Winding resistance measurement of reactor or any other similar electrical equipment determines the following information.
- Calculation of copper losses.
- Calculation of winding temperature at the end of the winding temperature rise test cycle.
- Indication of any damage occurs in the winding system.
Principle of Winding Resistance Measurement
Previously, winding resistance was measured by Kelvin Bridge method but it was quite laborious and time consuming. In present days, constant current resistance measuring kit is used for that. In this measurement constant current is injected between two terminals of the winding across which the resistance to be measured. It may be between two phase terminals or between phase and neutral terminals. The resistance is measured by the ratio of voltage measured between the terminals and that injected current. There are different resistance measuring instruments available. The basic principle is same. The instruments show resistance value directly on the display screen after calculation
Reactor, transformer or other similar equipment has high value of reactance. So, the DC resistance value can only be obtained after the injected current reaches its steady state constant value. So, the reading cannot be taken instantaneously, we have to wait for sufficient time to get constant injected current through the winding. This is why, prolong time is taken by the winding measurement test in the case of large transformer or reactor.
From, the above equation, it is clear that induction portion of the voltage only becomes zero when current gets its steady state constant value. Then, only we can get reliable value of resistance R from ration V/I.
When to Measure Winding Resistance?
There may be a chance of damage occurs in the winding (loose connection or open connection), tap changer (In case of transformer) or connectors during loading the equipment on the transport vehicle in factory, during unloading the equipment from the vehicle at site or during erection at site. This damage may lead a change in DC resistance of the path through them. That is why it is mandatory to conduct DC resistance measurement test before commissioning of the equipment. The result of the field test must be tallied with factory test result. To ensure each tap changer contact is intact after transportation and erection, the winding resistance measurement should be conducted at each tap position in case of power transformer. The test result should be kept as future reference.
Some time there is huge fault occurs in the equipment. During which there may be a chance of damage in the windings and its connections points. So, it may be recommended to perform winding resistance test after the fault to ensure about healthiness of the current carrying path of the equipment before re-energizing. It also helps to analyze the phase and location of the fault.