Top 5 Essential Testing & Maintenance Requirements For Switchgear

1.Typical Switchgear Routine Tests

There are several routine tests that are mandatory in the IEC62271–100 standard. This is one standard of many others that should be used as a reference to carry out tests. The ISO9001 standard ensures that the manufacturers switchgear units match the same performance for each unit.

The power systems reliability can only be maintained when a routine maintenance regime is in place and conducted routinely. Circuit breakers that fail to trip properly during a faulted condition can cause fires, equipment damage and increased Arc Flash Hazards to workers performing electrical maintenance or operations.

Certain additional tests or inspections may be required and stipulated by the manufacturers to ensure that the unit will function correctly in service and meet the declared ratings.

Mandatory routine tests include:

1. Product design checks.
2. Visual Checks.
3. Frequency voltage withstand tests.
4. Voltage withstands tests on the control and auxiliary circuits.
5. Measurement of the resistance of the main circuit.
6. Mechanical operating tests.

These are examined each in turn in the following:

Frequency Voltage Withstand Tests On The Main Circuit

The frequency voltage withstand test should have an identical procedure, measurement value, and test duration as the frequency type test. For example, 12 kV rated switchgear should be tested phase-to-phase and each phase to earth at 28 kV for 1 min.

Prior to carrying out the tests it is recommended that voltage transformers and surge suppressors are isolated or disconnected from the main primary circuits.

2. Insulation Resistance Tests

During insulation resistance tests, we have to take temperature readings to apply a correction of temperature (we do this because resistance is often very sensitive to temperature). During the tests, it’s important that we also apply the correct voltage to the insulation, and the voltage level is based on the voltage class specified by the equipment. We take multiple insulation readings, the first at 30 seconds and the next at 1 minute. If a 10 minute reading is taken of a winding, we will divide the 10 minute reading by the 1 minute reading. This will give us the Polarisation Index.

The next step is to calculate the Dielectric Absorption Ratio, which is the 1-minute reading divided by the 30-second reading. If we have access to take a 10-minute reading, we’ll calculate the Polarisation Index by dividing the 10-minute reading by the 1-minute reading. Once all readings and calculations are completed, we can correctly grade the insulation using our results.

3. Over-Potential Test (HI POT)

In a similar way to insulation resistance tests, over-potential tests are performed on a breaker. These tests are a lot more likely to be used on medium and high voltage breakers. A typical over-potential critical test analyses vacuum circuit breaker bottles to determine their integrity. This test involves applying a high voltage across the bottle opening to verify the bottles integrity. Without carrying out this test, we wouldn’t be able to find out whether the bottle still has a vacuum.

It’s important to note that some manufacturers of breakers require the use of an AC HI POT. During the test, it’s important to be aware of the possible production of X-Ray radiation in the event of there being an arc present within the bottle. Safety precautions for this test include standing at a distance of at least 3 feet away and ensuring that we have metal between ourselves and the bottle. Despite the potential radiation being produced being minimal, awareness is important.

4. Voltage Withstand Tests On The Control And Auxiliary Circuits

Voltage withstand tests use a 2 kV voltage to test the earth on the secondary wiring. During these types of tests electronic equipment may need to be disconnected.

Main circuit resistance measurements

The resistance of each phase main primary circuit should be measured using specialised equipment and compared with the type test values and the specified design acceptance criteria.

5. Mechanical Operating Tests

Evaluating the circuit breakers mechanical operation is critical, the mechanical operating time should be recorded and compared with the type test values. Not only the mechanical operating time is tested but the differences in operating times at a minimum, nominal, and maximum operating voltages, will confirm that the correct trip and close coils are fitted.

The test will also confirm that the correct mechanism charging motor or solenoid is installed by measuring the time it takes to charge the switchgear. Figure 1 shows a typical mechanical operation test of a circuit breaker.

The record traces shown in Figure 1 are as follows:

1. The vertical dotted lines are time markers, usually shown at 10 m/s intervals, increasing
2. from left to right.
3. The top trace shows the current flowing through the trip coil.
4. The next trace is similar and shows the current flowing through the closing coil.
5. The third trace from the top records the movement of the moving contact. This is usually obtained by attaching a linear transducer, or rotary potentiometer, to a mechanism component that moves with and, therefore, mirrors the movement of the contacts.
6. The bottom three traces measure the voltage across the main contacts and are usually obtained by applying a low voltage across the incoming and outgoing terminals of the switchgear. The purpose of these traces is to indicate when the contacts close and open.

Whether the circuit breaker is a Low Voltage or High Voltage should be considered when examining the timing of the breaker.

A Low Voltage Circuit Breaker is tested using either a Secondary Injection Test Set or a Primary Injection Test Set.

A secondary injection set applies a current (simulating the sensor or CT output) to the current input terminals of the overcurrent device on the breaker. The pick-up time is verified by using the correct characteristic curve for the overcurrent protection device. Every overcurrent device has its own characteristic curve.

The LV protection Long Time (overload condition), Short Time (Arcing Fault), Instantaneous (Bolted Fault) and Earth Fault (self explanatory) are tested.

Secondary injection enables us to verify the pickup, trip times and the function of the trip coil and the overcurrent device.

However, primary injection testing works to push current through the main circuit to trip the circuit breaker. This tests the same functions as the secondary injection, but the key difference is the high current that is used through the main circuit using a primary injection test set.

The test is timed in association with medium and high voltage circuit breakers, which is done for the same reason as the low voltage breakers but with different equipment being used. A circuit breaker travel analyser is used to perform the travel of the main current carrying contacts. The way that the travel analyser works is as follows:

1. With the breaker closed, a signal is sent to the trip coil which activates the said coil. At the same time, the analyser receives a signal that enables it to begin recording.
2. Once the trip coil is activated, the mechanical prop is knocked out from underneath the latch. Without the mechanical prop, the breaker is no longer closed, so it’s gravity and the trip springs that are pulling the contacts open.
3. The travel analyser is then connected to the breaker linkage using the transducer device, which moves with the linkage and sends a signal to the analyser. The signal starts as soon as the trip signal is applied, which begins the timing, which is measured between the trip signal starting and the contacts parting.
4. The received signal from the transducer is sent directly to the analyser, which is then displayed as the movement of contacts.
5. A travel trace is then drawn, which can be used to measure the overall travel of the contacts in order to get the following information:

• The opening speed of the contact
• The total distance of the contact travel
• Knowledge of any type of binding linkage that can affect the travel or the opening speed of the contact
• Any bounce of contact at the end of the opening stroke, which can indicate dashpot problems
• A dashpot is a device that is similar to a shock absorber which slows the linkage down at the end of the opening stroke.

The sequence of events shown in the record, starting at the left-hand edge, begins with the main contacts in the OPEN position, confirmed by the bottom three traces. The second from the top trace then shows current starting to flow in the closing coil. After that, the moving contacts are shown to move to the CLOSED position; the bottom three traces indicating contact touch, which for finger type contacts, is before the main contacts reach the end of their travel.

The measurements made from the mechanical operations record are as follows.

The Circuit Breaker Closing Time
This is the time for both contacts touch from the initiation of the closing coil current. The closing time can be measured with the trip coil voltage at its minimum, nominal and maximum rated value. During this test the tester would analyse the closing times between the phases to make sure that they all close simultaneously. The time difference time must not be greater than that measured in the prototype during the short-circuit-type tests.

Contact Closing Speed
The contact closing speed is measured at the instant of contact touch. When this is measured manually, it is normally taken as the straight-line slope joining a point at about 5 percent of the contact stroke before contact touch with the actual point of contact touch.

Contact Stroke
The contact stroke is simply the linear distance between the OPEN and CLOSED positions.

Circuit Breaker Opening Time
This is the time for the contacts parting from the initiation of the trip coil current. A spread in time between the first and last contacts to part is less important during an opening operation than during a closing operation.

Contact Opening Speed
This can either be measured similarly to that used for measuring the contact closing speed or measured from the contact part to the point where the mechanism dashpot starts to slow down the contact speed.

The method used should be the most appropriate for the type of interrupter used.

Contact Travel
Full contact travel to the OPEN position should be confirmed by measurement. Contact travel records often show some apparent oscillation before the contacts come to rest in the OPEN position. Quite often, this is not real and only reflects oscillations in the travel transducer drive linkage.

This concludes the review of the measurements usually made following mechanical operation tests.

Maintenance Costs
One of the major reasons to perform circuit breaker maintenance is to save money over a long period of time. One failure can cost many times more than the cost of normal maintenance and with much less down time.

Circuit breakers that have had normal maintenance performed are less likely to fail. If the breaker is less likely to fail then the danger to personnel and surrounding equipment is less likely to happen. Also a fault downstream of the breaker will be isolated quicker, generating less damage.