Series Capacitor Help Diminish Vdrop Due to Motor Starting???

I'm afraid your thinking in terms of a shunt connected capacitor, which is more of the garden variety type and supplies kvar to compensate for lagging power factor loads and the accompanying voltage drop. They also reduce losses.

Series capacitors are used to reduce the series reactance, as your equation describes. Since the typical impedance has a significant inductive component. The series capacitor introduces a negative X component which compensates for some of the inductive reactance. The net result is that the system X term as seen at the load side of the series capacitor is greatly reduced. This reduces the voltage dip as seen for any loads on the load side of the series capacitor. It doesn't do anything for the source side.

Series capacitors are usually made up of fairly low voltage capacitors compared to the line voltage. This is done to achieve more ohms per kvar. The capacitors themselves are made up of parallel 2-bushing units and are connected in series with the line. You can visualize this as simply cutting the phase conductors and reconnecting one end with one of the bushings of the capacitor and the other end with the other bushing.

They have been used on transmission and distribution circuits.

Hi magoo2. I agree with you. But these series capacitors connected to the line will not in anyway improve the voltage during motor starting (as my subject implies), right? I heard that they even provide parallel resistors on them to avoid sub-synchronous resonance when motor connected to the system is being started.

I have a paper published about 35 or 45 years ago from a respected engineering company that deals with the use of capacitors to reduce starting current of three phase induction motors.
The starting current of an induction motor is highly reactive and in theory this component of the starting current may be supplied by a (parallel) capacitor bank. As the motor accelerates, the reactive component of the current drops and the capacitors must be cut out in steps as the motor is accelerating. Although the theory is sound, the cost of a very large capacitor bank and switching arrangement will be high.
There is also the issue of switching.
I have never seen this theory put to use. It is interesting, but probably not practical.
A couple of quick figures. 5 HP @ 550 V = 5 amp
Starting current = 40 amp @ 0.2 PF
The real component of the starting current = 8 amps
The reactive component of the starting current = 39 amps
The capacitive reactance needed to offset the reactive component of the starting current = 37 KVAR
For comparison, the capacitive reactance to correct the power factor of the normally running motor from 0.9 to unity = 2 KVAR
Consider also if for some reason the switching fails and leaves the capacitors connected while the motor is running normally, the current drawn by the capacitor bank will be almost equal to the starting current that we were trying to avoid. This current will not drop as does the starting current but will continue until switched off.
Possible but not recommended.

Bill
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"Why not the best?"
Jimmy Carter

I'm not aware of series capacitors used at any voltage that would have motors connected. Series capacitors are typically used on long, high capacity, high voltage (EHV, UHV) lines for line compensation. If you had a series capacitor in a motor circuit you would have less voltage drop during starting simply because some portion of the source X would be compensated and there would be no voltage drop associated with the "missing" X. If you're looking for power factor correction you will find shunt capacitors much easier to deal with.

hey nightfox,
The series cap will improve the voltage during motor starting. Motor starting has a poor power factor, in the 20 to 30% range.

If you look at the voltage drop equation,
Drop = I (R * cos th + X * sin th)

Since cos th is low, the other term, X * sin th, becomes most important. By reducing X, you lessen the voltage drop to the motor. So the voltage to the motor is higher with a series cap than without one.

davidbeach
I agree you generally will have a transformer between the series cap and the motor. We have a fairly large water pumping station on our system with a 4000 or 5000 HP motor supplied from a 35 kV feeder. The motor is either 4 kV or 6.9 kV. The voltage drop with our existing substation transformer would have been excessive at the substation bus so we were forced to look at putting in a larger substation transformer or a series capacitor. We went with a series cap. The site still had additional drop through the service transformer, but the customer said they could live with it and the series capacitor was installed to keep the voltage drop at the substation bus under 5%.

And yes, the series cap has a damping resistor to avoid subsynchronous resonance.

I think that my friends davidbeach and magoo2 are in agreement here. Generally the voltage dip associated with motor starting is mostly local in the transformer and secondary feeders.
If the motor starting load is large in relation to the high voltage feeder or transmission line capacity then the reactive voltage drop of the line may become a significant part of the total voltage drop. Long heavily loaded transmission lines are often compensated with series capacitors to reduce the impedance and the voltage drop of the line. This is called line compensation.
The same technique may be used to reduce the line voltage drop when a very large motor starts and line impedance is causing a significant part of the voltage drop.
When we are comparing the price of a new substation with the cost of a high voltage series capacitor station, it becomes obvious that this is not a viable solution for the majority of motor starting issues. If there is very little voltage drop in the high voltage feeder during motor starting, then there may be little to be gained by series capacitors.

Bill
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"Why not the best?"
Jimmy Carter

There is a recognized procedure called "Capacitor Assisted Motor Starting" and was very popular for large motors some time ago. As mentioned above, the capacitors need to be much larger than what would be used in pf correction and they must be switched out of the circuit at maximum slip, but no later otherwise you end up over shooting and creating new problems.
This has been discussed here before.

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Correct me if i'm wrong gentlemen, based from the your inputs. A series capacitor is normally used on high voltage utility systems which normally at the line side of distribution transformers. If I have an industrial facility, series caps (line compensation) is normally applied to the primary of utility system. And these are already designed to based from system maximum loading where these caps are connected. If these caps are already there and existing, if a big motor starts at the secondary of the plant's main distribution transformer, then these caps will may or may not be factor of mitigating the voltage drop depending on how large is the starting kVARs compared with the primary system capacity (i,e. Primary line is significantly loaded. Am I getting the concept straight?

There are two different issues here.
1> The motor starting current is highly reactive. This reactive current may be supplied by parallel capacitors. This is theoretically possible but does have possible issues. This works by decreasing the actual motor starting current.
2> If the characteristics of the supply line are such that the motor starting current causes a reactive voltage drop in the line, the line reactive voltage drop may be compensated with series capacitors. This has more to do with the inductive reactance of the primary line than with the stateof load of the line. If there is little reactive voltage drop in the primary line, the gain with series capacitors will be little.
If there is a significant reactive voltage drop in the primary line then series capacitor compensation will benefit all loads, not just motor starting.

Bill
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"Why not the best?"
Jimmy Carter

In my experience, looking at the Pacific Northwest Grid, series caps are generally used at 500kV with the possibility of limited application at 230kV and above. I've seen nothing that would indicate any use of series caps below 230kV.

Series caps are a significant protection problem. Any fault current through the cap bank produces a damaging voltage rise across the cap so there needs to be some voltage control device in parallel with the caps, spark gaps, MOV, or etc. Then to protect the voltage control device you need a breaker to short out the cap and the voltage control device.

If you try to insert series capacitors in a motor circuit without that protection and you have a motor fault you may find that you have lost all of your caps. Quite likely that locked rotor current would be enough to require the bypass of the caps and then they wouldn't do much for motor starting, would they?

Thank you David. I have been wondering about protection issues and hoping you would comment on them.


Bill
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"Why not the best?"
Jimmy Carter

I have a question Davidbeach. If I am off base, please red flag this post to avoid misinformation and confusion.
As I understand line compensation, the reactive voltage drop due to the inductive reactance of the line is compensated by inserting a roughly equal amount of capacitive reactance.
As the current in the line varies the inductive reactive voltage drop varies but is offset by an equal amount of capacitive reactive voltage drop of opposite sign.
The more current flowing in the line, the higher will be the voltage drop (rise?) across the capacitors.
If we have an instance where the the addition of the motor starting current results in twice the line design current, the voltage rating of the capacitors must be doubled to allow safe operation.
Given that high voltage capacitor stations are built by connecting a large number of relatively small capacitors in series parallel, doubling the voltage rating would require that the number of capacitors be increased by a factor of four.
When motor starting may require four times the number of capacitors as normal line compensation, the idea gets very expensive very fast.
Please note. This is a hypothetical case. The actual numbers will vary widely depending on actual conditions and line loading.

Bill
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"Why not the best?"
Jimmy Carter

Bill, sounds good to me. I've never tried to design a series capacitor station and what I know about them is mostly theoretical anyway.

Thank you David.

Bill
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"Why not the best?"
Jimmy Carter

Waross,
Let me try to address your case:
If we have an instance where the the addition of the motor starting current results in twice the line design current, the voltage rating of the capacitors must be doubled to allow safe operation.

As I said before, they normally parallel fairly high kvar, relatively low voltage cans to obtain a reasonable reactance. You can see why this is so because X = kV^2/Mvar. I've seen 2.4 kV rated capacitors used on series capacitors on both 15 kV and 35 kV series capacitors. The voltage rating stays the same because there is MOV protection across the capacitors.

If the motor starting current causes the current to be twice the rated line current, so be it. After all, it's just motor starting duty and doesn't last that long.

I don't know if this helps, waross, but I hope it does.

Hi magoo2;
On the installations that I am familiar with the capacitor banks are constructed of series parallel connections of lower voltage rated capacitors. Doubling the current doubles the rated voltage required. If the voltage rating of a series parallel bank of capacitors is doubled, it needs four times as many units to maintain the same capacity or capacitive reactance.
I suspect that in some instances it would be cheaper to upgrade the line.

Bill
--------------------
"Why not the best?"
Jimmy Carter

Bill,

I think we're talking about 2 different situations.

I agree, if you go from a design for 600 A to one capable of handling 1200 A, the cost will more than double and probably more like 4 times as you suggest due to the number of capacitors.

I thought the earlier point was dealing with a 600 A rating that, due to motor starting, saw 1200 A. Since motor starting is in the order of seconds, I think the 600 A series capacitor could handle the higher current for that brief period without any special modification.

Are we on the same page now?