Large interconnected electrical networks require protection and monitoring of the electrical energy transmission networks. Electrical energy supply systems have three parts:
• Generating units, i. e. the power plants
• High voltage transmission networks moving large quantities of energy to distant consumers and maintaining synchronisation of the power system
• Medium voltage and low voltage distribution networks, supplying the customers.
Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy. The three classes of protective devices are:
• Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network
• Automatic operation, such as auto-reclosing or system restart
• Monitoring equipment which collects data on the system for post event analysis
While the operating quality of these devices, and especially of the protective relays, is always critical, different strategies are considered for protecting the different parts of the system. Very important equipment may have completely redundant and independent protective systems, while a minor branch distribution line may have very simple low-cost protection.
Protection of the generator sets
In a power plant, the protective relays are intended to prevent damage to alternators or of the transformers in case of abnormal conditions of operation, due to internal failures, as well as insulating failures or regulation malfunctions. Such failures are unusual, so the protective relay have to operate very rarely.
If a protective relay fails to detect a fault, the damage to the alternator or to the transformer may have important financial consequences for the repair or replacement of equipment and the value of the energy that otherwise would have been sold.
Protection of the high voltage transmission network
Protection on the transmission and distribution serves two functions: Protection of plant and protection of the public (including employees)
At a basic level protection looks to remove items of plant from service which experience an overload or a connection to earth. Some items in substations such as transformers may require additional protection based on temperature or gassing among others.
Overload protection
Overload protection requires a current transformer which simply measures the current in a circuit. If this current exceeds a pre-determined level a circuit breaker or fuse should operate.
Earth fault protection
Earth fault protection again requires current transformers and senses an imbalance in a three phase circuit. Normally a three phase circuit is in balance, so if a single (or multiple) phases are connected to earth an imbalance in current is detected. If this imbalance exceeds a pre-determined value a circuit breaker should operate.
Distance Protection
Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If this voltage falls below a pre-determined level and the current is above a certain level the circuit breaker should operate. This is useful on long lines where if a fault was experienced at the end of the line the impedence of the line itself may inhibit the rise in current. Since a voltage sag is required to trigger the protection the current level can actually be set below the normal load on the line.
Back Up Protection
At all times the objective of protection is too remove only the affected portion of plant and nothing else. Sometimes this does not occur for various reasons which acn include:
Mechanical failure of a circuit breaker to operate
Incorrect protection setting
Relay failures
A failure of primary protection will usually result in the operation of back-up protection which will generally remove both the affected and unaffected items of plant to remove the fault.
Protection of the low voltage networks
The low voltage network generally relies upon fuses or low voltage circuit breakers to remove both overload and earth faults.
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