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Line-commutated converter based high-voltage direct-current (LCC-HVDC) has been extensively applied in power system. LCC-HVDC is prone to continuous commutation failure (CCF), which consequently causes numerous active and reactive impacts, thereby resulting in various power system security and stability issues. Given that there is no CCF predictive method at present, the effect of CCF suppression measure is still limited, and the control margin of DC commutation station and power grid cannot be fully applied, so that the negative impact of CCF is difficult to reduce through advance control. In view of this situation, this paper aims at proposing a predictive method of LCC-HVDC CCF, which occurs again during the first CF recovery process. First, the predictability of CCF is demonstrated, and the idea for predicting CCF by comparing commutation voltage is proposed; Second, the calculation equation of CCF threshold voltage is derived by analyzing the CF process; Third, a predictive method of CCF based on threshold commutation voltage is proposed; Finally, the correctness of the theoretical analysis and predictive method is verified through the CIGRE HVDC standard system. Simulation results show that the proposed method has high accuracy and can provide sufficient time margin for emergency control to suppress the LCC-HVDC CCF.

The transmission lines between the sending and receiving ends of an interconnected power grid may be tripped under severe disturbance or equipment failure. The tripping will result in a large power imbalance in the sending grid, thereby causing frequency instability and even collapse. The effect of existing emergency control methods based on synchronous generators, such as generator tripping control, is limited due to the lack of available tripped generators, high cost and long recovery time. The high penetration and flexible controllability provide wind farms with the potential to participate in system emergency control. To evacuate excess power from the tripping of transmission lines and improve frequency stability, an emergency power balance control method based on the doubly-fed induction generator (DFIG)-based wind turbine (DFWT) is proposed in this study. A novel idea of the motor operation of DFWT is presented and applied to the proposed method. To realise steady and adjustable power consumption of DFWT under motor operation, a rotor virtual resistor control and supplementary pitch control are introduced under safety constraints. The validity of the proposed method is verified by case studies on single DFWT and DFWT-based wind farms.

Traditional distribution networks are transforming into active distribution networks (ADNs) with the advancement of distributed generators (DGs). Break faults that lead to voltage and current imbalances and fluctuations threat the safety of sensitive power electronic equipment in ADNs. However, locating break faults in ADNs remains a challenge under the influence of DG fault polymorphism. This paper proposes a new method to locate break faults by monitoring information of DG current based on the observability of ADNs. The DG equivalent model is established based on the DG output characteristics under the presence of a single-phase break fault. The characteristics of the fault current contributed by DG are analyzed. An identification method of break faults is proposed based on the variations in DG current. Thus, a fault-locating matrix algorithm is proposed combined with current information and location information of DGs. The simulation results demonstrate that the proposed method can locate break faults quickly and accurately. The method is not affected by topology and changes of DG output and load.

Large-scale wind farms are generally integrated by long-distance transmissions, but power grids cannot sufficiently support the access point voltage of these wind farms. The access point voltage undergoes a stability problem under wind speed variations. However, the reactive power compensation device cannot reconcile the requirements of response speed and compensation capacity. Despite their fast power decoupling control, the reactive power capability of doubly fed induction generators is restricted by active power output. To satisfy the reactive power demand of system under wind speed variations, coordinating the reactive power capability and active power output of wind farm is the key solution, based on which a novel active control idea is proposed. The reactive power capability of wind farm and the reactive power demand of system are both studied, and the controllable conditions of access point voltage are analyzed. Active voltage control strategies, including active adjustment of reactive power reference, active speed control, and active pitch angle intervention according to wind speed ranges, are proposed. In the simulation, the method is verified to adequately consider the reactive power demand, and excavate the wind farm reactive power capability. The method also effectively suppresses the change of grid voltage under wind speed variations.

The rotor angle stability of power systems under the influence of doubly fed induction generator (DFIG)-based wind turbines has elicited increasing attention as the penetration of wind power increases. The flexible and controllable power capability of DFIG makes it feasible to improve the stability performance of power systems. However, the effects of DFIG on rotor angle stability are not accurate because the phase angle variation of grid voltage has not been considered in existing studies. Current stability control methods may not be effective. This study proposes a rotor angle stability control method considering phase-amplitude characteristics of transient-grid voltage. Variations in both amplitude and phase angle of grid voltage are considered to analyse the effect of DFIG on the rotor angle stability of the DFIG-integrated power system. An evaluation index, which represents the accelerating area variation of the synchronous generator (SGs), is defined to quantise the effect of DFIG. In addition, the controllable power region of DFIG is established by considering DFIG's power constraints and effects on system stability. The control method of rotor angle stability is proposed based on the controllable power region and the evaluation index. Simulation results verify that the effectiveness of proposed method.

Wind speed fluctuations are extremely easy to cause the voltage change frequently. The existing voltage control methods are lacking in adjustment range and response speed. When wind speed fluctuates substantially or rapidly, voltage exceeding limit and the resulting grid safety problems cannot be avoided. Therefore, reasonably distributing and utilizing the reactive power of variable speed wind turbines in the whole network are an inevitable choice. However, the controllable reactive capacity of wind turbines is limited by converter capacity and active power, and thus, the grid demand may not be met. A new idea of active and reactive power-coordinated control for preventing voltage exceeding limit due to wind speed fluctuations was proposed. The effects of wind speed fluctuations on voltage were analyzed, and the controllable reactive capacity of variable speed wind turbines under wind speed fluctuations was studied. The relationship between voltage and the active and reactive power of wind farms was deduced. The theory and method of active and reactive power coordinated control before wind speed fluctuations were proposed based on the model predictive control theory. The control model based on multi-timescale was established. Finally, it was verified that the method can solve the voltage problems and maximize the system economy.

With a high proportion of wind power being transmitted through ac and dc lines, a new power grid pattern has gradually emerged. A dc inverter is prone to commutation failure, which may lead to cascading trip of ac lines for overload, resulting in serious consequences. However, existing control methods have problems of over- or under-shedding and are unsuitable for high-proportion wind power transmission systems. Accordingly, a new fault overload control method based on emergency acceleration of doubly fed induction generator (DFIG) is proposed. The fast power control capability of DFIG is fully utilized; the active power of power grid could be rapidly balanced. First, the fault characteristics of a wind power ac/dc transmission system under the fault of dc receiving-end grid are analyzed, and the power flow transfer after self-recovery from the dc commutation failure is derived. Second, by analyzing the power characteristics of DFIG, a fault overload control idea of the wind power transmission system is proposed based on emergency acceleration of DFIG. At last, the maximum controllable capacity of a wind farm is analyzed, and a fault overload control strategy of the wind power transmission system is proposed. The simulation results prove that the proposed method can rapidly reduce the output of wind farms and prevent the overload of ac lines.

A regional power grid containing high-density wind power is generally connected to a load centre through long-distance transmission lines. Voltage problems, caused by wind speed fluctuation, in the sending-end grid have become increasingly serious. The variable speed wind turbine has been used to compensate the deficit in reactive power. However, the available methods could not meet the requirement because the controllable range of reactive power contributed by wind farms cannot be adjusted. A new idea of reactive power control realised by the active control before wind speed fluctuation was proposed to prevent the voltage variation. Firstly, the influence of wind speed fluctuation on voltage was analysed, and the power controllable range of wind turbines was studied. The principle and strategy of the active control of reactive power were proposed based on the model predictive control theory. Then, the active control model was established according to the system dynamic demand for reactive power. According to wind speed forecasting, the reactive control capability of wind turbine was excavated to meet the grid demand by adjusting active power before wind speed variation, and the reduced active power of wind farms is optimally compensated by thermal powers. Finally, the method was proven to solve voltage problems.

With the rapid development of modern distribution network and the access of distributed generation, the network structure is becoming increasingly complex. Frequent single-phase break faults have seriously affected equipment and personal safety and stable operation of the power system. However, with the development and application of the composite neutral grounding modes, the protection of single-phase break fault is facing new challenges. This paper proposes a protection method of single-phase break fault for distribution network considering the influence of neutral grounding modes. The characteristics of neutral voltage and sequence current are analyzed under normal operation and single-phase break fault with different grounding modes. Following this, the protection criterion based on neutral voltage and sequence current variation is constructed. The protection method of single-phase break fault for distribution network is proposed, which is applicable for various neutral grounding modes. Theoretical analysis and simulation results show that the protection method is less affected by system asymmetry, fault location and load distribution. The method has higher sensitivity, reliability and adaptability.

Photovoltaic(PV) power system plays an important role in emergency control as well as fault restoration of power systems by its fast power control capability. Nevertheless, despite a lot of methods for low-voltage ride-through(LVRT) of PV power system, the disconnection of PV under certain degrees of failure and duration cannot be avoided due to the limitation of impact resistance of power electronic devices. In order to avoid the deterioration of the operating status of power grid and give full play to the control ability of PV, PV should be quickly reconnected to the grid after the grid impact is eliminated. Since there is a certain delay in restoring the power output after the shutdown of PV, the shutdown should be avoided after PV is grid-disconnected. Whereas, the operating status of the disconnected and non-stop PV changes continuously with the output of the PV panels, the control of the inverter, and the duration of grid disconnection. Its output may fluctuate greatly after reconnecting to the grid, causing an impact on the power grid. At present, the impact of PV reconnection and the measures to address it have not received much attention. Therefore, the disconnection and reconnection process of PV power system is theoretically analyzed in this paper. The mechanism and influencing factors of current impact caused by PV reconnection are studied. An impact suppression method for PV reconnection based on DC voltage control is proposed. The simulation results are used to verify the effectiveness of the proposed method. The proposed method realizes the stable restoration of power after PV reconnection by suppressing the DC voltage of PV during disconnection, and effectively avoids the current impact of PV reconnection.

With the development of large wind farms, the voltage problem caused by the power changes of wind farms occurs continuously because of the wind speed variation. The problem cannot be resolved by extra reactive power compensation because of the limitations in cooperation and response speed. A doubly fed induction generator (DFIG) possesses the power decoupling control capability, which can be used to prevent the voltage change rapidly. However, the dynamic processes of the power demands of the grid and the power limits of the DFIG have not been considered. Therefore, a vector equivalent model of the DFIG considering the controllability of the rotor voltage is established. The feasible power ranges that are limited by the electromagnetic process and controller response are deduced. The dynamic requirement and limitation of the reactive power under the wind speed variation are investigated. The emergency control strategy of reactive power contributed by the DFIG is proposed on the basis of the requirement and limitation. The simulation shows that the rapid adjustment capability of the DFIG can be fully utilised by the proposed method.

The complex interactions between AC/DC systems and different line-commutated converter-based high-voltage direct current (LCC-HVDC) expand the effect of commutation failure (CF). The rapid prediction of CF is crucial in ensuring the stable operation of such systems. The simultaneous CF caused by grid fault has received widespread attention in recent years. However, the coupling between the LCC-HVDC with CF and the adjacent normal LCC-HVDC is not considered. When the state of the main circuit suddenly changes and the control system subsequently starts, the LCC-HVDC with CF inevitably produces an effect on the adjacent LCC-HVDC through the coupling of grid voltage. This study describes a new phenomenon, where the former produces a successive CF in the latter under the fault of weak receiving-end grid. The dynamic reactive power characteristics of inverter station and the induced mechanism of successive CF are analyzed. And the dynamic reactive power model of LCC-HVDC is established. A fast prediction method for successive CF is then proposed by calculating the extinction angle of LCC-HVDC with consideration of the effect of adjacent LCC-HVDC with CF. The proposed method can improve the accuracy of CF prediction in multi-infeed LCC-HVDC systems through theoretical derivation and simulation verification.

Limited by the reserve capacity and control speed of synchronous generator (SG), the prediction error of wind power (PEWP) results in severe frequency problems in power systems with high-proportion wind energy. The deloading of doubly-fed induction generators (DFIGs) is an effective way to provide active power support for the grid frequency. However, the existing methods are usually based on a fixed deloading rate, which is difficult to match the uncertainty of PEWP. Accordingly, a new idea of dynamic deloading is proposed considering the influence of safe limit of DFIGs, prediction error of wind power, spinning reserve of SGs, and system operating costs. The positive and negative reserve capacities of DFIG are derived based on coordinating the rotor accelerating control and pitch angle control. The allocation principle of the reserve capacity of DFIGs and SGs for frequency regulation is built. Thus a partitioned scheduling mode of power systems with high-proportion wind power based on the optimal deloading of DFIGs to participate in frequency regulation control is proposed. The simulation shows that the proposed method can simultaneously satisfy the dynamic frequency regulation demand and operating costs of power systems.

The Integrated Electrical and Natural-gas System (IENS) based on Power-to-Gas (P2G) devices and gas-burning units with wind power is rapidly developing. The failure of wind turbines brings a greater risk to the safe operation of integrated system. The gas-burning units and P2G devices have fast and flexible power and gas control capabilities, providing an effective way to reduce the operational risk of integrated system. At present, the operation and scheduling of the IENS fails to take into account the risks of the system, and the influences of the control strategies of gas-burning units and P2G devices on the system risk cannot be ignored. The scheduling methods in independent power system are difficult to directly apply to the integrated system. Therefore, based on the operating characteristics of the IENS and the risk of wind turbines failure, a risk indicator of wind turbines failure considering the P2G devices was established. Based on this risk indicator, a multi-objective optimal scheduling model with minimum wind turbines failure risk and minimum coal consumption of coal coal-burning units was established, which is to solve the contradiction between wind turbines risk and cost during the system operation. The example shows that the multi-objective risk scheduling model of this paper can effectively reduce the operational risk of the IENS and improve the capacity of wind power consumption.

With the wide construction of the line-commuted converter-based high-voltage direct current (LCC-HVDC), a large-scale AC/DC hybrid grid pattern has gradually formed. LCC-HVDC is prone to subsequent commutation failure (SCF) during the recovery process of first commutation failure (FCF). This phenomenon poses a great threat to the safe and stable operation of the power grid. LCC-HVDC is equipped with several controllers to prevent SCF. However, these controllers adopt preset fixed parameters and ignore the fault severity and LCC-HVDC self-regulation. The controller effect may not match the dynamic demand of the FCF recovery and even become the driving force of SCF. Accordingly, a new method is proposed on the basis of the quantification of the FCF recovery consequence, and it can avoid SCF by dynamically adjusting the DC current command according to a variable slope of U d - - I d - ord curve. First, the causes and influencing factors of SCF are presented by analyzing the recovery process of CF under the AC grid fault. Second, the correlation factors of the FCF recovery path are analyzed, and the idea of adaptive control against SCF based on the critical slope is proposed. Third, the critical slope of the U d - I d - ord curve is derived with considering the influence of the initial state of the fault and constant extinction angle control. Then, a current-limit control method to prevent SCF based on the adaptive trigger voltage is proposed with considering the influence of the DC current variation on power angle stability. Finally, simulation results verify the effectiveness of the proposed method in preventing SCF.

The doubly fed wind turbine is the main equipment of wind generators. The fault transient characteristics of doubly fed induction generators (DFIG) have elicited the attention of many scholars. However, the short-circuit current (SCC) of power grid-contained DFIG under asymmetrical voltage drop cannot be accurately analyzed. The main difficulty is that the response and coupling of converters under partial voltage drop remain unclear. Thus, this paper presents an imperative study on SCC of DFIG, with particular attention to the transfer of negative-sequence voltage in the windings and rotor-side converter. The electromagnetic process and excitation control are simultaneously deduced in a uniform coordinate space by constructing positive- and negative-sequence vector models. The generating mechanism and analytical expressions of stator SCC are proposed. The positive- and negative-sequence equivalent models of DFIG during fault initial stage and steady-state process are constructed for practical short-circuit calculation. Simulation and physical testing are implemented to verify the theoretical analysis.

As loads connected to active distribution network (ADN) grow, ADN’s voltage safety issues are becoming more serious. At present, the solution is mainly to build more distributed generation (DG) or to adjust the reactive power in the whole network, but the former needs a lot of investment while the latter requires a large amount of communication equipment and it takes a long time to calculate the adjustment amount of reactive power and to coordinate reactive power compensation equipment. When the loads are heavy, there will still be drawbacks of insufficient reactive power. Therefore, this paper analyzes the relationship between the active power, reactive power, and the voltage in the ADN. Through the autonomous region (AR) division, a voltage control method based on the active power variation and adjustable power in the AR is proposed. According to the relationship between the amount of active power and the adjustable amount active power, the active power control, the reactive power control, and the coordinated control of active power reactive power control are adopted to adjust the DGs’ output to stabilize the bus voltage. The simulation results show that the proposed method can effectively improve the voltage control capability of ADN and can enable it to operate normally under greater power changes. Through the control method in this paper, the communication requirements are greatly reduced and the calculation time is effectively shortened and is more adaptable.

Continuous commutation failures (CFs) are serious malfunctions in line-commutated converter high-voltage direct current (HVDC) systems that cause the continuous and rapid sag of transmitted power and may threaten the stability of AC systems. The conventional emergency control strategies of AC systems exhibit difficulty in responding quickly and accurately. After suffering from continuous CFs, the forced blocking of direct current (DC) converter to prevent AC system instability might also cause other adverse effects. This study proposes a ride-through control method to improve the endurance capability of AC systems against continuous CFs. An active power output model of inverter station under continuous CFs is built, while considering the process and mechanism of CFs. The impact of continuous DC power sag on the stability of sending-end system is analyzed through a four-area AC/DC equivalent model. A rolling calculation model for the power angle and acceleration area variations of the sending-end system during continuous CFs is established on the basis of model predictive control theory. A calculation method for the emergency power control reference is obtained by using the aforementioned models. Lastly, a ride-through control method for continuous CFs is developed by utilizing the emergency control of adjacent HVDC link. Simulation results show that the proposed control method can improve the endurance capability of an AC system to continuous CFs and reduce blocking risk in an HVDC link.

As the VSC-HVDC system has attractive advantages of high stability and controllability, it becomes a key development direction of the HVDC. Dynamic operation and controllability features of VSC-HVDC system differ a lot from that of conventional AC and DC transmission. During the grid fault, the VSC-HVDC export various short circuit current to AC system which can be a threat to the correct operation of distance protection. Based on the control principle of VSC-HVDC converter station, the voltage and phase features of the VSC-HVDC short-circuit current is analyzed. The measured impedance variation of the VSC-HVDC distance protection in AC side is also analyzed. The influence and interrelated factors of VSC-HVDC to distance protection are proposed. Digital simulation on PSCAD/EMTDC is done to verify the validity of the analysis.

The large-scale application of doubly-fed wind turbines has significantly changed the fault transient characteristics of power systems. However, the transient state of a doubly-fed induction generator (DFIG) under large disturbances is difficult to accurately evaluate. The main difficulty lies in the failure to acquire the transient process of the DFIG excitation circuit because of the high orders and the strong coupling of excitation control. This paper presents a detailed transient analysis of a DFIG, with focus on the dynamic behavior and effects of the excitation circuit. The dynamic models of the rotor-side converter and the grid-side converter, which include the excitation regulation and the electromagnetic process, were constructed. The effects of the implementation of controllers on the transient behavior of the DFIG are analyzed by deducing the transitive relation of transient processes. Simplified expressions of DFIG electrical variables are proposed with the excitation regulation considered.

The transient characteristics of power systems with large-scale doubly fed wind turbines undergo profound changes. Doubly fed induction generators (DFIG) under non-deep voltage drop generate short-circuit currents, which involves complicated electromagnetism and control coupling. This phenomenon has only been partially studied in the literature. Thus, this paper presents a detailed analysis of the short-circuit current of DFIG, with particular attention to the influence of converter adjustment. The dynamics of generator and converter control were simultaneously analyzed by vector analysis in uniform coordinate space. The generation mechanism, composition, and analytical expressions of short-circuit current were proposed. DFIG models were constructed to calculate the initial and steady-state short-circuit current. Simulation and experiment were performed to verify the efficacy of the proposed analyses.

The neutral grounding mode of medium-voltage distribution network decides the reliability, overvoltage, relay protection and electrical safety. Therefore, a comprehensive consideration of the reliability, safety and economy is particularly important for the decision of neutral grounding mode. This paper proposes a new decision method of neutral point grounding mode for mediumvoltage distribution network. The objective function is constructed for the decision according the life cycle cost. The reliability of the neutral point grounding mode is taken into account through treating the outage cost as an operating cost. The safety condition of the neutral point grounding mode is preserved as the constraint condition of decision models, so the decision method can generate the most economical and reliable scheme of neutral point grounding mode within a safe limit. The example is used to verify the feasibility and effectiveness of the decision method.

Microgrids can provide a more reliable power supply and can enable the interconnection of renewable energy. The microgrid has been recognized as one of the most important directions of power systems. The transient characteristics of the microgrid are very important for the planning and operation of the whole power system. However, its mechanism still needs further exploration because the circuit structure, control feature, and even the operation mode of the microgrid are quite different from those of the traditional power system. In this article, a dynamic model is proposed for a microgrid according to the difference in transient processes between the microgrid and the traditional grid. The model is expressed as a differential-algebraic equation system, so the structure of the microgrid and the physical meaning of original variables are preserved. The focal point in this article is the fast control function of the inverter and the strong coupling between inverter and grid. The inverter is modeled...