PSM and TMS Settings Calculation of a Relay:

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PSM and TMS settings that is 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 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 how dangerous the fault is and in what time it should be cleared. Changing the position of plug changes the number of turns of the pickup coil.
  • Time Multiplier Setting used to change the value of operation of the relay. If it is more the relay will take more time to operate and vice versa. Changing the position of TMS setting, changes the distance between the contact of the rotating disk and the coil.

Calculation of TMS Setting:

PSM and TMS Settings of Relay Characteristics
Relay Operating Characteristics

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

Actual time of operation of the relay = (Time obtained from PSM- Operating time graph) * TMS

Calculation of PSM Setting:

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

An example relay settings shown in the figure below

PSM and TMS Settings of a relay
Position of Plug on a Relay

Plug position is set at 2.5 times or 250% of rated CT current.

This will give the value of pickup current.

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

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

Relay Plug used for PSM and TMS settings
Relay Plug

Let us consider few examples to understand what exactly PSM is?

Sometimes it is need to replace the old current transformers with new ones for upgrading system. In this case it is need to change the value of 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 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 new one our objective is to respond the relay within same time for either case for 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 even though CT is replaced its Pick up current should not change.

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