ON APPLICATION OF MICROPROCESSOR TO SUBSTATION RELAYING

Abstract

Reliability of operation of various power elements at substations has assumed wider importance with the advent of EHV and UHV transmission. The faults at these levels are more severe than those at lower voltage levels and must be isolated at relatively faster speeds. The conventional protective relays of electro-mechanical type used for protecting these elements have being gra dually been supplanted by wired-logic semiconductor based relays. The advent of microprocessors ushered in programmable protective relays based on microprocessor systems- These systems offer many functional and economic benefits over conventional electro-mechanical and solid state relays. The author has made an attempt to improve the performance of existing relays using 8085A micro processor for the protection of various power elements of substations. The author has developed a microprocessor-based equipment for shedding and restoration of loads at sub stations incorporating some novel decision logics. In practice, though loads are disconnected in multistages in response to underfrequency condition of the system, the present equipment incorporates freauency for trip decision alongwith under-voltage and light-load inhibi tion to avoid unnecessary frequent tripping of feeders. It also resorts to rotational sequencing of feeders to be disconnected and responds to power import/export state of a tie-line feeder. This approach benefits both the power-system and the consumers. The equipment is immune to temporary disturbances and voltage dips. Also incorporated is the self-checking feature for detecting through software any failures within it caused by individual device failures. A laboratory model of the equipment has been tested for all possible simulated conditions of operation of the system. The conventional second-harmonic restraint differenti.Pl relays are widely used for short-circuit protection of power transformers. It has been observed by some researchers that such relays have failed to block tripping under certain inrush conditions. ihe author has made an attempt to eliminate this undesired tripping of the differential relay using waveform approach. The important features of current waveform on inrush, over excitation and heavy through-fault conditions have been exploited to discriminate these conditions from internal short-circuits. Internal fault and inrush conditions are distinguished on the basis of prolonged zero current, unusual apparent half-time period and unequal consecutive peaks of the inrush current waveform after eliminating dc component. The same checks 3re utilised in the relay to yield restraining signal on severe through-faults since the current waveforms for this condition are similar to those for inrush. The relay alSo incorporates an additional check to detect double-topped Waves occurring on over-excitation of transformer and yields a blocking

signal on this condition. The prototype of the relay has been tested in laboratory with various types of simulated waveforms representing inrush, over-excitation, internal fault and through-fault conditions. By and large it has become the modern practice to provide overfluxing protection of grid transformers (as well as generator-transformers) apart from the other conventional protective schemes. The overfluxing relays used for the purpose detect overfluxing condition on the basis of v/f measurement assuming that iron losses are proportional to flux density. This is far from reality as the eddy current losses are proportional to the square of flux density and hysteresis losses to a power of flux density that lies between 1.6 and 2.4. The author has developed a practical oriented design of overfluxing relay based on the measurement of transformer iron losses computed by the microprocessor using a realistic iron loss m8thematic model for transformer. The prototype of the relay has been tested on-line on a small transformer and the test results are compared with the conventional relay operating characteristics. A review of literature on transformer protection reveals that most of the researchers have concentrated mainly on short-circuit relaying of power transformers. The author has made an attempt to develop a comprehensive relaying scheme for power transformer protection wherein vii • one 8-bit processor performs all high speed relaying functions and another similar processor provides all time-lag relaying functions. The high-speed relaying functions include inrush-re strained biased differential protection and restricted earth-fault protection, while the time-lag relaying includes overcurrent back-up, over fluxing, overload, tank leakage and standby earth-fault protection. The logic adopted in differential relaying is based on waveform characteristics and overfluxing relay logic is based on the measurement of transformer iron losses. Other protective functions are based on conventional approaches. In the present relaying scheme, except for biased differential and overcurrent back-up all other relaying functions are kept optional to the user. The relay has provisions to select the relaying functions through links whereupon software residing in microcomputer bypasses the relaying functions not opted for. A prototype was assembled and tested in laboratory on simulated signals for various normal and abnormal conditions of power transformer. The problem of bus differential protection is receiving increased attention by relay engineers because of serious consequences of a bus fault. In conventional relays problem becomes more involved if the ratio of maximum through fault current to minimum internal fault current is large. If the system is grounded through an impedance, the operating current may be quite limited viii for a phase to ground fault on the bus and a highly sensitive relay will be required to meet this situation. Upon the occurrence of heavy external short-circuit fault, an unfaithful current-transformer performance may result in large differential current in the relay circuit with a consequent undesired tripping. This limitation has been overcome in the relay developed by the author by implementing a variable percentage differential logic. This approach leads to a high sensitivity of the relay on light internal faults and low sensitivity at high currents to prevent false tripping on heavy through faults. In addition, a voltage check feature is also incorporated in the relay to provide high reliability and freedom from unwanted tripping of circuit-breakers. Simulated signals were used for testing the relay p ro to typ e. The aforesaid protective functions for various power elements at substations having been achieved through software, the hardware requirements are fairly simple.