About temperature rise test
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Question:What is the length of the connection row for temperature rise test?
Answer:The length of the external connection row for the temperature rise test is not less than 2 meters. According to the standard, the temperature rise should be measured 1 meter away from the test terminals of the switchgear cabinet and compared with the temperature rise of the nearest point inside the switchgear cabinet, which should not be more than 5k, in order to verify that there is no significant import or export of heat into or out of the switchgear cabinet by the external connection row.
Question:How can the temperature rise test meet the customer's requirement of 60 Hz?
Answer:According to IEC-200 standard, the temperature rise test according to 50Hz, need to have 5% margin can be considered to meet the requirements of 60Hz, there is no 60Hz test power supply, there are generally two practices, one is in accordance with the rated current, the calculation of the final temperature rise value of the margin. Another practice is to test according to 1.05 times the rated current, the temperature rise value needs to meet the standard requirements. Generally speaking, the first method is easier to pass the test.
Question:How to simulate the middle cabinet test?
Answer:Temperature rise test needs to be in accordance with the intermediate cabinet test, and the general customer test is often carried out on a single prototype, this will need to simulate the intermediate cabinet, in accordance with the standard needs to be used on both sides of the test cabinet 50mm thick foam board affixed to simulate the intermediate cabinet of the heat dissipation environment. Of course, there are also using multiple switchgear directly for the test, because the test cabinet on both sides of the cabinet generally do not have through current or rated current, so multiple cabinet test is often easier to pass than a single cabinet to paste the foam board.
Question: Double or multi-layer circuit breaker circuit breaker switchgear how to carry out temperature rise test?
Answer: and the current does not exceed 630A three-level switch can be used three-pole series connection, the same way, multi-layer circuit breaker switchgear can be used to test the multi-layer circuit breakers in series, you can also be the main bus into the line, according to the rated current of each circuit breaker through the current test.
About Dynamic Thermal Stability
Question: How to carry out dynamic thermal stability test for grounding contact of circuit breaker trolley?
Answer: Generally, the circuit breaker trolley is only used as protective grounding, according to the determination of resistance less than 0.1 ohm, there is no need to carry out dynamic thermal stability test. Special requirements such as IEEE C37.20.2 standard switchgear, the circuit breaker compartment needs to be used as the grounding contact connection of the grounding trolley G&T, then it is necessary to carry out the dynamic thermal stability test, and can be used together with the grounding trolley test to verify the grounding contact.
Question:How to verify the dynamic thermal stability of the switchgear main bus?
Answer:Since the dynamic thermal stability test is normally performed using one switchgear cabinet, the main busbar needs to be supported separately as an incoming line to avoid damage to the switchgear cabinet caused by peak withstand external connections. Unless more than one switchgear is used for the test, there is no dynamic thermal stability test verification of the main busbar for the general type test.
Question:Do I have to do the dynamic and thermal stability tests of the main circuit and the grounding switch together?
Answer:They can be done together or separately. Doing them separately ensures that the tests are passed one by one, while doing them together fails both the main circuit and the earthing switch.
Question:Can dynamic stability and thermal stability be done separately?
Answer:Can they be done separately, i.e., according to the standard, the short-time withstand current (thermal stability) and peak withstand current (dynamic stability) test values are tested separately to determine whether they pass or not.
Substations and switchgear in an electrical system perform the functions of voltage transformation, system protection, power factor correction, metering, and circuit switching. Electrical power apparatus, such as transformers, regulators, air switches, circuit breakers, capacitors, and lightning arresters, comprise the components necessary to perform these functions.
This guide provides a general overview of the inspection, testing, and maintenance techniques used on switchgear and switchboard assemblies, and their associated components.
Safety Considerations
Warning: Only qualified electrical personnel familiar with the equipment, its operation, and the associated hazards should be permitted to work on switchboards and switchgear. Always ensure that the primary and secondary circuits are de-energized before attempting any testing or maintenance.
This inspection involves visually inspecting the switchgear and its components, such as circuit breakers, disconnect switches, busbars, and control devices, to ensure they are properly installed, free from damage or deterioration, and in compliance with applicable standards and specifications.
Switchgear maintenance is essential for continued reliable operation. Photo: Twins Chip Electrical Industry
Inspect the physical, electrical, and mechanical condition of switchgear or switchboard, including its anchorage, alignment, grounding, and required clearances. When performing acceptance testing, verify that the equipment nameplate data matches project drawings and specifications. This is important because switchboards are designed and rated for specific applications and should not be used otherwise unless explicitly approved by the manufacturer.
The unit should be clean, with all shipping braces, loose parts, and documentation shipped inside the cubicles removed. Keep all documentation in a safe location for maintenance personnel in the future, while loose parts and switchgear tools should be safely stored outside of the enclosure for easy access. When performing maintenance programs, clean the assembly using industry-accepted methods of cleaning.
For initial acceptance, verify that fuse and/or circuit breaker sizes, types, and protective device settings match the project drawings and coordination study. Circuit breakers equipped with microprocessor-communication packages should be programmed with the proper digital address. All instrument transformer current and voltage ratios should also correspond to project drawings.
If corona occurs in switchgear assemblies, it is usually localized in thin air gaps that exist between a high-voltage bus bar and its adjacent insulation or between two adjacent insulating members. Corona might also form around bolt heads or other sharp projections that are not properly insulated or shielded. Corona in low-voltage switchgear is practically nonexistent.
Inspect for evidence of moisture or corona when performing maintenance inspections. On outdoor assemblies, roof or wall seams should be checked for evidence of leakage, and any leaking seams should be sealed with weatherproof caulk.
Prolonged leakage can be identified by rust or water marks on surfaces adjacent to and below leaky seams. The assembly base should be checked for openings that could permit water to drain into the interior, and any such openings should be caulked or grouted. Larger openings should be sealed to prevent rodent intrusion.
All interior and exterior lighting should be checked for proper operation. It is essential for personnel safety that the area be well-lit at all times for emergency response and other security reasons.
Bolted electrical connections should be inspected for high resistance, either by use of a low-resistance ohmmeter (DLRO), calibrated torque-wrench, or infrared scan. Loose bolted electrical connections can lead to higher energy consumption and eventual equipment failure if not properly addressed.
a. When using a low-resistance ohmmeter, investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
b. Bolt-torque levels should be in accordance with manufacturer’s published data. Use NETA Table 100.12 in the absence of manufacturer’s data.
Related: Infrared Thermography for Electrical Distribution Systems
Loose control wires can lead to catastrophic failure if they are part of a critical protective circuit, such as a protective relay for a circuit breaker. Other critical functions, like electrical charging and re-closing of circuit breakers, can be inhibited if poor connections overheat and lose integrity.
Key Interlock Scheme Example. Photo: Kirk Key Interlocks
Related: 3 Critical Lubrication Points for Power Circuit Breakers
Tracking is an electrical discharge phenomenon caused by electrical stress on insulation. This stress can occur phase-to-phase or phase-to-ground. Although tracking can occur internally in certain insulating materials, these materials, as a rule, are not used in medium- or high-voltage switchgear insulation. Tracking, when it occurs in switchgear assemblies, is normally found on insulation surfaces.
Accumulated dirt, oil or grease might require liquid solvents or other alternative methods to be removed. Photo: Wickens Dry Ice Blasting
Electrical insulators should be inspected for evidence of physical damage or contaminated surfaces. Damage caused by electrical distress is normally evident on the surface of insulating members in the form of corona erosion or markings or tracking paths.
Inspect barrier and shutter assemblies for proper installation and operation. All active components should be exercised, mechanical indicating devices should be inspected for correct operation.
Ensure that vents are clear and filters are in place. Screens covering ventilation openings should be in place to prevent entry of rodents or small animals.
Perform resistance measurements through bolted electrical connections with a low-resistance ohmmeter. Measure line/load bus resistance end-to-end and to each distribution section.
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Verify dual-source switchgear bussing is correct at the tie breaker. Compare resistance values to values of similar connections and investigate values that deviate by more than 50 percent of the lowest value.
Example
A-phase bus measures 109 microhms, B-phase bus measures 90 microhms, C-phase bus measures 135 microhms. Investigate values that deviate by more than 50% of the lowest value. In this case, 90 microhms (90 * 1.5 = 135 microhms is the max tolerance).
Related: Fastener Torque in Electrical Systems: Understanding the Basics of Mechanical Connections
Insulation-resistance tests should be performed with a megohmmeter for one minute on each bus section, phase-to-phase, and phase-to-ground. The test voltage to be used is dependent on the rating of the equipment and should be applied in accordance with the manufacturer’s published data. ANSI/NETA Table 100.1 can be used as a guideline if the manufacturer’s data cannot be found.
Insulation-resistance values of bus insulation depend on the voltage class and should be in accordance with the manufacturer’s published data or ANSI/NETA Table 100.1. Values of insulation resistance less than those specified in Table 100.1 or the manufacturer’s recommendations should be investigated.
Related: Insulation Resistance Test Methods, A Beginners Guide
Dielectric withstand testing helps identify potential insulation weaknesses, such as inadequate clearances, contaminated surfaces, or insulation deterioration, that could compromise the safety and reliability of the equipment. By subjecting the switchgear or switchboard to a higher voltage level, any insulation defects can be detected, allowing for corrective actions to be taken before the equipment is put into service.
Perform a dielectric withstand voltage test on each bus section, each phase-to-ground with phases not under test grounded, using a test voltage in accordance with the manufacturer’s published data. If no manufacturer recommendation for this test exists, reference ANSI/NETA Table 100.2.
Apply the test voltage for one minute. If no evidence of distress or insulation failure is observed by the end of the total time of voltage application, the test specimen is considered to have passed the test.
Photo: AC Hipots are recommended for dielectric withstand testing circuit breakers. Photo: HV, Inc.
Important: Dielectric withstand voltage tests should not proceed until insulation-resistance levels are raised above the recommended minimum values. Dielectric Withstand is an optional test when performing routine maintenance per ANSI/NETA-MTS Section 7.1.B.3.
The purpose of this test is to identify any insulation weaknesses, such as cracks, pinholes, or moisture ingress, which could compromise the integrity and safety of the control wiring.
Perform insulation-resistance tests on control wiring with respect to ground. Apply 500 volts dc for 300-volt rated cable and volts dc for 600-volt rated cable for one minute each.
Minimum insulation-resistance values of control wiring should be comparable to previously obtained results but not less than two megohms. This test is optional for both maintenance and initial acceptance. Refer to NETA-ATS/MTS Section 7.1.B.4 for more information.
Important: Units with solid-state components could be damaged if not properly isolated (via removal of plugs and/or fuses) before applying test voltage. Be sure to follow all manufacturers’ recommendations when performing dielectric tests on solid state components.
Solid-state components could be damaged if not properly isolated before applying test voltage. Photo: Square D.
Current transformers are just some of the many instrument transformers found in switchgear and switchboards. Photo: ABB.
The procedure for inspecting and testing instrument transformers is beyond the scope of this guide, as each type has its own procedure. Instrument transformers generally include current transformers, voltage transformers, and control power transformers. Conduct electrical tests on instrument transformers in accordance with ANSI/NETA Section 7.10. Where applicable, testing of instrument transformers generally includes:
Results of electrical tests on instrument transformers should be in accordance with ANSI/NETA Section 7.10.
Related: 6 Electrical Tests for Current Transformers Explained
It’s essential that circuit breakers be tested and maintained to ensure proper operation during electrical faults. Photo: Vacuum Interrupter Testing
The procedure for the inspection/testing of circuit breakers and switches is beyond the scope of this guide, as each type and voltage class has its own procedure. Conduct electrical tests on circuit breakers in accordance with ANSI/NETA Section 7.
Where applicable, testing of circuit breakers generally include:
Results of electrical tests on circuit breakers and switches should be in accordance with ANSI/NETA Section 7.
Related: Transfer Switch Testing and Maintenance Guide
These tests measure the resistance of the grounding system to determine if it is capable of safely dissipating fault currents, protecting personnel from electric shock and preventing damage to equipment.
Perform resistance measurements through bolted ground connections with a low-resistance ohmmeter, if applicable. Compare bolted connection resistance values to values of similar connections and investigate values which deviate from those of similar bolted connections by more than 50 percent of the lowest value.
Determine the resistance between the main grounding system and all major electrical equipment frames, system neutral, and derived neutral points by means of point-to-point testing using a low-resistance ohmmeter. Values which exceed 0.5 ohm should be investigated.
Switchgear 2-Point Ground Resistance Example. Photo: TestGuy.
Metering devices are verified using secondary voltage and current levels. Photo: EATON
Metering device inspections and tests are beyond the scope of this guide. Generally, metering devices are verified using secondary voltage and current levels supplied by a relay test set or other secondary source.
Where applicable, testing for switchgear metering may include the following measurements at various points of full scale:
Determine accuracy of all meters and calibrate watthour meters in accordance with ANSI/NETA Section 7.11.
Current-injection tests will prove current wiring is in accordance with design specifications. This is an optional test according to ANSI/NETA.
The procedure for System Functional Testing exceeds far beyond the scope of this document. System function tests should be performed in accordance with ANSI/NETA-ATS Section 8 during initial switchgear/switchboard acceptance. Results of system function tests should be in accordance with ANSI/NETA-ATS Section 8.
Also See: NETA ECS: Standard for Electrical Commissioning Specifications
Moisture accumulation is prevented by heat and air circulation. It’s important, therefore, to ensure that the heating and ventilating systems are functioning properly to reduce internal condensation.
Tip: Infrared cameras are the easiest way to verify heater functionality without making contact with energized electrical equipment.
Inspection and testing procedures for surge arresters exceed the scope of this guide. Surge arresters should be performed in accordance with ANSI/NETA-ATS Section 7.19. Testing these devices typically consists of applying an overpotential across the arrester to ground and observing the leakage current.
Related: Surge Arresters: Selection, Application and Testing Overview
Phasing checks should prove the switchgear or switchboard phasing is correct and in accordance with the system design.
Remember to always follow safe work practices when performing energized work!
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