Selection of a universal analyzer for the transformers, electric machines and switching equipment

In the market environment every company tends to achieve a higher profitability level and minimize its costs. The issue of work efficiency is also rather challenging. This problem is rather urgent for small maintenance and erection companies with limited number of highly qualified staff. Selection of adequate instruments for controlling the parameters of electric equipment and detection of its faulty nodes is a rather important factor for such companies.

One of the most simple and popular methods of assessing the condition of electric equipment nodes is measurement of DC electric resistance in them. Unfortunately, each type of electric equipment (transformers, electric machines, switching equipment) requires individual means for diagnosis. As to DC electric resistance measurement, you can use the existing universal instruments whose application would provide considerable saving as compared to procurement of the whole set of testing equipment for every type of electric equipment. Let us consider instruments' capabilities and types of measurements to be performed on different types of electric equipment.   

Transformers. 

Transformer testing implies check of resistance spread across all the identical branches of different phases. If spreading does not exceed 2%, then, according to RD 34.45-51.300.97 "The scope and standards of electric equipment testing" (6th edition, Moscow, NC ENAS, 2000), it is normal. Should measurement of phase resistances of windings with zero lead show that the resistance deviation of one phase exceeds 2%, it means that this phase has a fault.

Linear resistances are measured when a winding has no zero output. They should be converted into phase resistances using well-known formulas. Let us show applicability of this method on a simple example. Let us assume that we obtained the following resistance values for a three-phase winding connected following the "star" diagram without a zero output: RAB=2.04 Ohm; RBC= 2.04 Ohm; RCA=2.0 Ohm. It means that the maximum winding resistance spread does not exceed 2%, hence, windings are integral. Conversion into phase resistances gives: RAО=1.0 Ohm; RBО= 1.04 Ohm; RCО=1.0 Ohm. Converted data, first, localize a faulty phase, and, second, detect its 4% higher resistance, which evidences the availability of a fault in it.

Along with determination of winding resistance spread across different phases you must also compare measured values to reference ones or to the values obtained during the transformer commissioning tests. It is not recommended to compare the measured resistances to the results of previous measurements only. In this case you do not see the annual gradual increase in the winding resistance that evidences continuous degradation in some couplings or elements in the electric circuit of a transformer.

For making such a comparison you need a temperature value of the measured winding which is assumed to be equal to the temperature of the upper layers of transformer oil. Transformers, as a rule, have no thermometers, therefore, you need a thermometer and an option for entering the values of measured and reference temperatures into the instrument. Reduction to reference (passport) temperature is done automatically, providing this function is integrated into the instrument. Computation of relative spread of resistances between identical branches and conversion of linear resistances into phase ones are done automatically. If there are no such functions, the manual conversion for 10-20 leads of windings per phase becomes rather time-consuming and may produce a lot of errors.

Along with determination of winding resistance spread across different phases you must also compare measured values to reference ones or to the values obtained during transformer commissioning tests. It should be done for controlling the winding resistance as its increase evidences continuous degradation of some couplings or elements in the electric transformer circuit. For making such a comparison you need temperature value of the measured winding which is assumed to be equal to the temperature of the upper layers of transformer oil. Transformers, as a rule, have no thermometers, therefore, you need a thermometer and an option for entering the values of measured and reference temperatures into the instrument. The MIKO-2.3 reduces to reference temperature automatically; computation of relative spread of resistances of identical branches and conversion of linear resistances into phase ones are also done automatically. 

During overhaul, or should the internal contact defect be detected when the transformer tank is opened, the transient resistances of detachable and integral connections can be measured directly on high test currents (without winding resistance), thus getting an accurate idea on their condition. With the contactor tank opened the current-limiting resistors of LTC devices and transient resistances of a contactor and selector become accessible for integrity tests..

In the current transformers built-in into power transformers, in the busbar transformers, in the reference and through-type current transformers installed in switchgears, the resistance of secondary windings and resistance of their load resistors are measured.

Electric machines. 

In AC electric machines (synchronous generators and motors, asynchronous motors) and in transformers the resistance of a three-phase winding of a fixed coil is measured: phase resistance is measured at all the winding ends outside; linear resistance is measured at internal 'star' or 'triangle' connection of windings. The same formulas are used for linear resistances conversion into phase ones, for calculation of  resistance spread between phases, and for measured resistance reduction to the reference temperature.

In the rotors of synchronous machines the excitation winding resistance is measured. If it is a rotor with explicit poles, then resistance of each pole is measured separately or in couples, and transient contact between them is also measured. In the asynchronous motors with a phase rotor the linear resistances of windings are measured.

In DC electric machines the resistances of excitation winding on a stator, rotor resistance between commutator segments, and resistance of rheostats and control resistors are measured.

For connecting the test cables to the rotor windings leads having the form of rings, the universal clamps (G-clamps, 'crocodile' clips) do not suit, special clamps in the form of collars with a screw nut are needed. Clamps with double spring-loaded needle-type contacts (potential and current) are the most convenient ones for connecting to commutator segments.

Switching equipment. 

In the switching electric equipment the following resistances are measured:

• transient resistances of contacts of circuit breakers, separators, interrupters and short-circuiters;
• transient resistances of detachable joints of switchgear  equipment;
• transient resistances of bolted connections of wires of HV lines, buses and  wireways of outdoor switchgears;
• resistance of windings of electric magnets of drives and current transformers;
• resistance of bypass resistors of arch extinction devices of bulk-oil circuit breakers of MKP and U types, resistance of potential dividers and of bypass resistors of air circuit breakers.

When measuring the transient resistance of contacts the problem of the test current amperage arises as oxidized contacts produce higher result of measurements that depends on the current value. For eliminating the erroneous measurements, the international standards IEC 56 and ANSI C37.09 specify the current value in the range from the minimum permissible (50-100A) to rated current of a circuit breaker.

A range of measured resistances for the listed above equipment is rather wide and lies within 10^-5 - 10⁵ Ohm, and the test current range is from 10^-3 to 600A and higher. For this reason the market offers mainly special-purpose instruments with lower ranges: microohmmeters for measuring the transient resistance of contacts and connections, and milliohmmeters for measuring the winding resistance of transformers, electric motors, etc. For measuring the windings temperature there are different types of thermometers, and for control of bypass resistors of arch extinction contacts the kiloohmmeters are used.

As it follows from the electric equipment review, control of a transformer only requires a microohmmeter, a milliohmmeter, a thermometer, and, very often, a kiloohmmeter. That is, you will have to manipulate four instruments, which is not always convenient. Larger number of instruments raises the cost of a set, its weight and thus complicates its transportation to remote facilities. Moreover, analysis of the offered special-purpose devices shows that:

• their precision is not sufficient for on-site measurements;
• the major share of devices have no function of converting the measured resistances, and only a few have a function of reducing it to reference temperature;
• high-current microohmmeters have heavy weight and large dimensions.

Use of universal instruments that allow measurement of all the above parameters is their cost-effective alternative. They offer the advantage of lower cost and lighter weight than a set of special-purpose instruments with the same set of functions. A table of technical performances given further (information was taken from the sites of manufacturers) for universal and three special-purpose instruments vividly demonstrates benefits of a universal instrument #1. 

Use of the MIKO-2.3 MicroMilliKiloOhmmeter for all the listed above types of equipment is cost-effective. The instrument allows measurement of all the above listed parameters, but costs and weighs less than a set of required analyzers. Technical performances of MIKO-2.3 are much superior to those of the special-purpose instruments (information was taken from the sites of manufacturers). Owing to the lack of sensitivity towards induced voltage the MIKO-2.3 can be connected to the circuit breaker terminals. After that the circuit breaker connecting rod with the help of a jack is adjusted to a position when arch extinction contacts are open but the main contacts are still closed, and you can read out the resistance of one or two connected in series bypass resistors from the display. It is obvious that this method is less labor-intensive than the use of kiloohmmeters.

Comparison of performances of universal and specialized instruments

	Universal instrument #1	Universal instrument #2	Milliohmmeter, Instrument #3	Microohmmeter, instrument #4	Kiloohmmeter, instrument #5
Microohmmeter
Range of measurements	10-6 – 0.1 Ohm	10-6 – 0.1 Ohm	-	10-6 – 10 Ohm	-
Range of amperage	50 - 1000 А	200 - 600 А	-	0,25 - 200 А	-
Main error	±0.2%	±1%	-	±0.25%	-
Milliohmmeter
Range of measurements	10-4 – 103 Ohm	2∙10-3 – 2∙103 Ohm	10-4 – 2∙103 Ohm	-	-
Range of currents	5∙10-4 - 5 А	10-2 - 12 А	10-3 – 10 А	-	-
Main error	±0.2%	±1%	±0.2%	-	-
A formula for winding resistance conversion			Reduction to reference temperature only	-	-
Kiloohmmeter
Range of measurements	100 – 3∙105 Ohm	-	-	-	10-3 –105 Ohm
Allowable induced voltage	5 kV	-	-	-	? kV
Main error	± 0.5%	-	-	-	± 0.5%
Thermometer
Range of measurements	–20 – +120 ˚С	-	-	-	-
Distinctive features
Availability of the archive in the instrument
Communication with PC
Power supply	mains, from internal and car battery	mains	mains, from internal and car battery	mains	mains
Range of working temperatures	–20 – +40˚С	–35 – +50 ˚С	–20 – +40 ˚С	–5 – +35 ˚С	+5 – +40 ˚С
Weight	2.7 kg	25 kg	7 kg	5 kg	4 kg