When lightning strikes a transmission line the field intensity stressing the insulation may exceed the ionization field intensity level (roughly 30kV/cm) and create an arc from the line to ground. A path now exists for current flow.

The resulting discharge current flow from the lightning stroke is usually over within a few milliseconds but the ionized path has been established and a 60Hz “follow” current flows. This current must be detected and interrupted by deenergizing the line with circuit breakers.

For the ionization path to dissipate, the voltage must be absent for a sufficient duration. The time during which the voltage is absent is commonly called “dead” time.

For transient faults to be successfully cleared, an adequate time for deionization must be afforded. Table 1 shows the minimum time required by voltage level and by probability of successfully reclosing and energizing the line.

Table 1. Minimum De-Ionization Time for Reclosing Breakers

System Voltage                                                         Cycles on 60-Cycle Basis
(line-line kV)                                                       95% probability    75% probability
23                                                                                    4
46                                                                                    5                             3.5
69                                                                                    6                              4
115                                                                                 8.5                            6
138                                                                                 10                            7.5
161                                                                                 13                            10
230                                                                                 18                            14

If sufficient motor load is still connected during the dead time the ionization path can/will be kept intact and a fault reignition will result when the utility breakers reclose. This occurs even though the fault is phase-ground and there is an interposing delta winding between the motor load and the fault.

Tapped motor load holds up the voltage as it decays. At the time of the reclose the voltage is roughly 50% of nominal. Oscillographic data has been obtained in the past showing transmission line voltage being maintained by tapped motor load during reclosing dead time.

Effect on Motors
Unsupervised high-speed reclosing on islanded motors (induction or synchronous machines) before their “residual” voltage has subsided below 25% may subject the motors and other equipment to damage. The motor should not be subjected to a reclose when the phasor difference between the source volts/Hz and the motor residual volts/Hz exceeds 1.33 per unit volts/Hz.

The available literature clearly indicates that reclosing on motor load should be delayed long enough for their residual voltage to decay to acceptable levels (or their contactors drop out) to prevent damage which may be immediate or cumulative. Alternatively, some means to ensure the two voltages are in-phase would be needed.

Damage may include shifting of stator coils, loosening of rotor bars, distortion of coil ends, shaft damage etc. In some cases torsional resonance can be established with resulting torques as high as 20
times normal.

When a motor is disconnected from its power supply it starts to slow down depending on its inertia and the characteristics of its connected load. For an open circuited induction (asynchronous) motor the voltage at its terminals will be a product of its speed, open circuit time constant, and its trapped rotor flux.

For a synchronous machine with field forcing it may take much longer for its voltage to decay. If not open circuited, the motors will experience an electrical interaction with other motors bussed with them as well (an electrical to-and-fro of energy).
From the moment the motors are disconnected from the power system they begin to slip out-of-phase with the power system and their voltage magnitude begins to decay. The voltage impressed across them at reclose will be a function of this internal residual voltage and the power system voltage at time of reclosing. If the two voltages were equal in magnitude and 180° out-of-phase the resulting voltage difference would be 2.0 per unit.

Related post

No comments: