This manual will give plant operators, electricians, field technicians and engineers a better appreciation of the role played by power system protection systems. An understanding of power systems along with correct management will increase your plant efficiency and performance as well as increasing safety for all concerned.
The manual is designed to provide excellent understanding on both a theoretical and practical level. Starting at a basic level and then moving onto more detailed applications, it features an introduction covering the need for protection, fault types and their effects, simple calculations of short circuit currents and system earthing.
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Need for protection
1 Need for protection
1.1 Need for protective apparatus
A power system must be not only capable of meeting the present load but also requires the flexibility to meet the future demand. A power system is designed to generate electric power in sufficient quantity, to meet the present and estimated future demands of the users in a particular area, to transmit it to the areas where it will be used and then distribute it within that area, on a continuous basis.
To ensure the maximum return on the significant investment in the equipment, which goes to make up the power system, and to keep the users satisfied with reliable service, the whole system must be kept in operation continuously without major breakdowns.
This can be achieved in two ways:
A protective relay is the device, which gives instruction to disconnect a faulty part of the system. This action ensures that the remaining system is still fed with power, and protects the system from further damage due to the fault.
Hence, use of protective apparatus is very necessary in the electrical systems, which are expected to generate, transmit and distribute power with least interruptions and restoration time.
1.2 Basic requirements of protection
A protection system has three main functions/duties:
These requirements are necessary, firstly for early detection and localization of faults and secondly, prompt removal of faulty equipment from service.
In order to carry out the above duties, protection must have the following qualities:
To meet all of the above requirements, protection must be reliable which means it must be:
1.3 Basic components of protection
The protection of any distribution system is a function of many elements and this section gives a brief outline of the various components that go into protecting a system. The following are the main components of a protection system.
The above items are extensively used in any protective system and their design requires careful study and selection for proper operation.
Power system protection-main functions
1. To safeguard the entire system to maintain continuity of supply.
2. To minimize damage and repair costs.
3. To ensure safety of personnel.
Power system protection-basic requirements
1. Selectivity: To detect and isolate the faulty item only.
2. Stability: To leave all healthy circuits intact to ensure continuity of supply.
3. Speed: To operate as fast as possible when called upon, to minimize damage, production downtime and ensure safety to personnel.
4. Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly at its setting.
Power system protection-speed is vital!!
The protective system should act fast to isolate faulty sections to prevent:
· Increased damage at fault location. Fault energy = I2 ´ Rf ´ t, where t is time in seconds.
· Danger to the operating personnel (flashes due to high fault energy sustaining for a long time).
· Danger of igniting combustible gas in hazardous areas, such as methane in coal mines which could cause horrendous disaster.
· Increased probability of ground faults spreading to healthy phases.
· Higher mechanical and thermal stressing of all items of plant carrying the fault current, particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electro-mechanical forces caused by multiphase faults proportional to the square of the fault current.
· Sustained voltage dips resulting in motor (and generator), instability leading to extensive shutdown at the plant concerned and possibly other nearby plants connected to the system.
Power system protection-qualities
1) Dependability: It MUST trip when called upon.
2) Security: It must NOT trip when not supposed to.
Power system protection-basic components
1. Voltage transformers and current transformers–To monitor and give accurate feedback about the healthiness of a system.
2. Relays–To convert the signals from the monitoring devices, and give instructions to open a circuit under faulty conditions or to give alarms when the equipment being protected, is approaching towards possible destruction.
3. Fuses–Self-destructing to save the downstream equipment being protected.
4. Circuit breakers–These are used to make circuits carrying enormous currents, and also to break the circuit carrying the fault currents for a few cycles based on feedback from the relays.
5. DC batteries–These give uninterrupted power source to the relays and breakers that is independent of the main power source being protected.