MICROCOMPUTER BASED PROTECTION AND MONITORING OF LARGE GENERATORS

Abstract

Digital protective relaying for power systems has been a subject of wide research in many countries since the advent of the microprocessor, which combines the advantages of programmability with low cost and high performance.A Generator is one of the most important and expensive equipments in any power system and for this reason its protection and monitoring assume a special significance. Sudden outage of a generator may overload rest of the system and this may affect the overall performance of the system. Moreover, any failure of the protection gear to clear a fault promptly may cause extensive damage to the generator itself. Generator faults (short circuits) are considered to be the most serious abnormality since they can cause severe and costly damage to the windings and the core. Faults in the generator or near its terminals which produce high magnitudes of short circuit currents require some form of highly selective and high speed protection to isolate and shut down the machine in the shortest possible time in order to minimize any damage to the machine or the system. Differential relays are used to detect both phase and earth faults. In differential relaying the currents at the two ends of each phase winding are taken and the difference and through currents are obtained. If the difference current exceeds a preset percentage of the current flowing through the protected zone, the relay operates.

Out of the other abnormal conditions on the generator, the loss of excitation (field failure) is considered to be very serious. Operation without field excitation is dangerous because in such cases the generator will operate asynchronously. Asynchronous operation causes alternating currents of the slip frequency to flow in the rotor, resulting in excessive rotor heating. It also results in voltage reduction, which may lead to power swings and instability of the system. Prevention of thermal damage does not require instantaneous action, but to maintain stability a fast relay operation is essential. As such, protection on loss of excitation is generally comprised of two impedance relays with circular characteristics centred on the reactance axis and offset downward the origin of the R-X plane. The operation of the relay with the outer operating characteristic will give a delayed tripping whereas the relay with the inner operating characteristic will disconnect the generator instantaneously. While the protective relays should operate and isolat the generator on the actual occurrence of faults, it is al possible to detect some faults at the incipient stage itself and give an adequate warning so that unwanted and prolonged outages are avoided. This is achieved by monitoring certain vital parameters of the generator. Microprocessor holds great application potential in this area too. The main objective of the present work is to explore the use of microprocessor and digital signal processing for e so

the protection of generators on certain faults/abnormal conditions and monitoring of certain parameters of the generator. A primary requirement of microcomputer relaying is the availability of computationally simple algorithms for digital filtering and other digital signal processing functions. Filters based on discrete Fourier transform, where a distorted signal is cross correlated with sine and cosine waves of fundamental frequency to obtain expressions for the fundamental-frequency real and imaginary components of the signal, have been widely proposed in the literature on digital relaying. It is noted here that these filter algorithms are computationally complex and their real time implementation requires either a fairly fast processor or the assistance of a hardware multiplier. Therefore, cross correlation of distorted signal with other than sinusoidal wave is investigated in the present work. The frequency responses of the algorithms have been studied using Z-transform and compared with the frequency response of a Fourier-transform based filter (serving as the reference) using a mainframe computer. It is concluded that the cross-correlation of a distorted signal with heptagonal-wave yields a filter response matching very closely with that of the Fourier-transform based filter algorithm, whilst, this new filter algorithm is computationally much simpler. It, therefore, allows the implementation of relays on microprocessor without using any high cost microprocessor or adding any arithmetic coprocessor or multiplier and without sacrificing relaying accuracy. A three phase generator differential relaying has been evolved employing the aforesaid filter. It uses only real component (instead of both the real and imaginary components) of the fundamental frequency in defining the fault discriminant and the trip criterion. The fault discriminant is expressed as the ratio of the running averages of the real parts of the fundamental frequency components of the differential and through (sum) currents. This feature reduces the amount of computation drastically. The efficacy of the differential relaying algorithm has been evaluated offline on a mainframe computer DEC-2050 with assumed typical relaying signals and the signals obtained from a power system analysis using the electromagnetic transient package developed by the University of British Columbia. The operating time of the relay is measured as the time difference between the instant of fault inception and the instant at which the trip criterion is met. Since the trip criterion involves the sensitivity factor or bias setting of the relay, the operating time has been evaluated as a function of the bias and found to vary from 1.67 to 10.0 ms (1-6 samples at a 600 Hz sampling rate) as the bias is changed from 0 to 12.5 percent. The effect of variation of system frequency on the computed values of the fault discriminant has been studied for a variation of +2 Hz keeping the sampling rate constant at 600Hz and found to be negligible. Three phase generator differential relaying has been implemented using the foregoing algorithm successfully on a 16-bit microprocessor, INTEL 8086, working with a 5-MHz clock. vi Its salient hardware features are the use of two active second order low pass filters for curtailing frequency bandwidth of each relaying signal and a 10-bit analog to digital converter. The actual time of computations per phase is 0.415 ms against the estimated time of 0.4 ms on the basis of benchmark programs. The total processing time for the three phases is 1.25 ms against the available sampling interval of 1.67 ms. The balance processor time can be well utilized to save relevant data in RAM for fault analysis and statistical records or to perform some other relaying functions. Results of a real-time test conducted on the prototype relay with signals representing a 3-phase terminal fault occurring at voltage zero (characterized by a maximum dc offset in the fault current), reveal a performance that agrees closely with results of offline evaluation. Various techniques of loss of field (LOF) protection have been critically reviewed and the rectangular characteristics have been proposed as an alternative to and possible improvement over the conventional circular characteristics of the distance relays used for LOF protection. Single-relay and two-relay protection are put forth to meet different situation. A digital LOF protection scheme with rectangular characteristic relays has been developed. Its performance has been evaluated offline in regard to the efficacy of the digital filter used therein, the relay operating characteristic and the effect of large variations in the system frequency.

A digital LOF relay has been implemented in real-time on 8086 microprocessor. Its operating characteristic has been experimentally verified. An algorithm has been proposed and tested for incorporating undervoltage monitoring to minimize unwanted operation of the LOF protection. An 8085 microprocessor based monitoring system has been developed for an early detection of broken conductor strands and overheating of the stator and bearings. For detecting broken strands in the stator, radio frequency (RF) current in the neutral lead of the generator is monitored through a high frequency current transformer, a radio frequency amplifier, a bandpass filter and a peak detector. The peak value of the RF current is read into the microprocessor through a 10-bit ADC and compared against three predetermined thresholds for three levels of arcing at the broken strands. An alarm is issued as and when a threshold is exceeded. The level of arcing is also displayed. The temperatures in the stator winding slots or, in the case of a water cooled stator conductors, the temperature of the cooling water at outlets and bearing temperatures are monitored by using resistance temperature elements (RTEs). The RTE outputs are scanned by the microprocessor using reed relays. The RTE is connected in a Wheatstone bridge, output of which is amplified and read into the microprocessor through the analog to digital converter. A lookup table is used to convert the ADC reading into temperature thereby accounting for the nonlinearity of RTE and preprocessing circuits. An alarm is given as and when any temperature exceeds a predetermined threshold. The identity of the overheated spot/location is also displayed to help troubleshooting.