PSM and TMS Settings Calculation of a Relay:

PSM and TMS settings that are Plug Setting Multiplier and Time Multiplier Setting are the settings of a relay used to specify its tripping limits. To understand this concept easily, it is better to know about the settings of the Electromechanical Relays. If we clear the concept for these relays first then understanding the Numerical Relay settings becomes easy.

PSM and TMS Settings:

• Plug Setting Multiplier actually refers to how dangerous the fault is and at what time it should be cleared. Changing the position of the plug changes the number of turns of the pickup coil.

• Time Multiplier Setting is used to change the value of the operation of the relay. If it is more the relay will take more time to operate and vice versa. Changing the position of the TMS setting changes the distance between the contact of the rotating disk and the coil.

let us see how to calculate these PSM and TMS Settings of a relay.

Calculation of TMS Settings:

In the above figure, the over-current relay time characteristics are shown. By using these we can calculate

The actual time of operation of the relay = (Time obtained from PSM & Operating time graph) * TMS

Calculation of PSM Settings:

From the figure shown below, we can observe that, when the plug position is increasing, the time in seconds is decreasing.

An example relay setting is shown in the figure below

The plug position is set at 2.5 times or 250% of the rated CT current.

This will give the value of the pickup current.

I (Pick UP)= Plug position(PSM) * Rated CT current

PSM = I (Pickup)/ I (rated current)

Let us consider a few examples to understand what exactly PSM is.

Sometimes it needs to replace the old current transformers with new ones for upgrading the system. In this case, it needs to change the value of the Plug Setting Multiplier, and no need to change the TMS.

While changing Old CT with New CT we have to keep in mind that the relay pick-up current should not be changed.

Calculation of PSM Method – 1:

Example-1:

Old CT Ratio- 75/1 A, PSM – 0.75

New CT Ratio- 100/5A

To change the Old Current Transformer with the New Current Transformer the following procedure is taken.

75% of 75A =75 * 0.75

= 56.25A

It means for 100A–56.25%

Therefore the New PSM value is = 0.5625

Example-2:

Old CT Ratio- 75/1 A, PSM – 1.0

New CT Ratio- 100/5A

To change the Old Current Transformer with the New Current Transformer the following procedure is taken.

100% of 100A =75 * 1

= 75A

It means for 100A– 75%

Therefore the New PSM value is = 0.75

Example-3:

Old CT Ratio- 600/1 A, PSM – 1.05

New CT Ratio- 800/5A

To change the Old Current Transformer with the New Current Transformer the following procedure is taken.

105% of 600A =600 * 1.05

= 630A

630A is what percent of 800A

630/800= 0.7875

It means for 800A — 78.75%

Therefore the New PSM value is = 0.7875

Calculation of PSM Method – 2:

However, there is a simple formula to find out the value of a New PSM

Here New PSM = (Old CT Primary Current * Old PSM) / New CT Primary Current

By using this formula

For the above Example 1

PSM = 75 * 0.75 /100 =0.5625

For the above Example 2

PSM = 75 * 1 /100 =0.75

For the above Example 3

PSM = 600 * 1.05 /800 =0.7875

Calculation of Relay Pick up current:

While changing CT with a new one our objective is to respond to the relay within the same time for either case for the same value of fault current.

Case-1 for Old CT:

Old CT Ratio- 600/1 A, PSM – 1.05

Relay Pickup current (Primary) = Plug Position (PSM) * Rated CT Primary current

Relay pick up current Primary side = 1.05 * 600 = 630A

Case-2 for New CT:

New CT Ratio- 800/5A

We have calculated New PSM =0.7875

Relay pick up current Primary side = 0.7875 * 800 = 630A

Hence we can understand from the above discussion that even though CT is replaced its Pick up current should not change.