A current-based solution for transformer differential protection. II. Relay description and evaluation

For pt.I see ibid., vol.16, p.485-91 (2001). This paper describes a new approach for transformer differential protection that ensures security for external faults, inrush, and overexcitation conditions and provides dependability for internal faults. This approach combines harmonic restraint and blocking methods with a wave-shape recognition technique. We compare in the paper the behavior of some traditional transformer protection methods to that of the new method for real cases of magnetizing inrush conditions.     

Effect of waveform distortion on protective relays

The influence of harmonics on protective relays is described theoretically, and the results of laboratory tests are given. Representative relays using various operating principles were tested using fundamental currents and/or voltages, single-frequency inputs that were multiples of the fundamental frequency, realistic combinations of fundamental and harmonics. From the theoretical concepts and expectations discussed and the test results, it is shown that the influence of mixed-frequency harmonics (with magnitude decreasing with order) on the steady-state behavior of the protective relays studied is insignificant. Pure single-frequency inputs, above the fundamental, caused a distinct change in relay operations     

The universal overcurrent relay

A comprehensive list of nondirectional overcurrent relays would include thermal overload, inverse-time, definite time, and instantaneous relays. The list could be further classified by operating quantities including individual phase, residual, and negative-sequence current. Taken collectively and depending on the characteristic shape, pickup and time range, and dynamics, these relays span the applications for motor, feeder, and breaker failure protection. Because of the past necessity for using either discrete or specialized system relays, overcurrent characteristics for these applications may appear diverse and unrelated. However, microprocessor relay technology has advanced to where it is not only feasible, but it is of distinct economic advantage, to consider all these characteristics collectively as attributes of a universal overcurrent relay. This universal relay concept is used here to discuss the commonality, the differences, and the coordination of the elements required for feeder, motor, and breaker failure protection. The article goes on to discuss the rules for the coordination of negative-sequence overcurrent characteristics for sensitive phase-to-phase fault protection in feeders, as well as for unbalanced current protection of induction motors     

Integrated metering and protective relay systems

The process of converting sampled analog signals to digital numbers using a low-cost microprocessor and an A/D converter for motor protection is considered. Distribution fault records, relay performance with extremely saturated low ratio current transformers, and advanced methods of protecting high-inertia drive motors are presented. The discussion also covers: digital filtering; range of operation of a digital overcurrent relay; the function of the anti-aliasing filter; advances in motor protection; rotor thermal model; and maintainable structured software     

A current-based solution for transformer differential protection.I. Problem statement

This paper analyzes the problem of transformer differential protection. First, the authors review the concept of transformer differential protection. They then analyze magnetizing inrush, overexcitation and current transformer (CT) saturation phenomena as possible causes of relay misoperation. Finally, they summarize the existing methods for discriminating internal faults from inrush and overexcitation conditions     

High current low ratio

This article is one of the few references describing high current testing of protective relays using low ratio current transformers (CTs). Maximum fault currents on power plant auxiliary buses are routinely 40 kA and can be as high as 80 kA. Accurate measurement of high-current magnitudes requires high-ratio CTs. The rests show the limitations of Fourier and cosine filters used in microprocessor relays that extract the fundamental phasors and eliminate harmonics. The tests validate the operation of a cosine-peak adaptive filter designed to cope with the highly distorted saturated waveforms produced by the low-ratio CTs subjected to high current. The details of relay operation are shown in unfiltered event records of the test cases. This article reports on the results of primary high-current tests of overcurrent, motor, and distance relays using low-ratio CTs. The test currents ranging from 6-50 kA were used with CTs with ratios of 50:5, 300:5, and 600:5. This article compares the internal unfiltered event records with MATLAB simulations.     

Motor analysis and thermal protection

A motor starting study can be completed in a matter of minutes using a PC program written for the purpose. The study uses readily available nameplate, load, and thermal-limit data and determines the most significant motor protection criterion: rotor temperature rise plotted per unit of thermal limit. The parameters for the electrical, mechanical, and thermal models are determined from the source data and used by the program to produce plots of current, torque, speed, and rotor temperature versus time. The author identifies the necessary source data and explains the method of analysis using a 6000 hp, high-inertia draft fan motor as an example