Q3Power System - I

(a) Causes of Over-Voltages in a Power System

Over-voltages in a power system can be classified into two broad categories based on their origin: External (lightning) over-voltages, caused by direct or nearby lightning strikes on transmission lines or towers, inducing extremely steep-fronted, high-magnitude transient voltage surges that propagate along the line; these are essentially independent of the system's own operating voltage and depend on the lightning stroke's current magnitude and the line/tower surge impedance.

Internal (system-generated) over-voltages, arising from the power system's own operation, include: switching surges, caused by sudden switching operations such as line energization/de-energization, capacitor or reactor bank switching, and fault clearing, which excite the system's natural oscillatory (resonant) response; overvoltages due to sudden load rejection, where a generator supplying a large load suddenly loses that load (e.g., due to a downstream breaker opening), causing the generator's terminal voltage to rise sharply until governor/AVR action can restore normal conditions; the Ferranti effect, where a lightly loaded or open-circuited long EHV line's shunt capacitance causes the receiving-end voltage to rise above the sending-end voltage; resonance and ferroresonance conditions, where the system's inherent inductance and capacitance combine under certain switching or fault conditions to produce sustained, self-reinforcing high-magnitude oscillatory overvoltage, particularly problematic in systems with unloaded transformers or long cables; and arcing ground faults on ungrounded or high-impedance-grounded systems, where a repeatedly re-striking arc at the fault point can produce escalating transient overvoltages on the healthy phases through repeated charge-transfer and re-ignition cycles.

(b) Devices for Over-Voltage Protection

Surge (lightning) arresters: the primary and most widely used overvoltage protection device, connected between the phase conductor and ground at critical points (substation entries, transformer terminals); modern metal-oxide (zinc oxide) arresters exhibit a highly non-linear voltage-current characteristic — presenting extremely high impedance (negligible leakage current) at normal system voltage, but abruptly switching to very low impedance once the voltage exceeds a threshold, diverting the surge current safely to ground and clamping the voltage across the protected equipment to a safe residual level, then automatically returning to their high-impedance state once the surge has passed, without requiring any external switching action.

Ground (earth/shield) wires: one or two grounded conductors strung above the phase conductors along the top of transmission towers, intercepting most direct lightning strokes before they can strike a phase conductor directly, and providing a low-impedance path to conduct the lightning current safely to ground via the tower and its footing resistance, though a low tower footing resistance is essential to avoid dangerous 'backflash' (where high potential rise at the tower top during a shield-wire strike can flash over the insulator string back onto the phase conductor).

Spark gaps (rod gaps): simple air-gap protective devices connected in parallel with equipment insulation, designed to spark over (break down) at a voltage safely below the equipment's insulation withstand level, diverting the surge current momentarily to ground; simpler and cheaper than modern surge arresters but with the disadvantage of a relatively wide/inconsistent sparkover voltage tolerance, a follow-on power-frequency arc that must be separately cleared (unlike a self-restoring arrester), and generally coarser overvoltage protection performance.

Neutral grounding: while not a surge-diversion device in the same sense as the above, appropriate neutral grounding (solid, resistance, or reactance grounding as discussed elsewhere) fundamentally limits the magnitude of overvoltage that healthy phases experience during ground faults, forming an essential complementary part of the overall system overvoltage-control strategy alongside dedicated surge-protection devices.