Summary： Several electrical terms are used when describing protective relays and other types of relays. This article will introduce some of the special terms that an engineer or a technician should be equipped with while working with relays.
In part four of this series, we introduced the basic theories of protective relays in electrical systems. Several terms were used and some of us might find it difficult to understand them and what they mean. For proper comprehension of such terms, this article will build on the basic definition and purpose of such terms. Understanding these terms will allow an engineer to grasp simple and complex theories while working with the relay.
This is the first term I am going to introduce. Not all electrical relays have free-to-move contacts. The contacts remain in their designated normal position and this is made possible by some forces that are applied to them. The force is referred to as the relay’s controlling force which can be a spring force, magnetic force, or gravitational force.
To change the position of the relay’s moving contacts from normal, a force known as the relay’s deflecting force is applied. The deflecting force and the controlling force are in opposing directions and both of them are always present in any given electrical relay. The deflecting force is connected to the main live line but has a magnitude less than that of the controlling force.
Increasing the relay coil actuating current gradually, increases the electromechanical relay deflecting force. Immediately the deflecting forces become more than the controlling force, the relay moving parts are triggered and a motion is initiated that changes the position of the moving contacts. This current that allows the electromagnetic relay to initiate its moving contact motion is what we refer to as the relay’s pick-up current.
Figure 1: Relay Timing Circuit
A relay has a constant minimum pick-up deflecting force value. The coil’s deflecting force is directly proportional to the turns in the coil and the amount of current that is flowing in the same coil. Changing the number of turns that are active changes the amount of current that is required to achieve the deflecting force minimum pick value. This means that by reducing the number of active turns, in the same proportionality, more electrical current will be drawn to achieve the desired actuating force of the relay. On the other hand, increasing the number of active turns on the relay coil reduces the amount of electrical current that is drawn by the relay to achieve the desired relay’s deflecting force.
In industries, a similar relay model can be employed in different areas of use. The relay pickup current is always adjusted as per the system requirement. The adjusted current set-up is what is known as the relay’s current setting. It is made possible by providing the necessary number of coil taps. The taps are connected to the plug bridge. A plug is inserted at different locations in the system bridge to change the active turn numbers of the relay.
The relay’s current setting is determined through the pick-up current percentile ratio to the relay’s rated current.
The simplified formula is as shown below:
Let us assume that you want a protective relay to work immediately after the current crosses 125% of the rated current. The given relay has a current rating of 1A, a normal pick-up current of 1A which is expected to be equal to the connected current transformer that is connected to our electric relay rated-current Then the relay PICK-UP current will be
The current setting is also referred to as the current plug setting and the table below shows the ranges of the current setting in different types of relays.
Table 1: Ranges of Current Setting
This is the ratio of the relay’s fault current as compared to the pick-up current of the same relay. Its computational formula is listed below.
Let us take a simple example where we have a relay connected to the protection circuit breaker with a turn ratio given as 200/1 A and 150% current setting.
Relays Pick-Up current will be given by 1 x 150% =1.5 A
Now assuming that there is 1000A of circuit breaker primary fault current, the circuit breaker secondary fault current will be given by 1000 x 1/200 =5A
And our PSM will, therefore, be given by 5/1.5 = 3.333
Two factors determine the relay’s operating time and this is:
The action of adjusting the electromechanical relay travel time is what we refer to as time setting and the adjustment itself is referred to as the relay’s time setting multiplier. The relay setting dial has a calibrated scale that has a range of 0 to 1 which has intermediary numbers of steps of 0.05 s.
Figure 2: Time Multiplier
Figure 3: Time vs PSM Curve
The figure above is a graph of operation time and the relay’s plug setting multiplier. The horizontal scale represents the PSM and the vertical represents the operating time the relay should run. For PSM of 10, the operation time is given by 3. The relay’s actual time can be obtained through the multiplication of the time read from the graph with the time-setting multiplier.
From the article, we have defined several terms used in describing protective relays below:
