Electrical Power Distribution and Control
Electrical power is controlled and distributed by assemblies. Each assembly-main distribution assembly, power controller assembly, load controller assembly and motor controller assembly-is an electrical equipment container or box.
Circuit breakers
A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either manually or automatically) to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect an individual household appliance up to large switchgear designed to protect high voltage circuits feeding an entire city.
Low voltage circuit breakers for control supply
Low voltage (less than 1000 VAC) types are common in domestic, commercial and industrial application, include:
• MCB (Miniature Circuit Breaker)—rated current not more than 100 A. Trip characteristics normally not adjustable. Thermal or thermal-magnetic operation. Breakers illustrated above are in this category.
• MCCB (Molded Case Circuit Breaker)—rated current up to 2500 A. Thermal or thermal-magnetic operation. Trip current may be adjustable in larger ratings.
• Low voltage power circuit breakers can be mounted in multi-tiers in LV
Medium-voltage circuit breakers for Generator output
Front panel of a 1250 A air circuit breaker
manufactured by ABB. This low voltage power circuit
breaker can be withdrawn from its housing for
servicing. Trip characteristics are configurable via DIP
switches on the front panel.
manufactured by ABB. This low voltage power circuit
breaker can be withdrawn from its housing for
servicing. Trip characteristics are configurable via DIP
switches on the front panel.
Medium-voltage circuit breakers rated between 1 and 72 kV may be assembled into metal-enclosed switchgear line ups for indoor use, or may be individual components installed outdoors in a substation. Air-break circuit breakers replaced oil-filled units for indoor applications, but are now themselves being replaced by vacuum circuit breakers (up to about 35 kV). Like the high voltage circuit breakers described below, these are also operated by current sensing protective relays operated through current transformers. The characteristics of MV breakers are given by international standards such as IEC 62271. Medium-voltage circuit breakers nearly always use separate current sensors and protection relays, instead of relying on built-in thermal or magnetic overcurrent sensors.
Medium-voltage circuit breakers can be classified by the medium used to extinguish the arc:
• Vacuum circuit breaker—with rated current up to 3000 A, these breakers interrupts the current by creating and extinguishing the arc in a vacuum container. These are generally applied for voltages up to about 35,000 V, which corresponds roughly to the medium-voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.
• Air circuit breaker—rated current up to 10,000 A. Trip characteristics are often fully adjustable including configurable trip thresholds and delays. Usually electronically controlled, though some models are microprocessor controlled via an integral electronic trip unit. Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.
• SF6 circuit breakers extinguish the arc in a chamber filled with sulfur hexafluoride gas. Medium-voltage circuit breakers may be connected into the circuit by bolted connections to bus bars or wires, especially in outdoor switchyards. Medium-voltage circuit breakers in switchgear line-ups are often built with draw-out construction, allowing the breaker to be removed without disturbing the power circuit connections, using a motor-operated or hand-cranked mechanism to separate the breaker from its enclosure.
Control Relays
AC coil control relay
Relays are used in control panels to control a circuit by a low-power signal, or where several circuits must be controlled by one signal. A type of relay that can handle the high power required to directly drive an electric motor is called a contactor.
Latching relay & Reed relays
Types of control relays used in a power plant.
1. Latching Relay
2. Reed Relay
3. Contactor Relay
4. Solid-State Relay
5. Solid State
6. Mercury-watted relay
7. Polarized relay
Protection Relays
Protection relays used to protect equipments (Alternator, Transformer, and Engine) and power supply related issues.
Types of protection relays.
1. Over current relay
2. Earth fault relay
3. Reverse power relay
4. Under voltage and over voltage relay
5. Multifunctional relay
6. Differential relay
7. Over frequency and under frequency relay
8. Buchholz relay
No comments:
Post a Comment