APPLICATION OF MICROPROCESSORS TO SOME POWER SYSTEM PROBLEMS

Abstract

The mainframe computers are used off-line for power system analysis and the minicomputers for roal-time control amplications. The modern microcomputers are growing in speed and power to substitute minicomputers in many real time applications. The objective of the present work is to solve some problems related to nower system nrotection, metering and instrument testing. An U2iderfrequency relay, used for indicating power generation deficiency, is ideally required to give the actual final circuit opening at a preset level under decaying frequency conditions. To this end, a plain under and over frequency relay based on time period measurement was first realised. Then the frequency decay rate and the circuit breaker opening time were suitably fitted into an empirical relation to compute a variable time lag such that circuit clearing is achieved at a preset frequency level under all system conditions. Thus an adequate compensation is provided in the proposed underfrequency relay for usual variations of the frequency decay rates. The same principle is utilised to perform frequency level governed automatic load shedding and restoration. The load shedding nhilosonhy, its considera tions and frequency-setting coordination have been discussed at length. The prototypes have been tested under simulated power system conditions. -IVThe programmed approach of microprocessor based systems has been exploited to realise an overcurrent relay with universal characteristic. The characteristic, as dic tated by the protection requirements, is in the form of a lookup table which could be in the form of a preprogrammed memory module. A single phase overcurrent relay is realised without using an A/D converter, the current signal is con verted into a proportional pulse width by simple front end hardware. The width of the pulse is measured by the pro cessor and the time lag is accordingly computed with the help of the said lookup table. A3-phase time lag overcurrent relay incorporating an instantaneous unit has also been developed, which takes samples of the current signal in each phase at the peak of its waveform for the computation of its r.m.s. value. Spurious signals, surges, d.c. tran sients and harmonica are filtered out beforehand as the principle is based on instantaneous measurement, k direc tional relay has been developed, in which « block spike coincidence' principle is employed for dual phase measure ment. The transient overreach is considerably low. No memory circuit is used and yet the relay is capable of main taining selectivity on close-in faults. The relay operating angular range is made immune to system frequency variations by adjusting the block position ^nd its width in accordance to the frequency. The directional inverse time overcurrent relay is an ingenious combination of the hardware and soft ware used for the individual directional and overcurrent relays- Here the phase and current measurements are done iteratively. The quadrilateral distance relay which has a large application potential in the protection of heavily loaded transmission lines has been realised using the sampling technique as it offers several advantages. The quadrilateral is realised as a combination of restricted directional, re actance and an ohm characteristics. The hardware development time and cost as compared to hard-wired versions of the relay, have been drastically cutdown. A new semi-automatic synchroniser has been developed, which can be readily put to use in small power houses. It compares the phase sequence, voltage magnitude and frequency of the incoming machine with those of the bus bar and gives visual signals to assist correction of generator excitation and turbine speed. The novel feature of the synchroniser is that the rate of change of phase angle, its sign and the circuit breaker closing time are taken into account to ob tain final circuit closing at zero phase difference. A system for rigorous testing of frequency relays has been devised to predict the effects of frequency decay rates, voltage magnitude and its wave form on their perfor mance. The microprocessor generates the necessary signals and records the pickup frequency under steady and dynamic conditions. The operating time of the relay is also measured with high precision. The results are printed out enbloc at the end of complete testing which may serve as a good guide to the relay designers. A scheme for testing energy meters has also been evolved which is based on comparison with a rotary substandard. The speeds of the two meters are transduced into proportional pulse rates and then simultaneously counted by software for the computation of percentage error. Testing on static energy meter can also be conveniently carried out on the same test set gear. A tendency to under-read present in the existing Merz-Price integrating maximum demand indicators has been overcome by eliminating the effect of peak load splitting between two consecutive time blocks. A floating 30 minute integration time block is used to record the absolute maximum demand thereby leaving no margin for uncertainty or ambiguity. The proposed microprocessor based meter essentially incorporates an energy meter as well. The block timing is accurately provided by software. In conclusion, the micorprocessor can handle most of the complex and demanding problems rather easily as com pared to the conventional electromechanical and static hard-wired devices. It has been possible in the present work to give due consideration to the parameters like rate of change of phase angle, rate of change of frequency, circuit breaker opening and closing times, which are very difficult to account for without the use of microprocessor. Similarly automation in testing reduces the error caused by human involvement. Signal gon«rati on at a precisely controlled rate-of-change of frequency and subsequently recording the device response are the other difficult functions which have been possible through the use of a microprocessor. The realisation of a floating integration time block for demand meters would perhaps have been the most difficult task without the use of a programmable device.