Medium Voltage Surge Arresters

Surge arresters, also known as lightning arresters, are an accessory often ordered on medium voltage transformers and reactors.  They are typically connected between line terminals of a winding and ground.  They provide a degree of protection to the insulation from high voltage surges.  They are available for winding voltage ratings 2.4 kV and above.  A surge arrester is not to be confused with a Surge Protective Device (SPD), formerly known as a Transient Voltage Surge Suppressor (TVSS), which is applicable for voltage ratings 1000 V and below, not covered by this article.  This article is intended to familiarize the reader with surge arrester application and accessories.

What is a Surge Arrester

Modern arresters use metal oxide discs, composed mainly of zinc oxide.  These discs are enclosed in a polymer or porcelain housing with terminals and mounting provisions.  One end is connected to the winding line terminal and the other end is connected to ground.

Most metal oxide arresters are gapless.  In these, the full system line to ground voltage is applied to the metal oxide discs.  Some metal oxide arresters have a spark gap in series with the metal oxide discs.  In the gapped arresters, the system line to ground voltage is taken by the spark gaps.

At normal operating voltage, the resistance of the metal oxide discs is high enough that the leakage current is a few milliamps.  When a voltage surge reaches the arrester the resistance decreases, allowing the surge to be conducted to ground.  When the voltage returns to the normal operating voltage, the resistance increases again to limit the leakage current.  In a gapless arrester, the transition is seamless.  In a gapped arrester, the voltage needs to reach the spark over voltage of the spark gap before the arrester discharges the surge.

Arrester Characteristics

Surge arresters are typically designed and tested to IEEE Standard C62.11 – IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (> 1 kV).  This standard defines usual service conditions, performance requirements and test procedures for arresters.  HPS standard arresters comply with this standard.

The voltage rating of a surge arrester is most commonly referred to by the Arrester Duty Cycle Rating or Arrester Rating.  Arrester ratings are defined in IEEE C62.11.  More importantly, each arrester rating is assigned a Maximum Continuous Operating Voltage (MCOV).  This is the maximum 50/60 Hz voltage that may normally be applied across the arrester.  Short time voltages above the MCOV are allowed provided they are within the published Temporary Overvoltage Capability (TOV) of the arrester.  Exceeding the MCOV or TOV may cause the arrester to overheat and fail.

The Discharge Voltage is the voltage across the arrester while carrying a discharge current.  Arrester manufacturers publish discharge voltage tables for various discharge currents for use in verifying proper protection.  These are peak voltages and would be compared to the winding BIL rating.

The resistance of the zinc oxide discs decreases as the disc temperature increases.  If an arrester is overheated, it can enter a thermal runaway.  The temperature increases, causing the resistance to decrease, causing the current to increase and heat up the arrester more until the arrester fails.  An arrester failure results in an increase in pressure within the arrester housing.  On porcelain arresters, the pressure is relieved by pressure relief vents.  On polymer arresters, the housing splits open to relieve the pressure.  Arresters are assigned a Pressure Relief Rating indicating the maximum fault current they can safely handle during an arrester failure.  The pressure relief rating must be equal to or greater than the available fault level where the arrester is installed.

Arresters are divided into one of three categories.  Distribution Class arresters are normally used to protect distribution systems.  Distribution class arresters are available with ratings from 3 kV (2.55 kV MCOV) to 36 kV (29.0 kV MCOV).  Intermediate Class arresters are intended for moderate duty applications.  Station Class arresters are intended for heavy duty applications.  Intermediate and station class arresters are available with ratings from 3 kV (2.55 kV MCOV) up to the highest system voltage that HPS products may be used on.  A table showing a comparison of the characteristics for the three classes for the same MCOV level for HPS standard arrester offering is shown below.

Class	Relative Cost	Discharge Voltage	TOV Capability	Pressure Relief Capability	Ambient Temperature	Maximum Altitude
Heavy Duty Distribution	Lower	Higher	Higher	20 kA, 0.2 s	-50 oC to +60 oC	12000 ft
Intermediate	High	Lower	Lower *	40 kA, 0.2 s	-50 oC to +60 oC	12000 ft
Station	Higher	Lowest	Lower *	63 kA, 0.2 s	-50 oC to +60 oC	12000 ft

*TOV capability for intermediate and station class arresters is the same.

Arrester Selection

Guidance for arrester selection is found in IEEE Standard C62.22 – IEEE Guide for the Application of Metal-Oxide Arresters for Alternating-Current Systems.  Article 280 in The National Electrical Code also has some rules regarding arrester selection.  The process is to select the MCOV of the arrester and the class based on the system the winding is connected to then verify that the discharge voltage is sufficiently below the winding BIL to properly protect the winding.  Selection of the MCOV and class requires knowledge of the power system beyond what the transformer manufacturer typically knows and should be performed by someone with that knowledge

In some cases, an arrester with the proper MCOV to be installed on the system does not adequately protect the transformer.  This usually happens for ungrounded systems when the transformer has the lowest BIL rating for the voltage class.  If the selected arrester does not provide adequate protection, DO NOT reduce the MCOV to a level lower than the minimum required for the system conditions as this could lead to an arrester failure.  Instead, a different class arrester may provide adequate protection – intermediate class arresters have a lower discharge voltage than distribution class arresters and some station class arresters have a lower discharge voltage than intermediate class arresters for the same MCOV.  If the transformer has not been built, the winding BIL level could be increased so the selected arrester provides adequate protection.

Arrester Installation

Surge arresters protect the winding they are connected to from surges approaching the winding from the connected system.  If the surge approaches from the high voltage side, the arresters are installed on the high voltage.  If the surge approaches from the low voltage side, the arresters are installed on the low voltage.  Note that an arrester or surge suppressor installed on the low voltage side will not protect the transformer from a surge approaching from the high voltage side.

In most cases, the air temperature above the core and coil assembly exceeds the maximum allowed ambient for surge arresters. For this reason, surge arresters are normally installed at the bottom of the transformer or reactor.  In special cases where the temperature above the core and coil assembly is maintained below the maximum ambient for the arrester, they may be installed above the core and coil assembly.  Surge arresters may also be installed in terminal chambers if the transformer or reactor has one.  In a terminal chamber, arresters may be mounted high and special designs are available to hang the arrester from the roof if required.

Arrester Accessories

An isolator or disconnector is an option on distribution class arresters.  It blows the ground lead off the arrester in the event of an arrester failure, preventing a system fault.  HPS does not recommend using an isolator on HPS products to avoid issues of the disconnected ground lead being thrown on an energized conductor, fan blade, or some other component.  HPS standard distribution class arresters do not have an isolator.  An isolator is not available on intermediate and station class arresters.

Discharge counters are available that record the number of surges an arrester has conducted.  The arrester is installed on an insulating base.  The ground lead from the arrester is then run to the discharge counter then to ground.

Many customer specifications specify gapless metal-oxide arresters.  This is HPS standard offering.  Gapped metal-oxide distribution class arresters are available on request.  Gapped metal-oxide intermediate class and station class arresters are not available.

Special Applications

HPS standard offering is a polymer housed, gapless distribution class arrester built and tested to IEEE Standard C62.11.  They are suitable for use within a transformer or reactor enclosure at altitudes up to 12000 feet.

Some specifications reference CAN/CSA-C233.1 Gapless Metal Oxide Surge Arresters for Alternating Current Systems.  At time of writing, this standard has been withdrawn and few, if any, manufacturers offer arresters built to this standard.  When the HPS product is installed outside North America, distribution class and station class arresters built and tested to IEC 60099-4 are available on request.

Some specifications request surge arresters to be CSA Approved or UL Listed.  At time of writing, there are no known CSA Approved or UL Listed medium voltage surge arresters.

HPS standard distribution class arresters are suitable for use on systems where the system fault level is 20 kA or below.  If the system fault level exceeds 20 kA, intermediate or station class arresters should be used.

The industry standard maximum altitude for arrester application is 6000 feet.  Above this, special arresters may be required.  If HPS standard arresters are used, a special arrester is not required until the altitude exceeds 12000 feet.  These special arresters use a larger housing to increase the external clearances between live parts.

Some customers specify a silicone free product.  In these cases, porcelain housings are required as silicone grease is often used in the assembly of polymer housed arresters.  Note that porcelain arresters are typically more expensive than polymer arresters and require more space to install them.

Surge arresters alone are not sufficient to protect the transformer against some switching transient voltages that may occur when the transformer is switched in or out of a system.  This problem and mitigation is beyond the scope of this article.  Further information on this may be found in IEEE Standard C57.142 – IEEE Guide to Describe the Occurrence and Mitigation of Switching Transients Induced by Transformers, Switching Device, and System Interaction.

Conclusion

Surge arresters are an important component to help protect transformers and reactors from damage due to voltage surges.  They must be properly selected to withstand the normal and short time operating voltages of the system while protecting the winding.  They must also safely relieve the internal pressure due to any fault current that may flow through them in the event of an arrester failure.  HPS is able to supply and install surge arresters on all its medium voltage products.