microprocessor-based relays of ac motors

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INTRODUCTION

Microprocessor-based distribution relays contribute to improved reliability and reduced costs on electric power systems. Microprocessor-based relays, also called digital relays, have a proven track record of reliability, with over 100,000 relay-years of field experience. Microprocessor based relays provide technical improvements and cost savings in several ways:

• The relays use programmable logic to reduce and simplify wiring.
• The relays provide protection for bus faults, breaker failure, and high-side transformer blown fuse detection at no or minimal additional cost.
• The relays have metering functions to reduce or eliminate the need for panel meters and transducers.
• The relays reduce maintenance costs by providing self-test functions and high reliability.
• The relays provide remote targets and fault location information to assist operators in restoration of electrical service.



Simple Functional Block Diagram of Digital Relay

Microprocessor-based relays, also called digital relays, typically consist of an ac signal data acquisition system, a microprocessor, memory components containing relay algorithms, contact inputs to control the relay, and contact outputs to control other equipment. Figure shows a simple hardware block diagram.


Motor Protective Relay Characteristics

There are several problems that must be addressed with using a motor and Motor Protective Relay. The following section describes an open-phase voltage drop, motor current waveform distortion, capacitor installation to improve the power factor, and unbalanced motor currents.



Motor Current Waveform Distortion
Current should be supplied to the motor with the sine wave shown in the figure above, but highly distorted current waveforms have been observed in some battered, old submersible motors that are operating perfectly fine.
Motor Protective Relays are designed for sine wave input, so distortion of the waveform will increase activation current value errors with overload elements or cause open- and reverse-phase elements to operate improperly.


Stator protection
The stator protection from the faults is provided with help of thermal or dash pot type overcurrent tripping devices giving an inverse time-current characteristic and usually providing instantaneous tripping at highcurrent.
Phase fault protection is provided by two high set instantaneous relay elements; the setting is so chosen that it is well above the maximum starting current.