Programmable Scheme Logic or PSL is a kind of feature provided in Numerical Relays to implement the protection scheme of a particular type. This feature of Numerical Relays makes it easier to implement many protection schemes in a single Numerical Relay for example, in Distance Relay we can configure Distance protection, over voltage protection, Over Current Protection, Earth Fault Protection etc.

Now we will study about PSL. PSL is a logical block which is made from different but suitable DDB. Here DDB stand for Digital Data Bus. There are many DDBs offered in a Numerical Relay. Each DDB perform a unique function. Thus it is very important to have the knowledge of function of DDBs to implement a particular logic. Hope you got some idea of DDB but don’t worry I will go in detail with example to make it crystal clear.

Lets us begin with an example. Let us assume that we have an Alstom Relay P442 and we want to implement a protection feature called Local Breaker Back-up (LBB) Protection in the Relay. So we need to finalize our logic for the operation of LBB. The generalized logic for LBB protection is

Lock-out Relay Operated AND Current still existing

Under the above logic the LBB Relay shall initiate its timer and shall give the tripping command to isolate the fault after a fixed time delay say 200 ms. Assuming the above logic for LBB, we will design a logic using PSL in the Numerical Relay. But before designing the PSL, we need to give the input to the Relay, in our case there are two inputs, one will be the contact of Lock-out Relay and another Current Transformer (CT) input. So our first step will be to assign the inputs to the Relay and label a name to each input. In the second step, we need to configure the output of Relay and label a name to the Relay Output contact. Let,

Lock-out Relay contact Input is labeled as INPUT1. Mind that only digital inputs can be labeled. So we can not assign a name to CT input.

Likewise let the Relay output contact to trip Breakers to isolate fault be RL1.

Thus as per our logic when INPUT1 is high and over current still exists then RL1 shall get high after 200 ms to isolate the fault.


Carefully observe the figure above. We have used digital inputs and Over-current DDB I>1 (1stStage Over-Current) in an AND gate to start the timer and if the input status do not change for 200 ms then RL1 will change its status from low to high but in the time window of 200 ms, if the input status changes then the timer will reset and RL1 will remain low.

Therefore, making a PSL in a Numerical Relay is just a logic building while having knowledge of function of each DDB to be used. This is just a brief of PSL to have some idea of PSL used in Numerical Relay. Hope you enjoyed this post.

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