Softstart Tripping

[martin]

Hi there,

I have a problem with a newly installed softstart tripping aboard a vessel. While the vessel generators have more than enough power at 660Vac, the output goes via another 660:440 transformer to the softstart input 3 phase connections. The vessel is moored. The vessel generators are an ungrounded system and I do see the Bender insulation monitor going to 135Kohm sometimes.

Anyway, they had this softstart tripping and called me in. I then ran it nine times in one day without a trip!!

Whats the problem. I cannot guess what was on the 440Vac bus the previous day.

When I monitored the start, I noticed the voltage dropped from 440Vac to 420Vac. The softstart is rated at a particular current at 440Vac...but as its dropping on the voltage is suspect although the generators may have enough power the transformer is not stiff enough.

The transformer is old and has no name plate. I could go back to the manufacturer and find the specs, but what do I look or ask for in particular of the transformer ratings?

The output of the softstart goes to a 440:3.3Kv transformer and that's output is run via a long line (several km) to the ROV. I started the softstart on it ran nine times. I cannot see a problem on the output load side or I would trip as this is the only final load. The voltage on the input is dropping from 440 at first to 420 during starting to running at 430...this will effect the system rated current I know by the same %, but on the increase!

Thanks for your help and I can supply some information but not everything!!

Regards

Martin

[marke]

Hello Martin

 

Welcome to the forum.

 

I can not see any major problem with the installation as you describe, I have done this on a number of occasions without problems.

 

I assume that it is the soft starter that is tripping and not some other protection?

 

If the soft starter is tripping, what is the trip message/code?

The drop in voltage during start should not be an issue, I have worked on many installations with voltage drops greater than 15% during start without problems.

 

Any further information?

Best regards

[martin]

Hello Marke,

The softstart is not tripping and no alarm message.

It is the incoming breaker tripping or the mcb just before the softstart.

I believe that it has been set to the maximum current adjustment/setting. I cannot adjust higher.

If they change the breaker to another type with a higher current setting I believe it will work. The problem is that these breakers have been sized almost exactly based on softstart rating, but it is not 440, it is dropping to 420 during the start and hence the current will increase in proportion. I believe although the change in current is small the sizing done on the electrical breakers is very tight with no margin to increase

Regards

Martin

Hello Martin

 

Welcome to the forum.

 

I can not see any major problem with the installation as you describe, I have done this on a number of occasions without problems.

 

I assume that it is the soft starter that is tripping and not some other protection?

 

If the soft starter is tripping, what is the trip message/code?

The drop in voltage during start should not be an issue, I have worked on many installations with voltage drops greater than 15% during start without problems.

 

Any further information?

Best regards

 

[marke]

Hi Martin

 

If the breaker is set to the rated current of the motor, I would expect that there could be problems with tripping, especially with frequent starting.

Another issue to consider is the potential temperature rise of an enclosure with the soft starter fitted. If the soft starter is in the same enclosure as the breaker, ensure that there is adequate ventilation. If the temperature rise is too much, it could cause the breaker to trip earlier (assuming it is in the same enclosure).

 

I would never consider using a breaker rated at the motor ratings. If the motor operates continuously at rated current, it is not going to take much of a current rise to trip the breaker. This could be due to a change in load, or a drop in voltage while the motor is running. Check the frequency stability of the generators. If the motor is operating a pump or a fan, a small shift in frequency can result in a big shift in load.

 

Good luck,

Best regards,

[martin]

Hi Marke,

The breaker that is tripping has a maximum adjust value of 300A.

The mcb that is sometimes tripping is 350A rated

Both at the maximum allowed manual setting.

The inrush current can be measured at 500A, whilst settling on 115A.

Is this any help in this?

 

forgot to mention the softstart is 356A rated

Regards

Martin

[marke]

Hello Martin

 

What is the rating of the motor?

When are the breakers tripping? is it during the start?

What is the motor driving?

 

Best regards,

[martin]

Hello Martin

 

What is the rating of the motor?

When are the breakers tripping? is it during the start?

What is the motor driving?

 

Best regards,

 

the motor I don't have the specs but it is a ROV. I gave the normal running current after start. The breakers are tripping after start. They have 40% voltage pedestal and a 3 sec start.

My worry is I didn't see a trip....what was different....I suspect there is another load on this 440Vac transformer and it is dropping the voltage from 440 lower....maybe more....if I start then to start the softstart on this level the required current would be increased and it would perhaps just go into the breaker tripping region. The breaker is 300/350 a setting max....the softstart is 356A rated....the softstart never trips but the breaker does....it could be that close that the window of starting is too narrow.

I will ask them for the specs of the motor tomorrow.

Regards

Martin

 

the motor I don't have the specs but it is a ROV. I gave the normal running current after start. The breakers are tripping after start. They have 40% voltage pedestal and a 3 sec start.

My worry is I didn't see a trip....what was different....I suspect there is another load on this 440Vac transformer and it is dropping the voltage from 440 lower....maybe more....if I start then to start the softstart on this level the required current would be increased and it would perhaps just go into the breaker tripping region. The breaker is 300/350 a setting max....the softstart is 356A rated....the softstart never trips but the breaker does....it could be that close that the window of starting is too narrow.

I will ask them for the specs of the motor tomorrow.

Regards

Martin

 

I should have said it trips during the start ramp....about 1 second I am told......the ramp was set to 3 seconds and it is this...I connected a Hioki 3196 Power Analyser on it to capture the waveforms.

Regards

Martin

[marke]

Hello Martin

 

I understand that the breaker trips part way into a start, but only sometimes.

1. You are concerned about the voltage drop during start as being a possible cause.

During start, we intentionally reduce the voltage to the motor to reduce the current drawn by the motor. This is what the soft starter does. A voltage reduction in the supply, during start, will not increase the current provided that the soft starter behaves normally. I have used many soft starters in situations where the supply voltage has dropped by more than 15% during start and there has been no problems.

Some brands of soft starter however, do not work well if the the supply impedance is too high. I have had issues with particular brands in situations where they are operated from generators where the generator rating is not many times larger than the starter. (high enough for the start current, but not multiples of the start current.) With one brand, the problem was fixed by setting the initial start voltage greater than 50%.

As a possibility, some vessels that I have worked on, have a number of generators that can be synchronised and run in parallel, but usually the vessel runs on the minimum requirement only. Is there a possibility that the supply capacity is changing due to the number of generators operating at the time?

If the supply capacity is reduced and you have a soft starter that is affected by the supply impedance, then this may be the cause.

 

2. The start current required by the motor is a function of the motor size and design as well as the motor terminal voltage. While the motor (via the transformer) may well draw only 115A during run, its rated current may be much higher and so the start current may also be much higher.

You have measured the start current at 500A, but it depends on how you have measured this. I have commonly seen "measured" currents that reflect that average reading over a period of time that includes the start time and some run time, so the true start current is higher.

 

3. If you have the machine running for a period of time, then stop and immediately restart, the breakers trip circuits are already hot, and so the overload capacity is reduced. Likewise, if you restart frequently, you will ratchet up the effective trip temperature and thereby reduce the trip threshold.

 

4. Based on the figures given for start current and time, I would not expect the breakers to trip.

This leads me to suggest that:

• The start current is higher than believed
• The start operations are too frequent
• The soft starter is reacting badly with the supply impedance - perhaps a varying impedance due to generator configuration at the time of start.

Best regards,

[martin]

Hello Martin

 

I understand that the breaker trips part way into a start, but only sometimes.

1. You are concerned about the voltage drop during start as being a possible cause.

During start, we intentionally reduce the voltage to the motor to reduce the current drawn by the motor. This is what the soft starter does. A voltage reduction in the supply, during start, will not increase the current provided that the soft starter behaves normally. I have used many soft starters in situations where the supply voltage has dropped by more than 15% during start and there has been no problems.

Some brands of soft starter however, do not work well if the the supply impedance is too high. I have had issues with particular brands in situations where they are operated from generators where the generator rating is not many times larger than the starter. (high enough for the start current, but not multiples of the start current.) With one brand, the problem was fixed by setting the initial start voltage greater than 50%.

As a possibility, some vessels that I have worked on, have a number of generators that can be synchronised and run in parallel, but usually the vessel runs on the minimum requirement only. Is there a possibility that the supply capacity is changing due to the number of generators operating at the time?

If the supply capacity is reduced and you have a soft starter that is affected by the supply impedance, then this may be the cause.

 

2. The start current required by the motor is a function of the motor size and design as well as the motor terminal voltage. While the motor (via the transformer) may well draw only 115A during run, its rated current may be much higher and so the start current may also be much higher.

You have measured the start current at 500A, but it depends on how you have measured this. I have commonly seen "measured" currents that reflect that average reading over a period of time that includes the start time and some run time, so the true start current is higher.

 

3. If you have the machine running for a period of time, then stop and immediately restart, the breakers trip circuits are already hot, and so the overload capacity is reduced. Likewise, if you restart frequently, you will ratchet up the effective trip temperature and thereby reduce the trip threshold.

 

4. Based on the figures given for start current and time, I would not expect the breakers to trip.

This leads me to suggest that:

• The start current is higher than believed
• The start operations are too frequent
• The soft starter is reacting badly with the supply impedance - perhaps a varying impedance due to generator configuration at the time of start.

Best regards,

 

Hi Marke,

I have spoken with the customer, it seems they were running with two geneators in parallel when I tested!!

When they tripped it was only one. The generators are 1MW plus each.....I have trouble believing it cannot start this softstart or application....I do however doubt the impedance of the 660:440 transformer.

I suspect when they tripped not only were they running one generator on this line, but to make things worse, some large load was also on it at the time. This is a survey vessel and this transformer was original....I suspect it was sized for lots of small loads....computers and small UPS's and test equipment but not large so-called loads like a compressor or a softstart.

It seems strange but the engineer who saw the trips said the softstarter never gave a fault of any kind...it is rated at 356A. The incoming breaker is 300A but is tripping. What if the fault is big enough to trip the breaker but below the softstart sizing of 350A.

The settings was 40% pedestal time....the job proved difficult for me as many people were involved. The PE had set the softstart and NOBODY was going to touch it. Perhaps I will suggest a pedestal of 50% to the ship.

They have informed me it has started 3 times (each time) since my visit. It has now left Singapore.

Has the ship electrician known they were starting the ROV, and specifically kept another large load off the same 440V bus thus giving the system enough capacity to start? Remember the ROV electrician is one company, hired by another and the ship electrican is another, so there is maybe a political communication problem as well. Ah well, it was a good job, and we'll have to see how!

Thanks for your help Marke; it was good to have help in the same time zone almost!!

Regards

Martin