MV switchgear emperature rise experiment

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What is a Temperature Rise Test?

MV switchgear are electrical devices that carry current for long periods. Due to the resistance present in conductors, heat loss occurs when current passes through them. Additionally, alternating electromagnetic fields acting on ferromagnetic objects and insulating media around the conductors cause ferromagnetic loss and dielectric loss. All these factors contribute to the heat sources. The heat generated by these sources raises the temperature of the switchgear. Simultaneously, the switchgear dissipates heat to the surrounding media in different ways, maintaining a relatively constant temperature. The difference between this temperature and the test environment temperature is the stable temperature rise of the switchgear.

Conducting a temperature rise test on medium-voltage switchgear is one of the essential type tests for such equipment. This test primarily involves using a temperature rise test device to supply rated current to the switch for an extended period until the temperature rise stabilizes (the temperature change at all measurement points does not exceed 2°C within 3 hours, indicating stabilization). The measured temperature rise values are then compared with the standards to determine whether the temperature rise at various parts meets the required standards. The Chinese national standard GB11022-1999 “General Technical Requirements for Medium Voltage Switchgear and Controlgear” specifies the temperature rise limits and test methods for different parts of medium-voltage switchgear.

Temperature Rise Standards for Switchgear

1.Chinese national standards stipulate that three-phase devices should generally be tested with three-phase current. When the magnetic circuit of the three-phase device is integrated, and the influence of proximity effects is significant, a three-phase current test must be conducted. However, the following conditions allow for single-phase current testing:

• For switchgear with a rated voltage above 35kV, if the center distance between phases exceeds 460mm and the rated current is 1250A or below, or if the center distance between phases exceeds 1m, single-phase current can be used for single-pole testing.
• For switchgear with a rated voltage of 63kV and above, single-phase current can be used for single-pole testing.
• For multi-break circuit breakers with a rated voltage of 110kV and above, if there is no thermal interaction between units, a single-phase current unit test can replace the test for the entire circuit breaker.

Temperature Rise Test Methods for Switchgear

Test Conditions

The maximum deviation of the test current should not exceed +2%; the current waveform should be a sine wave; the power frequency deviation should be between -5% and +2%; the temperature rise of the wire 1m away from the test product terminal should not differ by more than 5K from the corresponding terminal temperature rise; the test environment temperature should be between 0~40°C; the wind speed in the test environment should not exceed 0.15m/s for indoor products and 0.5m/s for outdoor products. If the test conditions meet these requirements and the temperature rise test complies with the standards, the test is considered valid.

Test Circuit: Refer to the instructions for the temperature rise test device for the appropriate test circuit.

Adjusting the Temperature Rise Test Current

1. 1. When conducting a three-phase test with three-phase equipment, if there is current imbalance caused by voltage imbalance, a simple and economical solution is to use silicon steel sheets stacked into a gate-shaped iron core around each connected conductor, maintaining a proper distance from the test product terminal. Adjusting the number of silicon steel sheets and the air gap at the iron core closure can change the impedance to balance the three-phase current.
   2. When using three single-phase devices for a three-phase test, adjust the current balance using a voltage regulator for each phase.
   3. To shorten the test time, during the initial stage of energization, a current higher than the rated value can be used to heat the test product to about 80% of the expected temperature. Then, reduce the current to the rated value until the temperature rise at all parts stabilizes.
2. IEC 62271-1 Clause 6.5.2: (Cpoy from ietresearch) The temperature difference between the main circuit and the temporary test connections measured at a distance of 1 meter should not exceed 5K. To verify this technical aspect, a temperature rise test was conducted. Several arrangements were made according to the standard before performing the test. A medium voltage (MV) vacuum insulated switchgear was selected for the test, and the thermocouple method was implemented. The thermocouple points included the incoming busbar contacts, vacuum circuit breaker finger contacts, busbar compartment, and circuit breaker compartment. During the test, a 5A current from the load group was fed into the incoming line. The current then flowed through the conductors and contacts of the switchgear, generating heat. This, in turn, raised the temperature of the switchgear, especially at the thermocouple points. A digital thermometer was used to measure the temperature, and the test duration was seven hours.

This table copy from ietresearch

This table copy from ietresearch

Degatech eco-friendly switchgear Temperature rise test report

Degatech Electric remains dedicated to providing high-quality, reliable, and safe electrical solutions. The successful completion of the temperature rise test is a testament to our unwavering commitment to excellence and our continuous efforts to innovate and improve our products.

Degatech eco-friendly switchgear is testing

How to Address High Temperature Rise in Medium Voltage Switchgear

Key Reasons:

Laboratory Test Conditions:

The data obtained from type tests are usually completed in a laboratory environment with a duration not exceeding 8 hours. These conditions do not account for the cumulative effect of temperature rise and cannot be equated with equipment that operates continuously and generates heat over a long period.

Different Expansion Effects of Metals:

The expansion coefficients of different metals vary significantly. Steel bolts have a much smaller expansion coefficient compared to copper or aluminum busbars. Especially in bolt-type equipment with copper waterproof joints (JG-T-M), the expansion and contraction of aluminum or copper compared to iron will differ with changes in load current and temperature, causing creep. Creep is the slow plastic deformation of metal under stress and is highly influenced by the temperature at the joint.

When the operating temperature at the joint exceeds 80°C, the metal expands due to overheating, causing a misalignment of the contact surface and forming tiny gaps that oxidize. When the load current decreases and the temperature returns to the original contact position, the oxidized film on the contact surface prevents direct metal-to-metal contact as in the original installation.

Each temperature change cycle increases the contact resistance, which in turn generates more heat in the next cycle, further deteriorating the joint’s condition, creating a vicious cycle.

Improper Pressure of Connecting Bolts:

Some installation or maintenance personnel believe that the tighter the connection bolts, the better. However, this is not the case, especially for aluminum busbars, which have a low elasticity coefficient. When the nut pressure reaches a critical value, if the material strength is inadequate, further increasing the pressure can cause deformation and bulging of the contact surface, reducing the contact area, increasing contact resistance, and affecting the conductor contact performance.

Conductivity of Conductor Materials:

The selected conductor materials might not meet the required electrical conductivity standards, often due to the insufficient purity of the raw materials.

Other On-site Factors:

Improper installation and maintenance techniques, such as inadequate processing, connection, or installation of busbars. Issues include poor treatment of the busbar contact surface, unevenness, roughness, and the absence of specialized electrical grease, leading to reduced effective contact area, increased contact resistance, and heating.

Solutions:

• • Enhance Testing Conditions: Extend the duration of type tests to better simulate real-world operating conditions and account for the cumulative effects of temperature rise.
  • Use Compatible Materials: Ensure the use of materials with similar expansion coefficients to avoid differential expansion and contraction that can lead to creep and increased contact resistance. Consider using connectors that can accommodate differential expansion or include stress-relief designs.
  • Proper Tightening of Bolts: Follow manufacturer specifications for bolt torque to avoid over-tightening, especially with materials like aluminum. Use torque wrenches to ensure bolts are tightened to the correct pressure.
  • Improve Material Quality:Use high-purity conductor materials that meet the required standards for electrical conductivity.
  • Improve Installation and Maintenance Practices:Ensure proper processing, connection, and installation techniques. Treat busbar contact surfaces appropriately by making them smooth and flat, and apply specialized electrical grease to enhance contact and reduce resistance.
  • Regularly inspect and maintain connections to prevent oxidation and ensure consistent contact quality.

Thank you for reading.