Power (Quiz 4)

single line-to-ground fault

(SLG: backwards E) - one phase shorted to ground

kAIC

(kA-interrupting capacity)

A matrix

1 1 1 1 a^2 a 1 a a^2

State the ratings that are typically important for selecting circuit breakers

Circuit Breakers Voltage ratings Rated maximum voltage Rated low frequency withstand voltage Rated impulse withstand voltage Rated voltage range factor K Current ratings Rated continuous current Rated short-circuit current Rated momentary current Rated interrupting time

three types asymmetrical fault

line-to-line (short between lines(ionization, physical contact, broken insulation) line-to-ground(short to ground(lightning, physical contact) double line-to-ground(two lines touch ground and each other(storms).

flashover

near simultaneous ignition caused when an arc occurs

symmetrical fault

or a balanced fault that affects each of the three phases equally. 5% are symmetric.

transient period (quantity)

second period and has less than above

State the two types of open circuit faults that are possible on a three-phase system

single line open fault double line open fault

State the three types of unbalanced short-circuit faults on a three-phase system

single line-to-ground fault line-to-line fault double line-to-ground fault

self-impedance

the "thevinen" resistance of the system; the diagronal elements of the bus impedance matrix

Interrupting capability

the amount of kA of current that a switching device can open without catastrophic damage.

withstand capability

the amount of kA that equipment can be exposed to without catastrophic damage

dc offset current

the dc part of the fault current that starts at an initial magnitude based on the time that the fault occurs in conjunction with the voltage waveform. The dc part has a exponential decay from the highest point where the ac part will travel on.

mutual impedance

the ratio of the potential difference between either of two pairs of terminals to the current applied at the other pair of terminals when the circuit is open; the off-diagonal elements of the bus impedance matrix

time constant

the response time based on L/R

equal-area criterion

this method is applicable for one machine connected to an infinite bus or for two machines

double line-to-ground fault

two phases shorted to ground (LLG: parallel all 3)

line-to-line fault

when two phases are shorted together (LL: parallel)

List the assumptions that are used in representing systems using symmetrical component models

1. The power system operates under balanced steady-state conditions before the fault occurs. Thus the zero-, positive-, and negative sequence networks are uncoupled before the fault occurs. During unsymmetrical faults they are interconnected only at the fault location. 2. Prefault load current is neglected. Because of this, the positive sequence internal voltages of all machines are equal to the prefault voltage VF. Therefore, the prefault voltage at each bus in the positive-sequence network equals VF. 3. Transformer winding resistances and shunt admittances are neglected. 4. Transmission-line series resistances and shunt admittances are neglected. 5. Synchronous machine armature resistance, saliency, and saturation are neglected. 6. All nonrotating impedance loads are neglected. 7. Induction motors are either neglected (especially for motors rated 50 hp or less) or represented in the same manner as synchronous machines.

List the assumptions used in three-phase short circuit analyses

1. Transformers are represented by their leakage reactances. Winding resistances, shunt admittances, and D-Y phase shifts are neglected. 2. Transmission lines are represented by their equivalent series reactances. Series resistances and shunt admittances are neglected. 3. Synchronous machines are represented by constant-voltage sources behind subtransient reactances. Armature resistance, saliency, and saturation are neglected. 4. All nonrotating impedance loads are neglected. 5. Induction motors are either neglected (especially for small motors rated less than 50 hp) or represented in the same manner as synchronous machines.

asymmetrical fault

An unbalanced fault that affects the three phases differently; asymmetrical fault current has an ac and a transient dc component

Describe the effects of delta-wye transformer phase shifts on the magnitude and phase angles of currents on both the primary and the secondary

D-Y transformer phase shifts have no effect on the fault currents and no effect on the contribution to the fault currents on the fault side of the D-Y transformers. However, on the other side of the D-Y transformers, the positive- and negative-sequence components of the contributions to the fault currents are shifted by +/- 30, which affects both the magnitude as well as the angle of the phase components of these fault contributions for unsymmetrical faults.

Discuss design methods used to improve transient stability

Improved Steady-state stability => higher system voltage levels => additional transmission lines => smaller transformer leakage reactances => series capacitive transmission-line compensation => static var compensator and flexible ac transmission systems (FACTS) High-speed fault clearing high-speed reclosure of circuit breakers single-pole switching larger machine inertia, lower transient reactance fast responding, high-gain exciters fast valving braking resistors

Explain the behavior of zero sequence current and draw the zero sequence impedance circuit models for power systems

The zero sequence current changes based on the circuit setup. The setups were discussed in class and diagrams for short Xlines, and 2 / 3 winding Xfmrs should be in notes. Zero sequence circuit models: see section 8E

subtransient period (quantity)

first period and has highest current

damping torque

happens anytime the generator deviates from is synchronous speed

State the ratings that are typically important for selecting fuses

Voltage rating Continuous current rating Interrupting current rating Time response

sequence impedance matrix

Zs = A^-1*Z*A

permanent fault

a fault that does not disappear when power is disconnected.

transient fault

a fault that is no longer present if power is disconnected for a short time and then restored.

fuse

a one time use circuit breaker that uses a wire of specific gauge that is designed to break at a specific temperature based on current.

circuit breaker

a piece of switchgear that is able to break a circuit that experiences a higher than normal current; is a mechanical switch capable of interrupting fault currents and of reclosing.

steady-state period (quantity)

after all transients, small or normal current returns

arc

an electrical breakdown of a gas that produces an ongoing plasma discharge, resulting from a current through normally non-conductive media

single line open fault

break in one line

double line open fault

break in two lines

normalized inertia constant

called the H constant; stored kinetic energy at synchronous speed/generator voltampere rating

zero-sequence components

consist of three phasors with equal magnitudes and with zero phase displacement

positive-sequence components

consist of three phasors with equal magnitues, + or -120 degree phase displacement, and positive sequence

negative-sequence components

consits of three phasors with equal magnitudes, + or - 120 degrees, and a negative sequence

dc offset current

decays exponentially with time constant T = L/R