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Considering reactive power coordinated control of hybrid multi‐infeed HVDC system research into emergency DC power support

2019; Institution of Engineering and Technology; Volume: 13; Issue: 20 Linguagem: Inglês

10.1049/iet-gtd.2019.0138

1751-8695

Congshan Li, Yikai Li, Ping He, Jian Guo, Yan Fang, Tingyu Sheng,

Power System Optimization and Stability

IET Generation, Transmission & DistributionVolume 13, Issue 20 p. 4541-4550 Research ArticleFree Access Considering reactive power coordinated control of hybrid multi-infeed HVDC system research into emergency DC power support Congshan Li, Corresponding Author Congshan Li 543627767@qq.com College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorYikai Li, Yikai Li College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorPing He, Ping He College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorJian Guo, Jian Guo College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorYan Fang, Yan Fang College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorTingyu Sheng, Tingyu Sheng College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this author Congshan Li, Corresponding Author Congshan Li 543627767@qq.com College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorYikai Li, Yikai Li College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorPing He, Ping He College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorJian Guo, Jian Guo College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorYan Fang, Yan Fang College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this authorTingyu Sheng, Tingyu Sheng College of Electrical and Information Engineering, Zhengzhou University of Light Industry, Zhengzhou, People's Republic of ChinaSearch for more papers by this author First published: 18 October 2019 https://doi.org/10.1049/iet-gtd.2019.0138Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract According to the characteristics of line-commutated converter high-voltage direct current (LCC-HVDC) and voltage source converter high-voltage direct current (VSC-HVDC), respective additional emergency DC power support (EDCPS) controllers are designed, and a coordinated control strategy based on a hybrid multi-infeed HVDC system for EDCPS is proposed. Considering the differences in system recovery performance between LCC-HVDC and VSC-HVDC in providing EDCPS, the emergency power support priority issues and power allocation schemes for LCC-HVDC and VSC-HVDC are discussed in detail. To maintain the stability of the receiving-end AC system during boosting power process and to ensure the active power is raised to the specified value, this study also designs an additional reactive power controller that can make full use of the ability of VSC-HVDC to support the reactive power of the AC system. Finally, a hybrid three-infeed HVDC system consists of two parallel LCC-HVDCs and one VSC-HVDC is built on PSCAD/EMTDC and simulated. The effectiveness of the proposed approach is verified based on this hybrid three-infeed HVDC system. 1 Introduction Due to its advantages of cross-region, long-distance, bulk power transmission, line-commutated converter high-voltage direct currents (LCC-HVDC), especially in ultra-HVDC (UHVDC) systems, occupy a dominant position in the electric energy transmission field, which has an absolute advantage in the east-west power transmission of electric energy in China. They are also significant ways to solve the uneven distribution of energy resources and load demands. In recent decades, with the continuous advances in the manufacturing process and computer control technology in fully controlled power electronic devices represented by the insulated-gate bipolar transistor (IGBT), these have been recommended in the field of HVDC. Internationally, the converter valve composed of the IGBT is named the voltage source converter high-voltage direct current (VSC-HVDC), when it participates in HVDC transmission. Compared to LCC-HVDC, VSC-HVDC has the excellent properties of independent decoupling control of active power and reactive power [1], as well as no commutation failure. It is not only the main technology for elastic interaction of large-scale wind-solar complementary power generation, clean energy and energy storage power, which make it a stable and controllable power supply to access the receiving-end power grid but also is an indispensable brace for building robust smart grids. With the gradual application and popularisation of VSC-HVDC, there will be multiple LCC-HVDC and VSC-HVDC lines that transfer power from different energy bases to the same load centres, and thus a hybrid multi-infeed HVDC system scenario will emerge. For example, the Shanghai Nanhui VSC-HVDC wind farm demonstration project was put into operation in 2012, for which several LCC-HVDCs were located in the coastal areas near Shanghai, forming the hybrid multi-infeed HVDC system [2]. The hybrid multi-infeed HVDC system unites the benefits of LCC-HVDC and VSC-HVDC, which is a novel HVDC system topology. Currently, the research on the hybrid multi-infeed HVDC system and emergency DC power support (EDCPS) concentrate upon the following topics: (i) The authors of [3–6] explored and discussed the influence of different control modes of the subsystems with hybrid multi-infeed HVDC on the operational characteristics of the system, and studied the interaction between the LCC-HVDC and VSC-HVDC systems using the equivalent effective short circuit ratio as an evaluation index. Firstly, the small-signal model of the hybrid dual-infeed HVDC system was established, according to the Newton optimal power flow method, and the equivalent impedance of the subsystem under different control modes was calculated. Then the index of the equivalent effective short circuit ratio was proposed, which provided a theoretical basis for assessing the interaction between subsystems. (ii) The authors of [7–11] studied the coordinated control approach for suppressing LCC-HVDC commutation failure in a hybrid multi-infeed HVDC system. Among them, the authors of [7, 8] utilised the reactive power support capability of the VSC-HVDC for the receiving AC system, which evaluates the reactive power that must be supplied after the fault based on the extinction angle or the extinction area, and then attaches this value to the VSC- HVDC inverter side reactive control link to enhance the ability of LCC-HVDC to mitigate commutation failure. The authors of [9–11] utilised the commutation failure immunity index as an indicator to evaluate the commutation failure immunity level of a hybrid multi-infeed HVDC system, and studied the influence of the VSC-HVDC control mode on LCC-HVDC commutation failure. The coordinated control approaches of VSC-HVDC and LCC-HVDC were presented, which mitigates the commutation failure of LCC-HVDC and improves the fault recovery performance of the overall system during fault and post-fault periods. (iii) Xu et al. [12] proposed a coordinated control strategy for multi-DC emergency power support to improve system frequency stability. On the premise of ensuring the economic operation of the system, the optimal emergency power support strategy for the current operating mode is determined. Zhen and Xingyuan [13] designed a multi-circuit HVDC auxiliary frequency controller based on proportional–integral–derivative control, which can effectively improve the frequency quality of AC power systems on both sides of any HVDC links once a large load disturbance occurs. Cheng et al. [14] designed an auxiliary power/frequency controller, for which the power coordination control of a multi-loop DC system can be archived rapidly under the situation of AC–DC system faults, the improvements in power angle/frequency transient stability using the system were obvious. Li et al. [15] proposed a kind of emergency power support control strategy that generates positive and negative sequence current compensation through the deviation in the AC voltage RMS when symmetric or asymmetric faults occur in the AC side. The simulation results show excellent control performance. In reference to the above studies, this paper initially proposes an EDCPS strategy for hybrid multi-infeed HVDC systems. In this paper, EDCPS additional controllers and an additional VSC-HVDC emergency reactive power support controller are designed. Meanwhile, this paper proposes active power allocation and reactive power coordinated control on EDCPS. The validity of the control strategy is verified by simulations. 2 Electromagnetic transient model of hybrid multi-infeed HVDC The system considers a scheme in which a VSC-HVDC and two LCC-HVDCs feed into a close electrical proximity network as shown in Fig. 1. The electrical distance is indicated by L in Fig. 1, which is represented by a π-type equivalent line. The π-type equivalent circuit is shown in Fig. 2. Fig. 1Open in figure viewerPowerPoint Schematic diagram of the hybrid there-infeed HVDC Fig. 2Open in figure viewerPowerPoint π-type equivalent circuit In Fig. 1, Pd and Qd are the active power and reactive power of the LCC-HVDC subsystem; Ps and Qs are the active power and reactive power of the VSC-HVDC subsystem; Pac and Qac are the AC systems of the respective subsystems; T1, T and T2 are the ratios of the converter transformers; XT is the leakage reactance of the converter transformer; E1 ∠φ, Eδ ∠ϕδ and E2 ∠φ are the equivalent electromotive forces and impedances of the AC systems of the respective subsystems. In Fig. 2, Z and θ are the equivalent impedance and phase angle of L. 2.1 Subsystem of LCC-HVDC The LCC-HVDC subsystem is modified from the CIGRE Standard Test Model. The rectifier side is controlled by a constant current and minimum angle. The inverter side is controlled by a constant current and constant extinction angle, as well as a voltage-dependent current order limiter [16]. The primary system parameters are listed in Table 1. Table 1. Main parameters of the LCC-HVDC LCC-HVDC subsystem Rectifier parameters Inverter parameters AC system 382.87 kV 47.7∠84°Ω 215.05 kV 21.2∠75°Ω reactive power compensation 500 Mvar 500 Mvar converter transformer 600 MVA 590 MVA smoothing reactor 0.5968 H 0.5968 H 2.2 Subsystem of VSC-HVDC The VSC-HVDC subsystem employs the direct current control strategy in a dq synchronous reference frame. More concretely, the inner current loop controller use a dq decoupling control. The underlying principle is that the three-phase current in the abc three-phase stationary coordinate system is converted into a two-phase synchronous rotating coordinate system. The modulation degree and phase angle of the converter are controlled by regulating the AC current dq component of the current that flows through the reactor, such that the active and reactive power independent decoupling control is realised [17]. For the sake of preventing further deterioration of the AC bus voltage on the inverter side, constant active power and constant reactive power control are adopted on the rectifier side and a constant DC voltage and constant AC voltage are adopted on the inverter side [6]. The primary system parameters are listed in Table 2. Table 2. Main parameters of the VSC-HVDC VSC-HVDC subsystem Rectifier parameters Inverter parameters equivalent AC system 230 kV 4.372∠84°Ω 215 kV 6.370∠84°Ω converter transformer 500 MW 236/170 kV 500 MW 230/165 kV DC capacitance 300 μF 300 μF 3 Analysis and research on emergency DC power support to improve system stability 3.1 Mechanism of emergency DC power support The principle behind improving system stability by EDCPS has been described in many studies, the core of which is to employ the extended equal area criterion (EEAC) of the transient process under large disturbances to demonstrate [18–20]. Based on EEAC theory, the overall system can be converted into a double machine-unstable model, which is divided into severely disturbed groups (S groups) and remnant groups (A groups). The stability problem of the entire system is the relative stability problem of the two groups. The transient equations of motion for the critical group S and the remaining group A are as follows: (1) (2)where M is equivalent inertia time constant; δ is equivalent generator angle; ω is equivalent angular frequency; i is equivalent generator i groups; j is equivalent generator j groups; Pm is mechanical power of equivalent generator; Pe is electromagnetic power of equivalent generator; S is considered as S groups; A is considered as A groups. Formulae (1) and (2) are merged into a single machine infinite bus system to produce formula (3) as follows: (3)During normal operation of the HVDC system, , the generator power angle is in a steady state. In the HVDC system, using generator tripping caused by the sudden severe fault of the internal generator of the S groups as an example, the mechanical power in the S groups is less than the electromagnetic power, so the generator rotor will perform a deceleration motion. The additional power ΔPm of the EDCPS can be equivalent to the mechanical power of the generator, and the EDCPS measure is added to the formula (3); then, formula (3) becomes the formula (4), where ΔPm is the amount of EDCPS (4)From formula (4), it is concluded that after adding emergency power support measures, tends to zero, such that power angle stability can be achieved. 3.2 Constraints of emergency DC power support The overload capacity of the LCC-HVDC system refers to the overload capacity of the thyristor, which is the magnitude and duration of the DC current higher than the rated value. Generally, it has a long-term overload capacity of 1.1 times and 1.5 times short-time overload capability of 5 s [21], as shown in Fig. 3. Fig. 3Open in figure viewerPowerPoint LCC-HVDC overload capacity diagram The overload capacity of the VSC-HVDC system is mainly constrained by two conditions: (i) the maximum current Imax is allowed to flow through the converter valve, and Imax determines the maximum power that is exchangeable between the inverter and the AC system; and (ii) the maximum steady-state DC current Idc-max, which allows flow through the DC transmission line, and Idc-max determines the range of active power transmission. With regard to the LCC-HVDC system, whether it can meet the power boost requirements in accordance with the specified power boost command mainly depends on two factors: (i) the overload capacity of the LCC-HVDC system itself; or (ii) the AC bus voltage level of the inverter station, the essence of which is that the LCC-HVDC consumes more power than the normal full-load operation when the active power is boosted. If the reactive power is entirely borne by the AC system, the AC system voltage stability will be deteriorated; this is especially true for a weak AC system, which may cause commutation failure of the LCC-HVDC system. Additionally, the reactive power was provided by the filter and the compensation capacitor of the LCC-HVDC converter can be expressed by formula (5) as follows: (5)where U is the converter buses voltage and XC is the capacitive reactance. From formula (5), it can be concluded that as the U decreases, the reactive power generated by these devices will decrease, demonstrating a positive feedback process. In addition, the LCC-HVDC system also has a minimum power limit, which is determined by the minimum current limiting factor, when the current is below the minimum value, a discontinuous DC current will occur. 3.3 EDCPS strategy and controller design of LCC-HVDC EDCPS is a type of large-mode modulation. LCC-HVDC and VSC-HVDC as two different types of EDCPS control measures have different methods for suppressing the power angle swing between regions and improving frequency stability [22]. Therefore, this paper designs two kinds of active additional controllers, all of which are open-loop controls. Assuming that the system contains only the fundamental component, the simple mathematical model of the LCC-HVDC with a multi-bridge converter is as follows: (6)where Ud0 is secondary side no-load voltage of converter; n is the number of converter bridges; kT is the ratio of converter transformer; Ud is considered as DC voltage; γ is considered as extinction angle; Xc is equivalent commutation reactance; Id is considered as DC current; φ is the converter power factor angle; Uac is the AC voltage RMS of converter secondary side. It can be concluded from formula (6) that when the converter is operated under overload conditions, the DC voltage is increased only by reducing the extinction angle and the DC power boosted is very limited. Hence, this paper chooses to add an EDCPS controller on the constant current control side of the rectifier, and boosts the DC current to preform power support on the basis of the principle of equal-amplitude step-by-step increments. Each time boost adds 20% of the amount of EDCPS, the power boost signal is shown in Fig. 4. The EDCPS controller of LCC-HVDC is shown in Fig. 5. The current, voltage and power are all taken as the per-unit value. Fig. 4Open in figure viewerPowerPoint Output signal of power increasing controller Fig. 5Open in figure viewerPowerPoint EDCPS controller of the LCC-HVDC In Fig. 5, Ifault is a fault line current signal, and 1 − Ifault produces the initial current support signal Isc. C serves as a constant, and Isc divided by C produces the support current signal Is. The value of C can be 1 or Isc/0.5; when C is 1, Isc is the same as Is, indicating that LCC-HVDC operates within 1.5 times the overload range, and when C is Isc/0.5, this indicates that LCC-HVDC operates at a maximum of 1.5 times the overload. Is is multiplied by the step-by-step signal to obtain the current booster signal ΔI from the additional controller. When the system is in steady state, ΔI is zero, and the EDCPS controller does not work. In the event of a fault, this signal is superimposed with the non-faulted LCC-HVDC rectified side current command, and a trigger signal is sent to the valve control unit via the pole control stage to achieve power support. To ensure the unnecessary malfunction of the emergency power support controller, the limit link is set to ±5% of the normal transmission power. 3.4 Emergency power support strategy and controller design of VSC-HVDC Since VSC-HVDC can independently control active power and reactive power, VSC-HVDC can provide both emergency active power support and emergency reactive power support while it participates in emergency power support. 3.4.1 Emergency power support controller design of VSC-HVDC In this paper, the dq decoupling control method of double closed loop is adopted. The outer loop controller generates the current reference values of the d-axis and the q- axis. The inner loop controller realises the tracking control of the current reference value, such that it can control the d-axis and q-axis current to control active power and reactive power. The VSC mathematical model in the dq coordinate system is as follows: (7)According to feed-forward compensation decoupling control and instantaneous power calculation approach, and then the grid voltage vector is orientated by the d-axis [23], such that it can be derived from formula (8) as follows: (8)This paper designs the active additional controller based on the VSC-HVDC active control loop, as shown in Fig. 6. The current signal of the fault HVDC is measured, and the modulation signal Pmod is output by adding an emergency power support link. Then, the modulation signal Pmod is superimposed on the active power command value of the active power control loop of the d-axis, and the active power reference current of the d-axis is increased to achieve EDCPS. Fig. 6Open in figure viewerPowerPoint EDCPS controller of the VSC-HVDC In Fig. 6, Pfault is a fault line active power signal, and 1 − Pfault produces the initial power support signal Psc. C serve as a constant, and Psc divided by C produces the power support signal P. The value of C can be 1 or Psc/0.5; when C is 1, Psc is the same as P, indicating that VSC-HVDC operates within 1.5 times the overload range. When C is Psc/0.5, this indicates that VSC-HVDC operates at a maximum of 1.5 times the overload. P is multiplied by the step-by-step signal to obtain the active power boost signal Pmod from the additional controller. When the EDCPS is actually performed, the power support amount is not necessarily equal to the command value due to the power support restrictions. 3.4.2 Design of VSC-HVDC emergency reactive power support controller Taking the LCC-HVDC inverter as an example, a schematic diagram of reactive power exchange is shown in Fig. 7. Fig. 7Open in figure viewerPowerPoint Schematic diagram of reactive power exchange for the converter station The active power and reactive power calculated by formula (6) are as follows: (9) (10)If UacI is the RMS of AC voltage on the inverter side as follows: (11) (12)It is known from formula (11) that N has a non-linear relationship with QI and Pdc. As Id increases, N increases non-linearly. ΔQ refers to the amount of reactive power consumption as the LCC-HVDC boosts the DC power, which is the required reactive power compensation value. The specific parameter settings are listed in Table 3. Table 3. Main parameters of the LCC-HVDC inverter Parameter Ud Id UacI kT n XC γ value 500 kV 2 kA 230 kV 0.91 2 13.32 Ω 15° On the basis of the reactive power characteristics of the LCC-HVDC converter, the reactive power variation of LCC-HVDC is converted into the AC voltage compensation value when the active power is boosted, which is added to the outer loop reactive power class control link of the VSC-HVDC inverter. This utilises the reactive power rapid adjustment capability of the VSC-HVDC to improve the reactive power support of the receiving-end AC system. The following are specific emergency reactive power support strategies: (i) Derive the compensation amount ΔQ of the reactive power according to formulae (11) and (12), and determine ΔU through the proportional–integral controller with the limiting function. The compensation is taken into account in the AC voltage compensation, which is added to the inverter outer loop reactive control link. If the hybrid multi-infeed HVDC system demands emergency reactive power support, the output will be the corresponding ΔU. (ii) Based on the original reactive current calculation, the reactive current reference value iq_ref after adding the compensation ΔU is obtained, such that the VSC-HVDC subsystem can provide more reactive power to the AC system to maintain the inverter AC bus voltage stability under transient conditions. It cannot exceed the maximum restriction amplitude of reactive power. The control block diagram of the above emergency reactive power strategy is shown in Fig. 8. Fig. 8Open in figure viewerPowerPoint VSC-HVDC emergency reactive power control block diagram We emphasise that the additional controllers designed in this paper aim to enhance the transient stability of the hybrid multi-infeed HVDC system by taking full advantage of the rapid response characteristic of the non-fault HVDC. The power modulation of each HVDC is determined by the respective control signals, which can be independently operated without affecting each other. They can also coordinate and cooperate with each other to improve the stability of the system and ensure that the active power boost should be within the overload range of the converter. 4 Strategies of EDCPS for LCC-HVDC and VSC-HVDC Wang et al. [24] studied the interconnected power transmission system composed of VSC-HVDC and LCC-HVDC; it designed an EDCPS supplementary controller and presented a VSC-HVDC reactive power coordination control strategy. The power allocation of LCC-HVDC and VSC-HVDC on emergency power support is essentially the influence of power flow transfer on the support effect, and irrational power allocation will deteriorate the stability of the system. EDCPS for hybrid multi-infeed HVDC systems relates to two core issues: how to design EDCPS controllers to actualise the reposeful transfer of power and a power support allocation strategy that achieves the coordinated control of LCC-HVDC and VSC-HVDC and cooperates with emergency reactive power support. For these reasons, the main ideas in this paper are presented as follows. When the amount of power loss in the system is small, the bus voltage does not fluctuate much. If the power of the VSC-HVDC is preferentially boosted, the power may not be boosted because the power of the VSC-HVDC is less than the dead-zone of the reactive power regulation [22]. If the LCC-HVDC power is preferentially boosted, not only can the dynamic reactive power reserve can be retained, but also the system's ability to cope with subsequent reactive power shocks can also be improved. Hence, the EDCPS priority must be determined according to the power boost level. If the power boost is relatively small (the LCC-HVDC rated DC power is 1000 MW in this paper; by setting the power shortage value and performing simulation experiments one-by-one, this paper is set to 15% of the maximum power loss when the HVDC is blocked), the LCC-HVDC active power can be preferentially boosted. Supposing that the amount of power loss in the system exceeds the setting value and does not exceed the maximum overload capacity of the VSC-HVDC, the VSC-HVDC power can be preferentially boosted. When boost power of LCC-HVDC or VSC-HVDC alone cannot restore the stability of the system, it is necessary to consider the coordinated operation of the LCC-HVDC and VSC-HVDC. Furthermore, when the VSC-HVDC itself fails, if there are other VSC-HVDCs in the system, it should preferentially boost the power of the VSC-HVDC, or conversely boost the power of the LCC-HVDC system. In the case that the boost power of the LCC-HVDC cannot reach the command value, the reactive power can be utilised to increase the reactive power by the VSC-HVDC and provide emergency reactive power support to the LCC-HVDC. In this paper, the load model is based on a polynomial model and considers the static load model of the frequency term [25]. The load power is calculated by formula (13) as follows: (13)where m is the number of bus; PZ is the active power proportional coefficient of constant impedance term; PI is the active power proportional coefficient of constant current term; PP is the active power proportional coefficient of constant power term; QZ, QI and QP are the reactive power proportional coefficients. And the following: (14)where KDP and KDQ are the load frequency factors, which are the coefficient of load power as a function of frequency, U is the current voltage value; and U0 is the initial voltage value. If the system loss is ignored and it is assumed that spinning devices are not reserved, the primary frequency regulation of generator Ppf and load frequency control ΔPL are taken into account, where KG is the unit generator regulation power. Supposing that the power loss amount is set to balance power Pbalance, after a system fault, the amount P of emergency power support is demanded in the system as follows: (15)From the modulation priority and amount of EDCPS set above, the following coordinated control strategy is as follows: (i) The amount of the power loss is lower than the threshold, the amount PLCC of power is preferentially boosted for EDCPS, and the power support amount PLCC is calculated according to formula (15). (ii) If the amount of power loss exceeds the threshold, the VSC-HVDC is preferentially boosted for EDCPS, and the PVSC is calculated according to formula (15). (iii) If the amount of emergency power support exceeds the maximum overload capacity of VSC-HVDC, the LCC-HVDC is required to participate in tandem to maintain system stability. The LCC-HVDC participates in the power support according to the following: (16)where PLCC pertains to LCC-HVDC power support and PVSC pertains to VSC-HVDC power support. (iv) If the total power support amount of VSC-HVDC and LCC-HVDC cannot restore system stability, generator tripping or load shedding is required (17) (v) While LCC-HVDC participate