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Anonymous Poster #1

Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/08/2017 4:57 AM

Hi All,

We are designing a pumping station with 6 pumps driven by each induction motor with power 200 kw, 400v, we need to do power factor improvement to reduce reactive power which is waste energy:

1. which one do you think is better to be used vfd to each motor or capacitor bank to the whole system?

2. Is capacitor bank fitted with controller to control the switching based on measured P.F? to avoid leading effect?

Best regards

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#1

Re: Which one is preferred? VFD or Capacitor Banks for PF improvement?

10/08/2017 5:41 AM
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#6
In reply to #1

Re: Which one is preferred? VFD or Capacitor Banks for PF improvement?

10/09/2017 5:29 AM

Thanks, this is useful material, I checked it before.

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#2

Re: Which one is preferred? VFD or Capacitor Banks for PF improvement?

10/08/2017 7:29 AM

The capacitor bank is cheaper, and it will work in this case, so you should be fine with it.

However, if by wasting energy you mean you are wasting the reactive power, you should know that you are not.

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#3

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/08/2017 11:05 AM

Softstarter, VFD and Capacitor banks

Here's what member JRaef, an expert, says on ENG-TIPS.com:

jraef (Electrical)7 Feb 07 11:00

#1 - There is no need for power factor correction caps on a VFD application, the displacement power factor presented to the line by the VFD is alrerady as corrected as you need it to be. The distortion power factor is worse, but that is a different issue.
#2 - You can use capacitors on a soft starter motor application, but they can NEVER be connected to the load side (downstream) of the soft starter. The charging current of the capacitors looks like a short circuit to the SCRs, so it can cause them to self-commutate and misfire, causing damage to themselves and other equipment. In addition, the harmonics created by the phase angle firing of the SCRs can cause the capacitors to overheat, swell and sometimes even explode. it is usually a race to see which component, the SCRs or the caps< fail first. If you want to use PFC caps however, just wire them on the line side of the soft starter and have a separate capacitor contactor keep them off-line until after the soft starter has brought the motor to full speed and preferably bypassed. If you have (unfortunately) selected one of the phony baloney "energy saver" soft starters because someone fooled you into believing they will work, you MUST turn the "energy saver" feature off if you want the PFC caps. When that feature is in use, the SCRs are firing continuously and the SCR/capacitor damage will occur.

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#7
In reply to #3

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 5:35 AM

yes VFD's can improve the power factor but you may need active filters to eliminate harmonics effects, it depends on the system harmonics limits.

for the soft starter, how can we connect the capacitors in this case? if we cannot connect them in the load side down stream? if you can present this in line diagram so I can figure out will be much appreciated.

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#4

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/08/2017 8:49 PM

VFD is only applicable if you have a variation flow control on each pump. If your system is on-off pressured controlled I bet you can't use that.

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#8
In reply to #4

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 5:37 AM

Our application is in waste water treatment pumping station depends on level measurements to command the drivers start/stop the motors.

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#10
In reply to #8

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 9:14 AM

Are all 6 pumps operational during this period of pumping water the desired tank level? Or 2 clusters of 3 pumps each? How is it set up? I bet there are standby pumps, in case of other pump unit failure.

Get the log and see the trends of the 6 pumps (on-off trends)

How is the outflow water discharge from the pump is it dropped freely above the tank or connected underneath the bottom of the tank?

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#5

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/08/2017 11:16 PM

Since you have 6 pumps, it's not likely you can save much power cost by varying the speed of more than one pump, as Gutmonarch mentions. Variable speed drives can easily pay for themselves by matching centrifugal pump speed to hydraulic needs of the system.

There is actually very little lost energy due to poor power factor, the motors should be running close to full load, so power factor will probably be better than 85%. You really should do an investigation of the typical design losses you expect over the course of a year. The IsquaredR losses of your feeder wiring (copper heating) is one source of revenue to pay for capacitors, other losses are increased transformer losses, which may be buried in the total losses in a 2000kVA transformer. Your utility may have a power factor penalty, usually for installations presenting less than 95% at the utility metering point. I would not be surprised if the payback for power factor correction in your case was close to 8 years or more.

Your most inexpensive and easy to understand, troubleshoot and operate is to install a single 3 phase capacitor connected to each motor, preferably between the motor overload relay and the motor starting contactor. If below the overloads, you need to reduce the overload relay setting to account for the improved motor power factor/lower apparent motor current.

With the capacitors switched on and off with the motor, you don't have to worry about power system over-excitation, and the voltage surges that come with switching capacitors at the bus level. NEMA rated motor contactors are rated to switch capacitors & motors together. The size of the capacitor is set by the motor design, the maximum value is usually stamped on the motor nameplate, as this is a very common old school method for power factor improvement dating to the 1970s.

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#9
In reply to #5

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 5:46 AM

Yes we have 6 pumps in total separated each 3 pumps for 2 sumps, the maximum running pumps normally is 3, configured duty/duty assist/standby. the vfds bring the pump curve to the exact required head and flow with reducing the maximum speed rather than running at 50Hz at full load they will be 45 Hz, and this is at the peak with may happens rarely few hours per day. So it will be good advantage for the speed and P.F improvement and cost saving, specially in our regulation if the total station p.f less than 0.9 the KVAR will be counted in the bill.

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#11
In reply to #9

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 9:43 AM

I think you'll find that no power factor correction will be needed at all. Capacitors and non-linear loads like PWM drive front ends will not go well together and generally (without a highly engineered installation) will not be trouble free.

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#12
In reply to #9

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 11:59 AM

Fardan,

R Williams' answer has a glaring error--DO NOT use capacitors between any VFD and the motor. This is a standard warning from the VFD manufacturers. His answer is valid and good for motors operated DOL (across-the-line, you US people). Since VFD's have a rectifier head end they will introduce harmonics onto the power line. The magnitude of these increases as motor load decreases. In the last few years, VFD's have been marketed with active filters on their head end--I strongly recommend using them, as they will greatly reduce any harmonic distortion on the incoming power line and transformers. If you don't, then you should use 3% reactors on the line side or (better) a separate active harmonic filter. Unfortunately separate active harmonic filters are often expensive and their capacitors are known to fail. If the VFD does not have load side filtering, then you will want some 3% load side reactors there, as they greatly reduce the percent of harmonics the motor sees and will help protect the motor against the higher voltages the first few turns of their windings are exposed to because of the higher frequency harmonics' reflecting off the impedance mismatch between the power cables and the windings.

The information in your post suggests to me that each motor will be wired directly from a VFD and that you have a separate controller that alternates the assignment of motors between duty/duty assist/standby on a daily basis so their wear and hours of use are equalized. It also suggests that at periods of low flow (such as very early morning) for each sump you may have only one motor running at a reduced frequency; while at times of very high flow you can have the duty and duty assist motors both running at full speed and the standby pump also running at a speed as needed to maintain the proper sump level.

--JMM

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#14
In reply to #12

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 3:51 PM

The OP did not realize at first that modern PWM VFDs do not present poor power factors to the utility, and do not require power factor correction at all. However, the mention of existing VFDs did not come until much later in the time line.

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#19
In reply to #12

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/10/2017 4:22 PM

Hi,

Yes some VFD's types come with low harmonic distortion, there are 16pulse and 18 pulse types which I think the 18 pulse is better in eliminating harmonics, but if the harmonics with 16 pulse still fine then its O.K, I mean cost fortune point of view.

your suggestion is right to my information, speed variation is required in this application.

thanks

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#18
In reply to #9

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 10:52 PM

Refer to the figure. What exactly you want to express? See manufacturer's pump curve for the specific pump and review existing head and flow delivery.

Typical design consideration for the pump size should fall on the (red) efficiency curve of the figure above. Other than that is misappropriated design. It does say don't control the speed lower than 50hz because you are drastically wasting electrical energy.

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#20
In reply to #18

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/10/2017 4:28 PM

The VFDs will be set to minimum frequency/speed applicable in your case as 50 HZ, but here is one question, some time at the minimum freq. the hydrolic eff. will be poor, so you may set the minimum with considering the pump eff. as well, Am I right?

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#21
In reply to #20

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/10/2017 9:26 PM

Assuming you do have centrifugal pumps, there is some speed for the pump that will not produce any flow. That speed value is typically somewhere below 20% of the design pump speed. This is a personal rule of thumb I've observed in typical water or water like fluids, and with typical pump selection, observed when converting fixed speed pumps to variable speed operation.

The piping configuration also affects the minimum speed, if you consider piping that has a different static head from start of pumping to when the pipe is full.

To improve the response of a controller loop, I set the minimum pump speed where flow starts, so the PI controller, (level, pressure, flow, etc) doesn't wind up trying to establish flow. Your controller is likely controlling to level, so the speed will adjust to the required speed soon enough, no worry about inefficient operation. For two pumps in parallel, both VFDs, you need to bring one pump up to full speed at its BEP Best Efficiency Point on the curve, then control the speed of the other pump to keep the controller happy.

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#22
In reply to #20

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/11/2017 8:51 PM

Yep, above is just a straight forward plot of efficiency with respect to speed in Hz. Obviously, everybody doesn't want to operate on inefficient regions of the plot.

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#13

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 12:32 PM

If the VFDs are going to be used to VARY the flow, then in using them, your displacement power factor will be corrected to .95 BY the VFD itself, no need for further correction.

If your pumps run CONTINUOUSLY at 45Hz, not VARYING, then your pumps are over-designed and you would be better served by using a throttling valve or trimming the impellers to get the flow values you need. Using a VFD to trim flow (as opposed to VARYING flow) is wasteful because the VFDs introduce about 3-5% losses in the system. So if you the flow of each pump is fixed and never varies, use Soft Starters, then use PFC capacitors. What you do is to have a PFC capacitor contactor attached to each soft starter and only AFTER the soft starter has finished ramping the pump to full speed, the PFC Contactor closes to put the caps on-line. You do NOT want the caps on-line when the SCRs are ramping, and you should not put them down stream of the soft starter at all. So the separate PFC contactor keeps the caps off-line when the pump is not operating.

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#15
In reply to #13

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 4:09 PM

Soft starters are generally fixed frequency reduced voltage controllers, which are really only useful to reduce voltage dip and flicker on weak power systems. They have no other good use.

I see many applications where the soft-starter is applied to reduce mechanical torque applied to the shaft of the driven equipment. While this does work, the thermal toll it takes on the motor (rotor bars and stator winding heating), while the motor comes up to frequency is significant, and often not accounted for in the operation costs of the installation.

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#16
In reply to #15

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 6:11 PM

Slight correction. There in no net thermal difference in acceleration energy using soft starting vs across-the-line (DOL) starting. The area of the thermal curve is exactly the same size, only the time axis is longer and the amplitude axis is shorter. So soft starting has no negative effect on motor life. Common misconception, based on an erroneous concept that with insufficient accelerating torque, the motor overloads before it finishes accelerating. It's erroneous in that if the motor stalls like that, it was NOT a successful application!

There IS however a difference in the acceleration energy when using a VFD, because only with a VFD can one accelerate a load without exceeding the Full Load Current capacity of the motor, as long as you have enough time. But again, if not varying the speed, the VFD is going to thermally stress the motor at constant speed compared to a starter. Motor mfrs will tell you that if using a VFD on a motor with a 1.15 Service factor, the Service Factor becomes 1.0, because of the added thermal stress. If you are VARYING the speed, the long term benefits are there to justify it, but not if operating at a constant speed.

.

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#17
In reply to #16

Re: Which Is Preferred? VFD or Capacitor Banks for PF Improvement?

10/09/2017 10:09 PM

You are neglecting the thermal energy that enters the motor components during the time the motor rotor rotation does not match the applied frequency (slip frequency). The startup current that is reactive is extended as you increase the acceleration time, by reducing the available torque. This loss appears especially in induction motor rotor bars, and is a function of the slip frequency.

Soft starting at fixed frequency increases thermal stress on motor components.

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