T. Ganeshkumar Pappu Pawan Puthra

GAYATRI VIDYA PARISHAD COLLEGE OF ENGINEERINGVISAKHAPATNAM, INDIA

Abstract— This paper proposes a novel distribution-level voltage control scheme that can compensate voltage sag and swellconditions in three-phase power systems. Faults occurring in power distribution systems or facilities in plants generally cause the voltage sag or swell. Sensitivity to voltage sags and swells varies within different applications. For sensitive loads, even voltage sag of short duration can cause serious problems. Normally, a voltage interruption triggers a protection device, which causes shut down of entire load.. In order to mitigate power interruptions, this paper proposes a voltage sag support based on a pulse width modulated autotransformer and D-STATCOM. The proposed devices are quickly recognizes the voltage sag and voltage swell and it can correct the voltage by either boosting the input voltage during voltage sag events or reducing the voltage during swell events. Simulation analysis of these devices is performed in PSCAD/EMTDC and performance analysis of the system is presented for various levels of sag and swell. Simulation results are presented for various conditions of sag and swell disturbances in the supply voltage to show the compensation effectiveness.

Index terms—D-STATCOM, Pulse Width Modulation (PWM)

I. INTRODUTION

With an increase in the use of sensitive loads, the power quality issues have become an increasing concern. Poor distribution power quality results in power disruption for the user and huge economical losses due to the interruption of production processes. According to an Electric Power Research Institute (EPRI) report, the economical losses due to poor power quality are $400 billion dollars a year in the U.S. alone [1]. Many power quality surveys have been done, which show that voltage sags have been identified as the most serious power quality problem facing industrial customers today. Voltage sag is a momentary decrease of the voltage RMS value with the duration of half a cycle up to many cycles. Voltage sags are given a great deal of attention because of the wide usage of voltage-sensitive loads such as adjustable speed drives (ASD), process control equipment, and computers.

T. Ganeshkumar is a post graduate student at Gayatri Vidya Parishad College of Engineering, Visakhapatnam, India.

P. Pawan Puthra is Assistant Professor at Gayatri Vidya Parishad College of Engineering, Visakhapatnam, India

Sag can cause serious problem to sensitive loads that use voltage-sensitive components such as adjustable speed drives, process control equipment, and computers [2], [3]. Power systems supply power for a wide variety of different user applications, and sensitivity to voltage sags and swells varies widely for different applications. Some applications such as automated manufacturing processes are more sensitive to voltage sags and swells than other applications. For sensitive loads, even voltage sag of short duration can cause serious problems in the manufacturing process. Normally, a voltage interruption triggers a protection device, which causes the entire branch of the system to shut down. Various voltage sag mitigation schemes are based on inverter systems consisting of energy storage and switches. The D-STATCOM has emerged as a promising device to provide not only for voltage sag mitigation but a host of other power quality solutions such as voltage stabilization, flicker suppression, power factor correction and harmonic control [4].The D-STATCOM has additional capability to sustain reactive current at low voltage and can be developed as a voltage and frequency support by replacing capacitors with batteries as energy storage. The D-STATCOM, which consists of a thyristor-based voltage source inverter [5], can provide fast capacitive and inductive compensation and is able to control its output current independently of the AC system voltage. This feature of the compensator makes it highly effective in improving the transient stability In an effort to achieve the advantages of a fast response time, but at a significantly lower cost, the PWM switched autotransformer is proposed here [6]. The proposed system has only one PWM switch per phase with no energy storage, which is a very low cost solution for voltage sag mitigation. Any power electronic switch for a high voltage application is expensive, and the peripheral circuits such as gate drivers and power supplies are even more expensive than the device itself. The overall cost of power electronics-based equipment is nearly linearly dependent on the overall number of switches in the circuit topology. Hence, this paper suggests a scheme that uses only one PWM switch with no energy storage. Here the control circuit based on RMS voltage is used to identify the sag and swell disturbances. Simulation of the compensator is performed using PSCAD/EMTDC and performance results are presented.

Mitigation of Voltage Sag and Voltage Swell by Using D-STATCOM and

PWM Switched Autotransformer

II. DISTRIBUTION STATIC COMPENSATOR (DSTATCOM)

In its most basic function, the DSTATCOM configuration consist of a two level voltage source converter (VSC), a dc energy storage device, a coupling transformer connected in shunt with the ac system, and associated control circuit [7, 8] as shown in Fig 1. More sophisticated configurations use multipulse and/or multilevel configurations as discussed in [9]. The VSC converts the dc voltage across the storage device into a set of three phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the DSTATCOM output voltages allows effective control of active and reactive power exchanges between the DSTATCOM and the ac system.

Fig. 1. Schematic diagram of the DSTATCOM as a custom power controller

The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes [10]:

i. Voltage regulation and compensation of reactive power;ii. Correction of power factor;iii. Elimination of current harmonics.

The design approach of the control system determines the priorities and functions developed in each case. In this case, DSTATCOM is used to regulate voltage at the point of connection. The control is based on sinusoidal PWM and only requires the measurement of the rms voltage at the load point.

A. Basic configuration and function of D-statcom

The DSTATCOM is a three phase and shunt connected power electronics based device. It is connected near the load at the distribution systems. The major components of the DSTATCOM are shown in Fig 2. It consists of a dc capacitor, three phase inverter module such as IGBT or thyristor, ac filter, coupling transformer and a control strategy. The basic electronic block of the DSTATCOM is the voltage sourced converter that converts an input dc voltage into three phase output voltage at fundamental frequency. Referring to Fig 2, the controller of the DSTATCOM is used to operate the inverter in such a way that the phase angle between the inverter voltage and the line voltage is

dynamically adjusted so that the DSTATCOM generates or absorbs the desired VAR at the point of connection.

Fig. 2. Building blocks of DSTATCOM

The phase of the output voltage of the thyristor based converter, Vi, is controlled in the same way as the distribution system voltage, Vs. Figure 3 shows the three basic operation modes of the DSTATCOM output current, I, which varies depending upon Vi. For instance, if Vi is equal to Vs, the reactive power is zero and the DSTATCOM does not generate or absorb reactive power. When Vi is greater than Vs , the DSTATCOM ‘sees’ an inductive reactance connected at its terminal. Hence, the system ‘sees’ the DSTATCOM as a capacitive reactance. The current, I, flows through the transformer reactance from the DSTATCOM to the ac system, and the device generates capacitive reactive power. Furthermore, if Vs is greater than Vi, the system ‘sees’ and inductive reactance connected at its terminal and the DSTATCOM ‘sees’ the system as a capacitive reactance, then the current flows from the ac system to the DSTATCOM, resulting in the device absorbing inductive reactive power.

Fig. 3. Operation modes of a DSTATCOM

III. A PWM SWITCHED AUTOTRANSFORMER

The proposed device for mitigating voltage sag and swell in the system consists of a PWM switched power electronic device connected to an autotransformer in series with the load. Fig. 4 shows the single phase circuit configuration of the mitigating device and the control circuit logic used in the system. It consists of a single PWM insulated gate bipolar transistor (IGBT) switch in a bridge configuration, a thyristor bypass switch, an autotransformer, and voltage controller.

Fig. 4. Block diagram of the voltage sag/swell mitigation scheme.

A. Principle of operation

An IGBT is used as power electronic device to inject the error voltage into the line so as to maintain the load voltage constant. Four power diodes (D1 to D4) connected to IGBT switch (SW) controls the direction of power flow and connected in ac voltage controller configuration. This combination with a suitable control circuit maintains constant rms load voltage. In this scheme sinusoidal PWM pulse technique is used. RMS value of the load voltage VL is calculated and compared with the reference rms voltage Vref. Under normal condition when there is no voltage disturbance the power flow is through the anti parallel thyristors used as the ac bypass switch. Output filters containing a main capacitor filter and a notch filter are used at the output side to filter out the switching noise and reduce harmonics. During this normal condition, VL = Vref and the error voltage Verr is zero. The gate pulses are blocked to IGBT. A sag or swell occurs in the system due to sudden increase or decrease in the load, or due to faults. The supply voltage VS and hence VL decreases. When the sensing circuit detects an error voltage Verr greater than ±10% of the normal voltage the voltage controller acts immediately to switch off the thyristors.Voltage Verr applied to the pi controller gives the phase angle δ. The control voltage given in (1) is constructed at power frequency f= 50 Hz.. Vcontrol = ma * sin (wt+δ) (1) where ma is the modulation index.The phase angle δ is dependent on the percentage of disturbance and hence controls the magnitude of Vcontrol. This control voltage is then compared with the triangular voltage Vtri to generate the PWM pulses VG which are

applied to the IGBT to regulate the output voltage. Hence the IGBT switch operates only during voltage sag or swells condition and regulates the output voltage according to the PWM duty-cycle. To suppress the over voltage when the switches are turned off, RC snubber circuits are connected across the IGBT and thyristor.

B. Voltage sag compensation

The ac converter topology is employed for realizing the voltage sag compensator. This paper considers the voltage mitigation scheme that use only one shunt type PWM switch [11] for output voltage control as shown in Fig. 5. The autotransformer shown in Fig.5 is used in the proposed system to boost the input voltage instead of a two winding transformer. Switch IGBT is on the primary side of the autotransformer.

Fig. 5. Voltage sag/swell mitigating device.

The voltage and current distribution in the autotransformer is shown in Fig. 6. It does not provide electrical isolation between primary side and secondary side but has advantages of high efficiency with small volume. The compensator considered is a shunt type as the control voltage developed is injected in shunt. The relationships of the autotransformer voltage and current are expressed in (2),

(2)

where a is the turns ratio, VL= Primary voltage VH = Secondary voltage = Load voltage IL,IH = Primary and secondary currents, respectively IS= Source current

A transformer with N1:N2 = 1:1 ratio is used as an autotransformer to boost the voltage on the load side when sag is detected. With this the device can mitigate up to 50% voltage sag. As the turns ratio equals 1:2 in autotransformer mode, the magnitude of the load current IL (high voltage side) is the same as that of the primary current IL (low voltage side). From (2), it is clear that VL = 2VP and IS = 2IL. The switch is located in the autotransformer’s primary side and the magnitude of the switch current equals the load current. The voltage across the switch in the off-state is equal to themagnitude of the input voltage. When sag is detected by the voltage controller, IGBT switched ON and is regulated by the PWM pulses. The primary voltage VP is such that the loadvoltage on the secondary of autotransformer is the desired rms voltage.

Fig. 6. Voltage and current relations in an autotransformer.

C. Ripple filter design

The output voltage VP given by the IGBT is the pulse containing fundamental component of 50 Hz and harmonics at switching frequency. Hence there is a necessity to design a suitable ripple filter at the output of the IGBT to obtain the load voltage THD within the limits. A combination of notch filter to remove the harmonics and a low pass filter for the fundamental component as shown in Fig. 1 is used. Capacitor Cr1 in combination with source inductance and leakage inductance form the low pass filter. The notch filter is designed with a center frequency of PWM switching frequency by using a series LC filter. A resistor may be added to limit the current. The impedance of the filter is given by (3)

(3)

where R, Lr and Cr2 are the notch filter resistance, inductance and capacitances respectively.The resonant frequency of the notch filter is tuned to the PWM switching frequency. The capacitor is designed by considering its kVA to be 25% of the system kVA. Capacitor value (Ctotal) thus obtained is divided into Cr1and Cr2 equally. The notch filter designed for switching frequency resonance condition is capacitive in nature for frequencies less than its resonance frequency. Hence at fundamental frequency it is capacitive of value Cr2 and is in parallel with Cr1 resulting to Ctotal.

IV. SIMULATION ANALYSIS AND RESULTS

Simulation analysis is performed on 230/11kv three phase systems in PSCAD version 4.2 to study the performance of D-STATCOM and PWM Switched autotransformer. The system data as follows

TABLE ISystem Parameters Used For Simulation

(A). Voltage sag/swell mitigation by using D- STATCOM:

Fig. 7 shows the test system implemented in PSCAD to carry out simulations for the D-STATCOM. The test system comprises a 230 kV transmission system, represented by a Thévenin equivalent, feeding into the primary side of a 3-winding transformer. A varying load is connected to the 11 kV, secondary side of the transformer. A two-level D-STATCOM is connected to the 11 kV tertiary winding to provide instantaneous voltage support at the load point. A 750F capacitor on the dc side provides the D-STATCOM energy storage capabilities. The set of switches shown in Fig. 7 were used to assist different loading scenarios being simulated with ease. To show the effectiveness of this controller in providing continuous voltage regulation, simulations were carried out with and with no D-STATCOM connected to the system. A set of simulations was carried out for the test system shown in Fig. 7. The simulations relate to three main operating conditions.1) In the simulation the load is increased by closing Switch BRK3. In this case, the voltage drops by almost 27% with respect to the reference value2) The switch BRK3 is opened and remains so throughout the rest of the simulation. The load voltage is very close to the reference value, i.e., 1 pu.3) Three phase faults also applied in the study system to study the performance of the device.4) In the simulation Switch BRK1 is closed, connecting a capacitor bank to the high voltage side of the network. The rms voltage increases 27% with respect to the reference voltage

Supply 3-Phase100 MVA, 230KV, 50 Hz , AC supply

Transformer 230KV / 11KV / 11KV,100MVA,50Hz

Load1Load2

Capacitor Bank

R = 12.1 Ω, L = 0.1926HR= 0.05Ω, L=0.0059H

C=3uf

PI controller gain 200Switching frequency 2KHz

Duty cycle 50%

Fig. 7. Test system implemented in PSCAD for D- STATCOM simulation.

Fig. 8. Simulation results for load voltage during voltage sag without

D-STATCOM

Under normal conditions D-STATCOM continuous monitors the load voltage and generates the error voltage. The voltage sag can be created by using either load switching or by using three phase fault.The load voltage corresponding to sag is shown in Fig 8.The d-statcom can mitigate the sag as shown in Fig 9.

Fig. 9.Simulation results for load voltage during voltage sag with D-STATCOM

Fig. 10.Simulation results for load voltage during voltage swell Without D-STATCOM.

Voltage swell created by using a capacitor bank switching during a period of 0.3s to 0.6s.under this condition voltage swell is experienced.By using D-statcom it can be eliminated. The corresponding wave forms are shown in Figs 10 and Fig 11.

Fig. 11.Simulation results for load voltage during voltage swell with D-STATCOM

(B). VOLTAGE SAG/SWELL MITIGATION BY USING A PWM SWITCHED AUTOTRANSFORMER

Under normal condition, the power flow is hrough the antiparallel SCRs and the gate pulses are inhibited to IGBT. The load voltage and current are same as supply voltage and current. When a disturbance occurs, an error voltage which is the difference between the reference rms voltage and the load rms voltage is generated. The PI controller thus gives the angle δ. Control voltage at fundamental frequency (50 Hz) is generated and compared with the carrier frequency triangular

wave of carrier frequency 1.5 kHz. The PWM pulses now drive the IGBT switch. The simulation modeling of PWM switched autotransformer used as mitigating device along with its control circuit is shown in Fig. 12. The autotransformer rating in each phase is 6.35/6.35 kV (as line voltage is 11 kV) with 1:1 turns ratio. The effective voltage available at the primary of autotransformer is such that the load voltage is maintained at desired rms value (6.35 kV or 1 pu).The simulation results of load voltages are shown in fig 13-fig 16 during voltage sag and voltage swell disturbances.

Fig. 12.Test implemented in PSCAD for simulation of PWM Switched Autotransformer

PWM Switched autotransformer control circuit

Fig. 13.simulation result of load voltage during voltage sag without compensator

Fig. 14. Simulation result of load voltage during voltage sag with compensator

Fig 15. Simulation result of load voltage during voltage swell without compensator

Fig. 16.Simulation result of load voltage during voltage swell with compensator

TABLE IITHDS Of Load Voltage For Voltage Sag Of D-Statcom

TABLE III THDS Of Load Voltage For Voltage Sag Of Pwm Switched

Autotransformer

Tables II and table III summarizes the simulation results for

both these devices for various sag conditions and swells

V. CONCLUSION

A new Voltage sag mitigation topology called PWM

switched autotransformer is modeled and simulated with RMS

voltage as a reference. This topology requires only one PWM

switch per phase as compared to DSTATCOM requires two

switches per phase. The PWM switched autotransformer does

not require energy storage device for mitigation of voltage sag

as compared to DSTATCOM requires energy storage

elements. The voltage mitigation capability of D-STATCOM

depends on energy storage device. Hence it is shows that

PWM switched autotransformer is more economical than D-

statacom

The PWM switched autotransformer and D-STATCOM for

mitigation of voltage sag/swell could identify the disturbance

and capable of mitigating the disturbance by maintaining the

load voltage at desired magnitude and THD within limits.

VI. REFERENCES

[1] Electric Power Research Institute (EPRI), “Power quality in commercial

buildings,” Tech. Rep. BR-105018,

[2] M. F. McGranaghan, D. R. Muller, and M. J. Samotyj, “Voltage sags in

industrial systems,” IEEE Trans. Ind. Appl., vol. 29, no. 2, pp. 397–

403,Mar./Apr. 1993.

[3] M. H. J. Bollen, Understanding Power Quality Problems: Voltage Sags

and Interruptions. New York: IEEE Press, 2000.

[4] Gareth A. Taylor, “Power quality hardware solutions for distribution

systems: Custom power”, IEE North Eastern Centre Power Section

Symposium, Durham, UK, 1995, pp. 11/1-11/9.

[5] Hendri Masid, Norman Moriun, Senan Mahmud, Azah Mohamed and

Sallehuddin Yusuf, “Design of a Prototype D-Statcom for Voltage Sag

Mitigation,” in Proc. 2004 National Power and Energy Conf.,Kuallampur,

Malaysia, Nov. 2004, pp. 61-66.

[6] J. R. Rostron and D.-M. Lee, “Voltage Sag and Over Voltage

Compensation Device With Pulse Width Modulating Switch Connected in

Series With Autotransformer,” U.S. Patent 6 750 563, Jun. 2004.

[7] A. Hernandez, K. E. Chong, G. Gallegos, and E. Acha “The

implementation of a solid state voltage source in PSCAD/EMTDC,” IEEE

Power Eng. Rev., pp. 61-62, Dec 1998.

[8] L. Xu, Anaya-Lara, V. G. Agelidis, and E. Acha “Development of custom

power devices for power quality enhancement,” in Proc. 9th ICHQP 2000,

Orlando, FL, Oct. 2000, pp. 775-783.

[9] Y. Chen and B. T. Ooi, “STATCOM based on multimodules ofmultilevel

converters under multiple regulation feedback control,” IEEE Trans.Power

Electron., vol. 14, pp. 959-965, Sept. 1999.

[10] Dong-Myung Lee, Thomas G. Habetler, Ronald G. Harley, Thomas

L.Keister and Joseph R. Rostran, “A Voltage sag Supporter Utilizing a PWM-

Switched Autotransformer,” IEEE Trans. Power Electronics,Vol. 22, No. 2,

Mar. 2007, pp. 626-635.

VII. BIOGRAPHIES

T.Ganesh kumar was born in April 24th 1987. He graduated in 2008 from Sri

Sai Aditya institute of science and technology,

Rajahmundry, India in Electrical Engineering. He is

currently pursing M.Tech from GVP college of

Engineering, Visakhapatnam. His area of interest is

Power Quality.

P. Pawan Puthra was born in 15thNov 1983. He

graduated in JNTU Hyderabad from St.Therasa Institute

of Engg & Tech., Garividi India in the year 2006. He obtained his post

graduation from Vellore Institute of Technology (Vellore) with

Specialization Power Electronics & Drives in year 2008. Presently he is

working as an Asst. professor in GVP College of Engineering. His main area

of research is Power Electronic & Drives, FACTS, Non Conventional Energy

Sources.

Type of disturbance Vs (rms) VLoad (rms) THD (%)

LLLG Fault(sag)(VDC=30kv)

0.5 0.9645 3.445

0.55 0.9651 5.011

0.65 0.9766 6.291Inductive load switching(sag)

(VDC=20kv)0.73 0.975 2.456

Capacitive load switching(swell) 0.73 1.012 3.978

Type of disturbance Vs (rms)

VLoad (rms) THD (%)

LLLG fault(sag)0.5 0.9732 2.34

0.55 0.9673 2.64

0.65 0.9622 3.711Inductive load switching(sag) 0.73 0.9823 4.56

Capacitive load switching(swell) 0.73 0.95 4.76