FAQ


Get answers to frequently asked questions about DV power test equipment.

Micro Ohmmeters

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. 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
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

What is the maximum power of transformer that RMO-T series winding ohmmeters can measure?

Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (they have been successfully used on 900 MVA units).

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are 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 to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

Coil Tester & Power Supply units

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. 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
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

What is the maximum power of transformer that RMO-T series winding ohmmeters can measure?

Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (they have been successfully used on 900 MVA units).

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are 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 to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

Winding Ohmmeters

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. 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
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

What is the maximum power of transformer that RMO-T series winding ohmmeters can measure?

Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (they have been successfully used on 900 MVA units).

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are 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 to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

Three-phase Transformer Turns Ratio Tester

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. 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
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

What is the maximum power of transformer that RMO-T series winding ohmmeters can measure?

Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (they have been successfully used on 900 MVA units).

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are 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 to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

Three-phase Transformer Demagnetizer

Why the turns ratio deviation is sometimes different in single-phase and three-phase results?

When doing a true three-phase test, turns ratio obtained for different phases refers to the ratio of line voltages. For example, phase A is the ratio of 1U-1V/2U-2V, phase B is the ratio of 1V-1W/2V-2W, phase C is the ratio of 1W-1U/2W-2U, so each turns ratio involves 2 windings. For most of the vector groups, a single-phase test is performed in different way, and involves only 1 winding. In general, turns ratios obtained in the single-phase and true three-phase tests do not involve the same windings. There are certain vector groups (for example Dd0, Yy0) where the true three-phase and single phase turns ratios are performed on the same pair of windings. However, even in that case certain difference between turns ratio deviations in these two tests may exist, due to the TRT turns ratio accuracy.

How to measure contact resistance of dead tank circuit breakers?

Contact resistance measurement of dead tank circuit breakers differs from the live tank CBs because the DC current used for this measurement flows directly through the current transformers mounted on their bushings.

It is recommended to use the DTRtest mode for Contact resistance measurement of dead tank circuit breakers. By generating DC current for some period of time (e.g. 20 s) the current transformers will become saturated and will not affect the measurement results.

After a DC current test magnetic core of a current transformer may be magnetized (remanence magnetism). The remanence magnetism can cause various problems such as erroneous diagnostic electrical measurements on a current transformer or incorrect operation of protective relays due to magnetized CT cores. To eliminate this source of potential problems, demagnetization should be performed. Our RMO-D series of micro ohmmeters is specially designed for dead tank circuit breakers and the contact resistance measurement is followed by automatic demagnetization process.

When do you use the two RMO-T independent measuring channels?

Two independent measuring channels allow simultaneous testing of primary and secondary windings or measurement of two phases of a transformer at a time.
For simultaneous test of both windings, high and low, on a single-phase transformer, it is possible to speed up the measurement when two channels are used to test both windings of the transformer.

  1. 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
  2. For resistance measurement on a single-phase transformer with two sections
  3. This method is useful for testing transformers that contain two or more inputs at one winding, high or low, because it speeds up measurements.
  4. For quicker resistance measurement on a three-phase Wye winding.

For quicker resistance measurement on a three-phase open delta winding.

What is the maximum power of transformer that RMO-T series winding ohmmeters can measure?

Our transformers winding resistance meters RMO-T series are suitable even for the highest power-rated transformers (they have been successfully used on 900 MVA units).

Which of the RMO-T devices is appropriate for me?

All of our RMO-T devices are suitable for OLTC testing and for static winding resistance measurement. However, our RMO-TT and -TD devices have much better frequency sampling rate than the RMO-TW devices, which means that they can detect even minor changes in the OLTC operation. The RMO-TD devices can perform automatic demagnetization, with the additional ability to measure the OLTC motor drive AC current. The RMO-TT instruments can measure temperature at several points, which makes them ideal for a heat run test. Depending on the minimum resistance of windings you need to test, select the unit with appropriate features and sufficient output current. We supply RMO-TT instruments with up-to 100A capability.

Which test currents are available with RMO-T devices?

Different RMO-T series units generate different test current values. RMO10TW generates test currents from 5mA to 10A true DC with a resistance measurement range from 0,1µΩ to 2000Ω, while RMO60TD generates test currents from 5mA to 60A also true DC and with the same resistance measurement range as RMO10TW.
Using 60 A test current allows you to achieve more accurate results when testing windings of power transformers with inherently low resistance. Transformer test current can reach stable state faster when the current magnitude is higher. The inductance value of a transformer depends on the current injected into the windings. When the transformer is saturated the inductance is minimized. Power transformers are 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 to obtain more accurate results. Note also that the test current is NOT to exceed approx. 10-15 percent of the nominal winding current.

We have bought a power supply type POB30 last year and would like to know if this device is suitable for testing of switchgear or it is suitable only for circuit breaker test? Because when we supply one cell of MV switchgear (contains Siemens protection relay, control relay, signal lamp in LV chamber) with setting voltage of 30V, the voltage changes from 10 V to 40 V and is not stable. It happens especially when the voltage is applied to binary input of relay for testing of supervision function.

All our devices have protection schemes. If the load draws current in excess of 26 A, the voltage on our devices will start to decrease.

What is the difference between POB30D and POB30AD?

The POB30D can generate DC voltage at coils outputs. Besides DC voltage, the unit POB30AD can generate AC voltage, at coils outputs.

What is the procedure to do the minimal trip voltage test?

Both devices POB30D and POB30AD have built-in function intended for automatic test of minimal trip voltage. You can choose the triggering voltage of the closing coil (or the breaking coil) on the switch by setting the minimal voltage, the maximum voltage and the step voltage. The device will automatically examine the minimal trip voltage of the coil.

I wonder if the DC voltage of the power box POB30 can be adjusted continuously. For example, from 65 V to 66 V to 67 V to 69 V, and any value of voltage within the specs? For example, 35 V, 40 V, 134 V etc.

We have developed two variations of the power box, POB30D and POB30AD. The POB30D generates only DC voltage, and POB30AD generates both, DC and AC voltages. On our power box POB30D you can select any voltage from 10 V up to 300 V with resolution of 1V. On our power box POB30AD you can select any voltage from 10 V up to 300 V DC and from 10 V up to 250 V AC with resolution of 1 V.

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