Power systems must work without interruption even in an event of a disturbance.
Power distribution network consists of the following components:
• HV/MV power transformer(s) (secondary side),
• MV substation and switchgear,
• MV power cables (including termination),
• MV/LV power transformer(s) (primary side).
Surge Arrestors - Surge arrestors are used to connect overhead lines with generators and transformers to reduce maximum surge voltage.
They are placed near transformer terminals. The three types of surge arrestors in use are:
• rod spark gapped,
• multiple gapped arrestors,
• zinc (metal) oxide surge arrestors.
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Surge Capacitors - Surge capacitors are connected to generator terminals, used to stop the sharp transients, clipping the voltage at safe level.
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MV Switchgear Disconnector/Isolator - The device is a switch that is designed to disconnect the circuit under normal operating conditions. 'Off-load' disconnector opens the circuit when no current is flowing. 'On-load' disconnector can open the circuit when normal current is flowing.
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Circuit Beaker (CB) - A switch that opens the electric circuit under over current or fault conditions. When a high current is received a latch is pulled by a magnetic field generated by the tripping coil. The spring is released and contacts separate. The total tripping time is the sum of opening time and arcing time. Arcing time is the time it takes to for contacts to stop conducting current after they separate.
When fault occurs the current rises and an oscillation arises at the CB terminals. The oscillation occurs due to reactive components of power system impedances. The oscillation period is proportional to L/R ratio. When contacts open transient recovery voltage (TRV) flows across the CB terminals. MV CB life is only about 100 short circuit operations due high electrical and mechanical loads. CB lifetimes are usually limited to 20,000 load break operations and 100,000 mechanical operations.
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Contactors - Contactors are 'switchgear' or 'control gear' devices used for control operations such as starting and stopping of motors, switching capacitors banks between on- and off-line modes for power factor correction purposes. Vacuum contactors can sustain loads of 400 A per vacuum bottle at 1.25-13.8 kV. Those devices are made for frequent control operations, not to interrupt load current. They can stop overload currents of up to 120-140 % but not short circuit or fault currents. It is good engineering practice to use contractors with other load protection devices (eg. relay-CB combination or fuses) to prevent fault currents. Contactors are usually used for normal load switching and isolation applications, eliminating the need for CBs. Thus fuses are used in conjunction with contactors.
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Fuses - Fuses is the oldest and cheapest high protection component. The high temperature melts the fuse element that is used to conduct electricity. Once the element melts it breaks and the current stops flowing. The time it takes for the fuse to blow is shown on the time-current specification curve. The curve is exponential. Fuses blow only if input current exceeds the specified limit. Thus fuses are used for less critical circuits. Typical applications include remote Ring Main Units (RMU).
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Is Limiter
Is limiter interrupts high current very quickly. The device is consist of four main components:
1. Current transformer - detects short-circuit current
2. Measuring and tripping device - measures the current and outputs the triggering energy
3. Pulse transformer - converts tripping pulse to busbar potential
4. Insert holder with insert - conducts operating current and limits the short circuit current
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The Is limiter stops currents of 210 kA for 11 kV in 1 ms. It switches off early in the sinusoidal AC cycle.