Voltage Surges and Power System Grounding

Comprehensive vocabulary flashcards covering power system voltage surges, lightning stroke mechanisms, types of arresters, and various grounding methods and safety parameters.

Voltage Surge (Transient Voltage)

A sudden rise in voltage for a very short duration on the power system.

Internal Causes of Over-voltages

Causes stemming from within the system, including switching surges, insulation failure, arcing ground, and resonance.

Switching Surges

Over-voltages produced on the power system due to switching operations, such as switching an unloaded line or interrupting a loaded line.

Switching Surges Voltage Calculation (Unloaded Line)

When an unloaded line is connected to a supply voltage $E$ (r.m.s.), the instantaneous voltage it must withstand is $2 \times \text{peak value}$, calculated as $2\times\text{r.m.s.}\times\text{peak factor}$ or $2 \times \text{r.m.s.}\times\text{1.414}$ (2 \times \text{r.m.s.}\times\text{\sqrt{2}}), which simplifies to $2\times1.414\times E$ (2\times\text{\sqrt{2}}\times E).

Current Chopping

The phenomenon where the powerful de-ionising effect of an air-blast circuit breaker causes current to fall abruptly to zero before the natural current zero is reached, producing high transient voltage.

Arcing Ground

The phenomenon of intermittent arc taking place in a line-to-ground fault of a $3\text{-phase}$ system with an insulated neutral, producing severe cumulative transients $3$ to $4$ times the normal voltage.

Resonance

A condition in an electrical system occurring when inductive reactance of the circuit becomes equal to capacitive reactance ($X_{L} = X_{C}$), resulting in high voltages.

Lightning

An electric discharge between cloud and earth, between clouds, or between charge centers of the same cloud, occurring when the dielectric strength of the air is destroyed.

Leader Streamer (Pilot Streamer)

A streamer that starts from the cloud towards the earth carrying a low current ($< 100\text{ A}$) with a velocity of propagation about $0.05$% of the velocity of light.

Stepped Leaders

Luminescent points that travel in jumps towards earth with a velocity exceeding one-sixth that of light, covering distances of about $50\text{ m}$ in one step.

Return Streamer

A streamer that shoots up from the earth to the cloud following the ionized path of the leader streamer, resulting in the actual lightning spark.

Direct Stroke (Stroke A)

A lightning discharge occurring from the cloud directly to the subject equipment, such as an overhead line.

Direct Stroke (Stroke B)

An indirect lightning stroke resulting from a discharge between two clouds that suddenly releases a bound charge on a third cloud, which then discharges rapidly to ground.

Indirect Stroke

Surges resulting from electrostatically induced charges on conductors due to the presence of nearby charged clouds; these form the majority of surges in transmission lines.

Overhead Ground Wires

Wires placed above line conductors and grounded at each tower to intercept lightning strokes and protect transmission lines.

Protective Ratio

The ratio of the induced voltage on a conductor with ground wire protection to the induced voltage that would exist without such protection.

Protective Angle ($\alpha$)

The angle between a vertical line through the ground wire and a slanting line connecting the ground wire and the phase conductor; typically in the region of $20^{\circ}$ to $45^{\circ}$.

Earthing Screen (Shielding)

A network of copper conductors mounted over electrical equipment in a substation and grounded to protect against direct lightning strokes.

Lightning Arrester (Surge Diverter)

A protective device, consisting of a spark gap in series with a non-linear resistor, that conducts high voltage surges to the ground.

Rod Gap Arrester

A simple diverter consisting of two $1.5\text{ cm}$ rods bent at right angles with a gap; it is often used as 'back-up' protection.

Horn Gap Arrester

An arrester consisting of two horn-shaped metal rods where the arc travels upward and lengthens until it is extinguished at a point where the gap is too large to maintain it.

Expulsion Type Arrester (Protector Tube)

An arrester that uses the heat of an arc to vaporize fiber tube walls, creating high-pressure neutral gas that expels ionized air and de-ionizes the arc.

Valve Type Arrester

An arrester using non-linear resistors (e.g., Thyrite or Metrosil) that offer high resistance at normal voltage and very low resistance to high-surge currents.

Surge Absorber

A protective device that reduces the steepness of the wave front of a surge by absorbing its energy.

Ferranti Surge Absorber

A device consisting of an air-cored inductor in series with the line surrounded by an earthed metallic dissipator, functioning like a transformer with a short-circuited secondary.

Equipment Grounding (Safety Grounding)

Connecting non-current-carrying metal parts like enclosures to earth to protect personnel from electric shock during insulation failure.

System Grounding

The process of connecting electrical parts of the power system, such as a neutral point of a star-connected system, to earth.

Effective Grounding (Solid Grounding)

Connecting the neutral point directly to earth through a wire of negligible resistance and reactance; the coefficient of grounding is less than $80$%.

Ungrounded Neutral System

A system where the neutral is isolated from ground; it is prone to arcing grounds and over-voltages during single line-to-ground faults.

Resistance Grounding

Connecting the neutral point of a $3\text{-phase}$ system to earth through a resistor to limit earth fault current.

Resonant Grounding (Petersen Coil Grounding)

Connecting an iron-cored inductor (arc suppression coil) to the neutral to produce a current ($I_{L}$) that exactly balances the system capacitive current ($I_{C}$) during a fault.

Grounding Transformer

A core-type transformer with differentially wound identical windings used to create a neutral point on a $3\text{-phase}$, $3\text{-wire}$ system.

Step Voltage

The voltage between the feet of a person standing on the substation floor, with $0.5\text{ m}$ spacing (one step), during ground fault current flow.

Touch Voltage

The voltage between the fingers of a hand touching a faulted structure and the feet of the person standing on the substation floor.

Peterson Coil Inductance Equation

The value of inductance $L$ for resonant grounding is given by the formula $L = \frac{1}{3\omega^{2}C}$, where $"\omega"$ is the angular frequency ($2\pi f$) and $C$ is the capacitance of each phase to earth.