The choice of instrument for measuring resistance in electrical equipment

One of the most common and simple methods for assessing the state of crosspoint of high-voltage switches, disconnectors and separators, short-circuits, current distributors, transformers, electrical machines and similar electrical equipment is to measure the electrical resistance to direct current in them.

LLC SKB EP develops and manufactures instruments for monitoring and diagnosing high-voltage switching equipment. One of the assortment niches of the company are high-precision and mobile instruments for measuring electrical resistance:

  • microohmmeters (MIKO-1MIKO-10MIKO-21) for measuring the transition resistance of contacts of switching instruments and contact connections of current conductors;
  • milliometers (MIKO-7М(А)МIKO-8М(А)MIKО-9А) for measuring the active resistances of circuits with high inductance (transformers, electric motors, electromagnets, etc.);
  • micromillikilohmmeter (МIKO-2.3), which uses the tasks of previous instruments and temperature measurement of different environments.

The SKB EP equipment considers the peculiarities of the physical processes in electrical equipment and technical requirements to ensure the accuracy of measurements and the convenience of its users. Thus, we propose to descry the criteria and parameters that consider in the SKB EP instruments, as well as what you need to pay attention to when choosing one or another type of instruments:

For microohmmeters (instruments for measuring transient resistance), the main rule is to ensure a large current through the transition resistance. The oxide film and non-metallic inclusions of the contacts causes nonlinear dependence of the transition resistance on the flowing current. So, to get reliable values, we recommend taking measurements at high currents through the transition resistance, which reflect, for example, in IEC 56 (I≥50A) and ANSIC 37.09 (I≥100A) standards.

Bulk-oil high voltage circuit-breaker have built-in current transformers. So, most instruments for measuring resistance are unsuitable in such objects. Because the measuring current of the microohmmeter must flow for enough time, and the microohmmeter must work in conditions of powerful electromagnetic fields.

For milliometers (instruments for measuring the DC resistance of circuits with high inductance) the main requirements are to ensure a stable current in the winding and to limit the EMF of self-induction when it turns off or the current circuit breaks. Such electrical circuits as the primary side of a high-power transformer have considerable inductance. Therefore, the current in it establish quiet. Moreover, the total time costs are significant because according to RD 34.45-51.300-97, resistance measurements must be made for all branches in all positions of the switching instruments, and for all windings.

The accuracy requirements for milliometers can determin on the basis of the tolerance on the deviation of the resistance of the transformer windings. They can be both between themselves and from the passport values. Since this tolerance should not exceed 2%, the maximum basic measurement inaccuracy should not exceed (0.5 ... 1)%.

Instruments for measuring the resistances of shunting resistors of high-voltage circuit breakers can call kiloohmmeters. Since the upper limit of the range of these resistances reaches 100 kOhm (the shunt resistance of each chamber of the MKP-220 circuit breaker). A feature of this type of measurement is the presence of induced voltage at a frequency of 50 Hz on the measured resistors. Induced voltage occurs when capacitive current flows from lines under high voltage through a resistor to ground. The size of the induced voltage determines by the line voltage level, design, location and resistance value of the resistor, atmospheric conditions and can reach several kilovolts.

When measuring resistance, the interference current with a frequency of 50 Hz can be several orders of size higher than the measuring current of the instrument. During resistance measurement, the interference current with a frequency of 50 Hz can be several orders of size higher than the measuring current of the instrument. Due to the difficulties in ensuring the required level of noise suppression (about 120 dB) there are very few special instruments of this kind. At the same time, the use of kilo ohm meters makes it possible to measure the resistance of resistors under conditions of high noise on electrical equipment without extra labor costs.

As follows from the foregoing, we need several specialized instruments to measure all types of electrical circuit resistances, which is not always convenient. The provision of all these functions with one instrument is most preferable. Especially since the cost of a set of instruments with the same characteristics will be higher than the cost of one universal device. Back in 2008, SKB EP Company launched the МIKO-2.3 universal instrument. Device has since taken a stable position in the market. МIKO-2.3 also includes the fourth type — a thermometer (immersion type) for measuring various temperatures liquids, including oil.

Mode  Range (R)  Current (I) Accuracy (δR
Microohmmeter   0,1 μΩ ÷ 0,1 Ω
10 ÷ 1 000 А
Milliometer 100 μΩ ÷ 1 kΩ 0,5 ÷ 5 А
Kilohmmeter 0,1 kΩ ÷ 300 kΩ
- ±0,5 %
Thermometer -20 °C to +120 °C
- -
The mass of the device without cables - 2.7 kg
The mass of the device complete with cables - 9.5 kg

The use of modern components, special measurement algorithms and a thoughtful original design made it possible to increase the current in the microohmmeter mode to 1000 A. But to maximized battery life in the microohmmeter mode the measurement current is adjustable. Thanks to this mode, one battery charge is enough for 40 transition resistance measurements. So, one battery charge is enough for 40 transition resistance measurements. Autonomous power supply from the built-in battery and low weight of the instrument (2.7 kg) make it easy to climb switches and transformers with it. And the length and weight of the connecting cables can reduce.

In millimeter mode, the measuring current range of МIKO-2.3 coincides with the current range of many milliometers (0.5 ÷ 5A). Plus МIKO-2.3 have special technique is developed by SKB EP to determine the additional inaccuracy caused by non-zero current instability. Tests by the method revealed the maximum additional inaccuracy which does not exceeding 0.1% in the range of inductance of the circuit (0.01 ... 15) GN.

After measuring the active resistances of the windings, another cable with a temperature sensor can connect to МIKO-2.3 and the instrument will switch to digital thermometer mode. In this mode, the actual temperatures of the windings measure to convert their resistances to the rated temperature.

The safety of work when measuring resistance with a large induced voltage in kilometer mode ensure by the usual organizational measures. They provide for in the design and circuitry of МIKO-2.3. They are clamps in cable grips, voltage suppressor in the input circuit, etc.

A serious drawback of most universal instruments is the increased complexity of use. It arises due to the fact that the instrument must have all the controls and inscriptions necessary to perform all the functions provided. In МIKO-2.3, technical solutions a use to avoid the reverse side of the advantages of universality:

Firstly, it provides automatic inclusion of a certain type of measurement when connecting the input cable. As well as the conclusion of the relevant information on the display.

Secondly, all functioning processes are automating. For example, these are processes such as compensation of the initial level and thermal emf, determining the parameters of an object, selecting a measurement range and operating current, assessing the level of interference and establishing the necessary degree of filtration, and many others.

Thirdly, a special program monitors at what step is the implementation of each type of measurement. Then, a message appears on the display. And after connecting the cables to the object, it controls the integrity of the contact via potential and current wires. In case of violation, instrument issues a warning.

Universal МIKO-2.3 allows you to get by with the only instrument for all the tasks at hand, instead of three or four devices. Moreover, it has a record high current in the microohmmeter mode with a record low weight. All these advantages are especially relevant for installation and repair organizations. Such organizations, by the nature of their work, are forced to move large masses of instruments, appliances and tools over long distances.

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