By T. DAVIES
The security that's put in on an business energy method could be subjected to more challenging stipulations than the security on the other form of strength procedure. beginning with the numerous uncomplicated units that are hired and protecting the entire region of business energy method security, this booklet goals to assist in achieving a radical knowing of the safety invaluable. very important features comparable to the trendy cartridge fuse, kinds of relays, and the function of the present transformer are lined and the commonly used inverse definite-minimum time overcurrent relay, the idea of the Merz-Price safety process and the advance of the high-impedance relay method are seriously tested. This new version has occur based on the dramatic switch from using electro-magnetic relays to digital and micro-processor relays which determine in virtually all new installations. as a result, even though the idea and utilization are a similar, the appliance might be a lot more suitable due to the elevated variety and accuracy and the further amenities supplied with the fashionable relays. This ebook displays the switch and explains the technical benefits.
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Additional info for Protection of Industrial Power Systems, Second Edition
Example text
The importance of a sensitive relay with a high overload capability can be appreciated when the conditions of operation of a protection scheme are considered. A relay may be required to have a setting of, say, 10% normal current and yet be capable of carrying, say, 50 times normal current, which means that the relay must be capable of carrying 500 times setting current or 5002 times setting power. With the moving-coil relay with a setting of 20/xW the power at maximum fault condition is only 5 W.
On the other hand, a protection current transformer is required to operate over a range of current many times the circuit rating and is frequently subjected to conditions greatly exceeding those which it would be subjected to as a measuring current transformer. Under such conditions the flux density corresponds to advanced saturation and the response during this and the initial transient period of short-circuit current is important. It will be appreciated, therefore, that the method of specification of current transformers for measurement purposes is not necessarily satisfactory for those for protection.
To eliminate this it is usual to split the electromagnetic pole face and surround one-half by a copper loop. The current induced in this loop causes a phase delay in the flux passing through the loop compared with that passing through the other half of the pole and therefore the net flux is never zero. Alternatively vibration can be prevented by supplying the relay through a rectifier. In this case coil inductance maintains the flux during the idle portions of the cycle. c. operated relays residual flux is a problem and may prevent release of the armature.