Dimensioning, testing and application of metal oxide surge arresters in medium voltage systems

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Metal Oxide Surge Arresters | Source: ABB 

Metal oxide surge arresters (MO) are important components of medium voltage electrical systems because they provide protection against overvoltage events that can damage equipment, interrupt power supply, and cause safety hazards. Here are some reasons why MO's are crucial for overvoltage protection in medium voltage systems:

  1. Protection against overvoltage events - Overvoltage events such as lightning strikes, switching operations, and system faults can cause high transient voltages that can damage equipment and disrupt power supply. MO's provide a low-impedance path to ground for transient voltages, protecting equipment and preventing damage. 
  2. Energy absorption capability - MO's have high energy absorption capability, which means they can absorb and dissipate a large amount of energy during an overvoltage event. This is essential for protecting equipment from damage and ensuring that power supply is not interrupted. 
  3. Long service life - MO's have a long service life and require minimal maintenance. They are designed to withstand repeated overvoltage events and can protect equipment for many years. 
  4. Compact size - MO's are relatively small in size and can be installed in confined spaces, making them suitable for medium voltage systems where space is limited. 
  5. Cost-effective - The cost of installing MO's in a medium voltage system is relatively low compared to the cost of repairing or replacing damaged equipment. MOs can provide long-term protection against overvoltage events, saving the system operator money in the long run.

Overvoltages

Overvoltages in electrical supply systems result from the effects of lightning incidents and switching actions and cannot be avoided. They endanger the electrical equipment because for economic reasons the insulation cannot be designed to withstand all possible cases. An economical and safe on-line system calls for extensive protection of the electrical equipment against unacceptable overvoltage stresses. This applies generally to high voltage systems as well as to medium and low voltage systems.



Overvoltage protection can be basically achieved in two ways:
  1. Avoiding lightning overvoltage at the point of origin, such as through earthed shielding wires in front of the substation that intercept lightning. 
  2. Limit overvoltage near the electrical equipment, for instance through surge arresters in the vicinity of the electrical equipment.
In high voltage systems both methods of protection are common. The shielding wire protection in medium voltage systems is generally not very effective. Due to the small distance between the shielding wires and the line wires, a direct lightning stroke on the shielding wire leads to an immediate flashover to the line wires as well. In addition, induced overvoltage in the line wires cannot be avoided by shielding wires. 

The most effective protection against overvoltage in medium voltage systems is therefore the use of surge arresters in the vicinity of the electrical equipment. ~ ABB

For the following considerations it is meaningful to distinguish between three types of overvoltages:
  1. Temporary overvoltages occur, for example, during load rejection or because of faults with earth connection. The duration of these overvoltage, mostly with power frequency, can be between 0.1 seconds and several hours. Generally, they are not higher than √3 p.u. and are usually not dangerous for the system operation and the insulation of the equipment. However, they are decisive for the dimensioning of the arresters. 
  2. Switching overvoltages (slow front overvoltages) occur during switching actions and consist mostly of heavily damped oscillations with frequencies up to several kHz and a magnitude up to 3 p.u. In the case of inductive switching, the switching overvoltages can reach up to 4 p.u. 
  3. Lightning overvoltages (fast front overvoltages) originate in atmospheric discharges. They reach their peak value within a few microseconds and subsequently decay very rapidly. The magnitude of these unipolar overvoltages can reach values well above 10 p.u. in medium voltage systems.

Metal oxide surge arresters are electrical devices that are used in medium voltage systems for overvoltage protection. They are designed to protect equipment from transient overvoltages caused by lightning strikes, switching operations, and other voltage surges. In this answer, we will discuss the dimensioning, testing, and application of MOSAs in medium voltage systems for overvoltage protection.

Metal Oxide Surge Arresters


Dimensioning:
The dimensioning of MOs is critical to ensuring their proper functioning in a medium voltage system. The dimensioning process involves determining the appropriate size and number of MOSAs required to provide adequate protection against overvoltage events. This involves considering factors such as the system voltage, the expected maximum transient voltage, the energy capability of the MOSA, and the protective level required.




Testing:
Before installation, MOs should be tested to ensure that they meet the relevant standards and specifications. The testing process involves verifying the MOSA's energy absorption capability, its insulation resistance, and its ability to withstand high voltage stresses. Testing is usually done in specialized laboratories, and the results of the testing should be documented and made available to the system operator.

Application:
The application of MOs in a medium voltage system involves selecting the appropriate type of MO and determining the optimal location for installation. MOs are typically installed at key locations in the system, such as at transformer terminals, switchgear, and at the point of common coupling. The selection of the MO type is based on the specific application and system requirements. For example, a high-energy MO may be required in areas with high lightning activity, while a low-energy MO may be sufficient in areas with lower lightning activity.

Sample Installation of Surge Arresters 


The diagram above shows an overvoltage protection between phases and between phase and earth (Source: ABB).
  • a) 6-arrester arrangement with Uc ≥ Us for all arresters. 
  • b) Neptune design. A1, A2, A3 and A4 are 4 similar arresters, each with Uc ≥ 0.667 × Us. T is the transformer to be protected
  • for more detailed information, see the PDF below. 
In summary, the dimensioning, testing, and application of MOs in medium voltage systems is crucial for effective overvoltage protection. Properly sized MOs should be selected and tested before installation and installed in strategic locations to provide the required level of protection.

~ End

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  • Title: Overvoltage Protection: Dimensioning, testing and application of metal oxide surge arresters in medium voltage systems
  • Source: ABB

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