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Surge arresters in vital components that offer protection for electrical systems. To ensure continuous operation, one parameter you should consider is the Maximum Continuous Operating Voltage (MCOV). Amongst other things, this blog will explain how to calculate the MCOV of surge arresters.
Proper selection of MCOV guarantees that your arrester is resistant to continuous overvoltage.
MCOV is short for Maximum Continuous Operating Voltage. This refers to the maximum frequency of power voltage a surge arrester can handle. It is the function of the maximum line-to-line voltage and the system grounding parameters. It is usually expressed in kilovolts (kV).
It is important to differentiate the MCOV from the rated voltage of the arrester. The MCOV is typically less than the rated voltage of the arrester.
Here's why MCOV is critical in surge arresters:
Prevents premature failure: Arresters exposed to voltages above their MCOV may fail.
Proper Insulation: If the MCOV of an arrester aligns with the insulation level of the system, it will avoid flashover. Otherwise, there would be improper insulation coordination.
Ensures reliability: When the MCOV of an arrester matches that of the system, it will ensure that the arrester will activate in a surge. This will protect transformers, switchgear, cables, and other equipment against stress.
Thermal Stability: If the selected MCOV is too low, the arrester will not perform optimally. It will continuously conduct leakage current and generate heat.
Equipment Durability: Proper MCOV increases the life of the equipment and the arrester. It prevents excess stress on the arrester blocks, which would otherwise disrupt the system.
Protection Coordination: MCOV assists in coordinating surge protection and the system insulation levels. It ensures that the arrester does not activate on normal conditions; it only activates under surge or power outages.
It is the highest RMS (root mean square) voltage it can withstand under surge conditions. It also refers to the line-to-ground voltage it can withstand across its terminals for a specified duration (often several hours) without exceeding its thermal design limits.
The rated voltage does not equal MCOV. It is usually higher than MCOV. Although the MCOV accepts continuous operation, the rated voltage will ensure that the surge arrester is triggered during temporary overvoltages like faults, switching surges, or load rejections. Rated voltage is used to provide proper insulation coordination and surge protection.
An arrester that has an RMS value near the normal system voltage might not offer sufficient protection. Conversely, if the RMS is too high, the arrester's protective level rises, and it may activate prematurely and damage your equipment.
The nominal voltage of the system is the conventionally acceptable voltage designation of the system. It is often expressed as line-to-line voltage in three-phase systems (e.g. 6kV, 11kV, 33kV). It acts as the reference point in determining voltages line-to-ground.
However, MCOV must align with the maximum system voltage, not just the nominal voltage. Otherwise, the arrester may face continuous stress. You should always confirm whether the system operates closer to nominal voltage or maximum voltage. This is especially crucial in regions where voltage or surges are common.
Since MCOV is chosen based on line-to-ground, you can convert the maximum line-to-line voltage into line-to-ground voltage using the formula:
where:
VLG is line-to-ground voltage
VLL is line-to-line voltage
For example, if a system has a rating of 12 kV(line-to-line), the line-to-ground voltage is VLL3 = 6.93kV
Line-to-ground is the true reference for MCOV; that is, MCOV is typically selected based on it. This is because surge arresters are usually connected between the phase connectors and earth. The grounding system defines the distribution of voltage between the phases and ground when everything is operating normally. It's broken down to three:
Solid grounded systems: In this system, each phase to ground is stable and equal to VLL/√3. Here, MCOV can be selected close to the actual line-to-ground voltage. It is usually common in distribution networks, like 11kV and 33kV distribution systems.
Impedance Grounded Systems: In an unbalanced state, or under fault conditions, surges could occur in one or more phases. As a result, MCOV should be slightly increased (about 10% - 20%) to accommodate this.
Ungrounded Systems: If there is no direct path to the ground, the faulted phase can have sudden high voltage. MCOV must be increased by 73% or by a factor of 3 to ensure protection.
For example, in an ungrounded system with a 33kV rating, the MCOV of the arrester may need to be nearly equal to 33 kV to ensure safety.
Transformer tap settings may occur in a maximum voltage that is 5% or more above the nominal voltage. This should be considered when selecting MCOV. Transformer tap settings may cause variations in system voltage. Thus, it is crucial to get a higher MCOV.
TOVs are short-duration spikes that typically last from a few cycles to several seconds. This may be caused by switching operations, ground faults, or load rejection. Although the surge is brief, the overvoltage may damage equipment and electrical systems if not accounted for.
If the arrester is too close to system voltage, TOVs can push it into overheating, which would be damaging.
To ensure the arrester gives continuous protection, add a 10–15% margin to the calculated MCOV. Additionally, ensure the arrester's TOV withstand capability matches your system behavior.
Start by determining the system nominal voltage, which is measured in kV. This is the standard operating voltage of your electrical network, forming the baseline of all MCOV calculations. Common distribution voltages are:
11kV system, which is used in urban and industrial distribution
33kV system, which is used in regional transmission and distribution
132kV and 330kV systems, which are used in high-voltage transmission
Therefore, an 11kV system has a nominal voltage of 11,000 V. However, the system's nominal voltage is insufficient for calculating MCOV. This is because systems typically operate slightly above this value. It is advised that you check both nominal and maximum values.
The electrical systems experience voltage variations as a consequence of load variations, switching activities, etc. Electrical systems often run above nominal voltage to maintain stability in long feeders. Therefore, it is important to determine the maximum system operating voltage.
The maximum system operating voltage is usually 5% - 10% higher than the nominal voltage. For example, the maximum operating voltage of a 33kV system could be: 33kV × 1.10 = 36.3kV.
This rating will prevent the arrester from missing polarity, overheating, or conducting under normal voltage variations.
Surge arresters are usually connected phase-to-ground. Therefore, MCOV is calculated based on line-to-ground voltage (VLG).
The relationship between line-to-ground and line-to-line voltage is expressed as follows:
or 0.577 x line-to-line voltage
For a 33kV system, 33/√3 or 0.577 x 33 = 19.05kV. This forms the base voltage for MCOV selection.
The grounding pattern identifies how the line-to-ground voltages behave during fault conditions and normal operation in the system.
In solid-grounded systems, use the computed line-to-ground voltage directly.
For impedance-grounded systems, apply the correction factor. That is, for 6.93kV, it will be 6.93 x 1.1 = 7.92kV.
For ungrounded systems, use the maximum line-to-line voltage instead of line-to-ground voltage.
To protect the arrester from conducting during surge events, add a 10–15% margin to your calculated MCOV. Doing this ensures the arrester remains inactive during normal operations or brief voltage surges but conducts during surge events.
For example: Adjusted MCOV = 19.05kV × 1.15 = 21.91kV
Select a surge arrester with a slightly higher MCOV than what you have calculated.
If the rating is too low, your arrester will overheat and fail prematurely. If the rating is too high, the arrester protective level rises, and it will leave equipment at risk to surges.
For example, if your adjusted MCOV is 21.91kV, select an arrester rated at 22kV MCOV or higher.
Every manufacturer provides MCOV ratings, rated voltage, TOV withstand capabilities, and application guidelines in product datasheets. You need to familiarize yourself with these specifications and safety margins. Always check your arrester selection with the manufacturer's datasheet.
Check that the arrester's MOV matches your system's grounding profile.
Look at environmental conditions such as level of pollution, altitude, etc.
Worked Out Example for an 11kV Solid-Grounded System
Nominal system voltage = 11 kV
Maximum operating voltage = 12 kV
Line-to-ground = 12√3 = 6.93kV
Grounding = For a solid grounded system, no correction is needed.
Add TOV margin (5%) = 7.28 kV
Choose the nearest higher arrester MCOV = 7.65 kV arrester
Check manufacturer's datasheet = Rated voltage ≈ 10 kV, acceptable
Therefore, the final calculated MCOV surge arrester for an 11kV system is 7.65kV.
Rated voltage is not the same as MCOV. Hence, they should not be used interchangeably. While rated voltage is the arrester's withstand capability during TOVs, MCOV is the continuous allowable voltage. Mixing them up can lead to disaster, including under- or over-specification.
Grounding configuration affects line-to-ground voltage. If you ignore the grounding configuration, it can negatively affect your arrester. It can cause it to be under-rated, especially in ungrounded systems, where line-to-line voltages appear to earth. Mixing this can lead to dangerously low MCOV selection.
TOVs are crucial and can push system voltage beyond MCOV. If you must select a good MCOV for your arrester, you must account for potential TOV conditions. Otherwise, your arrester may fail during switching events. Furthermore, it may experience repeated stress and thermal instability.
Choosing an MCOV too close or equal to the exact line-to-ground voltage is dangerous. It leaves no margin for fluctuations. As a result, it may lead to premature arrester failure.
Not consulting the manufacturer's datasheet or not being able to understand it is a common mistake that can cause disastrous consequences. It is recommended that you have general knowledge of the vital information presented within to choose a good arrester.
The rating for a surge arrester of 33kV varies. It could be between 27kV, 30kV, or 36kV, depending on several factors.
Conduct a visual inspection for damage. You can perform electrical tests to check for leakage current and detect overheating.
You can check the surge voltage using an oscilloscope to detect high-energy, short-duration voltage spikes. This instrument can show you the waveform and magnitude of the voltage spike so you can understand the disturbance.
MCOV, or maximum continuous operating voltage, is the highest voltage the surge arrester can withstand continuously under normal conditions.
There is no single standard surge voltage, as it varies depending on the application environment. The standard for an office environment may require protection from surges up to 1kV, while outdoor environments may require protection from surges up to 4kV or more.
Knowing how to calculate MCOV of surge arresters is a crucial step that ensures the protection of your electrical systems. It influences your selection of good arresters for long-term performance. Need help selecting surge arresters for your project? Contact us at Haivol Electrical for an expert solution.
