Protective Relays
8.1 Introduction
Relays are devices intended to protect electrical systems and equipements against damages caused due to abnormal operating conditions. In more technical terms, relays are devices designed to produce sudden pre-determined changes in one or more physical systems on the appearance of certain abnormal conditions in the physical systems controlled by them. Protective relays act as sensors of abnormalities and actuate control gears when required. Relays may be suitably set to operate with the required discrimination between sections in order to isolate only the faulty section /sections or equipment / equipments. A relay will have one or more energising quantities and one or more characteristic quantities in terms of which the relay is calibrated (eg. voltage for over voltage relays, time for definite time lag relays, time and current for inverse time lag current relays, angle for directional relays, power for reverse power relays etc.) A relay should watch the system changes and operate when it is called for. Testing of protective relays before commissioning and thereafter periodically is very important as failure of relays can cause danger to life and damage to material.
The testing procedure of common types of relays used in industrial applications are explained in this chapter.
8.1.1 Classification of Relays
Relays are broadly classified into two, based on the principle of operation.They are
1. Conventional electro magnetic relays
2. Static relays
Microprocessor based PLC (Programmable Logic Controlled) multifunctional relays are now
available as a replacement for a large number of independent unifunctional relays. These are
intelligent relays programmed to the requirements.
Relays can also be classified based on the time of operation of the relay as
1. Instantaneous relays
2. Time lag relays
Almost all relays fall under the categories of either instantaneous relays or time lag relays. Instantaneous relays operate and reset without any intentional time delay. But instantaneous relays have inherent time delay and based on this inherent time delay (operating time) they are sub classified as given in table 8.1.
8.1.2 Accuracy of relays
The manufacturer of the relay shall specify the accuracy of the relay at specified setting
values. The standard accuracy classes which correspond to the maximum percentage error is given in
table 8.2.
8.1.3 Characteristics of Time lag relays
Time lag relays are relays, the operation or resetting of which are intentionally time delayed. The time delay of the relay may be fixed or adjustable. Time lag relays are intended to operate after a specified time on the appearance of the energising quantity. The time lag of operation depends on the designed characteristics of the relay. Usually standard inverse, very inverse, extremely inverse and long time delay relays are used in practice. Many of the relays have definite minimum time of operation which will help to attain proper time grading between sections.
· Current operated relays
The time-current characteristics of current operated Inverse Definite Minimum Time Lag (IDMTL) relays without directional feature is given in tables 8.3.a to 8.3.e and in figures 8.1 to 8.5.
1. IDMTL 3 second relay
2. IDMTL 1. 3 second relay
Written by John on September 22nd, 2008 with
3 comments.
Read more articles on Electrical Installations and Protective Relays.
sai
sita
#1. January 26th, 2010, at 5:01 PM.
IDMTL relay do have a hot and cold curve to check whether it still comply to the acceptable timing. Ps forward me the cold and hot curve if you have a copy. TQ !