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

FBG-based fibre-optic current sensors for power systems protection : laboratory evaluation

Conventional differential current unit protection schemes rely on a pair of electronic protection relays that measure current phasors separately at the boundaries of the protected zone. The scheme requires a separate, often optical, communications channel for the sharing of measurement information to enable the timely identification of and reaction to internal faults. The high voltage environment that the transducers must operate in poses a number of engineering problems stemming from the need for electrical isolation and requirement for transformation of high primary system current magnitudes. Additionally, when either the number of relays or distance between relays is increased, timing problems can arise due to the limited bandwidth, speed and changeable latencies of the communication channels and the increased computation requirements. Fibre-optical sensor systems are maturing as a technology and offer a number of advantages over conventional electronic sensor regimes, including the possession of inherent electrical isolation, chemical inertness, immunity to electromagnetic interference, and their small size and serial multiplexing capability. Fibre sensor systems are therefore experiencing increased uptake in industries that operate in harsh environments, such as oil and gas, or where specific requirements such as large step-out distances or resistance to radiation prohibit the use of electronic sensors. The Advanced Sensors Team within the Institute for Energy and Environment has developed fibre-optic point sensors for voltage and electrical current, based on fibre Bragg grating (FBG) technology, that have been applied successfully to power systems diagnostics. With the photonic systems capability to interrogate up to 100 km from source at kHz sample rates with up to 30 sensors in series, it is possible and highly desirable to adapt this technology for use in power systems protection, where immediate applications in unit and distance protection are clear. In this paper, the application of the FBG-based hybrid current sensor system to power systems protection is presented for the first time. Experimental tests of the response of an optical unit protection system to a range of internal and external fault scenarios are also reported. Secondary current inputs to the system are modelled using ATP and injected into the prototype test system via an APTS3 (Advanced Protection Testing System) unit. Fibre sensors, separated optically by 24 km of fibre, provide all measurement information via a single interrogation system situated at one end of the protected zone. Experimental results confirm high performance of the optical unit protection both in terms of sensitivity to internal faults and stability under external fault conditions. Therefore, the systems ability to overcome problems experienced in electronic relaying systems using conventional current sensing technologies is demonstrated. No separate communications channel is required in this configuration, with fault algorithms being deployed only at one location that need not be close to the protected zone. The fibre-optic current sensor systems capacity for long-distance interrogation and high sensor count qualify it for further applications in more complex protection schemes, or over larger distances, where a single fibre could form the basis of highly novel distributed protection schemes. This potential will also be discussed in detail in the paper

Methodology for testing loss of mains detection algorithms for microgrids and distributed generation using real-time power hardware–in-the-loop based technique

The effective integration of distributed energy resources in distribution networks demands powerful simulation and test methods in order to determine both system and component behaviour, and understand their interaction. Unexpected disconnection of a significant volume of distributed generation (DG) could have potentially serious consequences for the entire system [1], this means DG sources can no longer be treated as purely negative load. This paper proposes a method of testing loss-of-mains (LOM) detection and protection schemes for distributed energy resources (DER) using real-time power hardware-in-the-loop (RT PHIL). The approach involves connecting the generator and interface under test (e.g. motor-generator set or inverter, controlled by an RTS – Real Time Station[3]) to a real-time simulator (an RTDS – Real Time Digital Simulator[2]) which simulates the local loads and upstream power system. This arrangement allows observation of the interaction with other controls in the network beyond the local microgrid area. These LOM schemes are of increasing importance because with growing penetration levels of distributed generation the network operator has less visibility and control of the connected generation. Furthermore when the generation and load in a particular network area are closely matched (e.g. a grid-connected microgrid), it becomes increasingly difficult to detect a loss of grid supply at the generator. This work builds upon the existing LOM testing methodology proposed in [4]. By utilising RT PHIL and a laboratory microgrid, the testing environment has been brought to a new level of functionality where system integrity can be more rigorously and realistically evaluated

Assessing the reliability of adaptive power system protection schemes

Adaptive power system protection can be used to improve the performance of existing protection schemes under certain network conditions. However, their deployment in the field is impeded by their perceived inferior reliability compared to existing protection arrangements. Moreover, their validation can be problematic due to the perceived high likelihood of the occurrence of failure modes or incorrect setting selection with variable network conditions. Reliability (including risk assessment) is one of the decisive measures that can be used in the process of verifying adaptive protection scheme performance. This paper proposes a generic methodology for assessing the reliability of adaptive protection. The method involves the identification of initiating events and scenarios that lead to protection failures and quantification of the probability of the occurrence of each failure. A numerical example of the methodology for an adaptive distance protection scheme is provided

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

Translating proprietary protection setting data into standardized IEC 61850 format for protection setting validation

For smart grid development, one of the key expectations is that the data should be accessible to and readily interpreted by different applications. Presently, protection settings are represented using proprietary parameters and stored in various file formats. This makes it very difficult for computer applications to manipulate such data directly. This paper introduces a process that translates the proprietary protection setting data into IEC 61850 standardised format and saves the data as System Configuration description Language (SCL) files. A code generation process that allows rapid implementation of the translation process is proposed. Among various applications, the paper demonstrates how such a translation process and generated SCL files can facilitate the development of an intelligent system for protection setting error detection and validation

Enhanced power system stability by coordinated PSS design [Correction]

A step-by-step coordinated design procedure for PSSs and AVRs in a strongly-coupled system is described. It is shown that it is possible to separate the design of individual PSSs and to separate the design of individual AVRs. Thereby, the designs of AVR and PSS devices at a given machine can be coordinated to achieve near optimal overall power system stability performance, including oscillation stability performance and transient stability performance. The proposed coordinated PSS/AVR design procedure is established within a frequency domain framework and serves as a most useful small-signal complement to established large-signal transient simulation studies

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

Reachability analysis for the verification of adaptive protection setting selection logic

The testing of adaptive protection schemes is a problem that remains largely unaddressed. These schemes can be characterized by uncertainty in behavior due to the dynamic changes in their configuration to suit prevailing network conditions. This paper proposes a novel approach to formalizing this behavior using hybrid systems modeling. This unlocks the ability to verify the safety performance of the schemes using reachability analysis. In this paper, an adaptive setting selection logic for distance protection is verified for its safety, using reachability analysis, during changes in network conditions

Standardization of power system protection settings using IEC 61850 for improved interoperability

One of the potential benefits of smart grid development is that data becomes more open and available for use by multiple applications. Many existing protection relays use proprietary formats for storing protection settings. This paper proposes to apply the IEC 61850 data model and System Configuration description Language (SCL), which are formally defined, to represent protection settings. Protection setting files in proprietary formats are parsed using rule-based reasoning, mapped to the IEC 61850 data model, and exported as SCL files. An important application of using SCL-based protection setting files is to achieve protection setting interoperability, which could bring multiple compelling benefits, such as significantly streamlining the IED configuration process and releasing utilities from being “locked in” to one particular vendor. For this purpose, this paper proposes a uniform configuration process for future IEDs. The challenges involved in the implementation of the proposed approach are discussed and possible solutions are presented