voktage disturbances. Nowadays, modern industrial equipment typically uses a large amount of electTonic components, such as program logic control (PLCs), adjustable speed drives and optical devices, which can be very sensitive to such voltage disturbances. The must majority disturbance that causes problems for electronic equipments is voltage sags. Voltage sag is defined as a decrease between 0.1 and 0.9 p.u. in the m voltage at the network fundamental frequency with duration from 0.5 cycles to few seconds. This defrnition assumes that the sag can be defined by a magnitude and duration. The duration is usually associated with the time taken by the protection system to clear the fault, Voltage sags are huge problem for many industries [1,2] &d they have been found especially troublesome because h e y are random events lasting only a few cycles. However, they are probably the most pressing power quality problem facing many industrial customers today [33. Voltage A. F . Huweg, S. M. Bashi and N. Mariun are with Depamnent of Electrical and Elecmnic Engineering, Faculty of Engineering, Univcrsiti P u h Malaysia, 43400 Serdang, Selangor, Malaysia (e-mail: senan@ eng.upm.edu.my).

0-7803-8724-4104/$20.00 02004 IEEE.

Fig.1. Typical waveform of voltage sag

This paper engaged with voltage sag caused by induction motor. The induction motors are subjected to the voltage sag slow down, but usually do not stop operating, if not tripped by contactors. Problems can occur due to torque oscillations that can be associated with very deep sags or to tripping of over current reIays, due to the high currents drawn by the motor. During the voltage sag, an induction motor siows down and requests higher current. If the sag is unbalanced, it is also subjected to a negative sequence voltage and therefore it absorbs a large negative-sequence current component, because the negative-sequence impedance of the motor is usu$y low. High is also the current drawn by the motor after the voltage has recovered, necessary to rebuild the air-gap flux and reaccelerate the machine [5]. This phenomenon causes extended post-fault sag with a long duration (one second or more) if the motor load is large with respect to the system impedance. In [4] it has been shown that the induction motors influence to the voltage sags during faults. Some solution approached for compensation of voltage sag a shunt injection of reactive current and a series injection of voltage. Ambra Sannino, et a1 f5], in their paper has carried out research to examine of the operation of a series connected

149 VSC for voltage sag mitigation. Static series compensator SSC IS depending on a'qutt large dc source. STAICOM is one o f the available shunt compensation

devices. The STATCOM obtained by a voltage source convericr (VSC), DC baiik charged via bridgc rcciilicr and proper control, as shown in Figure 2. The proposal of S'I'A'I'COM is to injection reactive power io the syp~t when voltage sag occurs. I h e amount of reactive power could be corilrol by change firing angle of the thyristor 01 the DC valuc. This paper investigates in, the performance of the S'I'APCOM verified wlien supplying an induction motor. The aim of this work is to develop STATCOM module based on thyristor (SCR) and studies the compensation capability of this module due to starting an induction motor.

11. STATCOM MODEL In general, STATCOM use to generate or absorb reactive power. The active power generation or absorption capability of the STATCOM is normally used under special circumstances such as to enhance the steady state and transient voltage control, to improve the sag elimination capability.

A . Bosic operation The basic electronic block of the STATCOM is the voltagesourced inverter that converts an input dc voltage into a threephase output voltage at fundamental frequency. In its simplest form, the STATCOM is made up o f a coupling transrormer, a voltage-sourced inverter and a dc capacitor. In this arrangement, the steady-state power exchange between the device and the ac system is mainly reactive. A functional model of the STATCOM i s shown in Figure 2.

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reactive power. If the amplitude of the STATCOM output voltage is increased above ihe amplitude of the ac system voltage, the current flows through the transrormer reactance from the STATCOM to the ac system, and the device gcncralcs reactive powcr (capacitivc). l r the atnpIitutlc or Lhc STATCOM output voltage is decreased to a level below Ihat of the ac system, then the current flows kom tfie ac system tu tll'e ' STATCOM, resulting in the device absorbing reactive power (inductive). Since tlie STA'I'COM IS gerieialiiig/ absorbing only reactive power, the output voltage and the ac systeni voltage are in phase, when neglecting ciicuit losses. The current drawn from the STATCOM is 90'- shifted with respect to the ac system voltage, and it can be leading (generates reactive power) or lagging (absorbs reactive power). A capacitor is used to maintain dc voltage to the inverter. An uticonlrolled rectifier based six diorlcs uscd lo kccp lltc capacitor charged to the required levels.

B. PrincipIe of reactive power control The principle of control reactive power via STATCOM is well known that the amount of type (capacitive or inductive) of reactive power exchange between the STATCOM and the system can be adjusted by controlling the magnitude of STATCOM output voltage with respect to that o f syslem voltage. The reactive power supplied by the STATCOM is given by

Where Q i s the reactive power. VsrAXOhfis the magnitude of STATCOM output voltage. Vs is the magnitude of system voltage. X is the equivalent impedance between STATCOM and the system. When Q is positive the STATCOM supplies reactive power to the system. Otherwise, the STATCOM absorbs reactive power from the system.

DC Voltage S0rUre

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Fig. 2. STA'I'COM futictionnl model

Regulating the amplitude of the STATCOM output voltage controls the reactive power exchange of the STATCOM with tfie ac system. If the amplitudes of the STATCOM output voltage and the ac system voltage are equal, the reactive current is zero and the STATCOM does not generatel absorb

C. Sag mitigation The application of a shunt device such as a STATCOM for mitigation of voltage sag has some advantages when compared with a series device, a s a shunt devices can simultaneously be used for steady-state voltage control, load power oscillation damping and as a back up power source [6].Some applications of STATCOM for niitigation voltage sag are preseiilcd in 16 - 81.

D. Simulution Modeling A three plIase voltage-sourced inverter is typically niade o f six thyristor switches (SRC)to shape the output waveform and it is the heart of the STATCOM compensator. There are also

150 six uncontrolled switches (diodes) to maintain dc source energy charged. The inverter bridge and diodes bridge are connected together and connected to the grid via eansfonner as show inFigure 4. The inverter bridge, which is the heart of the STATCOM, is typicaliy injection current to the system When the STATCOM voltage i s greater than that of the system voltage the STATCOM will supply VARs to the system Otherwise, the STATCOM will absorb VARS from the system as show in Figure 3.

voltage when the STATCOM was operation with the system. The voltage sag improvement clearly shown in fig 5b it is 91% in magnitude and 0.13 sec. figure Sa RMS voltage without

STATCOM Figure 6 summarized the active and the reactive power injection by the STATCOM to the system when the voltage sag event. The figure illustrates the active and reactive powers are positive. That means the load absorbs both active and reactive power during voltage sag. bmdmprlholllh8TATCUd

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500

SIMULATION RESULTS AND DISCUSSION

The circuit shown in fig .4 was implemented in the Matlablsimulink software. The system was operated hvice. Once Without STATCOM and the second time with the STATCOM. In both condition the motor started working after

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Absorbs "Q" do01

0

O i

02

03

D4

0.3

06

0.1

Ob

09

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Fig .3. Generation and absorption reactive power

Where V is the STATCOM voltage. VT is the terminal voltage.

Fig 5a ,Line voltage without STATCOM Bvr~ffhhSTAlCOb!

600,

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Fig 5b Line voltage with STATCOM Fig.4. STATCOM simulation circuit

0.25 second of switching on the system Fig. 5 shows the system RMS voltage with and without STATCOM. Fig. 5 shows the result obtained during voltage sag. of 76% in magnitude and 0.53 sec duration Fig 5a shows the RMS voltage at load terminal during voltage sag before the STATCOM connected to the system Fig 5b shows the R M S

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151

Looking at figure 7b, from 0.25 sec up to 0.65 sec during this interval tlie voltage sag is occurring and also it is at this interval that the STATCOM in making currcnl iiijectioti into the system as shown in figure 8. Comparing figure 7b with 7a the interval of voltage sag occurs between 0.25 up Lo 0.78.

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~ ~ ~ ~ ~ ~ ! ~ ~ ~ i ~ , ~ , ~ , ~ ~ ~ ~ ~ ~ , ~ ~ , ~ i ~ ~ i i , ~ , ~ yn!

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0.1

01

03

04

05

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om

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Fig f~ilie aclivc and rcsctivc power gcncratcd by tlic SI'ATCOM

Tlic Pigtircs 7a, 7b and 8 show Ilic efTcct or corinuc~iiig STATCOM to the system, the first figure shows the system . , I helbre connecting STATCOM .the second figuic shows the 0 RI 02 01 1.4 05 Gb O! Ud Ft 1 system immediately the STATCOM was connected, atid third tm. I figure shows the current which drawn by the STATCOM. At Fig 8 S'IA'I'COM current this time the STATCOM voltage is higher than system voltage. I t can also be seen there is a small current drawn by the Hence, it can be seen that the addition o f STATCOM increases STATCOM that it has no effect on the system. the response of the systcm by 0.13 secoiitl. Tlic adtlilion o f STATCOM can thus improve the transient stability of the system The dc voltage during sag events is sliown iti Fig 9. It is show that the capacitor discharged during voltage sag. .

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IV. CONCLUSION In this paper, the simulation model of static synchronous compensator STATCOM based thyristor has been constructed on Matlab/simulink software. Reactive power generation was achieved by charging and discharging the energy storage capacitor. T h e amount of reactive power is depending upon the thyristor-firing angle as shown in'tables. 1

152 TABLE 1

V. REFERENCES

CHANGES,OF REACTIE POWER WITH RESPECT TO F W G

ANGLE

Firing angle (degrees)

Reactive power WAR)

140

380

145

700

150

960

155

1180

160

1360

165

1560

170

1780

175

2000

The magnitude of the STATCOM terminal voltage was controlled w ith respect to the system voltage. STATCOM model Tested on. Matlabisimulink has shown that it can improve the voltage sag vector (magnitude and duration). Furthermore, it has shown the fast response of the STATCOM to voltage sag phenomena. Simulation resdts shown that the voltage sag improvement offered by a STATCOM may significantly reduce the number of trips in the sensitive equipments.

M . M c h a g a n : “Egecfs of Volfuge Sags In Prucers Indusiry Applicnfiom”, Invited paper SPT IS 01-2, presented at thc IEEL‘KTH Stockholm Power Tech C o n f m c e , Stockholm, Sweden, June 18-22 1995. J. Lamom,D. Mueller, P. Vine& and W.Jones: “Voltage sug analysis cme studies”. IEEE Transactions on.Indusq Applications, volume 30, No4, July- August 1994. Pages: 1083- 1089. M. H. 1. Bollcn :”Voltage sugs: e-ects, rnirigufion w d prediction” Power Engineering Joumal. Volume: 10,3 June 1996,Pages: 129 -135. Yaleinkaya, G.;Bollen, M.H.J.; Crossley, P.A.:”Characterization of voltage sags in indusaial distribution systems” Industry Applications, IEEE Transactions on, Volume: 34, Issue: 4. July-Aug. 1998 Pages: 682 - 688 [51 Sannino, A.; Svmsson, J.;” Application of converter-based series device for voltage sag mitigation to induction motor load”, Power Tech PraceedinB, 2001 IEEE Porto,Volume: 2, 10-13 Sept. 2001 Pages: 6 pp. v01.2 Atputharajah A, Ekanayake J, Jenkins N “Application study of a STATCOM with energy storage”, IEE Proceedings Generation Transmission and Distribution, Vol. 150. No 3 May 2003 pp 368-373 P. Wang, N. Jenkins, M.H.J.Bollen, “Experimental investigation of voltage sag mitigation by an advanced static var Compensator“, IEEE Tramucfions on Power Delivery, Vo1.13, no.4, Oct. 1998, pp.1461-

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1467. M v i n a Uzunovic, Claudio A. Canizares,

John Reeve, “Fundamental Frpquency Model of Static Synchronous Compensator”, Waterloo, ON, Canada N2L 3G1.