A Modern Self-Defined Extinction Angle

8/12/2019 A Modern Self-Defined Extinction Angle

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A Modern Self-Defined Extinction Angle

Controller for CCC based Hybrid HVDC

SystemsM.Rajasekaran# andM.VenkataKirthiga #

#Department of Electrical and Electronics Engineering

National Institute of Technology [email protected]

[email protected]

ABSTRACT

This paper recommendsamodern self-defined extinction advance angle controller for Hybrid

HVDC systems and also investigates its suitability under different operating conditions. The

proposed self-defined controller suggests a new method for measuring extinction advance

angle which deals with the device level measurement. The proposed controller is dedicated

for the receiving end converter operated in inverter mode and the sending end converter

operated in rectifier mode is supplemented with a standard controller. A Hybrid HVDC

system with Voltage Source Converter(VSC) at the rectifier and Capacitor Commutated

Converter (CCC)is considered in this paper for analysis. The performance of the proposed

controller has been investigated under different operating conditions including post fault

recovery. Also a fault mitigation technique has been attempted for a DC link to ground fault

and the robustness of the controller has been confirmed in conjunction with the fault

mitigation technique. PSCAD/EMTDC simulation software has been used for validating the

proposed controller and the standard CIGRE benchmark system has been used for evaluating

the proposed controller in this paper.

Keywords- Hybrid HVDC systems, Capacitor Commutated Converter, Fault mitigation

technique

1. INTRODUCTION

HVDC technology has proven to be an efficient and flexible method to achieve bulk

power transmission over long distances [1]. Conventional HVDC transmission systems use

thyristors based Line-Commutated Converters (LCC) for power conversion and the modern

HVDC systems use IGBT based Voltage Source Converters(VSC) which enhance reactive

power support in case of weak ac grids. LCC based systems are well suited for very high

power transmissions and hence are widely used. LCC converters also posses less switching

losses and also the device being unidirectional such systems do not encounter reverse power

contributions during mid link to ground fault. However when the AC system to which the

LCC based converter is connected is weak, there are frequent chances for commutation

failureto occur at the devices. As an alternative, VSC based systems are mushrooming owing


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This configuration gives better reactive power support compared to the conventional Line

CommutatedConverter(LCC) and also improves the dynamically performance of the inverter

especially when connected to weak AC systems and/or long DC cables.

Capacitor in CCC configuration increases the commutation margin without any increase

in the reactive power consumption of the converter station[3]. This improvement incommutation margin reduces the chances of commutation failure and hence CCC is dedicated

to the inverter station, prone to commutation failure.

Fig. 3.AC bus voltage & valve voltage: (a) conventional inverter, (b) CCC inverter

As a matter of fact, it becomes inevitable to distinguish between the apparent extinction

angle () and the real extinction angle () as shown in Fig. 3. In the conventional HVDC

converter, the extinction angle is defined as the electrical angle spannedbetween the instant

at which the valve turns offand the positive zero-crossing of the line-to-line voltage at the ac

converter bus, as given by (1)

(1)

where is the inverter firing delay angle and is the overlap angle. Howev er, in the case of

CCC, this measurement does not take into account the capacitor voltage and does not

measure the actual extinction angle. Therefore it is referred to as the apparent extinction

angle () as given by (2)

(2)

where is the phase-lag angle between the AC bus voltage and valve voltage as shown in

Fig. 3. The commutation margin angle in the CCC inverter is the angle between the instant

of completion of commutation and the instant where the valve voltage crosses zero and grows

positive there onwards.

3. COMBINED CONTROL CHARACTERISTICS OF A HVDC

TRANSMSISION SYSTEM

Under forward power flow from the rectifier station to the inverter station, the CC and

CIA controllers need to be dedicated at the

rectifier end and the CEA controller is

inevitable on the inverter side of the system.

The combinedcontrol characteristics of a

long distance HVDC transmission system is

being shown in the Fig.4.A combined

control operation is required to enable both

forward and reversal of power flows.

Fig. 4.Combined Control Characteristicsis


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Under emergency control states viz., sudden reversal of power flow for control

requirements, inversion operation of both converters to de-energize the line completely,

emergency shutdown of the line etc., it is necessary to avail the CC and CIA controllers at the

inverter end and CEA controller at the rectifier end. Thus each converter needs to be

provided with all the three above mentioned controllers.

4. SELF-DEFINED EXTINCTION ANGLE CONTROLLER

Overall block diagram of the proposed modern self-defined controller has been shown in

the Fig. 5. This method is implemented digitally using firing pulse, zero-crossing instant of

valve current and AC voltage of each valve to determine the anode and the b node of

extinction angle , as shown in the Fig. 6.

Fig. 5.Schematic of the proposed controller Fig. 6.Voltage waveform to measure

Fig. 7.Overall simulation diagram of the modern self-defined extinction angel controller

The node a shown in Fig.6indicates the instant of current commutationfrom the

outgoing device to the incoming device. After the outgoing valve has completely de-ionized,

when the voltage across the valve and AC voltage,pass through zero axis, the valve would

stop conduction. This instant is denoted as b. The zero-crossing of valve current and ac

voltage of each valve, the nodes a and b have been identified, and a unit time-pulse, the

width of which is equal to the time of extinction angle isgenerated. This time-pulse is used

as an input signal for integrator. The output of the integrator is an amplitude-pulse which is

equal to extinction angle . This amplitude of thepulse is maintained until the next extinctionangle is measured. Throughthis method, theextinction angle of single valve in each cycle


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and hence the same is maintained constant throughout the inverter operation of the bridge

concerned. The inputs of the measurement module are voltage, current, firing pulse and ac

voltage of each valve and the output is the measured extinction angle of each valve. Then

the minimum of measured for all the valves is selected as the input of closed-loop control of

constant extinction angle control. This control scheme has been implemented usingPSCAD/EMTDC software as shown in Fig. 7.

5. RESULTS AND DISCUSSIONS

The efficacy of the proposed controller has been investigated for a Hybrid HVDC

systemwith power rating of 2000MW, at 1100kV mono-polar system derived from the first

CIGRE benchmark model with some modifications to facilitate the study of the system (6-

pulse converters), as shown in Fig.1.with a VSC rectifier and CCCinverter. The VSC rectifier

is supported by an Independent P & Q controller modeled using analytical expressions.The

capacitance of the CCC used in this model is determined to C = 53 F.

5.1 Normal operation of the Hybrid HVDC link with the proposed controller

The normal start up and steady state operation of the monopolar Hybrid HVDC transmission

link with the proposed controller is compared with that of the standard CIGRE controller as

shown in Fig. 8.


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Time(s) Time(s)

(a)Self-defined extinction advance angle controller (b)CIGRE controllerFig. 8.Performance of the Hybrid system under steady state start up operation

The waveforms for the DC link voltage, current, DC voltage and current at the rectifier and

inverter terminals respectively and the real and reactive power interactions with the ac grid on

either side are shown in the figure 8 for Hybrid HVDC system with the proposed controller

(a) and the CIGRE controller(b) respectively. It has been seen that the proposed controller, is

on par with the behaviour of the CIGRE controller during start up characteristics.

5.3 Fault mitigation technique for a DC link fault

The performance of the proposed controller has been analyzed for a DC link to

ground fault, at 1 sec that persists for200msecs. As the VSC has no control

over the fault current, the system has

not been able to resume appreciably

after the fault has been cleared.

HenceFig. 8. Fault mitigation technique a fault current mitigation

technique has

been attempted in this paper. This technique deals with either isolating the VSC rectifier

alone or isolating both the end converters (VSC & CCC) on occurrence of a dc link to ground

fault and both the converters being operated in inverter modes as shown in Fig. 8.

5.4 Performance analysis of the proposed controller with fault mitigation technique

A DC link to ground fault has been created at 1 sec &cleared at 1.2 sec. The fault

mitigation technique shown in Fig. 8 has been attempted. The performance of the proposed

controller has been compared with that of the CIGRE controller with and without

implementation of the fault mitigation technique. In both the modes of operation the proposed

controller has been found to outperform the CIGRE controller.In the first case, the inverter is isolated on occurrence of fault and the controller is

allowed to settle at appropriate firing delay angle to meet the required current order such that

the rectifier is at the inverted mode of operation. The proposed controller is found to

outperform the conventional CIGRE controller with respect to the post fault recovery time

and the waveforms of DC voltage, current, DC link power, real and reactive power

interactions with the grid are shown in Fig.9.


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Time(s)

Time(s) Time(s)

(a)Self-defined extinction advance angle controller (b)CIGRE controller

Fig. 9. Performance of the Hybrid system under fault mitigation operation-Isolating CCC inverter

As a second case, the DC circuit breaker at both the ends viz., the inverter (CCC) and

rectifier (VSC) are operated on occurrence of the fault. After fault clearance, both the

converters are switched ON and the post fault recovery performance of the proposed

controller is found to be superior to that of the conventional CIGRE benchmark controller.

Also the proposed self- defined controller helps the system to restore faster after the recovery

of the fault. Justification for this discussion has been shown in Fig. 10.


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Time(s) Time(s)

(a)Self-defined extinction advance angle controller (b)CIGRE controller

Fig. 10. Performance of the Hybrid system under fault mitigation operation of case 2

6. CONCLUSION

In this paper a Modern Self- Defined Extinction angle controller has been suggested for the

extinction advance angle control at the inverter end of a Hybrid HVDC system with VSC at

the rectifier end and CCC at the inverter end. It is seen that the proposed controller is able to

work on par with the standard CIGRE controller at post fault recovery instants as the system

recovers smooth and fast after the fault is removed and also the fault current is limited to one

third of the magnitude attained with the CIGRE controller. Also a fault mitigation technique

has been suggested in this paper and the performance of the proposed controller is found to

be better than the CIGRE controller when operated in conjunction with the suggested fault

mitigation technique. PSCAD/EMTDC simulation software has been used for validating the

proposed controller in this work.

7. REFERENCES

[1] K.R. Padiyar, HVDC Power Transmission Systems, New Age International(P) Ltd.,Publishers, 1990.

[2] M. O. Faruque, Yuyan Zhang, and VenkataDinavahi, Detailed Modeling of CIGREHVDC Benchmark System Using PSCAD/EMTDC and PSB/SIMULINKIEEE

Transactions on Power Delivery, Vol.21, No.1, January 2006.[3] K.Sadek, M.Pereira, D.P.Brandt, A.M.Gole, A.Daneshpooy, Capacitor Commutated

Converter Configurations For DC Transmission, IEEE Transactions On Power

Delivery, Vol.13,NO.4,pp.1257-1264,October 1998.

[4] Li Gengyin, Member, IEEE, Liang Haifeng, Zhao Chengyong, Yin Ming, Research onHybrid HVDC ,International Conference on Power System Technology - POWERCON

2004.

[5] Manling Dong, junjia He, Xiaolin Le, Ying Huang, Realization of Self-Defined ControlSystem For Constant Extinction Angle Control using PSCAD/EMTDCPower and

Energy Engineering Conference-APPEC 2009,pp.1-5, 2009

[6] AsfaqThahir M. and VenkataKirthiga M, Investigations on a Modern SelfDefinedExtinction Advance Angle Controller For HVDC Systems, International Conference

onProcess Automation, Control and Computing (PACC),IEEE, pp. 1-6, 2011.

[7] M.AshfaqThahir, M. VenkataKirthiga,Investigations on modern self-defined controllerfor hybrid HVDC systemsTENCON 20112011,IEEE Region 10 Conference,pp. 938-943,2012
http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=6111658http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=6111658http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=6111658http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=6111658http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854http://ieeexplore.ieee.org/xpl/mostRecentIssue.jsp?punumber=5978854

A Modern Self-Defined Extinction Angle

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