Solid State Starters and Capacitors

[Cammack]

Cutler Hammer brought up an issue on a project installing multiple solid state starters in a MCC. Three of the motors will have power factor correction capacitors that would come on-line once the starters were up to speed and the across-the-line bypass contacts were engaged. Cutler Hammer suggested that when any solid state starter was starting up in the MCC, to disengage ALL capacitors on motors during that time period.

A lesser known manufacturer on another project (on line already) said that this should not be an issue and they suggested that only the capacitor on the motor starting should be disengaged and the other caps could stay connected. This project was 4160v with 3x500hp motors.

So far, there has not been any problems at the facility but it concerns me that a major manufacturer has brought up this concern which seems to be a valid engineering concern.

Appreciate any feedback or experience on the issue.
Thanks,
Craig

 

[ghostbuster]

Capacitor bank energy dumping:

To prevent this from occurring during switching generally inrush inductors are installed in series with the capacitor banks to prevent this interaction.A good application engineer from the mfr. would be aware of this.

 

[weressl]

Cutler Hammer brought up an issue on a project installing multiple solid state starters in a MCC. Three of the motors will have power factor correction capacitors that would come on-line once the starters were up to speed and the across-the-line bypass contacts were engaged. Cutler Hammer suggested that when any solid state starter was starting up in the MCC, to disengage ALL capacitors on motors during that time period.

A lesser known manufacturer on another project (on line already) said that this should not be an issue and they suggested that only the capacitor on the motor starting should be disengaged and the other caps could stay connected. This project was 4160v with 3x500hp motors.

So far, there has not been any problems at the facility but it concerns me that a major manufacturer has brought up this concern which seems to be a valid engineering concern.

Appreciate any feedback or experience on the issue.
Thanks,
Craig

Eaton(Cutler Hammer) is right and that's why they ARE a well known manufacturer.

The damage to the SCR's will occur over time and there is a lot of variables, such as the X/R and X/R' of the machine itself, the cable type, size and length and also how 'stiff' is your system.

 

[Jraef]

In general, the potential risk in using PFC caps with soft starters is in having the caps downstream from the SCRs. There are two major issues and when ignored, it's generally a race to see which one causes a failure first.

The charging current of the caps looks like a short circuit to the SCRs and the rapid rate of rise in the current (di/dt) can cause adjacent SCRs to "self commutate" meaning they turn themselves on. When you are ramping voltage with a set of 6 SCRs the timing of the firing of each one is critical. If some of them turn on when they shouldn't, or don't turn off when they should, you get all kinds of trouble and high currents circulating around in the circuit, often high enough to damage the SCRs themselves.

Secondly, firing SCRs creates harmonics. Those harmonics are high when the voltage is phased back the most, steadily decreasing as voltage is ramped and non-existent once at full speed. The harmonics are generally not harmful to nearby equipment because in the case of a soft starter they are short lived since you are not phasing back the voltage for very long. But when PFC caps are involved, you can no longer accurately predict the interaction of the harmonics and the capacitors, especially if they are down stream of the SCRs. When you are starting a motor, the PF is extremely low so the caps are trying to supply all the VARs and therefore are at maximum current. Harmonics add to that and the combination can make the caps heat up, swell and maybe pop if subjected to them for extended periods. So for that reason it's mandatory to at least keep the PFC caps up stream of the SCRs and also a good idea to keep them off-line until the soft starters are done.

For these two reasons, mfrs recommend that caps be controlled separately by a PFC Cap Contactor that only closes once the starter is at full speed, i.e. when the bypass contactor closes.

But is it necessary to keep adjacent PFC caps off line during ramping of nearby starters? I have been in the soft starter business 20+ years, that's a new one on me. One major mfr does not even absolutely require a separate contactor for them, as long as they are up stream (but that means they are on-line all the time, which is also not good). Yes the harmonics they create go both ways so yes, the PFC caps already on-line when the starter next door is ramping will see them. But the interaction will be minimal at that point because the the overall cap current is relatively low since they are supplying VARs only to their own motor when that happens and it is already running at full speed.

As to C-H being a "major manufacturer", I will not disagree from a general stance, but as far as MV soft starters go they are the new kids on the block. For years they brand-labeled their soft starters from another manufacturer (Benshaw) and deferred all issues and designs to them. Their own new MV soft starter has only been on the market a couple of years. I think they are either doing some CYA because they have no idea if it's OK, of they have had an issue with their own design and are thinking that was what was the problem.

 

[weressl]

In general, the potential risk in using PFC caps with soft starters is in having the caps downstream from the SCRs. There are two major issues and when ignored, it's generally a race to see which one causes a failure first.

The charging current of the caps looks like a short circuit to the SCRs and the rapid rate of rise in the current (di/dt) can cause adjacent SCRs to "self commutate" meaning they turn themselves on. When you are ramping voltage with a set of 6 SCRs the timing of the firing of each one is critical. If some of them turn on when they shouldn't, or don't turn off when they should, you get all kinds of trouble and high currents circulating around in the circuit, often high enough to damage the SCRs themselves.

Secondly, firing SCRs creates harmonics. Those harmonics are high when the voltage is phased back the most, steadily decreasing as voltage is ramped and non-existent once at full speed. The harmonics are generally not harmful to nearby equipment because in the case of a soft starter they are short lived since you are not phasing back the voltage for very long. But when PFC caps are involved, you can no longer accurately predict the interaction of the harmonics and the capacitors, especially if they are down stream of the SCRs. When you are starting a motor, the PF is extremely low so the caps are trying to supply all the VARs and therefore are at maximum current. Harmonics add to that and the combination can make the caps heat up, swell and maybe pop if subjected to them for extended periods. So for that reason it's mandatory to at least keep the PFC caps up stream of the SCRs and also a good idea to keep them off-line until the soft starters are done.

For these two reasons, mfrs recommend that caps be controlled separately by a PFC Cap Contactor that only closes once the starter is at full speed, i.e. when the bypass contactor closes.

But is it necessary to keep adjacent PFC caps off line during ramping of nearby starters? I have been in the soft starter business 20+ years, that's a new one on me. One major mfr does not even absolutely require a separate contactor for them, as long as they are up stream (but that means they are on-line all the time, which is also not good). Yes the harmonics they create go both ways so yes, the PFC caps already on-line when the starter next door is ramping will see them. But the interaction will be minimal at that point because the the overall cap current is relatively low since they are supplying VARs only to their own motor when that happens and it is already running at full speed.

As to C-H being a "major manufacturer", I will not disagree from a general stance, but as far as MV soft starters go they are the new kids on the block. For years they brand-labeled their soft starters from another manufacturer (Benshaw) and deferred all issues and designs to them. Their own new MV soft starter has only been on the market a couple of years. I think they are either doing some CYA because they have no idea if it's OK, of they have had an issue with their own design and are thinking that was what was the problem.

C-H has been making SSRV starters for at least 35 years. Or do you claim that LVSSRV starter experience is not valid? I do not think that the advice was understood correctly. There are surge capacitors and power factor corrective capacitors that may be associated with a motor. I think that is what was meant by the comment; all and any capacitors connected to the motor to be controlled by the SSRV should be disconnected while starting the motor.

 

[Jraef]

... I do not think that the advice was understood correctly. There are surge capacitors and power factor corrective capacitors that may be associated with a motor. I think that is what was meant by the comment; all and any capacitors connected to the motor to be controlled by the SSRV should be disconnected while starting the motor.

But his question and their comment was specifically with regards to capacitors connected to OTHER starters.

Cutler Hammer brought up an issue on a project installing multiple solid state starters in a MCC. Three of the motors will have power factor correction capacitors that would come on-line once the starters were up to speed and the across-the-line bypass contacts were engaged. Cutler Hammer suggested that when any solid state starter was starting up in the MCC, to disengage ALLcapacitors on motors during that time period.

So either some technician misunderstood the issue or C-H is doing CYA. I have never seen this be an issue, nor have I ever heard of anyone else even mention it until now. I think it's a red herring.

And yes, I do not consider LV SSRV experience to be as valid as MV SSRV experience when discussing issues related to MV SSRVs. I went through the learning curve on MV SSRV design in the early 90s with two of the manufacturers and in spite of a long history with low voltage products in both companies, it was a steep and expensive curve. On the hundreds of MV soft starters I have been involved with or witnessed start-up on, including applications with lineups much larger than this, we never once had an issue with adjacent caps. The only issue I ever had was when the contractor didn't pay for star-up service and failed to check that the motor had surge caps connected in the motor connection box. They lost the SCRs and the caps in about 10 seconds of being initially energized.

 

[weressl]

But his question and their comment was specifically with regards to capacitors connected to OTHER starters.
So either some technician misunderstood the issue or C-H is doing CYA. I have never seen this be an issue, nor have I ever heard of anyone else even mention it until now. I think it's a red herring.

Usually 4160V starters are not called MCC's. So there is a disconnect between one part of the story and others. Eaton sell through lot of channels and sometimes store sales are mistaken for qualified Eaton representation.

And yes, I do not consider LV SSRV experience to be as valid as MV SSRV experience when discussing issues related to MV SSRVs. I went through the learning curve on MV SSRV design in the early 90s with two of the manufacturers and in spite of a long history with low voltage products in both companies, it was a steep and expensive curve. On the hundreds of MV soft starters I have been involved with or witnessed start-up on, including applications with lineups much larger than this, we never once had an issue with adjacent caps. The only issue I ever had was when the contractor didn't pay for star-up service and failed to check that the motor had surge caps connected in the motor connection box. They lost the SCRs and the caps in about 10 seconds of being initially energized.

In this context the issue is the same. In other aspects there are differences between between LV and MV.

 

[Besoeker]

The charging current of the caps looks like a short circuit to the SCRs and the rapid rate of rise in the current (di/dt) can cause adjacent SCRs to "self commutate" meaning they turn themselves on.

That's not actually quite right. The excessive di/dt is likely to cause failure in the device subjected to it. The mechanism is like this.
The SCR has a limited di/dt capability typically 200-300 A/us for a phase control device. The current spreads across the area of the chip relatively slowly. If the circuit is such that the current doesn't have time to spread sufficiently, the chip gets all the current in a relatively small area, overheats in that area and fails.
In fact, this is the mechanism for most of the common failures in SCRs. There are three ways a SCR can get turned on:

1. A normal gate pulse of the required current and rise time (<1us)
2. Excessive dv/dt
3. Overvoltage.

In case 1, the firing current spreads from the track that is the gate (see below) and, with good circuit design, the di/dt is limited to within the device rating.
In cases 2 and 3, the current starts at a single point, current flows locally and destroys the SCR.
The image below shows a typical overvoltage failure and you can see that the damage is limited to one area next to the edge of the chip. I put my phone on the scanner to give scale.

BTW commutate means to turn off.