DC Overvoltage Research Articles

Energy dissipation methods and protection device synergy strategies for offshore wind power receiving end surplus power

Abstract Firstly, a theoretical analysis of the fault ride-through scheme is carried out based on the land-side AC faults in the offshore wind power delivery system. According to the topological structure of the DC energy consumption device, the working mode and switching of the DC energy consumption device are analyzed and studied. Secondly, the DC protection action of offshore wind power flexible and direct transmission systems was studied, and the switching strategy of energy-consuming devices was determined. Then the impact of the switching of the energy-consuming device on the DC protection function was studied, focusing on the synergy between the DC overvoltage protection and the switching judgment voltage of the energy-consuming device. Finally, based on a typical offshore wind power flexing project, a system model is built in PSCAD/EMTDC to verify the effectiveness of the proposed synergy strategy.

DC overvoltage suppression method of wind farm connected via MMC‐HVDC system

AbstractA new integration strategy of grid‐forming‐controlled wind farms connected to the bulk power systems through high‐voltage direct current transmission based on the modular multi‐level converter (MMC) is proposed to solve the problem of traditional uncontrollable DC overvoltage during the short circuit faults at the receiving end. However, a new DC overvoltage phenomenon appears after the fault is cleared, and the interaction between the wind farm and sending‐ and receiving‐end MMCs makes the DC overvoltage mechanism more complex; further exploration shows that DC overvoltage during the sending‐end fault recovery stage occurs under the new integration strategy. Therefore, the evolution process and mechanism of the new DC overvoltage are analysed. It is found that affected by the interaction between wind farms and MMCs, the alternate saturation of the integrators in the PI controller of MMCs is the main cause. Based on this understanding, additional controls are proposed to suppress this DC overvoltage during the fault recovery stage. Simulations are carried out on a test with MATLAB/Simulink, and the results verify the efficacy of the proposed methods in suppressing DC overvoltage.

Voltage suppression strategy for multi-stage frequency regulation of DC-side energy storage batteries in doubly-fed induction generator

When DC-side energy storage batteries participate in frequency regulation, inconsistent inertia requirements exist for frequency deterioration and recovery stages. In addition, the frequency regulation power can lead to the DC overvoltage of the DFIG. To address these issues, this paper proposes a voltage suppression strategy (VSS) during multi-stage frequency regulation with the DC-side energy storage batteries. In the frequency deterioration stage, a SOC segmented feedback inertia control (SSFIC) is developed to reduce the frequency change rate. In the frequency recovery stage, to mitigate transient maximum frequency deviation, a maximum frequency differential inertia control (MFDIC) is constructed by incorporating the transient maximum frequency deviation into inertia control coefficient. In the frequency regulation stage, to decrease the steady state frequency deviation and avoid the secondary frequency drop, an exponential inertia droop control (EIDC) is created by considering the droop coefficient as an exponential function of the frequency change rate. To suppress the DC voltage fluctuation, a DC voltage compensation control (DVCC) is presented to transform the active power of SSFIC, MFDIC and EIDC to the d-axis current compensation increment of GSC. Extensive simulation results have proved that the proposed strategy can successfully enhance the frequency regulation and voltage suppression ability.

A P-Q Coordination Control Strategy of VSC-HVDC and BESS for LVRT Recovery Performance Enhancement

Voltage source converter (VSC)-based multi-terminal direct current (MTDC) transmission technology has been a research focus, and the low-voltage ride-through (LVRT) and recovery in receiving-end systems is one of the major problems to consider. A coordinated control strategy for a VSC-MTDC system is proposed to improve the frequency and voltage dynamics in the receiving-end system during the LVRT and recovery processes. A battery energy storage system (BESS) plays a significant role in providing frequency and voltage support with its flexible power control capability. During the LVRT process, the BESS can provide reactive current injection and active current absorption to improve system stability in the AC side, and during the recovery process, an adaptive current limitation method is proposed for the BESS converter to dynamically adjust the active and reactive power outputs according to the frequency and voltage deviation severity. Meanwhile, the coordination of the sending-end systems and DC chopper can reduce the power output to avoid DC overvoltage during LVRT, and it can also provide frequency support to the receiving-end system with the DC voltage transmitting frequency information during the recovery process. A simulation was carried out on the MATLAB/Simulink platform, and a three-terminal VSC-MTDC system was used to validate the effectiveness of the proposed strategy.

Research on dual-mode fault suppression strategy of AC isolation and current limiting based on hybrid MMC

Abstract The DC side fault of the high voltage DC transmission system based on modular multilevel converters high voltage DC (MMC-HVDC) is easily affected by the double influence of AC transient electrical quantity infeed and overvoltage and overcurrent, seriously threatening the safe operation of the power grid. Aiming at the problem above, a fault suppression strategy in dual mode of isolation and current limiting is proposed. Based on the half-bridge-full-bridge hybrid MMC, the fault transient mathematical model considering AC feed and sub-module switching feed is analyzed. AC isolation module with double conduction self-resistance structure and current-limiting control module is constructed, which can be adaptively adjusted to the system’s direct current parameters. The ±500 kV double-ended hybrid MMC system model is established to verify the scheme’s effectiveness. In addition, the influence of high AC modulation parameters on the suppression effect is investigated. PSCAD/EMTDC simulation results show that AC active and reactive power feed-in peaks are reduced by 388.69 MW and 119.82 Mvar, respectively, under dual-mode control. The peak value of DC overvoltage and overcurrent is reduced by 1211.53 kV and 22.5 kA, respectively, the fault self-clearing time is shortened by 43 ms, and the fault suppression effect is remarkable. In the high AC modulation ratio operating state, the modulation parameters are positively correlated with the AC power feed and negatively correlated with the overvoltage and overcurrent of the DC system. The research results provide support for the fault protection scheme of the hybrid MMC flexible and direct power grid.

Coordinated Control Strategy of Receiving-End Fault Ride-Through for DC Grid Connected Large-Scale Wind Power

This article proposes a comprehensive coordinated suppression strategy for DC overvoltage to achieve the receiving-end fault ride-through of DC grid connected large-scale wind power. For the monopolar DC overvoltage, by reasonably switching the control mode of bipolar modular multilevel converter (MMC), the power transfer capability of the healthy pole MMC can be improved while maintaining DC voltage stability, and the precise load-shedding control of wind farm and its coordination scheme with dissipative resistors are designed, which can effectively reduce the input capacity of dissipative resistors and the risk of wind turbines being cut off. For the bipolar DC overvoltage, the load-shedding control strategy and its parameter selection principles are proposed, which can make wind farm and capacitors in each MMC absorb excess power together, to reduce the change rate and amplitude of DC voltage. Then its cooperation scheme with the dissipative resistors is designed to achieve fault ride-through in the full power range of wind farm. Finally, based on the simulation model of four- terminal DC grid, the effectiveness of the proposed coordinated control methods is verified.

Open-circuit Fault Riding Through of Receiving AC Terminal in HVDC Transmission System for Offshore Wind Farms

Abstract Flexible high voltage direct current (HVDC) transmission system is indispensable for long-distance and high power transmission capacity in offshore wind farms. The AC open-circuit fault of the receiving terminal is quite severe, where the power transmission might be suddenly cut down to zero. As a result, a severe over-voltage will occur on the HVDC side and modular multilevel converter (MMC) submodule capacitor. In this paper, the over-voltage suppression mechanism is put forward to augment the AC open circuit fault at the sending and receiving terminal of the MMC-HVDC system. First, the formulation mechanism of DC over-voltage in the MMC-HVDC integrated wind farm is studied. Then, three different over-voltage suppression mechanisms, namely wind turbine pitch control, crowbar control, and submodule redundancy, are put forward to suppress the aforesaid over-voltage in the MMC-HVDC system. The simulations are carried out in PSCAD based on a test case, and the simulation results are used to confirm the efficiency of the proposed overvoltage suppression techniques.

Fault Current and Voltage Estimation Method in Symmetrical Monopolar MMC-based DC Grids

Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current (MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.

Transient DC Over-Voltage Protection for ITER PF AC/DC Converter

The International Thermonuclear Experimental Reactor (ITER) poloidal field (PF) AC/DC converters are composed by thyristor-based phase controlled converter modules. As the core component of ITER PF AC/DC converter, the thyristor is very sensitive to over-voltage and damaged in microseconds, therefore, the transient over-voltage protection strategy is desperately essential to ensure the converter safety operation. In this paper, a nanosecond respond and high reliability protection strategy which combined by Metal Oxide Varistor (MOV) and external bypass is proposed to protect the ITER PF AC/DC converter from transient DC over-voltage. The MOV is designed to certify the fast respond in nanosecond. Moreover, a bidirectional BreakOver Diode (BOD) circuit board is designed to activate external bypass to ensure the reliability of the transient DC over-voltage protection strategy. The performance-testing platform is built to study its performance. The experiments on ITER PF AC/DC converter test facility are carried out. According to the experiment results, the external bypass is triggered by BOD board effectively and the load current is transferred to the external bypass in 2 us when BOD suffers from an over-voltage. The effectiveness of the proposed transient DC over-voltage protection strategy is verified.

DC-side faults mechanism analysis and causes location for two-stage photovoltaic grid connected inverters

Due to the deep coupling of the DC faults for the two-stage photovoltaic (PV) inverters, it is very difficult to determine the specific causes of DC faults. In terms of this issue, the fault mechanism of different causes is analysed and the obvious fault features are selected to locate the causes. Furthermore, a complete set of fault diagnosis process is proposed for DC overvoltage and undervoltage faults. An experimental platform for PV power generation system is used to simulate the deterioration of operating conditions and obtains various fault data. The results show that the correctness of fault mechanism analysis and the effectiveness of the proposed diagnosis process.

Research on Overvoltage Suppression by Blocking Diode and DC Circuit Breaker in LCC-VSC Hybrid HVDC System

In this study, a LCC-VSC hybrid HVDC transmission system was taken as an example. The valve and neutral overvoltage of the hybrid HVDC system were simulated and then calculated at the VSC converter station by installing a blocking diode and a DC circuit breaker, as well as by grounding after neutral bus outage. The isolation effect of the blocking diode and the DC circuit breaker on the VSC converter station in the protection and the effect on the DC overvoltage were compared, and the difference was analyzed by complying with the mechanism of overvoltage. The effect of different locations of the blocking diode on the overvoltage suppression was compared, and it was concluded that different locations of blocking diode could raise different requirements for the insulation coordination of the converter valve. By analyzing energy sources of the neutral bus overvoltage at different stages, it was further concluded that the energy stored in the distributed inductance of the metal return line could be a vital source of neutral bus overvoltage energy. On that basis, an overvoltage suppression strategy for grounding neutral bus directly after the stop of the converter station was proposed.

A new AC fault ride-through strategy for HVDC link with serial connected LCC-VSC hybrid inverter

HVDC link with line commutated converter (LCC) as a rectifier at the power sending end, and with a serial connected LCC and voltage source converter (VSC) hybrid inverter (SLVHI) at the power receiving end, is adopted for future Baihetan HVDC engineering project in China. To realize the AC fault ride-through (FRT) of SLVHI, a new strategy based on DC chopper (DCC) is proposed in this paper. Firstly, the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure (CF) and related factors. Secondly, a modified DCC topology of SLVHI is designed to adjust unbalanced power dissipation based on voltage drop depth. Thirdly, an enhanced voltage-dependent current order limiter (VDCOL) is proposed to deal with the unbalanced power after DCC switching off. With the cooperation between modified DCC and enhanced VDCOL, the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously. Finally, the inverter side AC FRT performances of the HVDC link with SLVHI are studied by PSCAD/EMTDC. The simulation results validate the proposed strategy's effectiveness and superiority, with better CF mitigation and VSC DC overvoltage suppression abilities than other existing FRT strategies under different fault scenarios.

Preparation of PI porous fiber membrane for recovering oil-paper insulation structure

PI fiber insulation paper has excellent insulation and heat resistance, which can replace traditional insulation paper to provide a new high-performance material. In this paper, we adopt a two-step method to synthesize PI, that is, firstly synthesize polyamic acid, and then perform electrospinning and thermal imidization to prepare PI porous fiber membrane. The test results show that the breakdown field strength of the PI porous film is 50 kV/mm. Further research shows that the insulation structure has the characteristics of continuous multiple breakdown. When the fiber film is repeatedly broken down to 25 times at the same point, the field strength of each breakdown of the fiber film changes between 40 and 55 kV/mm, and the average breakdown field strength is 53 kV/mm. Nano-Al2O3 was introduced into the fiber membrane by in situ polymerization. The breakdown field strength of the composite fiber film can reach 74.89 kV/mm after doping 2 wt% nano-Al2O3, which is higher than that of the pure film. At the same time, this kind of fiber paper also has repeatable breakdown characteristics, and the breakdown voltage distribution width of multiple breakdown is reduced. This characteristic provides the maintenance free characteristic of DC over-voltage for the insulation structure composed of this fiber paper, and can improve the service life of transformer under DC over-voltage such as lightning stroke.

Cooperative strategy of SMES device and modified control for cost-effective fault ride-through enhancement and power smoothing of 10 MW class superconducting wind turbine

Superconducting wind turbine generators (SWTGs) are an effective option of 10 MW class WTG units integrated into offshore wind farms, owing to their potential advantage of reduced size and weight compared with conventional generators. Reliable grid-connection of SWTGs faces two main challenges including fault ride through (FRT) and wind power smoothing. Traditional solutions consist mainly of device-based and control-based schemes. However, the device-based scheme causes a high-cost burden due to the introduction of additional devices, while the control-based scheme is not suitable for FRT enhancement under serious grid faults. To solve these problems, this paper presents a cooperative strategy integrated with one cost-effective superconducting magnetic energy storage (SMES) device and two modified WTG controls. The utilization of SMES devices can achieve a favorable wind power smoothing effect during normal operations and overvoltage suppressing effect in dc-link of WTG during grid faults. Moreover, two modified WTG controls of power decreasing control method and reactive power control provide effective assistance for suppressing dc overvoltage, offering adequate reactive current support and reducing the capital cost of SMES devices simultaneously. Several test cases are conducted to demonstrate the feasibility of the proposed strategy.

Enhanced FRT and Postfault Recovery Control for MMC-HVDC Connected Offshore Wind Farms

Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) connected offshore wind farms could suffer serious challenges: DC overvoltage during a fault and post-fault DC voltage restoration. In this paper, to mitigate DC overvoltage, a two-stage voltage drop control scheme is proposed for offshore MMC to realize a fast active power reduction, by coordinating with a voltage-dependent active current control of individual wind turbines. The voltage drop depths are designed by considering the offshore de-loading efficiency and the stability of WTs. To improve the post-fault DC voltage recovery performance, an adaptive voltage rise control scheme is proposed to coordinate the recovery of onshore and offshore active power. The recovery trajectory of DC voltage is formulated and further employed to design the onshore active power ramp rate for uninterrupted operation of WTs. Case studies have been conducted on two test systems to validate the proposed method.

Advanced Fault Ride-Through Strategy by an MMC HVDC Transmission for Off-Shore Wind Farm Interconnection

In order to solve the problems brought upon by off-shore wind-power plants, it is important to improve fault ride-through capability when an on-shore fault occurs in order to prevent DC overvoltage. In this paper, a coordinated control strategy is implemented for a doubly-fed induction generator (DFIG)-based off-shore wind farm, which connects to on-shore land by a modular multilevel converter (MMC)-based high voltage direct current (HVDC) transmission system during an on-shore fault. The proposed control strategy adjusts the DC voltage of the off-shore converter to ride through fault condition, simultaneously varying off-shore AC frequency. The grid-side converter detects the frequency difference, and the rotor-side converter curtails the output power of the DFIG. The surplus energy will be accumulated at the rotor by accelerating the rotor speed and DC link by rising DC voltage. By the time the fault ends, energy stored in the rotor and energy stored in the DC capacitor will be released to the on-shore side to restore the normal transmission state. Based on the control strategy, the off-shore wind farm will ride through an on-shore fault with minimum rotor stress. To verify the validity of the proposed control strategy, a DFIG-based wind farm connecting to the on-shore side by an MMC HVDC system is simulated by PSCAD with an on-shore Point of Common Coupling side fault scenario.

CARACTERÍSTICAS TRANSITORIAS Y ESTRATEGIA ÓPTIMA DE CONTROL DE UN REACTOR CONTROLABLE SUPERCONDUCTOR

Superconducting controllable reactor (SCR) is a promising device for reactive power compensation to improve the voltage stability and reliability of power system. Given the special structure and complex magnetic circuit of SCR, the transient characteristics and control strategy, which have an important effect on the performance of SCR, lack comprehensive analysis. In this study, transient characteristics analysis and an optimal control strategy are proposed. First, the basic operating principle and the impedance of the analytical model of SCR were presented. Second, the transient characteristics on the DC component and the overvoltage of SCR were analyzed in detail, and an optimal control strategy based on circuit breaker and anti-parallel thyristors was proposed. Finally, the principle and performance of proposed methods were validated through electromagnetic simulation and experiments of 380V/30kVar prototype. The simulation and experimental results demonstrate that the DC component and overvoltage will be produced in the transient process. When the switching angle of superconducting winding is 90°, the DC component and overvoltage become small. The optimal control system proposed can avoid producing the DC component and overvoltage that damage the superconducting windings. The results obtained in this study can be applied to the optimal performance design of SCR. Keywords: Superconducting controllable reactor, Optimal control strategy, DC component, Overvoltage

Preventing DC over-voltage in multi-terminal HVDC transmission

This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power. Voltages and frequencies of offshore AC wind farm networks are used for transmitting control signals for the power reduction control. These methods do not require fast communication. Power reduction sharing among the offshore wind farms using the different control signals is analysed. The control systems are also compared against the DC chopper method to prevent a DC overvoltage. Simulation and experiments are carried out to evaluate the control systems.

Power Flow and Power Reduction Control Using Variable Frequency of Offshore AC Grids

In the “integrated strategy” for connecting offshore wind farms to onshore grids, an offshore AC grid is formed by interconnecting offshore wind farms and point-to-point VSC-HVDC links using an AC network. The advantage of this strategy is that in the case of a DC fault, AC circuit breakers are used to isolate the faulted HVDC link without the need for DC circuit breakers. This paper presents a control system that regulates power sharing between the HVDC links by varying the operating frequency of the offshore AC grid. Primary and secondary power regulation controllers are designed to achieve automatic power flow coordination between HVDC links. Increase in the offshore AC grid frequency during onshore AC grid fault or DC fault signals power flow imbalance. This signal is used to suppress the transient DC over-voltage through the coordination control between the wind farm side converters of HVDC links and the power reduction control of wind turbines. Importantly, because the inputs of the controllers are from local frequency signals only, fast communication is not required under the proposed control strategy. Simulation results are presented to verify the functioning of the proposed control strategy and illustrate variable frequency operation of an offshore four-node AC grid.

The Assessment and Development of Protection Models in HVDC DC side

The HVDC system consists of ac side and dc side through thyristor valve. ac side protection is similar to conventional ac system protection schemes but dc side protection is different from ac side of HVDC system. AC system don't have controller but HVDC system has controller that controls and protects system from faults and disturbance. This paper show protectional function of HVDC dc side such as asymmetry protection, ac/dc differential protection, dc overvoltage, dc overcurrent protection. Protection models is developed using RTDS software and assessment of protection models is also performed by RTDS system.