Using Voltage Source Converter (VSC) based HVDC

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developed that does not rely on the AC system voltage for the valves' commutations. With reference to power transmission, self-commutating. VSC offers the ...
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Using Voltage Source Converter (VSC) based HVDC Transmission Link for Supply a Passive Load M. Khatir, S A. Zidi, S. Hadjeri, M K. Fellah, R. Amiri

Abstract—The flexibility of the transmission system can be greatly improved by the use of voltage source converter (VSC). This technology permits the flow of active power, as well as the provision of reactive power, in either direction at each end of the link. Moreover, Self-commutated VSC does not need an AC system voltage source for the commutations and therefore the receiving end of the link may supply power to a totally passive system. In this paper, the scenario of VSC-HVDC link supplies power to an isolated (or passive) load is studied. Control strategy is implemented and its dynamic performances during disturbances are investigated in EMTDC/Pscad program. The simulation results have shown good performance of the proposed system under balanced and unbalanced fault conditions.

Index Terms—HVDC transmission, Voltage source converter (VSC), Isolated load, Control design

NOMENCLATURE UL = the sinusoidal AC voltage in the AC network UV (1) = the fundamental line to line voltage (valve side) XL = the leakage reactance of the transformer δ = phase shift between UL and UV (1) IV = source current C = DC side capacitance ω = source voltage angular frequency m = modulation index P, Q = AC active, reactive power inputs Ud, Id, Pd = DC side voltage, current, power * = reference value for controller

With reference to power transmission, self-commutating VSC offers the following advantages over conventional line commutated converter (LCC) [2]: • Each end of the link can be controlled to absorb or generate reactive power independently of the active power transfer. • The DC link can be connected to a weak, and even passive, AC network. • Reduced requirements for harmonic filters. • VSC transmission can be designed to provide a variety of ancillary services to the interconnected AC systems, such as reactive power compensation, harmonic and unbalanced voltage compensation, flicker elimination, etc. At the time of writing, self-commutating HVDC is only available in the form of PWM-VSC, a technology developed by ABB under the code name of HVDC Light [3, 4]. This technology provides the most flexible power transmission alternative in terms of power controllability. However, the high-frequency switching required by the PWM process results in substantially higher losses than those of line-commutated CSC transmission, a factor that has stimulated the development of the multi-level self-commutating VSC transmission. This paper presents the elements of VSC-HVDC where the inverter station supplying power to a passive load. The paper wills first give a brief description about the VSC based HVDC transmission system and its terminal control functions. Following that typical operating contingency scenarios are simulated in order to evaluate transient performance. The simulation results confirm that the control strategy has fast response and strong stability. EMTDC/Pscad program is used for the simulation studies.

I. INTRODUCTION

T

HE great advances made in the past two decades on power semiconductors, encouraged the manufacturers to investigate their possible use in HVDC transmission [1]. As a result, a more flexible VSC transmission technology has been developed that does not rely on the AC system voltage for the valves’ commutations.

II. FUNDAMENTALS OF VSC TRANSMISSION The basic configuration of a point-to-point VSC transmission link consists of two VSC units and a DC line as shown in Fig.1. Each end of the link may be connected to a separate AC system or to a different bus of a common grid. Moreover, the receiving end of the link may supply power to a totally passive system.

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Fig. 1. Basic VSC interconnection

If the VSC is connected to a passive network on the AC side, the power flows unidirectional from the DC input side to the passive AC load [5]. However, if the inverter is connected to an active AC system, the power can be made to flow in either direction by making the phase angle of the converter AC output voltage positive or negative with respect to that of the AC system voltage [6, 7, 8]. The switches must block a unidirectional voltage but need to be able to conduct current in either direction if bidirectional power flow is required. Fig.2 shows the fundamental frequency phasor representation of the VSC operating as an inverter and supplying active and reactive power to the AC system. In this operating condition the diagram shows that the VSC output voltage UV has larger amplitude and is phase advanced with respect to the AC system voltage UL.

according to the equation (2). The reactive power is fed from the voltage with higher magnitude towards the voltage with the lower magnitude. Theses features permit the independent control of the reactive and active power which is a major advantage for the VSC. If the converter is connected to an active DC system (e.g. to another converter as in the case of Fig.1) the AC current and voltage can have any phase relationship, and the converter can act as a rectifier or an inverter, and with leading or lagging reactive power (i.e. fourquadrant operation is possible as illustrated in Fig.3). Thus the VSC can be controlled to operate at any point within the circle of Fig.3, the radius of which represents the converter MVA rating.

Fig. 3. P-Q diagram of VSC power transfer

There are, of course, active and reactive power limits determined by the maximum allowable valve current and maximum allowable DC voltage respectively on the storage capacitor. For a given AC system voltage, the DC voltage rating is determined by the maximum AC output voltage that the converter must generate to provide the maximum required reactive power. Fig. 2. Phasor diagram of VSC operating as an inverter

The active and reactive power exchanges (P and Q) between two active sources are expressed as:

P =Ud ⋅ Id =

Q=

U L ⋅ U V (1) XL

sin δ

U L ⋅ (U L − U V (1) ⋅ cos δ ) XL

(1)

(2)

Equation (1) gives that the active power is proportional to the DC current and the DC voltage. Furthermore it is mainly determined by the phase-displacement angle δ. A positive phase-shift results in that the active power flows from the AC network to the converter. However the reactive power is mainly determined by the difference between the magnitudes of the AC bus voltage and the converter output voltage

III. SYSTEM DESCRIPTION A 100 MW (100 kV) based on a simple 6-pulse forcedcommutated converter (VSC) interconnection is used to transmit power from a 115 kV, 167 MVA, 60 Hz network to a passive load. In this paper, the simple load model (resistors and inductors) is used. The sinusoidal pulse width modulation (SPWM) switching uses a single-phase triangular carrier wave with a frequency of 27 time’s fundamental frequency (1620 Hz) at the rectifier and 33 time’s fundamental frequency (1980 Hz) at the inverter. The simulated system is shown in Fig.4. A. The AC systems The AC network at the rectifier end is modeled as infinite source separated from his commutating bus by system

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Fig. 4. VSC- HVDC link feeding a passive load

impedances. The impedance is represented as R-R/L having the same damping at the fundamental and the third harmonic frequencies. The impedance angle of the sending end system is selected to be 70 degrees. This is likely to be more representative in the case of resonance at low frequencies [9]. However, the AC network at the inverter end is modeled as a simple passive load (resistors and inductors). B. The Converters The converters are VSC employing IGBT power semiconductors, one operating as a rectifier and the other as an inverter. The rectifier and the inverter are simple 6-pulse voltage source converter VSC using close IGBT/Diodes. The two converters are connected by a 100 km DC cable. C. The transformers A 100 MVA converter transformer (Yg /∆) is used to permit the optimal voltage transformation. The 0.10 pu transformer leakage reactance permits the VSC output voltage to shift in phase to the AC system and allows control of converter power output. The tap position is rather at a fixed position determined by a multiplication factor applied to the primary nominal voltage of the converter transformers. The multiplication factors are chosen to be 1 and 1.05 at the rectifier and the inverter side with respect. D. DC System The DC system is composed of a 100 km DC cable. This model is based on the Bergeron’s traveling wave method. E. DC capacitors On the DC side there is a capacitor of (500 µF) at each converter. The objective for the DC capacitor is primarily to provide a low inductive path for the turned-off current and energy storage to be able to control the power flow. The capacitor also reduces the voltage ripple on the DC side.

IV. VSC-HVDC CONTROL STRATEGY A. Rectifier control The rectifier controls AC voltage by changing modulation index, and controls DC voltage by changing phase shift in PWM control. Fig.5 displays a block diagram of the AC voltage control loop. The measured AC voltage is filtered before being compared with the AC voltage reference to calculate the AC voltage error. The AC voltage error is then passed through a PI controller and a limiter to set the modulation index. By changes in modulation index it changes the magnitude of AC voltage generated by rectifier, and this changes the amount of reactive power generated/absorbed by rectifier.

Fig. 5. Rectifier AC voltage control principle diagram

The DC voltage controller is shown in Fig.6. It changes a phase shift between the AC voltage and AC voltage generated by rectifier, and this control real power. The difference between the measured DC voltage and the specified DC voltage reference is passed through a PI controller to set the phase angle shift for the PWM control signal.

Fig. 6. Rectifier DC voltage control principle diagram

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The latter is combined with the modulation index derived in Fig.5, as shown in Fig.7, to generate the firing instances.

B. Inverter control The AC voltage control loop is similar to that of the sending end except for the addition of the frequency control. The converter 2 controls the frequency and the AC bus voltage and consequently operates as a generator with the active power fed from the DC link.

Fig. 7. Rectifier control principle diagram

Fig. 8. Steps on the regulators references

V. SIMULATIONS RESULTS The dynamic performance of the transmission system is verified by simulating the: 1. Steady state, 2. Recovery from severe AC perturbations at the inverter side.

A. Steady-state As shown in Fig.8, the reference voltage at converter 1 side (Uabc 1) is controlled to 1.0 pu., while the DC reference voltage is set to 1.0p.u.. The reference voltage at converter 2

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Fig. 9. AC side perturbations at inverter side

side (Uabc 2) is also controlled to 1.0 pu and the frequency is controlled to 60 Hz; the load 2 is 100MW and 10MVar. It is being connected to the system by deblocking of inverter at 1.0s. From the simulation results, it can be seen that at steadystate the AC voltages are kept to the set reference values (1.0pu.) at both converters, and the DC voltage is also kept to the set reference value. B. AC side perturbations at inverter side. A single phase to ground fault was first applied at t = 2.5s during 0.084s (5 cycles) at station 2 in order to investigate the behaviour of VSC-HVDC during unbalanced faults. A second perturbation follows. A three-phase to ground fault is applied at station 2 at t = 3.5s and is cleared at 5 cycles after the fault, i.e., at t = 3.584s. Fig.9 presents the simulations results. From the simulation, it can be noted that before a

single phase to ground fault at station 2, the active power flow is 1.0 pu, transmitted from converter 1 to converter 2, and is kept constant during the fault. The DC voltage drops and it contains an oscillation during the fault. Consequently the transferred DC power contains also the oscillation. During the station 2 side fault the transmitted power can be kept constant except a small oscillation during the fault. All oscillations in voltages and currents at both systems, means that the phase voltages and currents at both systems are unbalanced. During the severe three-phase fault at station 2 at t = 3.5 s, the AC voltage at station 2 side is decreased to 0 pu during the fault and recovers fast and successfully to 1.0 pu voltage after clearing the fault. The transmitted power flow is reduced to very low value during the fault and recovers to 1.0 pu after the fault. The DC voltage, which can be controlled to 1.0 pu during the fault, has some oscillations at the beginning of the fault and at clearing the fault, and its

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maximum transient value is about 1.2 pu. On the other hand, the phase currents at station 1 side decrease to low values to reduce the power flow.

[5]

[6]

VI. CONCLUSION In this paper, we have presented the steady-state and dynamic performances of VSC based HVDC transmission system feeding a passive load. The results show that the VSCHVDC performance is adequate in transmitting the power flow as well as keeping the sending and receiving end voltages at the ordered magnitudes. It is apparently verified by the simulation results that the AC voltage controller of the inverter can guarantee rapid, precise adjustment of the controlled identities. It can be obtained also that during a single-phase fault the transmitted power can be kept constant except a small oscillation during the fault. However, during a three-phase fault; the decreased voltage at the converter terminals strongly reduces the power flow by the DC link. When the fault is cleared, normal operation is recovered fast. APPENDIX Station 1(Rectifier side)

Station 2 (Inverter side) Transformers

Main DC capacitor DC Cable Switching frequency

115 kV(70°), 167 MVA, f = 60 Hz Load (100 MW, 10 Mvar) Yg/∆, 115kV/62.5kV, 100 MVA, 10% 500 µF 100km (1620 Hz ) for Rectifier (1980 Hz ) for Inverter

REFERENCES [1]

[2]

[3]

[4]

Schettler, F.; Huang, H.; Christl, N. “HVDC transmission systems using voltage sourced converters design and applications” IEEE Power Engineering Society Summer Meeting. Vol 2, 2000 pp: 715 – 720. Vijay K. Sood, “HVDC and FACTS Controllers, Applications of Static Converters in Power Systems”, Kluwer Academic Publishers, Boston, 2004. Weimers, L. “HVDC Light: A New Technology for a Better Environment” IEEE Power Engineering Review, Vol 18, Issue 8, Aug. 1998. pp: 19 – 20. Asplund, G. “Application of HVDC Light to power system enhancement” IEEE Power Engineering Society Winter Meeting. Vol 4, 2000 Page(s):2498 – 2503

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[8]

[9]

Guibin, Z.; Zheng, X.; Hongtao, L. “Supply Passive Networks with VSC-HVDC”, IEEE Power Engineering Society Summer Meeting, Canada, 2001. Gengyin, Li.; Ming, Y.; Ming, Z.; Chenyong Z. “Modeling of VSCHVDC and Control Strategies for Supplying both Active and Passive systems”, IEEE Power Engineering Society General Meeting, 2006. Lie Xu; Andersen B.R.; Cartwright P. “Control of VSC transmission systems under unbalanced network conditions” IEEE Transmission and Distribution Conference and Exposition, PES, 7-12 Sept. 2003. Padiyar K. R.; Prabhu N. “Modelling, Control design and Analysis of VSC based HVDC Transmission Systems” IEEE International Conference on Power System Technology, POWERCON, Singapore, 21-24 Nov. 2004. Gole, A. M, HVDC course notes, Manitoba HVDC Research Centre, Canada, 2000.

BIOGRAPHIES KHATIR Mohamed was born in Ain Temouchent, Algeria, in 1977. He received the Eng. degree in electro technical engineering, and the Master’s degrees from the university of Djillali Liabes of Sidi Bel-Abbes (Algeria), in 2002 and 2006 respectively. He is now a PhD Candidate in the Electrical Engineering Department of Djillali Liabes University. His main field of interest includes HVDC and FACTS. Email: [email protected] ZIDI Sid-Ahmed was born in Sidi Bel Abbes, Algeria. He received the diploma of Electro technical Engineering degree from the university of Science and Technology of Oran, Algeria. The Master degree, from the university of Djillali Liabes of Sidi Bel-Abbes, Algeria in 1994. The PhD degrees from the university of Sidi Bel Abbes, Algeria, in 2005. He is currently interested by the HVDC link, FACTS and transient in power systems. Email: [email protected] HADJERI Samir received the Master's degrees in Electrical Engineering from the university of Laval, Quebec, Canada, in 1990. The PhD degrees from the University of Sidi Bel-Abbes, Algeria, in 2003. From 1991 to 2004 he was at the Faculty of Science Engineering, Department of Electrical Engineering, Sidi Bel Abbes, Algeria, where he was a teaching member. His research there focused on HVDC, FACTS and power system analysis. Email: [email protected] FELLAH Mohammed-Karim was born in Oran, Algeria, in 1963. He received the Eng. degree in Electrical Engineering from University of Sciences and Technology, Oran, Algeria, in 1986, and The Ph.D. degree from National Polytechnic Institute of Lorraine (Nancy, France) in 1991. Since 1992, he is Professor at the university of Sidi Bel Abbes (Algeria) and Director of the Intelligent Control and Electrical Power Systems Laboratory at this University. His current research interest includes power electronics, HVDC links, and drives. Email: [email protected] AMIRI Rabie was born in Ain Temouchent, Algeria, in 1979. He received the Eng. degree in electro technical engineering, from Djillali Liabes university of Sidi Bel-Abbes (Algeria), in 2002. The Master’s degrees from University of Sciences and Technology, Oran, Algeria, in 2006. He is now a PhD Candidate in the Electrical Engineering Department of Djillali Liabes University. His main field of interest includes High Voltage Engineering, HVDC links, and drives. Email: [email protected]

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