Y.T.R.Palleswari, B.Kali Prasanna and G.Lakshmi...2013/10/23 · Fig.11 shows a three phase five...

International Journal of Application or Innovation in Engineering & Management (IJAIEM) Web Site: www.ijaiem.org Email: [email protected], [email protected]

Volume 2, Issue 10, October 2013 ISSN 2319 - 4847

Volume 2, Issue 10, October 2013 Page 133

Abstract To improve the performance of power system, by reducing harmonics and managing reactive power, new power equipment is required. During the last decade, many control devices called” Flexible AC Transmission Systems” (FACTS) have been implemented. The new and dominant converters in FACTS controllers are synchronous condenser, static VAR compensator (SVC) and static synchronous compensator (STATCOM). STATCOM is one of the devices used for compensation of reactive power and harmonic reduction. This paper concentrates on harmonic reduction in two level, three level and five level diode clamped inverter based STATCOM. To verify the effectiveness, simulation will be done using MATLAB/ SIMULINK. Keywords: FACTS, Multi level inverter, STATCOM, Total Harmonic Distortion. 1. Introduction Transmission lines when travels over large distances, they suffer with considerable losses. In order to compensate these voltage losses, many FACTS (Flexible AC Transmission Systems) devices like static VAR compensator (SVC), synchronous condenser, Static Synchronous Compensator (STATCOM). In which STATCOM is a shunt compensation device also known as ASVG (Advanced Static VAR Generator) is coupled with a transformer and connected to a transmission line [1]. It is capable of generating and or/ absorbing reactive power. The STATCOM basically consists of a step down transformer with a leakage reactance, a three phase GTO or IGBT voltage source inverter (VSI) and a DC capacitor. The AC voltage difference across the leakage reactance produces reactive power exchange between the STATCOM and power system, such that the AC voltage at the bus bar can be regulated to improve the voltage profile of power system. Day by day, the demand for the power is goes on increasing. So, the converter/ inverter used within the STATCOM should be able to withstand the increased power levels. In power electronics, the most basic controllable device is the two level converter. But, this basic device injects unwanted harmonics into the power system [2]. So, in order to overcome this problem, the term multi level is brought out in 1981[3]. The very important thing of using multilevel inverters is that creating more output steps and to reduce the Total Harmonic Distortion (THD). Three different multi level inverter topologies have been proposed. Those are diode-clamped, flying capacitor and cascade H-bridge. Out of which diode clamped multi level inverter can compensate the reactive power and reduce the Total Harmonic Distortion effectively even though with the higher levels, capacitor voltage balancing is a problem. This paper introduces a STATCOM with three level and five level inverter and mainly concentrates on Total Harmonic Distortion. 2. Operation of STATCOM:

Fig 1.One line diagram of STATCOM

The STATCOM basically consists of a step-down transformer with a leakage reactance, a voltage source inverter (VSI), a capacitor in its DC side and a control system. The inverter in conventional STATCOM, switched with a single pulse per period and the transformer is connected in order to provide harmonic minimization and serve as a link between VSI and

MULTI LEVEL STATCOM FOR HARMONIC REDUCTION

Y.T.R.Palleswari, B.Kali Prasanna and G.Lakshmi

Assistant Professor, Shri Vishnu Engineering College for Women




the system. The leakage inductor limits the negative sequence currents. The capacitor is used to maintain DC voltage to the inverter. The inverter itself keeps the capacitor charged to the required level.

The AC voltage difference across the leakage reactance produces reactive power exchange between the STATCOM and the power system, such that the AC voltage at the bus bar can be regulated to improve the voltage profile of the power system, which is the primary duty of the STATCOM [4]. If the system voltage is greater than the inverter voltage, then the STATCOM absorbs the reactive power. If the system voltage is less than the inverter voltage, then the STATCOM generates reactive power. If the system voltage is equal to the inverter voltage, then there is no reactive power compensation [5].

3. Reactive current detection principle: The main component of STATCOM is the voltage source inverter. The voltage source inverter operation depends upon the working of semiconductor switches. Proper gating signals are required for the operation of these semiconductor switches. These signals are generated by using a proper control circuit. Here the control circuit required for generating pulses to the voltage source inverter is based on the synchronous reference frame theory. Before designing the control circuit, there is a need of modeling the equations required for constructing the control circuit.

Reference current generation:

Fig.2. Reactive current detection principle

Fig 3 shows the basic block diagram for generating the reference currents required for producing gate pulses to the switches. In this, shunt active power filter control is accomplished by monitoring the three phase line currents to the nonlinear load and the three phase line-to-neutral voltages at the load bus and then generating the three phase reference currents that should be supplied to the voltage source inverter [6].

Mathematical Analysis: In a, b, c coordinates, the a, b and c axes are fixed on the same plane, apart from each other by 2π/3. The instantaneous space vectors Vα and iα are set on the α- axis and their amplitude and direction vary with the passage of time. These space vectors are easily transformed into α, β coordinates as follows

(1)

(2)

The instantaneous active and reactive power in the α, β coordinates are calculated with the following expressions:

P(t) = Vα(t).iα(t) + Vβ(t).iβ(t) (3)

Q(t) = -Vα(t).iβ(t) + Vβ(t).iα(t) (4) It is evident that p(t) becomes equal to the conventional instantaneous real power defined in the a, b, c reference frame.




Q = Vα.iβ + Vβ.iα (5)

The expression of the currents in the α–β plane, as a function of the instantaneous power is given by the following equation:

= (6) The values of p and q can be expressed in terms of the DC components plus the AC

Components, that is: P = (7)

Q = (8)

(9)

The final compensating currents including the zero sequence components in a, b, c reference frame are the following:

(10)

4. Inverter Analysis: Comparing two-level inverter topologies at the same power ratings, MLIs also have the advantages that the harmonic components of line-to-line voltages fed to load are reduced owing to its switching frequencies [6]. Two level Inverter:

Fig 3.Two level Inverter

Table.1.Switching States and output voltages of two level inverter

S1 S2 S3 S4 Output Voltage ON ON OFF OFF Vdc OFF OFF ON ON -Vdc

From Fig.3, it can be observed that for two level inverter, the output voltages are Vdc and -Vdc. The switches turn on and off for every period according to the mode show in table 1. Simulation Results: A system with inductive load having R=70Ω, L=0.2H.Voltage and current waveform before switching in STATCOM are as shown in Fig 4.

Fig.4 voltage and current waveforms without STATCOM compensation with inductive load




From Fig.4, it can be observed that as the load has inductive part, the current waveform lag behind the voltage waveform with certain angle, therefore system power factor is not equal to unity. Hence there is a need of reactive power compensation for the system. A system with inductive load and after switching in STATCOM having R=70Ω, L=0.2H.Voltage and current waveform of A phase after switching in STATCOM are as shown in Fig 5.

Fig .5.voltage and current waveforms with STATCOM compensation with inductive load

From Fig 5, it can be observed that current and voltage waveforms are in phase after STATCOM compensation. The power factor of the system becomes unity as the reactive component is provided by the STATCOM.

Fig.6. THD analysis for 2 level with and without STATCOM.

Three level Diode Clamped Inverter:

Fig 7.Three phase three level Diode Clamped Multi Level Inverter

Diode clamped multilevel inverter is a very general and widely used topology. DCMLI works on the concept of using

diodes to limit voltage stress on power devices. A DCMLI typically consists of (m-1) capacitors on the DC bus where m is the total number of positive, negative and zero levels in the output voltage [7]. From Fig.7 one can say that for three level inverter, the output voltages are Vdc/2, 0, -Vdc/2. The switches turn on and off for every period according to the mode show in table 2. Switches Sx1, Sx3(X=1,2,3) and Sx2, Sx4 mix in pairs.




Table.2.Switching States and output voltages of three level Diode Clamped Multi Level inverter Sx1 Sx2 Sx3 Sx4 Output Voltage ON ON OFF OFF Vdc/2 OFF ON ON OFF 0 OFF OFF ON ON -Vdc/2

Simulation Results: A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase before switching in STATCOM are as shown in Fig 8.

Fig 8.voltage and current waveforms before STATCOM compensation with capacitive load

From Fig.8, it can be observed that as the load has capacitive part, the current waveform leading behind the voltage waveform with certain angle. Therefore system power factor is not equal to unity. Hence there is a need of reactive power compensation for the system.

A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase after switching in STATCOM are as shown in Fig 9.

Fig 9.voltage and current waveforms after STATCOM compensation with capacitive load

From Fig.9, it can be observed that current and voltage waveforms are in phase after STATCOM compensation. The power factor of the system becomes unity as the reactive component is provided by the STATCOM.

Fig.10. THD analysis for 3 level with and without STATCOM




Five level Diode Clamped Inverter:

Fig.11. Three phase five-level Diode Clamped Multilevel Inverter.

Fig.11 shows a three phase five level diode clamped inverter. The order of numbering of the switches for phase A is Sa1, Sa2, Sa3, Sa4, Sa1’, Sa2’, Sa3’ and Sa4’ and likewise for other two phases. The DC bus consists of four capacitors C1, C2, C3 and C4 acting as voltage divider. For a DC bus voltage Vdc, the voltage across each capacitor is Vdc/4 and voltage stress on each device is limited to Vdc/4 through clamping diode. The middle point of the four capacitors ‘N’ can be defined as the neutral point. The principle of diode clamping to DC-link voltages can be extended to any number of voltage levels.

Table.3.Switching States and output voltages of five level Diode Clamped Multi Level inverter.

Simulation Results:

Fig.12. Simulink model of a system with STATCOM with capacitive load

A system with capacitive load having R=10Ω, C=0.6e-3F.Voltage and current waveform of A phase after switching

in STATCOM are as shown in Fig 13.




Fig.13. Voltage and Current waveforms after STATCOM compensation with capacitive load

From Fig.13, it can be observed that current and voltage waveforms are in phase after STATCOM compensation.

The power factor of the system becomes unity as the reactive component is provided by the STATCOM.

Fig.14. STATCOM output currents.

Fig.15 THD analysis for 5 level with and without STATCOM

Table.4 Comparison table o total harmonic distortion for two level, three level and five level inverter based STATCOM.

Total Harmonic Distortion Two level Three level Five level

without STATCOM

with STATCOM without STATCOM

with STATCOM without STATCOM

with STATCOM

29.63% 28.29% 28.28% 11.44% 6.64% 0.8%

Table 5. Switching losses and dv/dt stress

For 2 level 3level 5level

Dv/dt stess 2Vdc Vdc Vdc/2

Switching losses 8Vdc 2Vdc Vdc

Conclusion: From the simulation results, it is observed that STATCOM can compensate the inductive and capacitive reactive power irrespective of the levels in the inverter. Compared to two level inverter and three level inverter, five level inverter has reduced harmonic distortion, less dv/dt stress and switching losses are reduced.




References: [1] N.G.Hingorani and L.Gyuyi, understanding FACTS-Concepts and technology of flexible AC Transmission Systems,

IEEE Press, 1999. [2] N.Choi, G.Jung and H.Gyu,” A General Circuit Topology of Multilevel Inverter,’ IEEE Power Electron.Spec.Conf.,

vol.PESC 91 Record., pp 96-103,1991. [3] A. Nabea, I. Takahashi, and H. Akagi, “A New Neutral-Point Clamped PWM Inverter,” IEEE Trans. Ind. Appl., vol.

17, no. 5, pp. 509–523, Sept/Oct 1981. [4] Amir H. Norouzi, A.M. Shard, “A Novel Control Scheme for the STATCOM Stability Enhancement”, 2003 IEEE

PES Transmission and Distribution Conference and Exposition, Sept. 2003. [5] S.P. Li and G.Y. Liu, “Static reactive power compensation technique,” China Electric Power Publishing House, pp.

101-121, 2006. [6] Yong Hua Song, Allan T. Johns, ”Flexible AC transmission systems FACTS”, IEE Power and Energy Series 30,

1999. [7] Rodriguez, J.; Jih-Sheng Lai; Fang Zheng Peng; , "Multilevel inverters: a survey of topologies, controls, and

applications," Industrial Electronics, IEEE Transactions on , vol.49, no.4, pp. 724- 738, Aug 2002 doi: 10.1109/TIE.2002.801052.

[8] S. Khomfoi, L. M. Tolbert, “Multilevel Power Converters,” Chapter17, Power Electronics Handbook, 2nd Edition, Elsevier, ISBN 978-0-12-088479-7, pp. 451-482, 2007.

Corresponding Author: Y.T.R.PALLESWARI

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Y.T.R.Palleswari, B.Kali Prasanna and G.Lakshmi...2013/10/23 · Fig.11 shows a three phase five...

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