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Novel Configuration and Transient Management Control Strategy for VSC-HVDC Ahmed Moawwad, Student Member, IEEE, Mohamed Shawky El Moursi, Member, IEEE, Weidong Xiao, Senior Member, IEEE, and James L. Kirtley, Jr., Fellow, IEEE

Abstract—This paper presents a novel system configuration for voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission connected to a large-scale offshore wind power plant (WPP). The proposed scheme is reconfigured at the onshore end to achieve shunt and series compensation, which is named as ‘Unified-VSC-HVDC’ (U-VSC-HVDC). A mathematical model of the proposed configuration is derived to determine the rating of the employed series and shunt converters. To achieve a flexible control strategy for balanced and unbalanced fault conditions, the proposed transient management scheme employs positive and negative sequence controllers for the series compensation. The negative sequence voltage components are determined in such a way as to minimize power oscillations caused by asymmetrical faults, and hence to reduce DC link voltage overshoots. A test system comprised of a detailed representation of the proposed configuration is simulated and evaluated using PSCAD/EMTDC. A comprehensive study validates the capability of the proposed configuration and transient management scheme for achieving smooth power transfer and superior transient performance of the electrical grid. Also, it minimizes the possibilities of electrical network propagations in response to symmetrical and asymmetrical grid faults. Index Terms—Offshore wind power plant (WPP), positive and negative sequence components, power oscillation mitigation, series compensation, smooth power redirection, VSC-HVDC control.

I. INTRODUCTION

H

IGH-VOLTAGE direct current (HVDC) technology has been widely applied for long-distance bulk power transfer in transmission networks [1], [2]. Modern HVDC systems use voltage source converters (VSCs), which are based on self-commutated switching devices. This enables a decoupled control of active and reactive power and allows the connection of weak or even passive networks [3]. Additionally, the high switching frequencies of approximately 1–2 kHz reduce the filter size, and the IGBT valves themselves have a smaller size compared with thyristor valves in classical HVDC systems [4].

Manuscript received September 08, 2013; revised November 27, 2013; accepted February 06, 2014. Paper no. TPWRS-01153-2013. A. Moawwad, M. S. El Moursi, and W. Xiao are with the iEnergy Center and the Electrical Engineering and Computer Science Department, Masdar Institute of Science and Technology, Abu Dhabi, UAE (e-mail: [email protected]; [email protected]; [email protected]. ae). J. L. Kirtley, Jr., is with the Massachusetts Institute of Technology, Cambridge, MA 02139 USA (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPWRS.2014.2305984

Recently, driven by the growing installations of large-scale offshore wind farms as well as the rapid evolution of power electronics technology, the VSC-HVDC technology, also called HVDC light or HVDC plus, is gaining wide acceptance [5]. The operational experiences for some projects regarding the use of controllability of VSC-HVDC transmission are discussed in [6]. The world’s longest HVDC light project (Sweden–Lithuania), to be commissioned in 2015, shows the vital role of VSC-HVDC in the coming transmission era [7]. One of the major challenges for HVDC transmission systems is fault ride through (FRT) capability during different grid faults. A reliability study in [8] analyzed the operational experiences of the first 660-kV HVDC link in the world rated as 4 GW. The analysis demonstrates the importance of keeping HVDC systems energized during fault disturbances to avoid bulk power interruption that may lead to huge stability problems. An alternative configuration is presented in [9] and [10] to assure uninterruptible energy transfer by using modular multilevel VSC-HVDC during internal faults of the VSC modules. However, the proposed solution does not consider the external grid faults that force converter stations to stop operating in order to protect switches from over currents. In [11], a new VSCHVDC system is proposed based on a hybrid multilevel converter with ac-side cascaded H-bridge cells. The proposed configuration can handle grid faults and avoid line shutdown due to faults. However, the controller relies on reducing the power transmitted from the sending end converter, therefore reducing system reliability when connected with an offshore wind power plant (WPP). Large-scale offshore WPPs have high ratings similar to conventional power plants, and hence they should ride through grid faults. Current grid codes stipulate an FRT capability of wind power plants down to zero voltage for fault durations up to 150 ms [12]. In [13]–[15]; the authors proposed different control strategies for FRT enhancement of WPPs, however these strategies are used on the level of a single wind turbine. The study in [16] proposed the reduction of the generated wind power during severe grid faults which may impose substantial mechanical stress on the wind turbines and other rotating components. To cope with these challenges, new configurations have been proposed in [17]–[19] to provide smooth power evacuation from the mechanical system. In [20], a unified power flow controller (UPFC) is used to enhance the FRT capability of wind turbines. However, this proposed configuration is not cost efficient as it adds two converters for each turbine. In addition to that, the proposed configuration did not support functionality in handling asymmetrical faults.

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Fig. 1. Conventional configuration of VSC-HVDC connected to an IEEE-9 bus system.

In [21], the authors utilized a static synchronous series compensator (SSSC) with VSC-HVDC to achieve a fast system recovery when faults are cleared. Nevertheless, this configuration provides only reactive power that may decrease the possible operating range of the converter, and hence may lead to increase in the size of the converter if more compensation is required. An important issue associated with asymmetrical grid faults is power oscillations that may cause severe dc voltage ripples that cause overshoots in the dc link of VSC-HVDC system. In [22], a negative sequence current controller is proposed to minimize the power oscillations and the associated DC link ripples. However, the end power sent from the WPP is reduced to zero, which is likely to cause severe stresses in the mechanical system and electrical power system propagations. In addition, the need for overloading capability to inject the addition negative sequence currents. This paper introduces a new configuration for the VSC-HVDC system to provide smooth power transfer during the fault condition. The solution increases system reliability, supports the grid during severe faults, and reduces the possibilities for voltage and frequency instabilities (oscillations). The proposed configuration relies mainly on series and shunt connection during faults. Positive and negative sequence components are controlled to handle symmetrical and asymmetrical faults. The proposed transient management scheme assures the minimization of power oscillations and dc link ripples due to negative sequence voltage injection. The detailed model of the system has been validated in the PSCAD/EMTDC environment. II. SYSTEM CONFIGURATION Conventional VSC-HVDC systems can have several configurations such as symmetric monopole, asymmetric monopole, bipole, or multi-terminal, as described in [23]. These configurations can be connected into meshed transmission networks as in [24]–[26]. The standard IEEE-9 bus system is employed to illustrate the configuration interconnecting offshore WPP through VSC-HVDC, as shown in Fig. 1. In this study, the multiterminal configuration is used, where the onshore VSC station consists of two independent converters

to deliver the generated power through two shunt transformers; and as in Fig. 1. Each one of these transformers should be capable to handle the total power of the HVDC system. A bus coupler; is used to allow delivering the total power through , or both of them. is the point of common coupling (PCC) between the HVDC system and the electrical grid. The wind power is transmitted through HVDC and is dispatched into the grid through and . To follow the typical ( ) transmission criterion obligation, both lines, and shall be capable to handle the rated power delivered from the HVDC network. The rated power of the HVDC can be handled by one converter as the offshore station or by two parallel converters sharing the rated power as the onshore station as shown in Fig. 1. Without losing generality, the onshore station shown in Fig. 1 can be reconfigured so that the grid connection accommodates both series and shunt transformers, and , respectively, as illustrated in Fig. 2. It is noticed that the series transformer replaces the shunt transformer . Therefore, the proposed configuration provides a practical solution without significantly changing the original network or splitting it, but supporting both series and shunt connections. It also indicates a cost-effective solution using the existing converters without installing additional converters. The series transformer used in the new configuration is known to be of lower leakage reactance than that of the shunt transformer which may reduce the costs further. In normal operations, the series transformer is disconnected by opening switch , (i.e. Switch is closed, as is the complementary of ). Hence, the leakage reactance of the series transformer is considered small enough to be neglected in comparison with the transmission line reactance, therefore the voltage levels of and are equivalent, so the grid connection can be considered as one-point. In this case, the series transformer splits the PCC point into two buses, emulating a bus coupler. However, during faults, the series transformer provides series voltage to transmit wind energy into the healthy part of the electrical system. For instance, if a fault happens at , the power is transmitted through , and this line should be able

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Fig. 2. New configuration of U-VSC-HVDC system.

to handle the total power generated from the wind farm. In addition to that, this process helps to improve the overall grid performance during fault conditions. The new configuration is named as a “Unified-VSC-HVDC” (U-VSC-HVDC) system. Fig. 3 shows the states of operation of the of the U-VSC-HVDC system to inject power into electrical grid during steady state and faulted conditions. Multiple operations can be achieved by opening and closing different switches according to the following states of operation. • State 1: When WPP is supplying rated power (i.e., 100%) and there are no grid faults, the onshore converters are connected through the shunt transformer to supply total wind power. This can be achieved by closing switches and , while and are opened (Fig. 3, State 1). When is opened (i.e., is closed), the leakage reactance of the series transformer is considered very small. Therefore, the voltage of and are equivalent and the grid interconnection can be considered as one point. • State 2: This can be useful to utilize the HVDC system increase power transfer capability of the electrical grid, where the series connection is used for series compensation functions and shunt connection is made to supply wind power when WPP is generating less than 60% of its rating. This state can be achieved by closing and , while and are kept open (Fig. 3, State 2). • State 3: The advantage of the U-VSC-HVDC configuration lies in its capability to provide fast power redirection during different grid faults through series voltage injection and therefore prevent grid propagations, which are caused by various fault impacts. For instance, if a voltage dip is detected at , series voltage is inserted through transformer to build up the voltage at . This operation can be achieved by closing switches and to guarantee transferring the active power of the HVDC system into the grid, while and are opened (Fig. 3, State 3). • State 4: For faults at , and are closed, while and are opened. The series voltage is injected in the direction of to direct the active power into the healthy part of the system (Fig. 3, State 4). Table I shows the operation sequences of the four switches , , , and , switching from one state to another. The converters should be blocked before opening or closing any of

Fig. 3. Different state operations for U-VSC-HVDC system.

the switches. These switches are GTO based switches to minimize the switching delays. The nominal switch-off delay is estimated to be several to hundreds of microseconds including the blocking time of the converters [27]. III. OPERATING PRINCIPLES OF THE PROPOSED SYSTEM The proposed configuration can also be applied to a two-machine system, as shown in Fig. 4. Through the series transformer, the system shows the flexibility to direct the active and reactive power into the electrical grid in any direction. This means that the two machines are considered as one simplified ac network to test the new configuration. Hence, two voltage sources and followed by their transmission impedances and are used to represent the electrical grid connected to the new configuration, as shown in Fig. 4. From the conceptual point of view, the series converter of U-VSC-HVDC can be considered as a generalized synchronous voltage source. It is represented at the fundamental (power system) frequency by voltage phasor with controllable magnitude and angle in series with the transmission line, as illustrated for the usual elementary two-machine system in Fig. 5(a). The transmission impedances and can be considered as pure reactances and have the same value of , for the sake of simplification. However, in reality, they can have different values. During fault conditions on one of the ac transmission systems (i.e., or ), the series connection provides the main function of the U-VSC-HVDC by injecting a series voltage with controllable magnitude and angle. Transmission-line current flowing through the series voltage source results in active and reactive power exchange; and between the series transformer and the ac system as shown in Fig. 5(a). is generated internally by the converter station, while is the real power generated due to the offshore WWP.

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TABLE I SWITCHES OPERATION SEQUENCES

phasor diagram shown in Fig. 5(b). Considering a voltage dip with some phase jump at the side, the new voltage during fault becomes . To compensate for this voltage dip, the series voltage is injected through the series transformer. Then, the total active and reactive power received at can be expressed as follows [28]: (1)

(2) Fig. 4. New configuration of U-VSC-HVDC with two-machine system.

Analyzing (1) and (2), the active and reactive power received at can be divided into three quantities such as: active and reactive power due to , active and reactive power due to the series voltage compensation , active and reactive power through the shunt connection of HVDC. The series active and reactive power can be separated as (3) Hence, the rating of the series converter can be obtained as follows:

Fig. 5. Conceptual representation of the U-VSC-HVDC in a two-machine power system.

Thus, the active power generated from the WPP can be delivered by the shunt and the series connections and , as shown in Fig. 5(a). The shunt active and reactive power and injection point is determined by the states described in Fig. 3. For illustration, if the voltage dip is at the side, then the system is at state 4. and are supplied at [Fig. 5(a)]. This guarantees proper power transfer from the HVDC system during different grid faults at different locations (i.e., at or at ). The general power control capability of the U-VSC-HVDC can be illustrated with the help of the two-machine system

(4) is maintained constant at 1.0 p.u. for a healthy power system, and reactance can be considered constant for a fixed transmission line. Therefore, the series converter should be rated based on the maximum series injected voltage during a fault. In the case study, it is assumed to be rated at 40% of the HVDC capacity with the possibility to apply 250% overloading capability during fault conditions [29]–[31]. Consequently, the shunt converter can be rated at 60% (with 150% overloading capability) to achieve 100% for the whole system. From the above discussion, it is noticed that the power rating of the new configuration (i.e. shunt and series) is similar to the traditional configuration. However the only difference is the utilization of the overloading capability of the converters

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B. Control Structure of the Series Connection of the Onshore VSC Station Here, we demonstrate the control approach for the series connection of the U-VSC-HVDC system. The main purpose of the series connection is to provide smooth power transfer during the (short) fault duration to protect WPP turbines and HVDC system from severe stresses and disturbances and to prevent harmful transient propagation in the electrical grid due to faults. To fulfill this requirement, a series voltage is injected through series transformer to build up the voltage at the point of the shunt connection. The series voltage is determined in such a way to restore the voltage level to the pre-fault value. This strategy assures the compensation for both; voltage magnitude dips and phase angle jumps. Referring to Fig. 5(b), the series voltage magnitude and angle can be determined as (5) Fig. 6. Conventional decoupled VSC station.

vector control structure for the offshore

during the FRT operation when deploying the new configuration. These overloading capabilities of the IGBTs during the short interval are utilized in many VSCs [29]–[31]. Some VSCHVDC projects have reported the benefits by applying the permanent and temporarily overloading capability [32], [33]. This feature is used mainly to enhance the FRT capability of the HVDC systems. Hence, the overloading feature of VSCs becomes more and more preferable and recognizable.

IV. CONTROL AND TRANSIENT MANAGEMENT SCHEMES Here, we explain the transient and control management schemes for the onshore and offshore VSC stations during steady state and faulted conditions. A. Control Structure of the Offshore and Onshore Shunt Connection of the VSC Stations The conventional control approach of the offshore and onshore shunt connection has a nested-loop structure. It consists of a faster inner current loop and a slower outer control loop as shown in Fig. 6. The outer control loop generates the -axis current references to the current loop controller. In this study, the offshore station is designed to send the generated active power from WPP and to support the internal WPP medium voltage (MV) grid with reactive power. Meanwhile, the onshore station is regulating the dc link and PCC voltages. Shown in Fig. 6, the transformation is used to get the direct and quadrature components of the offshore station voltage and the current passing through that point named as . The control diagram of the onshore VSC is similar to the one shown in Fig. 6, except that the outer control loops are considered to regulate the dc link voltage and the PCC voltage .

From (5), the magnitude of obtained as

and the phase angle can be

(6) (7) Expressions in (6) and (7) define the required series voltage for the positive sequence component. Fig. 7 shows the positive and negative sequence controllers for the series converter. The reference positive sequence voltage is determined according to (6) and (7) to achieve acceptable voltage quality control [34]. This reference is considered as the difference between pre-fault grid voltage; , and measured grid as shown in Fig. 7. voltage during fault The and/or indicate and/or sides. To determine which side is faulted, a directional element is used to detect the fault direction as shown in Fig. 8. This element triggers either forward or reverse fault direction detection to inject the compensating series voltage (i.e., ) in the proper direction as shown in Fig. 7 (fault direction). For instance, if a fault happens at side, then the reverse direction is activated, and therefore the voltage compensation is built in the direction of . Hence, becomes the pre-fault value of , is recognized as the faulted voltage of . and In general, the total active power delivered at the terminals of the series VSC station during fault conditions is a summation of three terms: average part , cosine part , and sine part . The last two parts are oscillating at twice the power system frequency:

(8)

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Fig. 7. Positive and negative sequence controllers for the series converter operation during symmetrical and asymmetrical faults.

Solving (10), the negative sequence voltage components can be obtained as

(11) Fig. 8. Fault detection using directional element.

These three terms can be expressed in [21]:

frame as follows

(9) Where is the component of the series voltage, and is the component of the current passing through the series transformer. Shown in Fig. 7, the reference negative sequence components of the series voltage are generated in such a way to cancel the oscillatory terms; and . The cosine and sin terms of (9) can be written and equated to zero as following:

reference frame The measured series voltages in the are regulated through positive and negative sequence voltage regulators respectively as illustrated inFig. 7. are transThen the compensating voltages formed into positive and negative sequences phasor frame. Finally, the series positive and negative sequences three phase voltages are added to command the PWM control circuit and produce the converter firing signals as shown in Fig. 7. The series transformer draws a high inrush current during severe voltage dips due to sudden full voltage compensation. This inrush current is caused by flux saturation associated with the curve of the series transformer material. To tackle such problem, the positive reference grid voltage is passed through soft energization topology as shown in Fig. 7. This gives a deliberately slower response for the applied transformer voltage with a lower risk of high saturation of the transformer core. V. SIMULATION STUDY A simulation study based on the U-VSC-HVDC illustrates the effectiveness of the proposed configuration to provide fast power transfer during symmetrical and asymmetrical fault conditions. A. System Under Consideration

(10)

A power system network resembling the two-machine system, U-VSC-HVDC configuration and an offshore WPP of Fig. 4, is considered. The voltage of both machines is rated as 400 kV. The HVDC system has a rating of 300 MVA, which is equivalent to the capacity of the WPP. It is considered that

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Fig. 9. Instantaneous voltages waveforms of phase fault.

,

and

7

during three-

both shunt and series converters are equipped with overloading capability of 250% and 150%, respectively. The voltages and represent the PCC voltages at machines 1 and 2, respectively. , , and are the active powers injected or absorbed by offshore and onshore stations, respectively. and represent the coefficients of the oscillating powers injected by the series converter. The active power sent from offshore station (i.e., ) is shown as negative quantity, while powers being injected from onshore station (i.e., and ) are represented as positive quantities.

Fig. 10. Positive sequence voltage. (c)

- components of (a)

, (b)

, and

B. Evaluation Here, we evaluate the feasibility of the proposed configuration to ride through different fault conditions as well as the robustness of the controller to mitigate dc-link double-frequency ripples caused by unbalanced faults. The fault is applied at the moment 1.0 s, it lasts for 300 ms, and, finally, it is cleared at 1.3 s. Two scenarios are proposed for this evaluation: 1) three-phase fault without phase jump at ; 2) double-line to ground fault with phase jump at with and without power oscillation mitigation. 1) Three-Phase Fault Without Phase Jump at : Fig. 9 shows the instantaneous voltages of , and . During the fault, the series converter starts to inject compensating voltage to maintain at 1 p.u. The series voltage is controlled in a slew rate in order to reduce unexpected inrush current of the series transformer (referring to Fig. 7). The positive sequence components of , , and are shown in Fig. 10(a)–(c), respectively. Before the fault, it is shown that , while . This means that the angle between the two voltages (i.e., and ) is 15 . During fault, this angle is kept

Fig. 11. U-VSC-HVDC system active power flow during steady state and fault conditions.

constant (i.e., the fault occurred without phase jump in ), as shown in Fig. 10(a). The controller starts to inject series voltage to compensate for voltage dip [Fig. 10(b)] and to maintain the angle at constant during the fault, as shown in Fig. 10(c). In both cases of Figs. 9 and 10, the state transformation is indicated as 1-3-1, which is defined in Fig. 3. Fig. 11 shows the average active power sent from the onshore station (i.e., ), injected by the shunt offshore converter (i.e., ), and active power injected by the onshore series converter (i.e., ). It is noticed that, before fault, 200 MW (the negative sign indicates sending power), 200 MW (the

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Fig. 12. DC link voltage of U-VSC-HVDC system during three-phase fault conditions.

positive sign indicates receiving power), and 0 MW (i.e. No active power is injected through series transformer). In this case, the U-VSC-HVDC is configured as State 1, which is defined in Fig. 3. During the fault, the sent active power is maintained constant at 200 MW. While this power is shared between shunt and series onshore converters, 150 MW and 50 MW, as shown in Fig. 11 (time interval: 1 to 1.3 s). The magnitude of is determined by the series injected voltage to compensate for the voltage dip. Then, the remaining active power received by the U-VSC-HVDC system is injected through the shunt converter . For the duration of the fault, the system can be realized as the State 3 configuration as shown in Fig. 3. After the fault is cleared, the system returned to State 1. Fig. 12 shows the dc link voltage during the compensation of three-phase fault. It is noticed that the dc voltage is maintained constant without any increase or overshooting in its magnitude. This is due to the fast power evacuation provided by the proposed U-VSC-HVDC system. 2) Double-Line to Ground Fault With Phase Jump at : To inspect the effectiveness of the proposed U-VSC-HVDC system during asymmetrical faults, a double-line to ground fault with phase jump is applied at . The instantaneous voltages of , and during faulted and nonfaulted conditions are shown in Fig. 13. It is noticed that during a fault, the series converter starts to inject compensating voltage to maintain at 1 p.u. Fig. 13 shows that the series-injected voltage is unbalanced due to the injection of the negative sequence component. The positive sequence components of , , and are shown in Figs. 14(a)–(c), respectively. During steady state, it is noticed that , while (i.e., the angle of is and of is 0 ). During the fault, the positive sequence components of are shown as . This means that the angle is changed to (i.e., the fault occurred with phase jump at ), as shown in Fig. 14(b). The positive sequence controller starts to inject series voltage to compensate for the voltage dip at [Fig. 14(b)], and to maintain the angle at constant (i.e., 0 ) during fault as shown in Fig. 14(c). Fig. 15(a) shows the negative sequence components of . During fault, the negative sequence components of , [Fig. 15(a)] are mitigated by injecting negative sequence series voltage as shown in

Fig. 13. Instantaneous voltage waveforms of double-line to ground fault.

Fig. 14. Positive sequence voltage. (c)

,

- components of (a)

and

, (b)

during

, and

Fig. 15(b) and (c). In Fig. 15(b), the negative series components

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Fig. 17. DC link voltage during double-line to ground fault with and without mitigating sine and cosine power oscillations.

Fig. 18. U-VSC-HVDC system active power flow during steady state and double-line to ground fault conditions without mitigation of oscillating powers and .

Fig. 15. Negative sequence voltage. (b) . mitigation. (a)

- components without power oscillation

Fig. 19. U-VSC-HVDC system active power flow during steady state and double-line to ground fault conditions with mitigation of oscillating powers and .

Fig. 16. Magnitude of oscillating active powers mitigation.

and

with and without

are: . These components are injected without mitigating the oscillatory active powers and . The values of these powers are as shown in Fig. 16 ( 20 MW, 30 MW). Fig. 15(c) shows the values of the negative sequence voltage injected with mitigating power oscillations . It is noticed that

the magnitude of the oscillatory active powers are diminished as shown in Fig. 16. The zoomed part of Fig. 16 shows that the values of these powers do not exceed 1 MW. Fig. 17 shows the dc link voltage during the compensation of double-line to ground fault with and without the power oscillation mitigation. It is noticed that, without mitigating the oscillatory active powers, the dc voltage is influenced by double frequency ripples within the fault period. When the fault is cleared, overshoots show at the dc link voltage. With the application of power oscillation mitigation, the ripples and overshoots of the dc link are very much minimized as shown in Fig. 17. Fig. 18 shows the average active power sent from WPP (i.e., ), injected by the shunt offshore converter (i.e., ) and active power injected by the onshore series converter (i.e., ) without power oscillation mitigation. It is noticed that the average power injected from the shunt converter suffers from

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TABLE II POWER SYSTEM PARAMETERS

double frequency oscillations during fault conditions (30 MW peak-to-peak, as shown in the zoomed part of Fig. 18). These oscillations are generated due to the double frequency dc link ripples (Fig. 17). Fig. 19 shows the average active powers , , and with power oscillation mitigation. It is noticed that the average power injected from the shunt converter is ripple-free due to the absence of dc link voltage ripples (Fig. 17). VI. CONCLUSION A double-ended universal power flow controller is configured for VSC-HVDC system in connecting offshore WPPs. The configuration is realized at the onshore VSC station to achieve shunt and series compensation, which is named as a U-VSC-HVDC system. The novel configuration allows the smooth power transfer from WPPs during symmetrical and asymmetrical faults in ac power system networks. It also reduces the possibilities of severe power network propagations that may occur due to sudden power reduction. The states of operations for the new U-VSC-HVDC are presented and realized to handle various fault location and types of compensation. Positive and negative sequence controllers are developed to handle symmetrical and asymmetrical faults. In addition, the paper shows the concept to control the negative sequence voltage component of the series transformer in order to mitigate power oscillations and DC link voltage ripples caused by asymmetrical faults. Hence, this prevents damage to the dc link and WPP components. A comprehensive simulation study proves the concept and demonstrates the proposed advantages. APPENDIX Table II shown the power system parameters.

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This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. MOAWWAD et al.: NOVEL CONFIGURATION AND TRANSIENT MANAGEMENT CONTROL STRATEGY FOR VSC-HVDC

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Ahmed Moawwad (S’10) received the B.Sc. degree in electrical power and machines engineering from Ain Shams University, Cairo, Egypt, in 2008, and the M.Sc. degree from Masdar Institute of Science and Technology, Abu Dhabi, UAE, in 2012, where he is currently working toward the Ph.D. degree. He was with ABB Power Systems and Automation Technologies, Egypt, for two years starting in 2008, where he was a Control System Engineer for power generation plants. His research interests include applications of power electronics in power systems, power system stability and control, wind energy generation, FACTS, and HVDC systems.

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Mohamed Shawky El Moursi (M’12) received the B.Sc. and M.Sc. degrees from Mansoura University, Mansoura, Egypt, in 1997 and 2002, respectively, and the Ph.D. degree from the University of New Brunswick, Fredericton, NB, Canada, in 2005, all in electrical engineering. He was a Research and Teaching Assistant with the Department of Electrical and Computer Engineering, University of New Brunswick, Fredericton, NB, Canada, from 2002 to 2005. He joined McGill University as a Postdoctoral Fellow with the Power Electronics Group. He joined Vestas Wind Systems, Arhus, Denmark, in the Technology R&D with the Wind Power Plant Group. He was with TRANSCO, UAE, as a Senior Study and Planning Engineer and seconded as Faculty member in the Faculty of Engineering, Mansoura University, Mansoura, Egypt. He is currently an Associate Professor with the Electrical Engineering and Computer Science Department, Masdar Institute of Science and Technology, Abu Dhabi, UAE, and a Visiting Professor with the Massachusetts Institute of Technology, Cambridge, MA, USA. His research interests include power systems, power electronics, FACTS technologies, system control, wind turbine modeling, wind energy integration, and interconnections.

Weidong Xiao (M’07–SM’13) received the M.Sc. degree and Ph.D. degree from the University of British Columbia, Vancouver, BC, Canada, in 2003 and 2007, respectively. He is currently an Associate Professor with the Electrical Engineering and Computer Science Department, Masdar Institute of Science and Technology, Abu Dhabi, UAE. In 2010, he spent one year as a Visiting Scholar with the Massachusetts Institute of Technology, Cambridge, MA, USA. Prior to his academic career, he was with the MSR Innovations Incorporation in Canada as an R&D Engineering Manager focusing on projects related to integration, research, optimization, and design of photovoltaic power systems. His research interests include photovoltaic power systems, dynamic systems and control, power electronics, and industry applications.

James L. Kirtley, Jr., (F’91) received the Ph.D. degree in electrical engineering from the Massachusetts Institute of Technology, Cambridge, MA, USA, in 1971. He has been a member of the faculty of the Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, since 1971, where he is currently a Professor of electrical engineering. He was also an Electrical Engineer with the Large Steam Turbine Generator at General Electric, General Manager and Chief Scientist with SatCon Technology Corporation, and was Gastdozent at the Swiss Federal Institute of Technology. He is a specialist in electric machinery and electric power systems. He is a member of the editorial board of the journal Electric Power Components and Systems. Prof. Kirtley is a Registered Professional Engineer in Massachusetts. He is a member of the U.S. National Academy of Engineering. He was the recipient of the Nikola Tesla Prize in 2002 and the IEEE Third Millennium medal. He was editor-in-chief of the IEEETRANSACTIONS ON ENERGY CONVERSION from 1998 to 2006 and continues to serve as an editor for that journal.