Present-Day Market of Instruments for Electric Resistance Measurements

New approaches to software development and design, development of energy industry and growing requirements on the part of energy specialists stipulate creation of new types of milliohmmeters that shall meet those requirements, while their choice is rather wide as they differ in the principles of operation, metrology characteristics, degree of automation, weight, dimensions, and, certainly, price.  Well-known and well-proven devices of MIKO group manufactured by SKB EP Company some performances of which have no analogues at the foreign market have occupied an appropriate niche at the Russian market.

The Company moves forward and continuously updates its products, which is evidenced by three new milliohmmeters to be manufactured in 2018: MIKO-7M, MIKO-8M, and MIKO-9.

Similar to their predecessors MIKO-7 and MIKO-8, new devices MIKO-7M, MIKO-8M and MIKO-9 have been designed for measuring:

• DC resistance of windings of power and measurement transformers, of electric motors, generators, straight-line compensators, and of windings of other high-inductance equipment;
• transient electric resistance of selector contacts of OLTC devices, earthing devices, isolating links and other detachable and non-detachable contacts;
• DС resistance of resistors, wires, buses and other inductance-free circuits;
• Transient electric resistance of selector contacts through an open OLTC tank of PC-3 and PC-4 types, which allows elimination of electric resistance of power transformer windings from the results of measurements thus providing more accurate information on the technical state of those contacts.

Moreover, MIKO-8M and MIKO-9 milliohmmeters similarly to MIKO-8 allow oscillogramms recording without switching over the contacts of a fast-acting OLTC device both without its opening by connecting to transformer terminals and with its opening by direct connection to closing switch contacts (DRM-test method)..

MIKO-9 allows performance of absolutely new tests, namely:

• Measurement of electric resistance of windings during cooling test as per GOST 3484.2-88;;
• Demagnetization of transformer’s magnetic circuit.

Apart from functional differences, MIKO-7M, MIKO-8M and MIKO-9 differ from their predecessors and similar devices by availability of a data base of standard facilities (a power transformer, a current transformer, a voltage transformer, a resistive object, a generator, etc.). Owing to special in-built modes of measurement for a specific object the device can automatically take into account its specific features thus allowing the user to avoid additional adjustment.

Table 1. Comparison of instruments developed by SKB EP Company

	MIKO-9	MIKO-8M	MIKO-7M
Selection of the object to be tested
A number of in-built objects for diagnosis	10	10	5
Microohmmeter mode
Milliohmmeter mode
The number of channels for measuring the electric resistance that work in parallel	4	1	1
Диапазон измерений электрического сопротивления	10 mkОhm ÷ 30 kOhm	10 mkОhm ÷ 10 kOhm	10 mkОhm ÷ 1 kOhm
Range of electric resistance measurements	0.00050 ÷ 10А	0.00050 ÷ 10А	0.00050 ÷ 10А
Automatic three-phase mode 1
Two-winding mode 2
Automatic processing of the results of measurements
DRM-test of OLTC devices
"Heat test" mode or cooling test
Demagnetization mode of transformer’s magnetic circuit

Detailed comparison of functions and performances see at ref

When analyzing the data on new developments of the Company that are given in Table 1 special attention should be paid to MIKO-9 milliohmmeter that notably stands out among its analogues, e.g., due to availability of four channels for electric resistance measurement that work in parallel thus adding to the device qualitatively new properties:

• It allows reduction in the number of climbing on the transformer’s tank (input) for reconnecting the test cables.  Test cables are connected simultaneously to all the power transformer’s windings taps of the same voltage class, and switching over between windings is performed from the ground using the device;
• Full time of measurement is reduced:
  
  1) Owing to the option for specifying the required direction of test current flow when switching over from one winding to the other one, thus avoiding extra re-magnetization of the transformer core;
  
  2) Owing to simultaneous measurement of electric resistance of two windings connected in series.  For example, when measuring the electric resistance of windings of the same voltage class that are located on different cores (HV windings of A and B phases); when measuring the electric resistance of HV and MV windings in the single-phase transformers; and when measuring the electric resistance of LV winding connected to HV winding in series;
  
• Now you can explicitly identify the winding whose resistance is higher than that of the other one and compute the ratio between them without performing the whole set of linear resistance measurements and their subsequent conversion into phase values.  In case of simultaneous measurement of two linear resistances of power transformer windings with triangular-form connection the accuracy increases dramatically;
• Direct measurement of electric resistance of phase windings connected as ‘star with isolated neutral’ allows elimination of the entire procedure of measuring the linear resistances and their subsequent conversion into phase values.

All the instruments are currently under tests and undergo pilot operation for being entered into the RF State Registrar of Measurement Instruments. Their production is scheduled for the second quarter of 2018.

Bookmark: #trexfaznyy The device allows measurement by three phases simultaneously and automatically stops measurements upon reaching the specified stopping criterion.

Bookmark: #obmot This mode allows measurement of electric resistance of powerful power transformers, particularly, with triangular-form connection of the secondary windings, when conventional methods do not produce a reliable result.