Unbalanced Voltage

[me2ss]

Hi. I'm a new member of LMPF Forum. I'm working with a Generating Power company.

 

On one of our client, we have 4 - 500kW, 480V generator operating as a primary power supply. We run 2 generator as a base load and during peak hours, the third generator kicks in. The fourth generator serve as a back up.

 

Last month, there were more than 10 units of condenser fan motor burned out. These motors are connected directly to the power supply (the client doesn't use motor protection).

 

One of our engineers conducted voltage logging, during the power plant operation, on the data collected, on the generator side, the minimum phase to phase voltage on line 1 and line 3 are 413.3V and 416.1 V respectively ( Line 2, 454.7V, FYI) while on the Chiller (load side) during this time, the data collected were 413.3V and 416.1V on the same line (Line 2, 454.7V). But when the client is supplied by the local utility company, the minimum phase to phase supply voltage were 468.9V and 469.4V on line 1 and line 3 respectively (Line 2 470.7V) while on the Chiller (load side), the data collected during this time were 464.3V and 464.2V on the same line (Line 2, 467.2V). Note that the chiller nameplate data indicate a voltage range of 414 - 506; the generators are less than a year old.

 

The said engineer suggested to increased the output voltage of the generator but someone said that harmonics causes the unbalanced or under voltage.

 

What I understand is we encountered undervoltage as per the chiller nameplate but I don't know why we encountered undervoltage.

 

Seeking for your professional advice.

 

thank you and best regards,

 

me2ss

[kana]

Hi me2ss,

 

Is the phase to phase voltage of the generator measured at no-load?

 

Regards,

kana

[me2ss]

Hi me2ss,

 

Is the phase to phase voltage of the generator measured at no-load?

 

Regards,

kana

 

it was measured with load

 

me2ss

[me2ss]

Hi guys,

 

In addition to my query above.

 

If the client use a motor starter, does burning of motors be AVOIDED? Or by installing a motor starter, the motor starter will be the one destroyed then?

 

thank you all for your inputs.

 

warm regards,

 

me2ss

[kana]

Hi me2ss.

 

The no-load test is essential to ensure that the generator outputs the rated phase to phase voltage. By the way, are these generators being protected by undervoltage relays? If it is, then relay should have protected the system by tripping the circuit breaker.

 

The increase in load can increase the voltage drop in the generator by theory. You can also check on your power factor. Increase in reactive power due to the installation of inductive loads such as motors draw more reactive power.

 

The fundamentals of motor starters are to protect the motors from overload problems and to start and stop motors safely.

 

For this problem, I don't think that the motor starter will be damaged.

 

 

Regards,

kana

 

 

[me2ss]

Hi me2ss.

 

The no-load test is essential to ensure that the generator outputs the rated phase to phase voltage. By the way, are these generators being protected by undervoltage relays? If it is, then relay should have protected the system by tripping the circuit breaker.

 

The increase in load can increase the voltage drop in the generator by theory. You can also check on your power factor. Increase in reactive power due to the installation of inductive loads such as motors draw more reactive power.

 

The fundamentals of motor starters are to protect the motors from overload problems and to start and stop motors safely.

 

For this problem, I don't think that the motor starter will be damaged.

 

 

Regards,

kana

 

Hi Kana,

 

I checked the Commissioning Report, on the Alarm Setting Portion, the Voltage fault, for the low (voltage) was set to low<406. I guess we have to adjust it.

 

Thank you Kana for your helpful inputs, it enlightened me.

 

warm regards,

 

me2ss

[yuri]

Hi. I'm a new member of LMPF Forum. I'm working with a Generating Power company.

 

On one of our client, we have 4 - 500kW, 480V generator operating as a primary power supply. We run 2 generator as a base load and during peak hours, the third generator kicks in. The fourth generator serve as a back up.

 

Last month, there were more than 10 units of condenser fan motor burned out. These motors are connected directly to the power supply (the client doesn't use motor protection).

 

One of our engineers conducted voltage logging, during the power plant operation, on the data collected, on the generator side, the minimum phase to phase voltage on line 1 and line 3 are 413.3V and 416.1 V respectively ( Line 2, 454.7V, FYI) while on the Chiller (load side) during this time, the data collected were 413.3V and 416.1V on the same line (Line 2, 454.7V). But when the client is supplied by the local utility company, the minimum phase to phase supply voltage were 468.9V and 469.4V on line 1 and line 3 respectively (Line 2 470.7V) while on the Chiller (load side), the data collected during this time were 464.3V and 464.2V on the same line (Line 2, 467.2V). Note that the chiller nameplate data indicate a voltage range of 414 - 506; the generators are less than a year old.

 

The said engineer suggested to increased the output voltage of the generator but someone said that harmonics causes the unbalanced or under voltage.

 

What I understand is we encountered undervoltage as per the chiller nameplate but I don't know why we encountered undervoltage.

 

Seeking for your professional advice.

 

thank you and best regards,

 

me2ss

 

413/416/454 Vs is a too big disbalance - some 10%. I see a motor running with the 1.5 or 2 % disbalance in voltage leading to about 40% disbalance in current - the highest phase reading exceeding the nominal current while the motor only runs at half-load.

[marke]

Hello me2ss

 

Welcome to the forum.

 

The voltages that you quote are definitely not suitable for running induction motors. This level of imbalance will cause motors to fail. The voltage imbalance should be less than 1%. As you increase the voltage imbalance, you must significantly derate the motors to prevent failure.

 

The high level of imbalance indicates that something is seriously wrong in this installation.

I would measure the voltages at the output of the generators. If the imbalance is only at the load end, and the voltage at the generators is OK, then you have a high impedance somewhere. If the voltage imbalance is at the generator output, this could indicate that you have an excitation poblem which could be due to a faulty AVR.

If the two generators are not correctly synchronised, it can result in a load imbalance, but I would expect this to affect all three phases equally.

 

Harmonics can affect an AVR, but I would not expect this type of performance from harmonics. If you have harmonics and/or transient loads, I would suggest that you use a three phase averaging AVR rather than a single phase (across two phases) peak reading AVR ( cheaper option.

 

Are the generator suppliers able to help??

 

Best regards,

Mark.

[me2ss]

413/416/454 Vs is a too big disbalance - some 10%. I see a motor running with the 1.5 or 2 % disbalance in voltage leading to about 40% disbalance in current - the highest phase reading exceeding the nominal current while the motor only runs at half-load.

[me2ss]

Hello me2ss

 

Welcome to the forum.

 

The voltages that you quote are definitely not suitable for running induction motors. This level of imbalance will cause motors to fail. The voltage imbalance should be less than 1%. As you increase the voltage imbalance, you must significantly derate the motors to prevent failure.

 

The high level of imbalance indicates that something is seriously wrong in this installation.

I would measure the voltages at the output of the generators. If the imbalance is only at the load end, and the voltage at the generators is OK, then you have a high impedance somewhere. If the voltage imbalance is at the generator output, this could indicate that you have an excitation poblem which could be due to a faulty AVR.

If the two generators are not correctly synchronised, it can result in a load imbalance, but I would expect this to affect all three phases equally.

 

Harmonics can affect an AVR, but I would not expect this type of performance from harmonics. If you have harmonics and/or transient loads, I would suggest that you use a three phase averaging AVR rather than a single phase (across two phases) peak reading AVR ( cheaper option.

 

Are the generator suppliers able to help??

 

Best regards,

Mark.

[yuri]

It is as it is, I'm saying what I have seen: even a seemingly insignificant V disnbalance in AC el. power supply may lead to significant A diff accross leads in the power consuming induction motors.