Power System Security

Author: Vaishnav Chathayil is pursuing his B.Tech. in Electrical and Electronics engineering at National Institute of Technology, Calicut.

The sole objective of any power system network is to transmit power within its acceptable limit. The network should also work or stay under its limits event under any contingency. Contingencies may be of any type:

  1. Generator outage
  2. Line outage

Thus, irrespective of the issue evolved in a network, the power transfer between generating units and its loads must be “secure”.
The following article deals with what power system security means and how a power system network is classified into various states of operation.

What does Power System Security mean?

Power System Security may be defined as: The ability of a power system network to withstand contingencies(changes) and remain in its secure state or operate within its acceptable limits.

Various parameters may be taken into consideration and each have their own constraints. Violation of any such constraints may deviate the network’s secure operation. The constraints to be met for a secure operation are:

Inequality Constraints:
Pgimin ≤ Pgi ≤ Pgimax
Vimin ≤ Vi ≤ Vimax
Qgimin ≤ Qgi ≤ Qgimax
fmin ≤ f ≤ fmax
Pgi - real power generated from ith unit
Vi - voltage magnitude
Qgi - reactive power generated from ith unit
f - frequency

Equality Constraints:

Total Generated Power from all the units = Power demand + Power loss

If these constraints are met irrespective of any disturbance or change in the system, then the network is in secure operation.

Functions of Power System Security

The two functions that are taken care of under power system security are:
  1. Security Control: Make sure all the parameters are within their limits.
  2. Security Assessment: Detects the change in the parameters and identifies in which state the system is operating in.

System State Classification

Dyliacco’s classification:

In 1968, Dyliacco was the first one to introduce the classification of states in power system security. The operating states were classified into:
  1. Preventive State: this state basically highlights the secure operation of the system. It states that the system is working under its parameter limits and is also capable of withstanding the contingencies that occurs. Thus the operator on analyzing the situation should take preventive measures in advance and let the system not deviate from its state even during the contingency.
  2. Emergency State: This state indicates that the system constraints has been violated, I.e., it is not operating in its limits.
  3. Restorative State: In this state power transfer does not take in some parts due to outage I.e., contingency occurs in some parts of the system. Thus necessary action is to be taken to deviate it back to the normal state.

Dyliacco’s classification Power System Security

L.H. Fink and K Carlsen’s classification:

In 1978, the next type of state classification was suggested by Fink and Carlsen. In their classification the operating states are of 5 types:
  1. Preventive
  2. Alert
  3. Emergency
  4. Extreme
  5. Restorative

The following state transition diagram can be used to explain the flow that take place.

Normal00The system is secure and all constraints are met
Alert0 0 When the constraints are on the verge of violation,I.e., the operator is notified to foresee contingency
Emergency0 1 When inequality limits are violated
Extreme1 1 When both equality and inequality limits are violated
Restorative1 0 The inequality limit violation has been sorted but the load demand is yet to be satisfied.

*EC= 0 (equality constraint is met)
IC=0 (when inequality constraint is met)
Each becomes 1 when both are not met

In the extreme state it is the situation where blackouts occur and restoring it back to normal state may take hours or even days. Thus maximum the operator tries to deviate it from that state.

L.H. Fink and K Carlsen’s classification Power System Security