Power system protection is a vital process that ensures the safety of electrical energy production, transmission, and consumption by mitigating the
impact of faults. This is achieved by disconnecting the faulty parts from the rest of the electrical network.
What is a Power System?
An electric power system consists of several components interconnected to form a large, complex system that generates, transmits and distributes
electrical energy over wide areas. Electric Power System is also called Electrical Grid (or Power Grid).
The image below depicts a part of a power system from the generation location to the various places of use of electricity. The power station generates
electricity at a high voltage of any of the following voltages: 10.5 kV, 11.0 kV, 15 kV or 16 kV. As power stations are usually far from the load centers,
the voltage generated in the range mentioned above is first stepped up to higher voltage to minimize losses arising from conveying electricity through the
long distance between place of generation and load centers.
A portion of the power system
In Nigeria, for example, generation voltage is stepped up to 330 kV through a step-up power transformer and transmitted through a long distance of over
100 km from where the voltage is gradually stepped down to 132 kV, then to 33 kV, 11 kV and finally to 0.415 kV residential voltage. Along the way,
industrial loads are supplied at 132 kV, 33 kV or 11 kV as may be required.
Electricity Generation Resources
Electrical energy is generated at the resource’s locations. The resources used for electricity generation may be fossil fuel, renewable
(hydro, wind, solar, biomass); transmitted to the various load centers and then distributed to the several customers.
Example of Electricity generation Resources
Power System Protection
Power system protection is a vital process that ensures the safety of electrical energy production, transmission, and consumption by mitigating the
impact of faults. This is achieved by disconnecting the faulty parts from the rest of the electrical network.
A single line diagram of a typical, simple power system is shown on the diagram labelled “Power System Protection”. The transmission line CD is protected
on both sides of the line by the circuit breakers 5 and 6. There was an earth fault indicated in red colour between stations C and D. Both circuit
breakers 5 and 6 tripped to isolate the faulty line from the rest of the power system. Hence, the remaining portion of the power system remained stable.
Power System Protection Single Line Diagram
Protection Goal
The primary goal of system protection is to quickly isolate the problem area in the power system, ensuring that the rest of
the system can continue to operate as much as possible. The term 'protection' does not imply that the equipment can prevent issues such as faults, equipment
failures, or electric shocks from accidental human contact.
However, possibility of faults can be minimised ahead through regular electrical equipment maintenance, repairs, replacement of faulty components before
causing problems, environmental hygiene and such.
The diagram below summarises the goal of Power System protection.
Power System Protection Goal
Protective Relay in Power System
Protective relays are protective devices that only act once an abnormal or intolerable condition has occurred. They isolate the faulty parts so that
the rest of the system continues to receive power and prevents further damage from the fault (s). Hence, protection is not about prevention, but rather
about minimizing the duration of the fault and limiting the resulting damage, outage time, and related problems.
To maximize the return on the significant investment in the equipment that comprises the power system and to ensure users remain satisfied with reliable service, it is crucial to maintain continuous operation of the entire system, avoiding major breakdowns.
Protection devices play a crucial role in enhancing system reliability and preventing widespread disruptions. A protection device serves three primary
purposes:
a. Safeguarding the entire system to ensure continuous supply
b. Minimizing damage and repair costs upon detecting faults
c. Ensuring the safety of personnel
These functions are essential for the early detection and localization of faults, as well as the swift removal of faulty equipment from service.
Typical Protective Relays
Qualities of Protection Scheme
The following qualities are necessary for a protection scheme:
1.
Selectivity:To detect and isolate the faulty item only.
2.
Speed: To operate fast when it is called upon to do so, thereby
to minimize damage and ensure personnel safety.
3.
Sensitivity: To detect the smallest fault current or system abnormalities
and operate correctly at its setting before the fault causes irreparable damage.
4.
Simplicity: Use minimal protective equipment and circuitry to achieve protection objectives.
5.
Economics: Provide maximum protection at the lowest total cost.
6.
Stability: It is the quality of any protection system to remain stable within a set of defined operating scenarios and procedures.
7.
Reliability: Assurance that the protection will perform correctly.
To be reliable means that it must be:
i. Dependable: It must trip when called upon to do so.
ii. Secure: It must not trip when it is not supposed to.
