Commutation Failure Immunity Index Research Articles

A General Self-Adaptive DC Current Control of LCC-HVDC for Eliminating Subsequent Commutation Failure

With more renewable resources are integrated into the receiving-end AC grid through the LCC-HVDC link, a single-infeed LCC-HVDC (SIDC) system is facing a critical subsequent commutation failure (SCF), which stems from the poor dynamic reactive power support capability of the existing LCC-HVDC link's control strategies. This letter proposes a general self-adaptive DC current control (GSDC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) of LCC-HVDC to eliminate the SCF, based on the dynamic reactive power interaction mechanism. Then, multiple comparative studies with the existing control strategies are implemented to verify the validation of the proposed control strategy by PSCAD/EMTDC <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> . A fast evaluation index, called general commutation failure immunity index (GCFII), is also defined to show the performance of the proposed control strategy in eliminating the SCF.

An Analysis on the VSC-HVDC Contribution for the Static Voltage Stability Margin and Effective Short Circuit Ratio Enhancement in Hybrid Multi-Infeed HVDC Systems

Over the years, high voltage transmission of large energy blocks over long distances has widely developed from the Line Commutated Converter (LCC) technology. However, the continuous expansion of the AC network and the increase in renewable energy penetration leads to the weakening of Electric Power Systems (EPS), causing operational problems for the LCC-HVDC. This paper evaluates the degree of contribution of the VSC-HVDC on the LCC-HVDC dynamic performance when the infeed is in a hybrid multi-infeed HVDC system. Through a steady-state mathematical framework, the new concept of Hybrid Power Voltage Interaction Factor (hPVIF) is proposed to assess the transient stability improvement of LCC. Additionally, this article introduces two key contributions from hPVIF: the complementary Improved Commutation Failure Immunity Index (iCFII), as a measure of the effective short circuit enhancement, as well as the apparent line length, which emulates the line lengths adjusting the power dispatch in the VSC-HVDC. PSCAD/EMTDCTM time-domain dynamic simulations are performed to assess the indexes, and the Matlab® software will be used to assist in mathematical operations.

Correcting the Calculation Method of Commutation Failure Immunity Index for LCC-HVDC Inverters

The commutation failure immunity index (CFII) recommended by CIGRE is an useful indicator for quantifying the immunity of LCC-HVDC inverters to the CF. It should be calculated by the electromagnetic transient (EMT) simulations under the worst fault condition that doesn't cause the CF as per its definition. Moreover, the symmetrical three-phase line-to-ground fault has been empirically claimed by CIGRE as the worst one in the previous calculation method. However in this letter, it is firstly found that the unsymmetrical double-phase fault rather than the three-phase one is actually the worst as clearly observed from large numbers of the EMT simulations. This is achieved by the comprehensive analysis of the simulated CF characteristics when taking into account the diversified fault conditions such as fault type, property, severity, initiation time. Secondly, the above interesting finding motivates a corrected CFII calculation method to be proposed considering the double-phase fault. The proposed method is further shown to be more superior than the previous method by the case studies under various ac grid strength.

Analysis on Local and Concurrent Commutation Failure of Multi-infeed HVDC Considering Inter-converter Interaction

This paper provides a comprehensive analysis of local and concurrent commutation failure (CF) of multi-infeed high-voltage direct current (HVDC) system considering multi-infeed interaction factor (MIIF). The literature indicates that the local CF is not influenced by MIIF, whereas this paper concludes that both the local CF and concurrent CF are influenced by MIIF. The ability of remote converter to work under reduced reactive power enables its feature to support local converter via inter-connection link. The MIIF measures the strength of electrical connectivity between converters. Higher MIIF gives a clearer path to remote converter to support local converter, but at the same time, it provides an easy path to local converter to disturb remote converter under local fault. The presence of nearby converter increases the local commutation failure immunity index (CFII) while reducing concurrent CFII. Higher MIIF causes reactive power support to flow from remote converter to local converter, which reduces the chances of CF. A mathematical approximation to calculate the increase in local CFII for multi-infeed HVDC configurations is also proposed. A power flow approach is used to model the relation between MIIF and reactive power support from remote end. The local and concurrent CFIIs are found to be inverse to each other over MIIF; therefore, it is recommended that there is an optimal value of MIIF for all converters in close electric proximity to maintain CFII at a certain level. The numerical results of established model are compared with PSCAD/EMTDCsimulations. The simulation results show the details of the influence of MIIF on local CF and concurrent CF of multi-infeed HVDC, which validates the analysis presented.

Study on Dynamic Characteristics of UHVDC System under Hierarchical Infeed Mode with STATCOM

This paper focuses on the investigation of the dynamic characteristics of Ultra High Voltage Direct Current (UHVDC) system Under Hierarchical Infeed Mode (UHVDC-HIM) with Static Synchronous Compensator (STATCOM). The STATCOM at the inverter side of UHVDC-HIM is developed in PSCAD/EMTDC, and their power flow equations are derived. Based on these equations, the control schemes for rectifier and inverter side of UHVDC-HIM and STATCOM are properly designed. Several dynamic characteristics indices such as Commutation Failure Immunity Index (CFII), Commutation Failure Probability Index (CFPI), fault recovery time and Temporary Overvoltage (TOV) are evaluated in detail. The impact of different sizes of STATCOM and Short Circuit Ratio (SCR) of AC sources on dynamic characteristics of UHVDC-HIM system during single-phase and three-phase faults are examined. The analysis shows that higher SCR values and greater capacity of STATCOM can make the UHVDC-HIM system less vulnerable to Commutation Failure (CF), considerably enhance the fault recovery time and effectively reduce the TOV.

AC system’s strength evaluation of UHVDC system under hierarchical connection mode

The inverter’s end of ultra high voltage DC (UHVDC) system under hierarchical connection mode (HCM) is connected to 500 and 1000 kV AC buses through high end converter and low end converter transformers, respectively. The two AC buses are interconnected via coupling transformer and the tie line. This complex structure of interconnection at inverter’s end can make the understanding unclear about each AC system’s strength of UHVDC-HCM. Therefore, this paper covers the system’s strength evaluation by developing the “Hierarchical mode effective short circuit ratio” (HMESCR) index. The HMESCR incorporates the impact of neighborhood AC networks, impedance of coupling transformer, the tie line and strength of the objective AC network. Based on the proposed index, an equivalent model for UHVDC-HCM system is developed. An equivalent model is verified under the steady state conditions such as maximum available power and the transient conditions such as commutation failure immunity index under AC faults. In addition, three different cases are considered in order to get the in-depth and comparative performance analysis of an equivalent model with UHVDC-HCM system. MATLAB and PSCAD/EMTDC are used as analysis tools for illustrating the steady state and transient state conditions. The analysis indicates that the presented equivalent model has similar behavior as UHVDC-HCM system under both steady state and transient state conditions, which verifies the accuracy of proposed index in assessing the strength of both AC systems of UHVDC-HCM.

Impact of synchronous condenser on the dynamic behavior of LCC based UHVDC system hierarchically connected to AC system

Hierarchical connection (HC) is a very attractive mode for ± 800 kV line commutated converter based ultra high voltage direct current (LCC-UHVDC) system connected to different AC voltage levels because of its ability to reduce the scale factor of a converter transformer. Faults in the HC-UHVDC system can cause commutation failure (CF). In this paper, impact of synchronous condenser (SC) to mitigate CF in HC-UHVDC system is analyzed. A ± 800KV HC-UHVDC system along with synchronous condenser is built in PSCAD/EMTDC. Transient performance analysis of HC-UHVDC for single and three phase to ground faults is investigated. Commutation failure immunity index (CFII), commutation failure probability index (CFPI), fault recovery time (FRT), and transient overvoltage (TOV) are used as measures to evaluate the effects of SC at HC-UHVDC system design. The simulation results show that SC can make the HCUHVDC system less susceptible to CF, effectively improve fault recovery performances of the overall system, and reduce transient overvoltage when single or multiple converters are blocked. The results of this research can provide technical assistance in real world HC-UHVDC projects.

Coordinated Control for Operating Characteristics Improvement of UHVDC Transmission Systems under Hierarchical Connection Scheme with STATCOM

Ultra-high voltage direct current (UHVDC) systems under hierarchical connection schemes (HCSs) linked to AC grids with different voltage levels (500 and 1000 kV) have been a great concern for power utilities to transfer bulk power. They have some operating issues like cascaded commutation failures and longer fault recovery time under certain fault conditions. Since STATCOM has the ability to effectively regulate AC busbar voltages, thus it is considered in this paper to improve the operating characteristics of UHVDC-HCS systems. To further improve the operating characteristics, a coordinated control between an UHVDC-HCS system and STATCOM is presented. To validate the effectiveness of coordinated control, the comparison between different control modes such as reactive power control (Q-control) and voltage control (V-control) in the outer loop control of STATCOM are conducted in detail. Various indices like commutation failure immunity index (CFII) and commutation failure probability index (CFPI) are also comprehensively evaluated in order to investigate robustness of the adopted coordinated control. An UHVDC-HCS system with multiple STATCOMs on the inverter side (500 kV bus) is developed in PSCAD/EMTDC. The impact of coordinated control on commutation failure phenomena and fault recovery time during single and three phase AC faults is analyzed. The analysis shows that coordinated control with V-control mode of STATCOM exhibits better performance in enhancing the operating characteristics of UHVDC-HCS system by improving the CFII, effectively reducing the CFPI and fault recovery time under various AC faults.

A Quantitative Index to Evaluate the Commutation Failure Probability of LCC-HVDC with a Synchronous Condenser

Since thyristor cannot turn off automatically, line commutated converter based high voltage direct current (LCC-HVDC) will inevitably fail to commutate and therefore auxiliary controls or voltage control devices are needed to improve the commutation failure immunity of the LCC-HVDC system. The voltage control device, a synchronous condenser (SC), can effectively suppress the commutation failure of the LCC-HVDC system. However, there is a need for a proper evaluation index that can quantitatively assess the ability of the LCC-HVDC system to resist the occurrence of commutation failures. At present, the main quantitative evaluation indicators include the commutation failure immunity index and the commutation failure probability index. Although they can reflect the resistance of the LCC-HVDC system to commutation failures to a certain extent, they are all based on specific working conditions and cannot comprehensively evaluate the impact of SCs on suppressing the commutation failure of the LCC-HVDC system under certain fault ranges. In order to more comprehensively and quantitatively evaluate the influence of SCs on the commutation failure susceptibility of the LCC-HVDC system under certain fault ranges, this paper proposes the area ratio of commutation failure probability. The accuracy of this new index was verified through the PSCAD/EMTDC. Based on the CIGRE benchmark model, the effects of different synchronous condensers on LCC-HVDC commutation failure were analyzed. The results showed that the new index could effectively and more precisely evaluate the effect of SCs on commutation failures. Moreover, the proposed index could provide a theoretical basis for the capacity allocation of SCs in practical projects and it could also be utilized for evaluating the impact of other dynamic reactive power compensators on the commutation failure probability of the LCC-HVDC system under certain fault ranges.

Comparative Performance Study of Dynamic Reactive Power Compensators on Operating Characteristics of LCC Based UHVDC System Under Hierarchical Connection Mode

This paper compares the performance of different dynamic reactive power compensators (DRPCs) on transient characteristics of LCC based UHVDC system under hierarchical connection mode (LCC-UHVDC-HCM). The different DRPCs considered are Synchronous Condenser (SC), STATCOM and combination of both SC and STATCOM (hybrid dynamic reactive power compensator, HDRPC). Various transient indices like commutation failure immunity index (CFII), commutation failure probability index (CFPI) and fault recovery time (FRT) of LCC-UHVDC-HCM system are undertaken in order to evaluate the comparative performance of various DRPCs. The LCC-UHVDC-HCM system along with different DRPCs is developed in PSCAD/EMTDC and numerous simulations are done to observe the transient phenomena. The results indicate that performance of DRPCs changes during various AC disturbances such as single phase to ground fault and three phase to ground fault for different fault severity. It is examined that STATCOM is better than other dynamic compensators in improving CFII and CFPI. SC and HDRPC show almost same behavior in improving CFII. However, in case of CFPI, HDRPC is superior to SC. The performance of SC is better than STATCOM and HDRPC in improving FRT under severe fault conditions. However, under low fault conditions, all DRPCs have equal performance in reducing FRT.

Brief analysis of Jiangsu grid security and stability based on multi-infeed DC index in power system

The impact of Multi-infeed HVDC has gradually increased to security and stability operating in Jiangsu power grid. In this paper, an appraisal method of Multi-infeed HVDC power grid security and stability is raised with Multi-Infeed Effective Short Circuit Ratio, Multi-Infeed Interaction Factor and Commutation Failure Immunity Index. These indices are adopted in security and stability simulating calculation of Jiangsu Multi-infeed HVDC system. The simulation results indicate that Jiangsu power grid is operating with a strong DC system. It has high level of power grid security and stability, and meet the safety running requirements. Jinpin-Suzhou DC system is located in the receiving end with huge capacity, which is easily leading to commutation failure of the transmission line. In order to resolve this problem, dynamic reactive power compensation can be applied in power grid near Jinpin-Suzhou DC system. Simulation result shows this method is feasible to commutation failure.

A Novel Coordinated Control Approach for Commutation Failure Mitigation in Hybrid Parallel-HVDC System with MMC-HVDC and LCC-HVDC

This paper proposes a novel coordinated control approach for the hybrid parallel-HVDC system that is composed of a line commutated converter-based high-voltage direct current (LCC-HVDC) link and a modular multi-level converter-based HVDC (MMC-HVDC) link in parallel. With the presented control approach, the reactive power of MMC-HVDC is regulated flexibly based on the extinction angle of LCC-HVDC to mitigate the commutation failure of LCC-HVDC and improve the fault recovery performances of the overall system during fault and post-fault periods. A hybrid parallel-HVDC system is developed in PSCAD/EMTDC to evaluate the effects of the proposed control approach. The transient performances of the hybrid parallel-HVDC, under single-phase and three-phase to ground faults at the inverter AC busbar are investigated. Commutation failure immunity index (CFII) and fault recovery time are used as measures to evaluate the effects of the proposed control approach. The results show that the proposed coordinated control approach can make the LCC-HVDC less susceptible to commutation failure in hybrid parallel-HVDC system and effectively improve the fault recovery performances of the overall system.

Efficient approach for commutation failure immunity level assessment in hybrid multi‐infeed HVDC systems

Commutation failure (CF) of hybrid multi-infeed high voltage-direct current (HMIDC) systems, comprising line commutated converter-based high-voltage direct current (LCC-HVDC) and voltage source converter-based high-voltage direct current (VSC-HVDC) links, has been mainly analysed by electromagnetic transient simulation tools, resulting in heavy workload and long computational time. This study proposes an efficient approach for CF immunity level assessment based on CF immunity index in HMIDC systems. Thus, an equivalent dual-bus analytical model of the HMIDC systems is first established, which then provides a theoretical basis for defining an equivalent voltage stability indicator (EVSI). It is then shown that such an EVSI index can be employed in the proposed approach. Moreover, the proposed approach is validated by a hybrid dual-infeed HVDC system. In particular, an unexpected phenomenon is observed that the CF characteristic of the LCC-HVDC link is somewhat deteriorated, due to the incorporation of the VSC-HVDC link when the VSC-HVDC link feeds into the alternating current system with both active and reactive power transferred in relatively high levels close to their limits.

Commutation failure and voltage sensitivity analysis in a hybrid multi-infeed HVDC system containing modular multilevel converter

Summary A typical hybrid dual-infeed high-voltage direct current (HVDC) system includes two types of links—line-commutated converter (LCC) HVDC and voltage-source converter (VSC) HVDC in proximity. When faults occur in the LCC-HVDC side, the VSC-HVDC link sufficiently can compensate power to the LCC-HVDC system; thus, voltage fluctuation is suppressed. In recent years, the modular multilevel converter (MMC) has been a promising type of VSC. This paper presents a novel hybrid dual-infeed HVDC system consisting of an LCC-HVDC link and an MMC-HVDC link, and the two links are interconnected through a tie line between buses in the inverter side. The positive contribution of the MMC-HVDC link to the LCC-HVDC link in the area of commutation failure immunity index, effective short circuit ratio, voltage sensitivity factor, temporary overvoltage, and DC line fault recovery are investigated. Investigations show that the stability of system has been enhanced significantly with MMC, and the contributors are physical structure change and the control strategy of the MMC-HVDC system. The modelling and simulation of the steady state and transient performance of hybrid HVDC system are conducted in PSCAD/EMTDC. Finally, the simulation results verify the enhancement in sensitivity and stability of the dual-infeed HVDC system. Copyright © 2016 John Wiley & Sons, Ltd.

Efficient approach to quantify commutation failure immunity levels in multi‐infeed HVDC systems

Commutation failure immunity index (CFII) is a useful indicator to quantify the immunity level to commutation failures in line commutated converter-based multi-infeed HVDC systems. Instead of using simulation tools which inevitably lead to heavy workload and long computation time, the analytical expressions of ‘local CFII’ (LCFII) and ‘concurrent CFII’ (CCFII) are derived based on the minimum extinction angle criteria. Using these expressions, the impact of the system parameters on commutation failure immunity levels in the inverter side can be determined quickly and effectively. Moreover, the derived analytical expressions will make the parametric sensitivity analysis more comprehensive and complete compared to the traditional simulation-based methods. Furthermore, in order to give an insight into how inter-inverter interactions influence the concurrent commutation failure behaviours, an improved index ‘weak coupling multi-infeed interaction factor’ (WCMIIF) is proposed. The validity and accuracy of the proposed index and analytical expressions are verified by the simulated results based on a dual-infeed HVDC system in PSCAD/EMTDC, which indicate that LCFII has an approximately linear correlation with the local effective short-circuit ratio (ESCR) of the faulted ac system, and that with the WCMIIF unchanged, CCFII has an approximately linear correlation with the local ESCR and an inverse correlation with the multi-infeed interaction factor.

Analysis of Dual-Infeed HVDC With LCC–HVDC and VSC–HVDC

The impact of a voltage source converter-based HVDC (VSC-HVDC) infeed on a line-commutated converter-based HVDC infeed (LCC-HVDC) at the same busbar is investigated. Steady-state aspects such as the maximum available power (MAP) as well as transient aspects such as temporary overvoltage (TOV) and commutation failure immunity index are considered in the analysis. The results show that VSC-HVDC improves the maximum available power, reduces TOV, and makes the LCC-HVDC less susceptible to commutation failure. The resulting improvement in performance of the LCC-HVDC is quantified by a proposed “apparent increase in short circuit ratio” (AISCR) index. AISCR is a measure of the increase in short-circuit ratio for the LCC-HVDC that would be necessary in the absence of the VSC-HVDC link to obtain similar performance. The analytical results are validated by a detailed electromagnetic transient simulation.

Commutation Failure Analysis in Multi-Infeed HVDC Systems

The commutation failure vulnerability of multi-infeed HVDC converters to ac side faults is analyzed. Faults in multi-infeed systems can cause commutation failure events in the nearby local converter or concurrently in both converters. The immunity of the system to both these events is investigated and suitably parameterized. “Commutation Failure Immunity Index” is calculated for multi-infeed systems with different ratings. Anomalous failure in valve current commutation due to voltage distortion has been investigated.