How to test the contact resistance of disconnect switches in 110KV/220KV substations?

The loop resistance test work is mainly used to test the contact resistance of the dynamic and static contacts of the disconnect switch. The conductive loop resistance of the disconnect switch mainly depends on the contact resistance of the dynamic and static contacts. The existence of the contact resistance increases the conductor when it is energized. The loss of the contact point increases the temperature at the contact point, and its value directly affects the current-carrying capacity during normal operation, and to a certain extent affects the short-circuit current cut-off capacity, and is also an important data reflecting the quality of installation and maintenance work.

1. Comparison of test methods

At present, there are three main test methods for loop resistance testers:
a. Bridge method: When using a double-arm bridge to measure the resistance of the conductive loop of the disconnect switch, since the measurement loop passes through a weak current, it is difficult to eliminate the oxide film with a large resistance, and the measured resistance value is too large. When the current is small, it is difficult to form shrinkage at the contact point of the contact, that is, the shrinkage resistance cannot be measured.
b. Voltage drop method: When a DC current is applied to the circuit under test, a voltage drop will be generated on the contact resistance of the circuit, the current value and voltage value passing through the circuit are measured, and the contact resistance is calculated. The test process is cumbersome, and there are certain errors in the manual calculation of the measurement results.
c. Micro-ohmmeter method (loop resistance tester): The principle is the voltage drop principle, but the measurement, calculation and other aspects are all processed by a single chip, which greatly reduces the workload.

Most of the maintenance personnel use the method of measuring the loop resistance to judge the quality of the contact of the conductive part, and think that as long as the loop resistance is within the qualified range, the maintenance work of the conductive part is successfully completed. In fact, the circuit resistance value given in the product manual is the value of the entire conductive circuit, which includes the overall resistance and contact resistance of the terminals, conductive tubes, conductive joints, contacts and other parts, and the range is large, generally 20% The error range cannot directly reflect the change of the contact resistance of the contacts. The actual test proves that the loop resistance of 2 pairs of contacts and 4 pairs of contacts is only a few microohms, which are all within the qualified range given in the product manual. Therefore, experienced loop resistance testers can often find important data clues in the small differences in test results, and further analyze the health status of the equipment.

The test of loop resistance is a different concept from the test of DC resistance. Both tests are used to test resistance, and the DC voltage drop method is also used. Many people think that they are the same and can be used interchangeably. In fact, the two are still different. The main is resistive load, and they are all micro-resistance tests. The DC resistance tester mainly tests inductive loads, and the resistance range is relatively large. The two cannot be used interchangeably. It may damage the equipment. At present, there is no composite type of two instruments on the market, because if two kinds of equipment are combined into one device, it needs to meet both high current and inductive load with relatively large test resistance, and the cost is very high. , the device is also very large.

2. Requirements for test equipment

Contact resistance includes the additional resistance that occurs when static and dynamic contacts are in contact with each other. It consists of two parts: shrinkage resistance and surface resistance of the dynamic and static contact parts.

There are various reasons for the unqualified contact resistance of the circuit breaker, including burnout of the contact when breaking a large short-circuit current; due to the poor adjustment of the switch structure and the inability to fix it, resulting in changes in the stroke, when the overtravel is seriously unqualified, it will cause Changes in contact pressure or contact area; after the circuit breaker is debugged and installed, it has not been put into operation for a long time, causing the surface of the dynamic and static contacts to oxidize, and the contact surface resistance increases; long-term operation will deform the spring and reduce the contact pressure; long-term operation of the mechanical part The mechanical wear caused by the oil-less circuit breaker may also cause an acid reaction due to the unqualified acid value of the insulating oil, which may erode the contact surface. Or floating impurities in the oil, after the short-circuit current is broken between the dynamic and static contacts. Residual particulate carbonaceous, metal powder increases the contact resistance.

pressure or contact area; after the circuit breaker is debugged and installed, it has not been put into operation for a long time, causing the surface of the dynamic and static contacts to oxidize, and the contact surface resistance increases; long-term operation will deform the spring and reduce the contact pressure; long-term operation of the mechanical part The mechanical wear caused by the oil-less circuit breaker may also cause an acid reaction due to the unqualified acid value of the insulating oil, which may erode the contact surface. Or floating impurities in the oil, after the short-circuit current is broken between the dynamic and static contacts. Residual particulate carbonaceous, metal powder increases the contact resistance.

There are many factors affecting contact resistance of disconnect switches, which need to be considered comprehensively. Including, material properties: resistivity, hardness, chemical properties, mechanical strength and resistivity of metal compounds; contact form: point contact, line contact, surface contact; contact surface condition: when the contact surface forms an oxide film (except for silver) oxide The membrane has a much greater electrical resistance than the metal itself; the contact pressure; the roughness of the contact surface.

Therefore, the comprehensive requirements for test equipment should consider the following aspects:

a. High current: Using the latest switching power supply technology, it can continuously output large current for a long time, overcome the drawbacks of the instantaneous current of the pulsed power supply, and can effectively break down the oxide film of the switch contact to obtain good test results.
b. High stability: Under severe interference conditions, the last digit of the LCD screen data can be stable within the range of ±1 word, the reading is stable, and the repeatability is good.
c. High precision: Adopt dual-channel high-speed 16-bit Σ-Δ AD sampling, the latest digital signal processing technology, the highest resolution can reach 0.01μΩ, it needs to be able to reach 0.01μΩ resolution and the output is stable, and its performance exceeds that of imported high-current micro-ohmmeters.
d.Intelligent: Using imported high-performance CPU, the system automatically switches the range according to the signal size during measurement, which ensures the test accuracy of the product. The over-temperature protection circuit can automatically stop the output current when the instrument exceeds the set temperature to ensure the safe use of the instrument.
e. High quality: The key components are all imported components. The influence of the ambient temperature on the measurement results is effectively eliminated through the cleverly designed temperature compensation circuit. The use of military connectors enhances the anti-vibration performance.
f. Powerful functions: the current can be freely selected between 50A and 100A, and the test time can be arbitrarily set within 5s~599s, which overcomes the defects of the instrument not being able to set the measurement time or the continuous working time is too short, and has high performance.

The above analysis illustrates the importance of ensuring reliable measurement of the circuit resistance of the isolation switch, and the necessity of detecting the circuit resistance. No matter when the product leaves the factory, before the product is put into operation or during regular maintenance, it is necessary to measure the loop resistance.

3. Summary

Due to the different structures and rated currents of the disconnect switch contacts, the circuit resistance of the contacts is not uniformly specified. In order to make the maintenance work of the isolating switch scientific and standardized, it is recommended that the isolating switch manufacturer indicate the rated value and allowable deviation of the contact loop resistance in the product installation and operation manual. For products that have been delivered, if there is no such parameter in the manual, you can ask for it from the manufacturer separately, or you can use the detector to test the new product before it is put into operation, and record the pressure value in the book for future use. At the same time, it is hoped that the manufacturer can indicate the contact resistance value of the contact parts of various rated current levels as much as possible, so as to facilitate the comparison before and after maintenance. In addition, it is recommended to add battery power to the detector to facilitate measurement in places without AC power.

The test items are required for 110kV/220kV substation acceptance and maintenance testing(with corresponding tester types):

Test Object	Testing Item	Tester Type
Cable	Cable AC Hipot Testing	JYCX	Details
	Cable Insulation Resistance Testing	JYM KYORITSU	Details
Transformer	Insulation Resistance of Winding	JYM KYORITSU	Details
	DC Leakage Testing in 1min	JYDHV	Details
	Winding Resistance Testing	JYR50S	Details
	Winding Deformation Testing	JYP	Details
	Turns Ratio Testing	JYT-A	Details
	AC Hipot Testing	JYCX	Details
	Short Circuit Impedance Testing	JYW6300	Details
	Dan Delta Testing	JYC	Details
	Oil Breakdown Voltage (BDV)Testing	JY6611	Details
	Oil Tan Delta Testing	GTD-61A	Details
Switchgear	Circuit Breaker/Disconnector Resistance	JYL-200B	Details
Relay	Relay Performance	JY7003G	Details
On-load	On-load Switchgear Testing	JYK-I	Details
	Tap Changer Dynamic Testing	JYK-I	Details
Instrument   Transformer	Current Transformer (CT)Testing	JYH-C	Details
	Potential Transformer (PT)Testing	JYH-C	Details
Surge Arrestor	Zinc Oxide Characteristics Testing	JY6800	Details
	1mA DC Reference Voltage	JYDHV	Details
Capacitor	Capacitor Current	JY6700	Details
	Capacitor Inductance	JY6700	Details
Ground Grid	Grounding Down Lead Earth	JYD	Details

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