An Improved VDCOL Control Strategy for Mitigating Continuous Commutation Failure in HVDC Transmission

The fault of the receiving-end AC power grid of the high voltage direct current (HVDC) transmission system may cause the HVDC subsequent commutation failures. In order to dynamically adjust the DC current command during the recovery process of the first HVDC commutation failure to achieve the purpose of suppressing the HVDC subsequent commutation failures, a new method is proposed based on improved genetic algorithm (IGA) of the voltage dependent current order limiter (VDCOL) link optimization method. This paper analyzes the influencing factors of HVDC subsequent commutation failures and the reactive power interaction between the inverter converter station and the external power grid. Taking VDCOL as the research object, the work principle and limitations of conventional VDCOL are introduced. In order to overcome the single linear defect of the conventional VDCOL link, an IGA was used to optimize the parameters of the multi-segment inflection point of the VDCOL link, which realized the dynamic reactive power demand adjustment of the inverter side converter station when the receiving-end AC power grid was faulty, and then suppress the HVDC subsequent commutation failures. Finally, the simulation results demonstrate the effectiveness of the proposed method.

Power grids with multi-DC high voltage direct current (HVDC) commonly face challenges such as inadequate voltage support, exacerbated interactions between AC and DC, and complex and variable operation modes. Severe AC faults in these grids can easily trigger consecutive failures in DC commutation, potentially leading to a collapse in the voltage across the entire network. This study introduces a synchronized optimization technique aimed at improving voltage stability within receiving-end power systems by adjusting the parameters of the Voltage Dependent Current Order Limiter (VDCOL). Firstly, principles for adjusting VDCOL parameters are proposed grounded on voltage instability mode. Derived from the sorting outcomes of the multi-infeed short-circuit ratio, parameters of VDCOL are adjusted to ultimately suppress commutation failures and enhance the capability of receiving-end power grids to accept external power. Finally, the simulation result verifies that optimizing VDCOL parameters according to the proposed method significantly increases the capability of receiving-end power grids to accept external power.

An improved reactive power control strategy for LCC-HVDC to mitigate sending end transient voltage disturbance caused by commutation failures

In this study, the influences of the signal delay duration between rectifier and inverter in ultra-high voltage direct current (U-HVDC) transmission systems have been analyzed. The simulation results with different signal delay time under the same fault condition show that the signal delay time has a significant influence on the control system response during the fault and recovery periods. In the worst condition, it may lead to a commutation failure in the inverter. To eliminate the influence of the signal delay time on the DC control system, a fast calculation method for computing the voltage and current has been proposed by establishing the equivalent capacitance and inductance of the DC line. A voltage dependent current order limiter is added in the rectifier control system, which can calculate the DC current order value. Based on this concept, this paper proposes an improved control strategy to eliminate the influence of signal delay. The analysis and simulation results show that the calculation method can effectively improve the response speed and shorten the fault recovery time of the HVDC systems.

Offshorewind farms inherently need alternating current (AC) grid connection withundersea cables. However, this requires reactive power compensation, which iscostly and not practical. Moreover, it threads the stability of the system. Forthese reasons, high voltage direct current (HVDC) transmission is a promisingsolution since no reactive power is needed. Furthermore, the DC power flowbetween offshore and onshore locations as well as the reactive power at eitherside can easily be controlled by voltage source converters (VSCs). In thispaper, the modeling and control of an offshore wind farm connected to themainland grid power with HVDC transmission is proposed. A control strategy isproposed to control active and reactive power at the offshore wind farmconverter and DC voltage at the onshore converter. The overall system,including wind farm, offshore and onshore converters, HVDC transmission, and ACgrid side, were tested and verified in a simulation environment. The designedcontrol systems of the VSCs were tested by various case studies.

The continuous commutation failure (CCF) of high-voltage direct-current (HVDC) transmission system will cause consecutive impacts to the AC-DC hybrid power system. This paper proposes an improved firing angle control strategy with the DC power recovery rate and AC fault severity considered to suppress CCF. The dominant factors and mechanism of CCF are analyzed. Then, the method of dynamically adjusting firing angle according to the DC power recovery and AC voltage drop is introduced. During the fault recovery after first commutation failure, firing angle of the inverter side can be adaptively reduced to improve commutation margin. Case studies based on the CIGRE benchmark model validate the effectiveness of the proposed control strategy.

Commutation failure is one of the common faults in LCC-HVDC transmission systems, threatening safety and stability of power grid. To focus on the key issues of commutation failure and reduce commutation failure frequency, a review of commutation failure in LCC-HVDC transmission systems is given in this paper. Firstly, the mechanism of commutation failure is analyzed from both device and system aspects. Since commutation failures are mostly caused by AC system faults on the inverter side, commutation failures are classified into three categories: first commutation failure, continuous commutation failure during the fault period and commutation failure after the fault. Then, the influencing factors and prevention methods of commutation failure in each category are summarized. Finally, the problems to be solved in the field of commutation failure are pointed out.

The commutation failure of high-voltage direct current (HVDC) transmission system will have a huge power impact on the AC system. It is necessary to accurately identify the commutation failure of the HVDC for the security and stability control device (stability control device). At present, the stability control device which has already been applied in engineering for HVDC identifies the commutation failure of HVDC depends on the command of DC control and protection system, and there is a certain risk. The valve side current of converter transformer and the DC current of valve group that can be directly collected by the engineering are studied. Starting from the self-demand of the stability control device, new HVDC commutation failure and continuous commutation failure criteria are proposed based on the original commutation failure protection principle. Also, RTDS test verification was carried out.

The continuous commutation failure (CCF) of a line-commuted converter (LCC) based high voltage direct current (HVDC) transmission system will lead to long-term power imbalance of AC systems, which seriously threatens the safe and stable operation of AC systems. At present, the short circuit ratio (SCR) is widely used to evaluate commutation failure recovery characteristics of HVDC systems. In order to verify the effectiveness of the SCR in evaluating the risk of the CCF, the factors influencing the accuracy of the SCR are studied based on a simplified single-infeed HVDC system model. The analysis results show that the accuracy of the SCR is seriously reduced if there are a large number of loads near the LCC.

Commutation failure is one of the commonest events in high voltage direct current (HVDC) systems. In the process of commutation failure, the DC voltage, current and power change drastically and impact the sending-side grid. At present, the related research lacks the quantitative analysis of transient overvoltage, which is caused by commutation failure in sending-side DC systems. Aiming at this problem, the equivalent model of the sending-side system is established. Considering characteristics of reactive power losses caused by the rectifier during commutation failure, analytical expression of transient voltage at the commutated bus is derived, then a quantitative calculation method of transient overvoltage and influencing factors are obtained. The results show that the weaker the AC system, the higher the transient voltage, and the transient over-voltage rises nonlinearly with the increase of the steady-state transmission capacity. Finally, the accuracy and effectiveness of the method are verified with the HVDC digital-physical hybrid simulation model based on Hypersim.

This paper focuses on the fault blocking analysis and operational issues associated with MTDC systems incorporated in an AC network. The dynamic modelling of a line-commutated converter based bipolar multi-terminal direct current (LCC MTDC) system are shown, and the dynamic response of the converter during a DC converter fault is discussed. The converter controller design for both rectifiers and the inverters system was modelled for a realistic active power and extinction angle (γ) control with consideration to the VI characteristics of all the converter stations. An overall power controller was modelled for both converter pole. Two operational scenarios of converter fault were simulated using PSCAD EMTDC. The converter firing angle and extinction angle, as well as the voltage-dependent current order limiter, was monitored and plotted on a graph. Results show that the MTDC link became unstable during the full deblocking stage with a continuous occurrence of commutation failure. Furthermore, the results presented in this paper show that during partial converter de-blocking showed a favourable performance, as the power system remains stable and commutation failure of the MTDC system is prevented.

Lingzhou to Shaoxing (LS) ± 800 kV ultra‐high voltage direct current (UHVDC) transmission project is the first rectifier side access 750 kV power grids. 750 kV bus voltage fluctuation in Lingzhou side is bigger because the rectifier side supporting the power generator has not been put into production. The system commissioning test program was researched according to the above characteristics. The contents of the system commissioning test program include system test aim, the conditions of the alternating current (AC) and direct current (DC) system, test contents and operation steps, the measurement signals, the test results acceptable criteria, and safety and stable precautions during the UHVDC project system test. Referring the Haminan to Zhengzhou (HZ) UHVDC transmission project system test and operation experience, the system commissioning test items were put forward and confirmed, addition test items included some control function verification tests. The system commissioning test program and measurement program that included the transformer DC magnetic bias current measurement, wind generator sub‐synchronous oscillation monitoring near Lingzhou side area was prepared based on the analysis results of the bus voltage strategy in the rectifier side, the system security and stability strategy in the rectifier and inverter side, and AC and DC line ground fault analysis in this study. The results of the system commissioning test indicated that the system commissioning test program met the demand of verifying LS project technical specification and guaranteed LS project to put into operation in time.

Accurate detection and effective control strategy of commutation failure (CF) of high voltage direct current (HVDC) are of great significance for keeping the safe and stable operations of the hybrid power grid. At first, a novel detection method for consecutive CF is proposed. Concretely, the 2nd and higher orders’ derivative values of direct current are summarized as the core to judge CF by analyzing the physical characteristics of the direct current waveform of the converter station in CF. Then, the Daubechies wavelet coefficient that can represent the 2nd and higher order derivative values of direct current is derived. Once the wavelet coefficients of the sampling points are detected to exceed the threshold, the occurrence of CF is confirmed. Furthermore, by instantly increasing advanced firing angle β in the inverter side, an additional emergency control strategy to prevent subsequent CF is proposed. Eventually, with simulations of the benchmark model, the effectiveness and superiorities of the proposed detection method and additional control strategy in accuracy and rapidity are verified.

6 - High Voltage Direct Current (HVDC) electric power transmission systems

Chinese energy resources and load are in reverse distribution, and it is necessary to vigorously develop UHV AC and DC technologies suitable for long-distance and large-power transmission. The pattern of UHV AC-DC hybrid power grid has been initially formed at present. However, the faults of UHV DC with large transmission power and complex fault forms have great impact on the power grid in both active and reactive power. In this paper, based on the analysis of the current situation and characteristics of UHV AC/DC power grids in China, the impact of UHV DC restart and commutation failure on the stability of the power grid is analyzed. The different types and triggering factors of DC commutation failures are analyzed, and the effects of single or multiple DC commutation failure types on the transient angle stability, the voltage stability and the transient overvoltage are simulated. The UHV DC restart fault time sequence is studied, and the effects of different restart modes on the system transient angle stability and transient overvoltage are demonstrated through simulation. The research results have laid the foundation for grasping the UHV DC fault morphology mechanism and the influence law on the stability of the power grid, and provided support for the safe and stable verification of the UHV AC/DC power grid.