Impact of distributed generation mix on the effectiveness of islanded operation detection

Distributed generation can be understood as a process where large scale power generation is gradually replaced by smaller power generation facilities with reduced power yield, and mostly connected at the system distribution level. One of the most important requirements for interconnecting distributed generation to healthy power networks is the Loss of Mains (or Islanding) detection. During a Loss of Mains (LOM) event a part of the grid (including distributed generation) losses physical connection with rest of the grid. A condition like this should be detected and actions to disconnect distributed generation should be initiated, in order to protect life and property. A very common passive method used to detect an islanding event, is the Rate of Change of Frequency (ROCOF). Since distribution networks nowadays are accommodating a great amount converter-interfaced generation, there is a risk that such methods may fail to successfully operate or operate spuriously, putting system stability at risk. Most of the existing LOM protection performance studies, consider only a single generator within the islanded part of the network. While historically such approach was reasonable, rapidly increasing numbers of DG connections lead to high probability of islanding with more than one generator in the mix. Therefore, this paper, considers various mixes of generation to investigate how this impacts LOM detection performance. In particular studies are undertaken with a few identified most likely combinations of distributed generators

Performance of loss-of-mains detection in multi-generator power islands

This paper presents an investigation of the impact of multi-generator power islands on the performance of the most-commonly used anti-islanding protection method, Rate of Change of Frequency (ROCOF). In particular, various generating technology mixes including Photovoltaic panels (PV), Doubly Fed Induction Generators (DFIGs) and Synchronous Generators (SG) are considered. The Non-Detection Zone (NDZ) for a range of ROCOF setting options is assessed systematically and expressed as a percentage of generator MVA rating. It was discovered that ROCOF protection becomes very ineffective when protection time delay is applied. In the majority of islanding situations the generator is disconnected by frequency-based G59 protection

Protection of microgrid with high amounts of renewables : challenges and solutions

Microgrid is a small-scale network including generators, loads and storage system, which provides a friendly way for the penetration of renewables and releases the burden of transmission system arising from the increased energy demand. Moreover, since microgrid can operate in islanded mode, it can provide backup power to local consumers when the main grid is disconnected. However, the utilization of microgrid causes serious problems in the area of power system protection. The main issues comprise varied fault levels in different operating modes and fault detection in islanded microgrid particularly when the microgrid is dominated by inverter based DGs (IIDGs). In addition, to avoid non-necessary power losses raised from multi-stage power conversion of DC loads and generators, DC microgrid becomes another attractive choice, which further increases the difficult on designing protection system for the futuristic microgrid. In this paper, a comprehensive review of the existing issues and protection methods for AC and DC microgrids is presented. Furthermore, to facilitate better understanding to readers, the benefits and limitations of each method are discussed in depth. Potential protection tools for future microgrid are suggested at the end of this paper

Impact of VSC Converter Topology on Fault Characteristics in HVDC Transmission Systems

This work presents the outcome of a comprehensive study that assesses the transient behaviour of two high voltage direct current (HVDC) networks with similar structures but using different converter topologies, termed two-level and half-bridge (HB) modular multilevel converter (MMC). To quantify the impact of converter topology on DC current characteristics a detailed comparative study is undertaken in which the responses of the two HVDC network transients during dc side faults are evaluated. The behaviour of the HVDC systems during a permanent pole-to-pole and pole-to-ground faults are analysed considering a range of fault resistances, fault positions along the line, and operational conditions as a prerequisite. Fast Fourier Transform (FFT) has been conducted analysing di/dt for both converter architecture and fault types taking into consideration sampling frequency of 96 kHz in compliance with IEC-61869 and IEC-61850:9-2 for DC-side voltages and currents

Non-unit protection for HVDC grids : an analytical approach for wavelet transform-based schemes

Speed and selectivity of DC fault protection are critical for High-Voltage DC (HVDC) grids and present significant technical and economic challenges. Therefore, this paper proposes a non-unit protection solution that detects and discriminates DC faults based on frequency domain analysis of the transient period of DC faults. The representation of a generic HVDC grid section and the corresponding DC-side fault signatures in the frequency domain form the basis of a generalized approach for analytically designing a protection scheme based on Wavelet Transform (WT). The proposed solution is adaptive within its design stage and offers general applicability and immunity to system changes, while the protection settings are configured for optimized performance. The scheme is validated through offline simulations in PSCAD/EMTDC and the technical feasibility of the algorithm in the real world is demonstrated through the use of real-time digital simulation (using RTDS) and Hardware-in-the-Loop (HIL) testing. Both offline and real-time simulations demonstrate that the scheme is able to detect and discriminate between internal and external faults at a significantly high speed, while remaining sensitive to high impedance faults and robust to external disturbances and outside noise

Challenges, advances and future directions in protection of hybrid AC/DC microgrids

Hybrid microgrids which consist of AC and DC subgrids interconnected by power electronic interfaces have attracted much attention in recent years. They not only can integrate the main benefits of both AC and DC configurations, but also can reduce the number of converters in connection of Distributed Generation (DG) sources, Energy Storage Systems (ESSs) and loads to AC or DC buses. In this paper, the structure of hybrid microgrids is discussed, and then a broad overview of the available protection devices and approaches for AC and DC subgrids is presented. After description, analysis and classification of the existing schemes, some research directions including communication infrastructures, combined control and protection schemes, and promising devices for the realisation of future hybrid AC/DC microgrids are pointed out

A novel protection scheme for inverter-dominated microgrid

Protecting an inverter-dominated microgrid is challenging for the traditional overcurrent protection scheme owing to the suppressed fault current from the inverter interfaced DGs (IIDGs). In this paper, a protection scheme based on the Discrete Wavelet Transform is developed in MATLAB/SIMULINK to detect the faults in the microgrid. The input voltage of the proposed scheme is first transformed into dq0 frame using the Park Transform. A filtering system based on the wavelet denoising approach is then implemented to reduce the sampling frequency and reject the switching noise generated by the inverters in the microgrid. The performance of the proposed scheme is evaluated in transient simulation by systematically applying different types of faults, including varied fault positions and impedances. Additionally, a high impedance arcing fault model is implemented to test the proposed protection scheme under nonlinear fault impedance conditions

Enhanced DC voltage control strategy for fault management of a VSC-HVDC connected offshore wind farm

This paper proposes a DC voltage control strategy for fault management taking into advantage the operation of the master controller located in the offshore AC substation platform. The issue resolved via the proposed controller relates to over-voltages caused in the HVDC links when the power transfer onshore is disrupted due to faults occurring at the AC side of the onshore grid. The control strategy presented in this paper proposes an effective way of maintaining the DC over-voltage within safety limits via reducing the connected wind farm power output. The operation of the aforementioned control strategy requires small computational power and no communication

Assessment of fault location techniques in voltage source converter based HVDC systems

This paper investigates fault location techniques in high voltage direct current (HVDC) transmission networks utilizing voltage source converters (VSCs). The subject has been extensively researched due to the fault locating actions associated with the supply restoration and the economic loss, and also because of the trending employment of VSC-HVDC transmission systems. However, the fast operation of HVDC protection has made fault localization more challenging as limited measurement data can be extracted. By broadly researching the existing fault locating approaches in such systems, a comprehensive literature review is presented. Then, two selected methods, active impedance method and travelling wave method (using Continuous Wavelet Transformation) are tested. These fault location techniques together with the power system models have been developed using Matlab/Simulink. The results are summarized and systematic comparative analysis of the two fault location techniques is performed

Fault current characterisation in VSC-based HVDC systems

The DC-side line faults in high-voltage direct-current (HVDC) systems utilising voltage-source converters (VSCs) are a major concern for multi-terminal HVDC systems in which complete isolation of the faulted system is not a viable option. A number of challenges are posed by both pole-to-pole and pole-to-ground faults including the presence of very fast and high amplitude discharge current from the DC-link capacitance, the lack of suitable DC current breaking devices, and the lack of highly discriminative fault detection techniques. Therefore, faults occurring along the interconnecting DC cables are likely to threaten system operation. In order to better understand the system under such faults, this paper analyses the behaviour of HVDC systems energised by the conventional two-level VSC. This investigation provides a systematic evaluation of the nature of a DC fault in HVDC systems during a permanent pole- to-pole and pole-to-ground fault taking into consideration a number of influencing parameters including fault position, fault resistance and other operational conditions. To quantify these dependencies on DC voltage and current characteristics a systematic simulation study is undertaken in which the natural responses of the HVDC networks transients during DC side faults are examined. The outcome of this paper lies the necessary knowledge foundation for developing future DC protection methods