Power loss measurements in medium voltage switchgear (RMU – Ring Main Unit)

Steady state temperature rise

Switchgear is exposed to electrical and mechanical stress. Stress is a mechanism that reduces the lifetime of the switchgear. Typical stress factors are abnormal environment or abnormal operating conditions. Temperature is a concern when talking about operating conditions. The higher the temperature, the accelerated aging are expected.

For all switchgear, a type test is executed. The type test shall provide the necessary characteristics of the switchgear, and includes measurements of resistance of the main circuit, mechanical tests, making and breaking tests and temperature rise test.

The temperature rise test demands from the IEC standard 62271-1 are given by very strict guidelines and requires relatively large resources of the manufacturer to execute. It is beneficial to have easier methods to predict and verify the temperature rise of switchgear. Such a method could benefit when dealing with the early stages of designing switchgear.

Cigré (International Council on Large Electric Systems) is an association where experts from all around the world work together in working groups to improve electric power systems.

The result of this thesis will be compared with the simulation results of the respective companies involved in the working group.

Thesis Objectives

The aim of the thesis is to assemble a realistic test device. The test device should have a design that is close to a real switchgear. The test device should consist of three phases. The resistance of the path should be within reasonable limits to get a proper power loss and temperature rise.

The results found by measuring and tests should be compared to relevant results from similar experiments.

To fulfill this aim, the following objectives are performed:

• The switchgear is to be assembled to a test device.
• Calculate theoretical bulk resistance.
• Measure bulk and contact resistance.
• Estimate and measure resistance during full load conditions.
• Measure the temperature rise at full load conditions.
• Compare the steady state temperature with relevant IEC limits
• Estimate and measure total power loss during full load conditions
• Investigate the influence of magnetic bolts in the bushing-connection
• Calculate the heat transfer coefficient

System description

In this chapter, a description of the test device and the components used to conduct the experiments will be given.

RMU enclosure

The RMU enclosure forms the closed enclosure of the test device. The RMU enclosure is used for the temperature rise tests. Figure 1 shows a sketch of the RMU enclosure with dimensions and definitions of the surfaces.

In this report, the side where the switchgear normally is operated from is defined as the back wall.

Figure 2 shows a picture of the enclosure seen from the front. The content in the red circle in Figure 2 are components used to make the current path stable. These components are not expected to not affect the results of the experiments.

Their only purpose is to make sure the current path is stable.

The test device in Figure 2 differs from a real design of switchgear. There are less components involved in the current path and less equipment used for supportive purposes. The mechanisms to operate the switchgear are neither involved in the test device.

These differences between the test device and a real switchgear are most likely to affect the heat transfer within the enclosure.