DEVELOPMENTS IN ADAPTIVE RELAYING FOR TRANSFORMER AND LINE PROTECTION

Abstract

Adaptive digital relaying is relatively a new area of research in power system protection. The concept lies in providing the relay with an ability to automatically change its settings, parameters, function or characteristic in accordance with the changes in the condition of the system protected by it with the objective of securing an optimal protection. Two important areas of protection, viz., differential and distance protection of transformer and line, respectively, were identified for the development of this concept by the author. A large number of digital techniques and algorithms are available for differential protection of transformers and distance protection of lines, mostly non-adaptive. More important ones are reviewed in the thesis. Certain methods and algorithms are developed here for providing adaptive functions to the harmonic-restraint transformer differential protection. Their purpose is to overcome some lacunae of conventional harmonic-restraint differential relays. To overcome the problem of varying CT mismatch due to a change in transformer operating tap, an adaptive function in the form of current correction is provided. The relay automatically detects the operating tap and corrects the currents sensed through CTs, to obtain true line currents. This feature ensures the relay same sensitivity on all tap positions. A second problem is the change in the dynamic errors of CTs with current and it is tackled by making the bias to automatically change on the basis of actual through current. This provides to the relay stability on external fault without sacrificing its sensitivity to internal faults. A third adaptive function is meant to provide restraint on overexcitation condition by monitoring the volt age-to-frequency / / ratio and adequately lifting the relay characteristic. Lastly, harmonic restraint is used to restrain relay operation on magnetizing inrush condition alone. This too is adaptively enabled — on the basis of a sudden rise in the terminal voltage of the transformer. As a result, the unnecessary delay in issuing trip signal on internal faults, that would be caused by the filters used to extract harmonics for harmonic restraint function, is avoided. Performance of each adaptive function on different types of faults is investigated using transient relaying signals obtained from simulation of the protected system on Electromagnetic Transient Package/Alternative Transient Program. The relay incorporating these features, and called current-correction adaptive-bias relay with voltage and frequency supervision, is able to detect all internal faults and is stable on even heaviest external fault. The problem of loss of accuracy of distance relays, used for EHV line protection, due to collapse of voltage on close-up faults has been tackled by providing the relay with adaptive-amplification feature. The relay is set to automatically amplify the voltage signal whenever it falls below a preselected threshold. This feature considerably improves the relay accuracy on high source-to-line impedance ratio and has a definite operation on close-up faults. Another adaptive function considered for EHV line protection is the automatic selection of reach in relation to the status of the circuit breaker at the remote end of the protected line or to the actual current infeed at the remote terminals in case of a multiterminal line. The relay has a quadrilateral characteristic and measures resistance and inductance. For the calculation of resistance and inductance seen by the relay, a computationally simple and fast, yet accurate, algorithm has been developed. Both the relays are tested by simulating the relay functions and faults. Convergence of the resistance and inductance as measured by the relay has been studied. Distance protection of traction overhead equipment is a special case of protection of EHV lines. The differences arise from the complexity of the traction supply system, feeding methods and the nature of load. An adaptive-reach distance protection scheme is presented. The relay characteristic proposed is a composite of two quadrilaterals meant to protect the overhead equipment on earth fault and wrong-phase-coupling. The relay sets its reach automatically by monitoring the power feeding condition. Another adaptive function of this relay is that, after making a preliminary check common for both protections, it automatically selects between its two protective functions. This relay also measures the resistance and inductance for its operation. The performance of the relay is evaluated by simulation. All the relaying techniques and algorithms are also implemented in real-time. The hardware is developed around a 16-bit microprocessor, Intel 8086, working as the central processing unit of the relay. To convert the analog relaying signals into digital form a 12-bit ADC is used.The relay hardware is common to all the relays, namely, the adaptive transformer differential relay, the adaptive distance relay for EHV line protection and the composite adaptive distance relay for the protection of traction overhead equipment. The software for each relay was written in assembly language of 8086. For the real-time evaluation of the relays, analog relaying signals were obtained by digital-to-analog conversion of the same digital data as used for simulation study. The results obtained with simulation and real-time tests are critically examined in the thesis. Contributions of the author and conclusions of the work are brought out at the end of the thesis. Some suggest ions are put forth for carrying on the work further.