When it comes to the cohesive protection of man, machine and infrastructure from electrical mishaps, Circuit Breakers (CBs) play a linchpin role, especially in the high-voltage (HV) environment of Substations.
With rapid advancements in technology, while the quality of HV CBs has definitely improved over the last few years leading to a significant drop in CB failures, CBs in essence, are mechanical devices that are prone to breakdowns if timely performance-assessment tests are not conducted to ensure their smooth functioning.
While various Utilities and Test & Measurement (T&M) service providers have different approaches to testing HV CBs, here is a quick snapshot of the 9 fundamental must-do tests as prescribed by IEC 62271-100 standard to ensure the optimal performance of HV CBs.
1. Main Circuit Dielectric Test
Popularly known as ‘Dielectric Withstand Test’, this test is primarily conducted to check the CB’s withstand capacity against a range of power frequencies and impulse voltages.
As a best practice, a dry, power frequency voltage test should be applied for continuous 60 seconds. Adhering to IEC 60061-2 standard, the voltage to be applied should be 1.5 – 1.8 times higher than the CB’s rated voltage at a frequency range of 45 to 65 Hz.
Alternately, unless otherwise specified by the concerned Technical Committee, the ideal values of the test voltage should be maintained within ±1% of the specified level throughout the test.
For test durations exceeding 60 seconds, the test voltage values should be maintained within ±3% of the specified level throughout the test.
2. Main Circuit Resistance Test
As prescribed by the IEC 62271-100 standard, the minimum current across the CB should be 50 Amps. The DC millivolt drop should be measured across it to calculate the main contact resistance value based on the Kelvins 4-Wire principle.
At any given point of time, the main contact resistance value should not exceed 150 micro-ohms.
3. Auxiliary and Control Circuit Tests
Testing Auxiliary and Control Circuits primarily consist of three unique tests:
1) Inspection and verification to determine whether the Auxiliary and Control Circuits conform to the prescribed Circuit and Wiring diagrams,
2) Functional testing of all low-voltage circuits to determine if Auxiliary and Control Circuits are functioning smoothly with the other parts of the Switchgear and Controlgear, and
3) Visual inspection to ascertain if the Auxiliary and Control Circuits can be safely accessed without risking an electric shock that may occur due to their direct contact with the Main Circuit.
4. Tightness Tests
Manual in nature, these tests should be conducted to ascertain the tightness of the CB’s connections.
Depending on the type of the Switchgear, the below three tightness tests should be conducted:
1) Controlled-pressure and Closed-pressure tests for Gas-insulated Switchgears,
2) Sealed-pressure test for Gas-insulated and Vacuum-insulated Switchgears, and
3) Liquid tightness test to identify leakages, if any.
5. O-C Timing Test
When it comes to preventing an electrical mishap, the opening and closing time of the CB hold pivotal importance. In the event of an electrical fault, the CB should promptly open and trip the current and immediately close when the fault is cleared.
Industry best practices recommend that HV CBs should open (and trip) within 30 to 50 milliseconds of the fault occurring and close within 60 to 80 milliseconds of the fault getting resolved.
However, if the fault continues to exist even after the CB has closed, the CB should immediately reopen, re-trip the current and close again promptly. It is precisely because of unpredictable incidents such as these that make testing the Opening-Closing (O-C) timings of CB crucial.
6. Coil Resistance Test
CBs are equipped with dedicated coils for opening (and tripping) and closing. Depending on the situation, the relay sends a ‘coil current’ that instantly activates and actions the concerned coil. To ensure that a CB continues to work faultlessly, testing the resistance values of each coil becomes critical.
7. DCRM Test
Dynamic Contact Resistance Measurement (DCRM) is an advanced test for assessing the condition of a Circuit Breaker’s main and arcing contacts.
While one can determine the condition of the main contact from its contact resistance test, it is practically impossible to know the condition of the arching contact as the CB is closed when arcing occurs.
To assess the condition of the arcing contact, a current of 100 Amps should be injected across the CB during its Close-Open (C-O) action to measure the corresponding voltage drop of the current. A DCRM test should never be conducted during the CB’s Open-Close (O-C) action.
In dynamic conditions, millivolts across arcing contacts are measured to calculate the arcing contact resistance using Ohm’s Law (V/I = R).
Arcing contact resistance values are always higher than the main’s contact resistance values because Silver Alloys or few other alloys (that have higher conductivity properties) are used as the primary materials for main contacts whereas Tungsten Alloys or few other alloys (that have relatively low conductivity values) are used as the primary materials for arcing contacts.
8. Travel Tests
Every CB’s interrupter includes two mechanisms – fixed and moving. When a CB closes, the moving component travels towards the fixed component to complete the loop.
To ensure that both these components perform unhindered, travel tests are undertaken to measure their efficacies. There are four broad parameters to be considered when conducting travel tests:
1) Stroke Length: The total travel distance of the contacts from their resting position in a specific state. E.g. In a closed state, the travel distance from their resting position to the open state, and vice versa.
2) Damping: This is measured directly on the travel curve by calculating the velocity in the damping zone or by calculating the time between two predefined points on the travel curve in the damping zone.
3) Contact Wipe: The total length that is measured from the initial contact touch to the final resting position after the concerned operation.
4) Over-Travel: Measured directly from the travel curve, over-travel refers to the maximum displacement that the contacts reach past their resting position.
5) Rebound: The minimum displacement that occurs (after the over-travel) to the final resting position of the contacts.
9. Aesthetic Tests
Finally, we come to aesthetic tests that are equally important as the above-mentioned 8 tests.
Aesthetic tests include a thorough verification of the Switchgear and Controlgear to ascertain compliance with their respective purchase specifications. These tests include a meticulous check of the language and information used on the respective nameplates, identification of auxiliary equipment, if any, the colour and quality of the paint and corrosion-protection materials used for metallic surfaces and the values of the resistors and capacitors connected to the main circuit.
In today’s age where transmission networks have become increasingly complex, the optimal functioning of CBs is of paramount importance to prevent transmission system breakups and equipment destruction.
As India’s leading T&M instruments manufacturer, SCOPE has been offering state-of-the-art CB testing solutions for the past three decades to leading power companies in India. Please visit http://www.scopetnm.com/test-and-measurements/circuit-breaker-testing-equipment/hisac-ultima to know more or get in touch with us on email@example.com.