Electrical Equipment and Devices

WEEK 16: POWER SYSTEM PROTECTIVE DEVICES

Sections: Protection Systems | Circuit Breakers | Example Problems

Definition. Protection Systems are apparatus or devices that removes short circuit process automatically from power systems.

Undesireable Effects of Short Circuits.
1. Reduced stability margin of power systems
2. Equipment damage -- heavy currents, unbalanced circuits, low voltages
3. Hazard to personnel -- explosion, personnel injury
4. Disruption of the Ppower system service area.

Zone of Protection is a portion of a power system such that for a fault anywhere within that zone, the power system responsible for that zone takes action to isolate everything within that zone from the rest.

Rules of Zones of Protection
1. Each zone consists of one or more power systems components
2. Each breaker is included in two neighboring zones of protection, which defines the boundaries of the the zone of protection.
3. Neighboring zones always overlap. If a fault should occur within the overlapped region, a much larger portion of the power system will be isolated and lost for service, thus overlap should be as small as possible.

Sections: Protection Systems | Circuit Breakers | Example Problems

Circuit Breakers

Definition. Circuit breakers are mechanical switching devices, capable of making, carrying and breaking currents under normal and abnormal circuit conditions.

Rating. Circuit breakers are rated by:
a. Voltage, usually nominal
b. Insulation level
c. Current - the root-mean square [rms] value existing during the first half-cycle of arc between contacts during the opening stroke.
d. Interrupting Capability - in KVA, is the product of the phase-to-ground voltage, kV, of the circuit and the interupting ability, in amperes, at stated intervals and for a specific number of operations
e. Transient recovery voltage
f. Interrupting time
g. Trip delay
h. Short-circuit duty - the maximum short-circuit current that the rotating machine, connected to a system at the time of the short-circuit, can pass through the breaker to a point just beyond the breaker, at an instant the brekaer contacts open.
i. Voltage range factor, k - range of voltage over which rated SCC times operating voltage is constant. For breakers greater than 115 kV, k is equal to 1.0.

Classification [by Media]
1. Air CB - uses the good insulating and arc-quenching property of dry and clean compresed air.

Types.
a. Air-Blast CB - air pressure ranges from 300 to 400 PSI. The continuous current carrying capacity is about 42 kA or higher operating at a nominal rating of 38kV.
b. Magnetic Air CB - uses a combination of strong magnetic-field (coil or soft-iron plates) with a special arc chute to lengthen the arc until the system voltage cannot maintain the arc circuit any longer. Nominal voltages ranges from less than to equal to 15 KV.

2. Gas CB - uses either sulfur hexaflouride, SF6, a very stable compound, inert up to 500 degrees Centigrade, non-flammable, non-toxic, odorless and colorless. Nominal voltages: 800 kV, continuous current up to 400 amperes, symmetrical interrupting ratinf up to 63 kA.

Types:
a. Indoor Design - metal-clad switchgear (dead-tank design) with a service pressure ranging from 20 PSI to 250 PSI.
b. Self-Blast Design - arc energy heats the gas, resulting in a pressure rise in the static volume. This pressure then quenches the arc at an ensuing current. Available on 169 kV, 242kV up to 362 kV ratings.

3. Oil CB - uses either a circuit breaker oil, similar to that of transformer oil Inerteen. Operates at over 362 kV. The oil characteristics have a flash point equal to 133 deg. C, burning point at 148 deg. C, freezing point of -40 deg. C, and a viscocity of 57.

Types:
a. Dead Tank - steel tank partially filled with oil with covers carrying porcelain or compensating bushings.
b. Live Tank - capable of up to 69kV only.

4. Vacuum CB - contacts are mounted on an insualting envelope from which virtually all air has been evacuated. Continuous current rating up to 3000A at 38 kV.