Review and evaluation of control-based protection solutions for converter-dominated power systems

With the rapid and massive increase of converter-interfaced resources (e.g. renewable generation, HVDC systems, etc.), the dynamic behavior of future power systems is expected to change significantly and will be predominantly determined by converters' control strategies. As a result, the performance of conventional protection schemes can be severely compromised due to the new and different fault characteristics introduced by converters. Recent studies reveal that, converter control could be designed in a manner, e.g. to intentionally inject certain fault signature or sequence components, that can effectively facilitate the protection operation, offering a promising solution to address the aforementioned protection challenges. While the control-based protection solutions have increasingly been investigated by the research community, there is no comprehensive comparison and evaluation of different proposed approaches. Therefore, this paper presents a comprehensive state-of-the-art review and evaluation of converter control-based solutions for addressing protection challenges, which covers approaches based on active signal injections, symmetrical component control, and integrated control and protection. Case studies of selected control-based solutions are presented, where the virtual impedance-based Grid-Forming (GFM) control is compared with dual-sequence current-based Grid-Following (GFL) control for distance protection, and it was found that GFM control can more effectively support the distance protection operation

Evaluation of travelling wave protection performance in converter-dominated networks

This paper presents a comprehensive study to investigate the travelling wave protection performance in networks with high penetration of Converter-Based Resources (CBRs). Representative network models are developed in the sub-step environment of the Real-Time Digital Simulator (RTDS) and realistic Hardware-in-the-Loop (HiL) tests are conducted on a pair of travelling wave relays (TWRs) to emulate realistic system and fault scenarios. The studies have been conducted in two stages: firstly, systematic tests with a total number of 300 cases have been conducted to have a high-level assessment of TWRs’ performance under different fault conditions, e.g. varied fault types, fault positions, Fault Inception Angles (FIAs), and fault resistances; and secondly, detailed investigation has been conducted on selected and representative cases to understand the impact of various factors, e.g. the line-end transformer, system fault levels, converter controls and external faults, on the protection performance. Based on the test results and analysis, it was found that the performance of the TWR is largely unaffected by the variation of system fault levels and converter control strategies, but the connection of the line-end transformer and the low FIAs present challenges for the healthy operation of TWRs in some conditions. Although the tests were conducted for a pair of specific commercially available TWRs, the results have been analysed in a generic manner, so the findings will not only facilitate understanding the benefits/limitations of travelling wave protection in the network with high amounts of CBRs, but also provide a valuable reference for system operators to select the protection solutions in the future

Impact of converter equivalent impedance on distance protection with the MHO characteristic

With the rapid increase of Converter-Based Resources (CBRs) and the decommission of conventional Synchronous Generators (SGs), recent studies have found that distance protection can face significant challenges. One of the key factors that can impact the protection performance is the CBRs’ fault behaviour being different from SGs, leading to the different characteristics of their equivalent internal impedance, which will have a particular impact on distance protection with the memory-polarised Mho characteristic. This paper presents a comprehensive investigation of the characteristics of the equivalent internal impedance of CBRs with different control strategies, based on which, their impact on the memory-polarised Mho distance protection is analysed in detail. In the paper, the equivalent impedance of CBRs with virtual impedance-based Grid Forming Control (GFM) and balanced current injection-based Grid Following Control (GFL) are calculated and plotted against time throughout the faults. These are then compared with the internal impedance of a reference voltage source. It is found that, unlike the SGs, which can be considered as voltage sources and have constant source impedance, CBRs’ internal impedance have dynamic and time-varying characteristics, which are governed by the implemented Fault-Ride-Through (FRT) control strategies. Such characteristics will lead to dynamical changes in the expansion levels of the Mho protection zones, which could lead to increase risks of protection failure and/or maloperation. It is also revealed that, by better understanding the internal impedance characteristics of CBRs, the control strategies of CBRs could potentially be refined to mitigate the negative impact on Mho distance operation, thus presenting a potential solution to mitigate the risks of compromised performance of memory-polarised Mho distance protection

Transient wavelet energy based protection scheme for inverter-dominated microgrid

When faults occur in the microgrids, high frequency transients will be superimposed on the system currents and voltages. The magnitude of those transients will attenuate as it encounters the discontinuity points in the network such as busbars, or any other impedance discontinuity points. This phenomenon can also be quantified by wavelet energy, which provides a useful tool to detect faults and locate the faulted feeder in the microgrid. In this paper, a novel protection scheme based on the transient wavelet energy of the superimposed current extracted by the Maximal Overlap Discrete Wavelet Transform (MODWT) algorithm is developed to detect faults and locate the faulted feeder in microgrids. Compared with existing protection schemes, the proposed protection scheme has the advantage of being largely immune to the changes in system fault level, fault types and positions, microgrid operating status and the control strategies deployed on the inverters, while presenting much lower requirement on the sampling frequency (10 kHz) compared with travelling wave-based methods. Unlike the conventional differential protection, the proposed scheme does not require synchronized measurement or high bandwidth communication channels, and thus, it can be considered as an economical and promising solution for microgrids

Vulnerability assessment of line current differential protection in converter-dominated power systems

Line current differential (LCD) protection is traditionally considered to be highly dependable and secure. However, the increasing penetration of converter interfaced sources (CIS) (e.g. wind, PV, HVDC systems, etc.) could significantly reduce the system fault level and change the fault characteristics, thus presenting challenges to the reliable operation of LCD protection. In this paper, the impact of the integration of CIS on LCD performance is investigated comprehensively. Analytical expressions representing LCD relay operation in the presence of converter-driven fault currents and weak infeed conditions have been developed. A test network, comprising of a CIS model equipped with a typical converter fault-ride through strategy that is compliant with the GB Grid Code, has been built in a Real-Time Digital Simulator (RTDS). Simulations of LCD performance for different fault and system conditions are performed and presented. It is demonstrated that the dependability of the LCD relay can be compromised during internal phase-to-phase faults. The results also show that with the synchronous generation being displaced by CIS, the increasing CIS penetration and fault contribution from the CIS can lead to an increased phase angle difference between the fault currents contributed from the two ends of the protected line, which will increase the risk of the compromised protection performance

Evaluation of Grid-Forming Converter's impact on distance protection performance

Motivated by the net-zero carbon emission target, the GB transmission system has seen a massive increase in the amount of Converter Based Resources (CBRs). Grid-Forming Converters (GFMs) have attracted significant interests for supporting the future system operability with high penetration of renewables due to their various more desirable properties compared with Grid-Following Converters (GFLs), e.g., stronger capability to operate in weak grids. Recent research work has found that Fault-Ride Through (FRT) strategies of CBRs have significant impact on the distance protection performance, and comprised protection operation was observed when synchronous generation sources were replaced with CBRs. However, existing research work has mainly focused on the impact of the GFLs’ FRT on distance protection, while the impact of GFMs, which could have very different FRT strategies, has not been comprehensively investigated. In this paper, a GFM with two typically used FRT implementations, i.e., the current control based FRT and the virtual impedance based FRT, is developed in the Real-Time Digital Simulator (RTDS) and the impact of the two FRT methods on distance protection is investigated for both balanced and unbalanced fault conditions. By comparing the relay performance with two FRT strategies, it is found that the distance protection appears to have better performance in terms of faulty phase selection, accurate impedance measurement and impedance measurement stability when the virtual impedance-based FRT is adopted by the GFM

Comparative evaluation of dynamic performance of virtual synchronous machine and synchronous machines

Increasing penetration of converter-interfaced renewable generation has led to significant operational challenges for power systems. Such challenges are mainly caused by the different capabilities and dynamic responses of the converters compared with synchronous machines, e.g. converters do not naturally provide inertia to the system and contribute limited fault level with very different fault characteristics. Virtual Synchronous Machines (VSM) and Synchronous Condensers (SCs) are both considered as promising solutions to address the challenges in operating converter-dominated power systems. This paper presents comprehensive studies for evaluating and comparing the dynamic performance of VSM, SC and Synchronous Generators (SGs), under a range of grid contingency events, which include short circuit faults, frequency disturbances, voltage depression, etc. The studies aim to offer insights on the level of support VSMs can offer to the system as compared with SCs and SGs, and their advantages, potential issues and limitations that need to be considered for a wider application in the system. From the studies, it is found that the VSM system appears to have comparable performance and support to the system from the perspective of fault ride-through (FRT), provision of inertial response and reaction to voltage steps. However, while VSM can potentially provide a fast fault current injection through the implementation of appropriate control, a key limitation is on the magnitude of fault currents, so it is unlikely to be capable of offering the same level of support compared with SCs and SGs

Impact of system strength and HVDC control strategies on distance protection performance

This paper presents comprehensive studies and tests for evaluating the impact of reduced system strength and different control strategies used by HVDC systems on the performance of distance protection. A Hardware-In-the-Loop (HIL) test setup is established to enable realistic testing of physical relays being used in the system, where simulated voltage and current waveforms are injected into the distance protection relay via an analogue amplifier, and the relay tripping signal is fed back to simulation and recorded for protection performance analysis. In the simulation, a reduced but representative transmission network model, which includes a Modular Multilevel Converter (MMC) based HVDC system, a synchronous condenser (SC), and a two-level converter representing non-synchronous generation (NSG), is developed in RSCAD for the RTDS simulator. The model can be flexibly configured to reflect different levels of system strength and synchronous compensation applied at the HVDC site. The HVDC system is implemented with a flexible controller, which can replicate typically used control strategies during faults (e.g. balanced current mode to eliminate negative sequence current, and constant active and reactive power modes to suppress the oscillations on the active and reactive power respectively), allowing the user to inject different levels of negative sequence current. From the studies, it was found that with decreased system strength, the impact of the HVDC system on the distance protection becomes apparent, i.e. protection performance could be compromised with delayed operation, and such impact, to some extent, is subject to the control strategies applied in the HVDC system. It was also observed that the installation of SC could facilitate the protection response, and such support is dependent on the SC capacity

Evaluation of HVDC system's impact and quantification of synchronous compensation for distance protection

This paper presents a comprehensive evaluation of the HVDC system's impact on distance protection operation via systematic and realistic experimental tests, along with the theoretical analysis of the root causes of the identified compromised protection performance. A methodology for quantifying the impact of synchronous compensation in supporting the distance protection operation is also established. In this work, the performance of two widely used physical distance protection relays from different manufacturers have been evaluated using a realistic Hardware-In-the-Loop (HIL) testing environment, where a total of 480 cases have been tested to obtain a comprehensive understanding of the distance protection performance under a wide range of system scenarios, including different fault conditions, HVDC control strategies, levels of system strength and protection characteristics. Based on the testing results, a set of representative cases where the compromised protection performance has been observed are selected for further analysis. It was found that the integration of HVDC systems (and converters in general) will introduce under/over-reach, faulted phase selection and impedance measurement issues to the distance protection, thus significantly compromising the protection performance. Furthermore, a method for quantifying the required level of synchronous compensation to address the distance protection under-reach and over-reach issues resulting from the integration of HVDC systems (and converters in general) is presented. The method provides insights on the relationship between the angle difference of fault infeed from both ends of the protected line (a key contributor to the failure of distance protection) and the level of synchronous compensation. Based on this relationship, the required capacity of Synchronous Condenser (SC) to constrain the angle difference within a targeted limit can be estimated. Case studies are presented to validate the SC quantification method. The paper offers a useful tool for system operators to appropriately size the capacity of the SC with additional valuable insights from the distance protection perspective

A flexible real time network model for evaluating HVDC systems' impact on AC protection performance

This paper presents a reduced but reprehensive real time network model constructed in RSCAD for RTDS to evaluate the impact of HVDC systems and Non-Synchronous Generation (NSG) on the protection performance in the AC grid. The proposed network model could be flexibly configured to evaluate key factors that could affect the protection performance, including the level of system strength, different control strategies adopted in the HVDC system, different levels of synchronous compensation installed at the HVDC site, etc. The developed network model contains a Modular Multilevel Converter (MMC)-based HVDC system, a NSG unit representing the converter-interfaced generation and a Synchronous Condenser (SC) representing the level of synchronous compensation. A flexible controller is designed for the HVDC system to realise various typically used control strategies, including balanced current control, constant active power control and constant reactive power control, and inject a desired level of the negative sequence current as required. The NSG employs the widely-adopted PQ control strategy. Three typical controllers, comprising the Automatic Voltage Regulator (AVR), constant reactive power and droop controller, are implemented for the SC to realistically emulate the SC behaviour under different control modes. Case studies on the application of the model for testing distance protection performance are presented. The developed model is suitable for both pure simulation-based studies and also hardware-in-the-loop test when connected to an external physical relay, thus providing an ideal testing platform for identifying the potential critical protection issues and the potential solutions in future power networks with high penetration of renewables