PROTECTION OF DISTRIBUTION SYSTEMS DURING DISTRIBUTED GENERATORS ENVIRONMENT

Abstract

During the last decade, penetration of Distributed Generators (DGs) in the Distribution Network (DN) has been increased tremendously. In this span, the cost of DG technology has been significantly reduced, which makes it more economical to utilize at the local level to mitigate excessive load demand. The DG plants are utilizing Distributed Energy Resources (DERs) with an inverter or rotating machine based generators. These plants cater the electrical energy demand of the area and, at the same time, distribute the surplus energy to an existing utility grid network. The utilization of DG leads to various advantages such as reduction in land, infrastructure, network integration and losses in power transmission. Along with this, it provides clean, reliable, and environment friendly energy. Besides these advantages, it causes several technical problems such as degradation of power quality, and loss of control over the existing grid network. There are several other issues such as safety of the utility personals/customers, protection of existing network/consumer’s property, protection against the abnormal operation of DG, and proper coordination among various protective devices. Among these issues, protection of DN during DG interconnection is the prominent one for the power system engineers. As the DG is utilized along with the main grid power supply, it is interconnected with the existing utility grid network. In case of fault/abnormal condition, the DG can be disconnected suddenly. This sudden disconnection of DG deviates the voltage magnitude and frequency from its nominal operating range which in turn damage the DN or end user’s equipment. Moreover, it is well known that the conventional protection schemes used for the DN are designed for the unidirectional power flow. Nevertheless, an integration of DG into DN changes the direction of current flow in the network. This makes the protective device to mal-operate or in some cases, it may remain unoperated during various abnormal/fault conditions. Therefore, the major protection problems raised by the integration of DG with an existing DN are (i) sudden disconnection of DG widely known as islanding and (ii) loss of existing protection coordination among various protective devices. In case of an unintentional islanding, the deviation in electrical parameters is large enough to damage the equipment at the consumer end. Hence, in order to protect consumers and DN, the islanding situation must be detected as quickly as possible. According to various standards, the DG in an island network must be disconnected within minimum time (less than 2 s) which can be derived by utilizing anti-islanding protection. This protection can be designed by adopting one of the methods (communication assisted, active, passive and hybrid) of vi islanding detection. Though the communication based method gives satisfactory results, it is not economically viable. At the same time, active and hybrid methods require an external signal to be injected into the network which degrades the power quality. Conversely, the passive method is less expensive and easy to implement. However, it suffers from Non- Detection Zone (NDZ) and nuisance trip. It is well known that all the protective devices such as Overcurrent Relay (OCR), Recloser, and Fuse used in the radial DN are coordinated with each other. However, an integration of DG makes the bi-directional current to flow into the network. Hence, there is a possibility of mis-coordination among the various protective devices. Further, in the presence of DG in the DN, an additional fault current is supplied by the DG. This makes the fuse to blow prior to the recloser for every temporary fault. This is against the fuse saving principle. Therefore, in order to provide a proper solution to the problems of sudden disconnection of DG and mis-coordination between fuse and recloser in the presence of DG in the DN, the proposed research work has been carried out. The main objective of the thesis is to design and develop effective protection strategies for the islanding detection and to maintain proper coordination between recloser and fuse in the presence of the synchronous based DG in the DN. In the first stage of the research work, an attempt has been made to present an effective and accurate islanding detection scheme. In this attempt, four different islanding detection schemes by acquiring current or voltage signals from the terminal of the target DG have been presented in the proposed work. These schemes have been tested on standard networks/hardware setup by developing a simulation model or laboratory prototype. Further, in order to check the authenticity of the presented technique, a Hardware-In-Loop (HIL) testing has also been carried out. The developed islanding detection techniques have utilized different approaches such as pattern recognition, sequence components, modal components, and autocorrelation. In a pattern recognition based islanding detection technique, synchronous and inverter based DGs have been utilized which are connected to various buses of the IEEE 34 bus network. A large number of simulation cases of islanding and non-islanding events have been generated on the IEEE 34 bus network in Real Time Digital Simulator (RTDS/RSCAD) environment. Further, in order to proficiently discriminate between islanding situation and non-islanding events, a precise feature selection is very important. Therefore, the process of selection of feature and formation of feature vector has been discussed. In this work, a negative sequence component of currents has been utilized for the formation of the feature vector. Afterwards, the generated test cases have been utilized for the training of the classifier model. Here, in the proposed work, an efficient classifier modal (Relevance Vector Machine (RVM)) vii has been chosen for its high discrimination efficiency. Furthermore, the minimum percentage (40%) of the acquired data of islanding and non-islanding cases has been utilized for the training of the RVM model. The results obtained in terms of accuracy of the RVM model show that the presented technique efficiently discriminates between islanding situation and nonislanding events. Moreover, it is capable to detect islanding situation with lower or zero mismatch of active/reactive power between load and generation. In addition, it remains immune to the non-islanding events. At the end, the comparative assessment proves that the proposed scheme is superior in discriminating islanding situation with non-islanding events. Afterwards, sequence component of voltage based islanding detection technique has been presented. In this work, the developed simulation model of IEEE 34 bus network has been utilized for the generation of various islanding and non-islanding test cases. Here, the voltage signals have been acquired from the terminal of the target DG. The sequence components of the voltage signals have been calculated from the phasor values of acquired voltages. These derived sequence components have been further utilized for the derivation of the Islanding Detection Factor (IDF). An effective discrimination between islanding situation and non-islanding events have been carried out by comparing the value of the IDF with the pre-set threshold value. The results obtained in terms of IDF for various islanding and nonislanding situations indicate that the proposed scheme accurately distinguishes islanding situation with the non-islanding event. Further, it senses islanding condition quickly even in case of a very critical islanding situation with perfect power balance condition. Furthermore, during a non-islanding situation, the presented scheme remains stable and does not issue a trip signal. Moreover, the comparison of the proposed scheme with a published scheme establishes its superiority in the detection of islanding situation. Then, an approach based on modal components of the acquired voltage signals has been presented in the proposed work. In this work, the value of IDF has been calculated by utilizing phasor values of acquired voltages. Further, in order to verify the authenticity of the proposed scheme, a prototype has been developed in the laboratory environment. In this prototype, a synchronous generator based DG has been utilized which is connected to the developed model of the DN. Furthermore, on the prototype, various non-islanding events such as faults with variable fault parameters, switching/starting of capacitor/induction motor and sudden change in load/power factor and islanding situations with different mismatches of active/reactive power have been generated. The experimental results obtained from the prototype show that the presented algorithm is able to detect islanding situation accurately and quickly (within 3 cycles). Moreover, it is capable to detect islanding situation with lower or zero mismatch of active/reactive power between the load and generation. Subsequently, it remains stable during viii non-islanding events. At the end, comparative evaluation of the proposed scheme with the existing schemes clearly indicates its superiority in distinguishing islanding situations with non-islanding events. Thereafter, a new islanding detection approach based on auto-correlation of the acquired voltage signal has been presented. In this work, a Discrimination Factor (DF) which is derived from an Auto-Correlation Factor (ACF) of the voltage signals is utilized for the detection of islanding situation. The value of DF is calculated from the most affected lags of the ACF during various islanding situations and non-islanding events. Discrimination of an islanding situation with the non-islanding events has been achieved by comparing the value DF with the pre-set threshold value. Further, the validation of the presented scheme has been carried out by developing a Hardware-In-Loop laboratory setup. In this setup, a power distribution test network has been virtually developed in RTDS/RSCAD environment which is physically connected to the Digital Signal Processor (DSP) controller. During testing, various islanding and non-islanding cases have been generated on the developed HIL model. The experimental results show that the presented algorithm is able to differentiate between the islanding situation and non-islanding events efficiently. Moreover, the proposed scheme is capable to identify islanding situation even in case of the perfect power balance condition. In addition, it provides better stability against various non-islanding events and hence, it avoids nuisance tripping. In the second stage of the proposed research work, an effort has been made to provide an effective coordination between recloser and fuse used in the DN along with DG. In this case, an adaptive relaying technique to restore the proper coordination between recloser and fuse based on the network impedance is presented in the proposed work. This technique is based on the positive sequence impedance (Z1cal) of a network seen by the recloser. This impedance varies with respect to the interconnection of DGs in the network. In the presented work, in order to calculate the value of Z1cal, the phasor values of the acquired current and voltage samples have been utilized. Further, the acquisition of the samples of current and voltage signals has been performed at the recloser location. The calculated value of Z1cal is utilized for the derivation of Modification Factor (MF). In the presence of the DG into the DN, the recloser changes its fast operating Time-Current Characteristic (TCC) with respect to the value of MF. The performance of the proposed scheme has been evaluated by modelling an IEEE 34 bus network in RTDS/RSCAD environment. At the same time, appropriateness of the proposed scheme has been verified on a prototype developed in the laboratory. The results obtained from simulations as well as from the laboratory prototype clearly indicate that the proposed scheme restores appropriate coordination between recloser and fuse even in case of ix a high as well as a low penetration of DG in the DN. The presented work is likely to contribute significantly to the area of protection of a radial distribution network in the presence of DG. The different techniques developed will be particularly useful for providing anti-islanding protection and recloser – fuse coordination in the electrical power distribution network. Some suggestions based on observations and simulations in this area are proposed at the end of the thesis for the benefit of potential researchers.