Technical Fundamentals of Surge Arresters
This section is intended to provide essential elements for the technical understanding of Metal-Oxide Surge Arresters for AC systems above 1kV. It includes all outdoor designs for HV substations, distribution systems and overhead lines applications.
Surge Arresters are connected from phase to ground (with some exceptions).
They act as a "static switch" to limit overvoltages and divert energy to the ground
Their active part is made of Metal Oxide Varistors (MOVs) stacked on each other
The microstructure of MOVs represent a multitude of miniature electronic switches
They have an essential function to ensure a proper Insulation Coordination and therefore protect against :
> Critical transient overvoltages that can breakdown the dielectric strength
> Lightning impulses coming for atmospheric conditions
> Switching impulses coming for breaker reclosing operations
Terms & Definitions
Main electrical ratings for gapless types
Rated Voltage (Ur)
Duty-Cycle Voltage (IEEE)
Specific to surge arresters, should not be compared with other equipment's ratings. It is basically a TOV rating for 10 sec with prior duty that is verified during a thermal recovery period.
IEC: Maximum permissible 10 s power frequency r.m.s. overvoltage that can be applied between the arrester, as verified in the TOV test and the operating duty test.
Continuous operating voltage (Uc)
Basically the maximum phase-to-ground voltage that the surge arrester can withstand across itself over its service life without degradation of performance and power losses.
IEC: designated permissible r.m.s. value of power-frequency voltage that may be applied continuously between the arrester terminals in accordance with 8.7
Temporary Overvoltages TOV
for 1sec / 10 sec
You must pay attention to those values, there are often underestimated.
First, the better you control the TOV in your system, the better is your protection level with surge arresters. It even helps reducing clearances in some cases.
Then, you need to consider carefully if you define TOV with prior duty or without. The "prior duty" rating is a proper verification of the performance under worst case preconditioning and prestress, clearly defined in the standards.
Nominal Discharge Current (In)
Classifying current (IEEE)
A standardized lightning current amplitude/waveform to help classifying the different Metal-Oxide Varistors/Surge Arresters designs. A Nominal Discharge Current should not be considered as a maximum lightning current discharge capability. It is rather a soft lightning impulse associated with its protection level.
IEC: peak value of lightning current impulse, which is used to classify an arrester
Residual Voltage (Protection Level)
Discharge Voltage (IEEE)
Residual voltages can be considered as the most essential values for surge arresters. There are often misunderstood since there are very specific to the non-linear characteristics of the Metal-Oxide Varistors. The residual voltage is the resulting voltage across the surge arresters when standard discharge currents are applied to it. Each type of transient overvoltage (fast-front, lightning, switching) is limited by the surge arresters. The protection margin is the difference between the protection level (residual voltages) and the withstand levels of the equipment to be protected. Therefore, the lower the residual voltage, the higher is the protection margin.
IEC: peak value of voltage that appears between the terminals of an arrester during the passage of
Repetitive Charge Transfer Rating (Qrs)
Single impulse charge transfer (IEEE)
A recently introduced concept of energy withstand classification. It is a statistical approach to verify the capacity of varistors to absorb an electrical charge in a repetitive manner. The objective is to guarantee its physical integrity and its electrical performance after undergoing a succession of discharges.
IEC: maximum specified charge transfer capability of an arrester, in the form of a single event or group of surges that may be transferred through an arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors.
Thermal Energy (Wth)
Thermal energy withstand (IEEE)
An important energy rating to verify the thermal stability of the arresters after injecting critical current impulses. The idea is to consider the worst case scenario in the context of the Operating Duty Test (high temperature, provoked ageing, temporary overvoltages and max phase to ground voltage) where we look for its best energetic performance while avoiding a thermal runaway.
IEC: maximum specified energy, given in kJ/kV of Ur, that may be injected into an arrester or arrester section within 3 minutes in a thermal recovery test without causing a thermal runaway.
The non-linear Voltage-Current characteristics of gapless Metal-Oxide Surge Arresters
Station Class Arresters
Protection of valuable assets as power transformers
To protect valuable equipment such as power transformers. Primary winding is universally protected since the transformer is the highest value asset in a substation and often has the lowest surge withstand voltage.
Both primary and secondary transformer bushings are protected by default using the same arresters that protect transformer windings. Essential to protect neutral points bushing going through neutral grounding resistor.
System Voltage Parameters
Neutral grounding is essential for the voltage ratings selection
Example 420 kV System :
Solidly grounded neutral
Earth fault factor = 1.4
UL-L = 420 kV
UL-G = 420 kV / √3 = 242 kV
Uc ≥ 1,05 × Us ⁄ √3
=1,05 × 420 kV ⁄ √3 = 255 kV
Uc ≥ 1,05 × Us ⁄ √3 = 1,05 × 420 kV ⁄ √3 = 255 kV
Ur1 = Uc × 1,25=255kV×1,25 = 319kV
1,25 is inherent to MOV manufacturing
TOV consideration 10 sec:
Ur2=TOV⁄T_10s =(1,4×Um⁄√3)÷T_10s=(1,4×420kV⁄√3)÷1,075= 316 kV
Typical Ur value is 336kV or 360kV
Ur Rated Voltage = TOV capability 10sec with prior duty
Protection of sensitive transition points
Often protected with arresters. For such applications, the protection in generally dedicated to the cable since the cable itself is not a self-restoring insulation. Potential damages on the cable could be significantly costly