Surge Arrestor

Surge Arresters

• A device designated to protect electrical apparatus from high transient voltage and to limit the duration and frequently the amplitude of follow-current.
• The term “surge arrester” includes any external series gap which is essential for the proper functioning of the device as installed for service, regardless of whether or not it is supplied as an integral part of device.

Types of Surge Arresters

At present the following types of surge arresters are used:

1. Gapped silicon-carbide surge arresters called valve type or conventional gapped arresters. These consists of silicon carbide discs in series with gap units.

2. Zinc-oxide gapless arresters called ZnO arresters or metal-oxide arresters. These are gapless and consist of zinc-oxide discs in series. ZnO arresters have superior V/I characteristic and higher energy absorption level. They are preferred for EHV and HVDC installations.

•  Surge arresters are the primary protection against different types of overvoltages (atmospheric or switching).
• Surge Arresters are usually connected between phase and ground in distribution system; near the terminals of large medium voltage    rotating machines and in HV, EHV, HVDC sub-stations to protect the apparatus insulation from lightning surges and switching surges.
• The active elements (blocks) of surge arresters are manufactured using a highly non-linear ceramic resistor material composed of the most part of ZnO mixed with other metal oxide.
• The resistor blocks in the surge arrester offers low resistance to high voltage surge to ground.
•  Surge Arrester discharges current impulse surge to earth and dissipates energy in the form of heat.
•  After discharging the impulse wave to earth, the resistor blocks in the surge arrester offers a very high resistance to the normal power frequency voltage and acts as open circuit.

Maximum system voltage (U_m) is the highest r.m.s. phase to phase voltage which occurs under normal operation conditions at any time and at any point in the system.

Continuous operating voltage (U_c), often abbreviated as COV or MCOV, is the designated permissible r.m.s. power frequency voltage that may be applied continuously between the arrester terminals.

Temporary overvoltage (TOV) as differentiated from surge overvoltages, are oscillatory overvoltages of relatively long duration and which are undamped or only weakly damped.

TOV frequencies range from a few Hz to some hundreds of Hz and duration from some milliseconds to many hours (depending upon fault clearing time).

Rated voltage (U_r) as per IEC means that an arrester fulfilling the IEC standards must withstand its rated voltage for at least 10 seconds after being both preheated to 60⁰C and subjected to a high energy injection as defined in standard.

Equipment insulation withstand characteristic is a general term for the equipment insulation withstand voltages and comprises:

1.            Chopped-wave withstand level

2.            lightning impulse withstand level

3.            Switching impulse withstand level

4.            Power frequency withstand

Residual voltage (U_res) is the voltage that appears between the terminals of an arrester during passage of discharge current through it. It depends on the magnitude as well as the waveshape of the discharge current and is expressed as a peak value. 

Arrester protective characteristic is the combination of its residual voltages for different current impulses. For good protection, the arrester characteristic should lie well below the equipment insulation withstand characteristic at all points.

lightning impulse protection level (LIPL) of the arrester is the residual voltage for the nominal discharge current.

Switching impulse protection level (SIPL) of the arrester is the residual voltage for a specified switching impulse current.

Protection ratio is the ratio of the equipment insulation withstand level to the corresponding protection level of its arrester.

Protective margin is the protective ratio minus 1 and expressed as a percentage. The margin should cover the voltage increase due to the connection between the arrester and the protected equipment as well as the increase in the residual voltage due to the discharge amplitude.

Continuous current (I_c) is the current that flows through the arrester at U_c. This current is predominantly capacitive and is generally expressed as a peak value.

Reference current (I_ref) is the peak value of the power frequency resistive current at which the reference voltage is measured. The reference current is in the range 0.4 to 10 mA peak.

Reference voltage (U_ref) is the peak value divided by Ö2 of the voltage measured across the arrester at reference current (I_ref)_.

Nominal discharge current (I_n) is the peak value of the 8/20 ms impulse current used to classify an arrester.

Single impulse energy capability is the maximum permissible amount of energy, expressed in kJ, which the arrester is able to absorb in one single impulse with a specific duration. When expressed in kJ/ kV (U_r), it is called specific single impulse energy capability.

Earth fault factor (k_e) is the ratio of the voltage in the healthy phase during and prior to earth-fault conditions. If the system neutral is directly earthed then k_e £ 1.4 while a resonant earthed or isolated system gives k_e as 1.73 approximately.

Pressure relief capability is the ability of the arrester, in the event of its overloading due to any reason, to conduct the resulting system short-circuit current through it without a violent explosion which may damage nearby equipment or injure personnel. After the pressure relief, the arrester must be replaced.

• Obtain system parameters: Obtain the highest system voltage (U_m).
• Check for abnormal service conditions: Abnormal conditions such as ambient temperatures below -40⁰ C or above +45⁰ C, frequencies under 15 Hz or above 62 Hz, presence of heat source (e.g. furnaces) near the arresters may lead to selection of higher U_c and/ or U_r and hence need to clarify such conditions.
• Check for other reasons for TOV occurrences: The TOV may arises during non-simultaneous operation of breaker poles
• Select the continuous operating voltage: In a 3-phase system with the arresters connected phase-ground U_c=U_m/Ö3. If the system does not have abnormal conditions U_c therefore should be equal to or higher than U_m/ Ö3.
• Select suitable TOV capability:

                TOV_e = k_e ´ U_m/ Ö3, where

                k_e £ 1.4 for directly earthed neutral system

                k_e = 1.73 normally for resonant earthed and isolated neutral systems