Overvoltage Protection – Objective, Methods & Equipment

The purpose of the protective equipment in a power system is to isolate the faulty section from the healthy system by initiating tripping for appropriate circuit breakers. This whole process must be carried out with minimum of delay and disturbance. Overvoltage, switching and other phenomena like lightning produce overvoltages on transmission and distribution systems. Therefore precautions against consequential damage and system outage must be taken by: preventing overvoltages from being impressed on the system and protecting the vulnerable apparatus from voltage surges.

Protective Techniques & Equipment used against Overvoltage

Protection against Lightning

These preventive measures only apply to overhead lines since underground cables are virtually immune from direct lightning strikes.

Earth Wire

Shielding the line conductors by an earth wire is quite effective in preventing a direct stroke to the conductors provided that the conductors lie within a segment subtending angle of about [45° or preferably 25° for towers above 50 m high] from the earth wire to the ground. However, this type of protection may sometimes fail; therefore in cases of certain importance such those near a major substation or where lightning is particularly predominant, two earth wires may be installed.

Earthing Screen

The earthing screen provides protection against direct lightning strikes by providing earthing screen. This consists of a network of copper conductors (referred to as shield or screen) mounted all over the electrical apparatus in power stations and substations. This shield is appropriately connected to earth on at least two points through low impedance.

On a direct lightning strike on the station, the screen provides a low resistance path by which lightning surges are conducted to ground. In this manner, equipment in the station is protected against damage. Nonetheless, the shortcoming of this technique is that it does not provide protection against travelling waves which may get to the power station apparatus.

Tower Footing Resistance

A lightning stroke to earth wire can produce a back-flashover to the line conductors resulting in a line  surge voltage unless the tower footing resistance is very, for example, not greater than 1 Ω per 100 kV of impulse level. In difficult cases, a buried counterpoise earthing, consisting of wires radiating from the foot of the tower to a distance of 30-60 m, or continuous wires from tower to tower, assist in reducing the footing resistance.

 Overhead Ground Wires

This is the most effective technique of providing protection to transmission lines against direct lightning by using overhead ground wires. The ground wires are placed above the line conductors at such positions such that all lighting strokes are intercepted by these ground wires. The ground wires are grounded at each tower or pole through a lowest resistance.

Lightning Arrestors

While earthing screen and ground wires provide protection against direct lightning strikes, they fail to offer protection against travelling waves which may reach the terminal equipment. To protect against surges due to lightning strike, lightning arrestors or surge diverters are employed.

A basic lightning arrestor/surge diverter is illustrated below:

The lightning arrestor above consists of spark gap in series with a non-linear resistor. One end of the arrestor is connected to the terminal of the equipment being protected and the other end is effectively grounded. The length of the gap is set such that the normal line voltage is not enough to cause an arc across the gap but a seriously high voltage will break down the air insulation and form an arc.

The operation of the lightning arrestor is as follows:

1. Under normal operation, the lightning arrestor is off the line, that is, it conducts no current to earth or the gap is non-conducting.
2. When an overvoltage incident happens, the air insulation across the gap breaks down and an arc is formed, providing a low resistance path for the surge to the ground. In this manner, the excess charge on the line due to the surge is conducted to through the arrestor to the ground without risk.
3. As the gap sparks over due to overvoltage, the arc will be short-circuit on the power system and may cause power-follow current in the arrestor. The non-linear resistor prevents the effects of short-circuit by providing high resistance to high voltage (or current). Thus after the surge is over, the resistor offers high resistance to make the gap non-conductive.

 Protection against Switching Surges

Even though switching surges can’t be entirely avoided, it is desirable on lines of 300 kV upward to limit them by shunting across circuit breaker contacts, during closure, resistors of value approximating to the surge impedance of the line (300-400 Ω)

Surge Attenuation

A typical travelling wave of voltage attenuates, mostly by corona loss as it propagates along a line, and its magnitude and wave-front steepness is lessened. The inductive coupling with the earth and with earth or counterpoise wires helps in this process. A reduction by one-half may occur in a line length of 5-8 km.

A length of underground cable between the terminal of an overhead line and the substation plant also reduces the magnitude of an incoming voltage surge, owing to the lower surge impedance of the cable. Nonetheless, additional reflections from the junctions may partially offset this advantage, making the cable itself vulnerable. Whereas such cable is every so often desirable for service or other reasons, it is seldom installed solely for protective purposes.

Surge Arrestor

The purpose of the surge arrestor is direct a surge to earth before it reaches a vulnerable plant. The surge arrestor must be situated as near as is practicable to the terminals of the plant.

An ideal surge arrestor has the following features:

1. Takes no current at normal system voltage.
2. Establishes an instantaneous path to earth for any voltage that is abnormally high.
3. Is capable of carrying the full discharge current.
4. Deters subsequent power frequency current.

A typical surge arrestor consists of an assembly of small gaps and non-linear resistors in series, the whole system being contained in cylindrical porcelain housing. The use of multiple instead of single gaps gives the most rapid breakdown. The resistor elements offer a low resistance to surge currents (limiting the voltage across the arrestor) and a much higher resistance to the power frequency follow current. Careful gap design ensures that the follow current does not restrike.

Rod/Spark Gaps

Rod or spark gaps are easy and cheap to install. They are usually installed in parallel between the live equipment terminal and earth. The gap distance setting is arranged such that the spark-over occurs at overvoltages well below the breakdown insulation level of the plant which the gaps are protecting.

Under normal operating conditions, the gap remains non-conducting. When there is an incident of a high voltage surge on the line, the gap sparks over and the surge current is conducted to earth. Thus the excess charge on the line due to the surge is conducted to earth without harm.

A plain air-gap is not costly and it satisfies requirements 1, 2, 3, of an arrestor as aforementioned. But, it doesn’t fulfill condition 4. Even though, gap breakdown protects the plant from overvoltage, there may be a power-frequency follow current which must be cleared by a circuit breaker operation, involving an outage.

A plain rod gap, connected between line and earth has the following typical gap lengths giving breakdown at about 80% of the plant impulse level:

System voltage (kV)	36.2	72.5	145	300	420
Gap Length (m)	0.23	0.35	0.66	1.22	1.70

Rod or Spark Gaps have the following shortcomings:

• Sudden reduction in voltage during gap operation places high stress on transformer inter-turn insulation.
• When they operate, a short circuit is created which will cause protection to operate and isolate the circuit. But, the alternative of insulation failure of the plant being protected is much more severe.
• The breakdown of plant insulation varies with the duration of the overvoltage. A gap has a relatively slow response to fast rise time overvoltage surges and performance is influenced by polarity and atmospheric conditions.
• Short distance gaps applicable at the lower distribution voltages are vulnerable to malfunction due to wind-borne debris, birds, and so forth.

Despite the above shortcomings, the rod gap is extensively used for the protection of small distribution transformers and as a back-up protection for transformers protected by surge arrestors.

Arc Suppression Coils (Petersen Coils)/Resonant Grounding

Due to economic factors and/or safety reasons usually the neutral point of a system is earthed directly or through a resistor. Any earth fault results in fault current and necessitates a circuit outage to clear it.

If the neutral is earthed through a high inductance reactance (several hundred ohms), the resulting lagging current can, neglecting losses, precisely neutralize the leading capacitance current (which flows through the fault and which may cause damaging voltage surges especially if it involves an arc) as illustrated in the figure below. Thus there will be no fault current and the system can be operated with the fault until such time as it can be conveniently be repaired.

As illustrated in the figure above, the arc suppression coil is connected to the neutral earthing circuit and creates a lagging current that is of opposite of the leading cable capacitive/charging current during fault therefore neutralizing it.

Arc suppression coils are effective on 12 kV, 36 kV and occasionally up to 245 kV systems; however at the higher voltages and with long lines the resistive and other losses prevent precise opposition of I_f1 and I_fc so that there is a resultant current in the fault which, if it involves an arc, is inextinguishable and damaging.

Auto-Reclose Circuit Breaker

These are circuit breakers with automatic re-closure. A transient flashover seldom causes damage if the fault is cleared by the normal protection equipment. After an interval of 0.4-0.8 s, enough for the natural de-ionization of the arc path, the circuit breaker can be reclosed with safety.

Circuit breakers with automatic re-closure are required to do the following:

• Provide time delay switching in the event of a permanent fault.
• Permit the normal fuse discrimination to limit the area of interruption.
• Operate swiftly on the initial passage of a fault current.

The figure below illustrates three applications of a typical auto-reclosing system:

In (i) all transient faults are cleared by the recloser, and in the case of permanent faults the recloser sequence provides a time delay trip to blow the fuse only on the faulty sub-circuit. In

 (ii) Sectionalizers provide the disconnecting means for faulty sub-circuits. Method (iii) works as (i) with the exception that on permanent fault the recloser holds closed and backup protection operates.

A typical current operated recloser consists of a normally closed oil circuit breaker for pole mounting. The breaker is held closed by springs. When current exceeds, for instance, twice full load value, movement of the plunger of a series solenoid causes the recloser to open. The plunger is then spring-reset and the reclosing is automatic. Relay features control the reclosing and openings times, and a means is provided to facilitate the recloser to lock open or hold closed at the end of its operating sequence.

Related articles:

• Surge Suppression in Power Systems
• SF6 Circuit Breakers: Features, Operation & Applications
• Air Circuit Breakers: Features, Operation & Applications
• 5 Causes of High-Voltage Switchgear Failures
• Power System Protection