| Frequently asked questions for Winding Ohmmeter | Print this page  |
| Q: How do you choose appropriate RMO-T unit from the series for the application? In other words, how do customers interested in testing windings of power transformers choose a device from RMO-T series? | |
| A:
Different RMO-T series units produce different test current value.
RMO10T generates test currents from 5mA to 10A true DC and with a resistance
measurement range from 1µΩ to 2000Ω, while for example RMO50T generates test
currents from 5mA to 50A also true DC and with the same resistance measurement range as RMO10T. Using 50A alows you to achieve more accurate results testing windings of power transformers which have inherently low resistance. Transformer can reach saturation faster when the current is higher. The inductance value of a transformer depends on the current injected in the windings. When the transformer is saturated the inductance is minimized. A transformer is normally designed to reach saturation when the current is 1,2 times the peak value of the no-load current. The no-load current is normally in the range of 0,2 to 5 percent of the nominal winding current.When measuring DC resistance the test current should be at least 1,2 times the no-load current of the transformer. This is to ensure that the transformer core is saturated. Note also that the test current is NOT to exceed approx 10-15 percent of the nominal winding current. |
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| Q: What is the maximum power of transformer that RMO-T series winding ohmmeters can measure? | |
| A: Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (more than 500 MVA). We have tested succesfully 300MVA transformers. | |
| Q: What are common failure modes of instruments, and is it possible to repair on site or we must ship back to you? | |
| A: Defective units have to be shipped back to us. Customers will lose warranty if they open our devices. Our instruments can be repaired only by DV Power authorized service. In case that customer tries to open or open any of our devices unauthorized, DV Power is released from every responsibility due to possible safety danger to people who use or are near the device that is turned on. | |
| Q: Are RMO25W and RMO10W suited to test OLTC? | |
| A: No, they are not. However, new devices RMO-T series are designed to test on-load tap changers. These devices will measure resistance change from tap to tap and also, they will measure current change while changing from tap to tap. This allows the operator to take measurements very quickly without discharging, then recharging the transformer for every tap. The winding resistance unit will rebalance after every tap change. In the moment the tap is changed from tap to tap, the device detects a sudden, very short drop of the current value. A properly working tap changer differs from a malfunctioning one, e.g., an interruption during the tap change, and by the magnitude of the ripple value. An interruption will result in much higher ripple value than a properly functioning tap changer. | |
| Q: When do you use two independent measuring channels of devices RMO-T series? | |
| A:
Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of transformers at a time. -For simultaneous testing both windings (high and low) on a single-phase transformer. In such a way, it is possible to speed up the measurement when two channels are used to test both windings of the transformer. -The dual reading characteristic will speed up the measurement when it is used to test windings on Delta-delta connected windings on three-phase transformers -For resistance measurement on a single-phase transformer with two sections. This method is useful for testing transformers which contain two or more inputs at one winding (high or low), because it speeds up measurements. -For quicker resistance measurement on a three-phase Wye winding. -For quicker resistance measurement on a three-phase open delta winding. |
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| Q: How long does it take to measure windings of a 63MVA transformer? | |
| A: Measurement takes from 10 to 15 sec for transformer's primary coil (63MVA; 110kV) and from 2 to 5 sec for secondary (10kV). Discharging time takes from 15 to 20 sec for primary and 2 to 3 sec for secondary winding. | |
| Q: Do RMO-T devices have RTD input for winding temperature monitoring? | |
| A: Our devices can generate continuous test current and they can continuously measure changing of resistance. That way it is possible to observe changing of winding temperature. | |
| Q: Do RMO-T devices have automatic temperature compensation features | |
| A: No, they still do not have. However, the temperature can be set manually and temperature compensation can be done on 25°C or on other desired temperature, for example 80°C. | |
| Q: What is a need for an earth lead to ground a winding of transformer to transformer case? | |
| A: For safety reasons you always must connect to earth one end of the measured winding of transformer to transformer case. High voltage, dangerous even lethal, may occur at sockets if you didn't earth one side and if you open the measuring circuit accidentally or purposely during current discharging. | |
| Q: Can we transfer all or a selection of memorized data from the RMO-T series devices or only the last test result? | |
A: We use PC two ways. The first way, PC is connected supplementary, after the tests are finished it is possible to transfer only results of the last 100 test. The second way, PC is connected during the test. Then, during the test results can be transferred in Excel spreadsheet form, from 1sec to up to 300 sec intervals. The maximum test duration time is now up to 999 sec.
Our devices RMO-T series can have that test duration time, but only for the following maximum test currents: |
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| Q: Do you produce Winding Ohmmeters with rechargeable battery? | |
| A: Unfortunately, we do not produce Winding Ohmmeters with the rechargeable battery. In case of power transformers' winding resistances measurement, the microohmmeter with rechargeable battery has only 12V test voltage value. Our Winding Resistance Micro-ohmmeter injects current with the voltage as high as 60V. This ensures that the test is as quick as possible, and that the desired test current is reached as soon as possible. Even other competitors' devices designed for the same application provide at least 30V test voltage at the output. | |
| Q: What would happen if during a measurement of transformer winding resistance a current lead got disconnected from Transformer? Is RMO30T instrument protected for this accident or not? | |
| A: It is strongly recommended that RMO30T equipotential connector is always connected to protective ground (PE) before measurement .
If during a measurement any current lead gets disconnected from the transformer, then the energy stored in the transformer magnetic circuit will be automatically discharged through voltage sense cables and built-in discharging circuit in our RMO30T device, also sometimes through the ground cable. However, it is strongly recommended to ensure all connections in order to avoid such accidental disconnecting since the RMO30T can be damaged. This is valid for all devices of the RMO-T series. You should note that our devices RMO30T, RMO40T and RMO50T have mechanical protection from accidental disconnecting of current cables. If you plug-in current cables into sockets of RMO30T or RMO40T or RMO50T devices and turn them (lock them) in clockwise direction, current cables can not be disconnected accidentally from the instrument. |
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