Dear All,

We have a motor of size 520KW and we have decided to install 150KVAR capacitor banks (ungrounded Delta) to improve the power factor.

1.Do we have to install series reactors on the line side?

2.Should the reactors be designed for Inrush current limiting or for controlling the harmonics?

3.What are the design considerations for sizing the reactor for limiting the Inrush & for controlling the harmonics?

4.We don’t have any Electronic Converters in our System. We have Auto transformer Starting for motor.

5.What is the formula to find the maximum inrush drawn by the capacitors?

6.How do we know which order the harmonics will affect our system? (Do we have analyse with some analysers).

 

Charz... can you provide detail:

1. Often the connecting cable is sufficient to reduce inrush current to a capacitor-bank!

2. How do you know you will have harmonics?

3. Insure that the shunt capacitance does not result in resonance, that is, inductive-reactance should not be close to capacitive-reactance.

4. If you are using AT-Starting why do you need the capacitors?

5. What are its rated namplate parameters?

6. What is the supply transformer's rated capacity and percent impedance?

7. What is the distance between the motor and its AT-starter?

8. What is the motor driving?

9. Sizing of PF-correction capacitors is critical to insure safe operation of the motor. The capacitor should only offset the about 80% of the no-load kVAr of the motor, and not the full-load kVAr. Also, it is dependent on voltage-level, construction-type (open-frame, totally-enclosed, etc). In addition, the capacitor size is inversely proportional to rated-speed!

Regards, Phil Corso

 

Thanks Mr.Phil,

1. How do you know you will have harmonics?

I don't Know.I know that a detailed study/analysis have to be made.In case if resonance occurs will it draw harmonic current from the grid?

2.If you are using AT-Starting why do you need the capacitors?
For Motor running PF improvement.

3.What is the supply transformer's rated capacity and percent impedance?
3 MVA & 6.46%

4.What is the distance between the motor and its AT-starter?
40 Meters

5. What is the motor driving?
Pump

6.I'm not able to get that the detuning reactor shifting the frequency or preventing the harmonics? could you help me on this?

Regards,
Charz

 

Charz... what is the motors rated voltage?

Phil

 

6.6KV

 

Charz... re your query, I will respond shortly!

Phil

 

Charz... following are the results of my review of your situation:

1) The voltage at the motor, during a DoL start, is approximately 87%!

2) The voltage at the motor, during an AT start, is approximately 95%!

3) PFC capacitors should not exceed 50-75kVAr!

4) A series-reactor reduce inrush current is unwarranted!

5) Unable to assess harmonic problem! Is the 6.6kV distributionsystem comprised of very long runs of cable?

Basis for above:

a) Motor is 520kW, or about 700 Hp!

b) Starting kVA per Hp is 6!

c) AT Tap set to 50%!

d) Motor is TEFC, 2-4 pole|

e) Feeder-cable sized for about 1.25x60 => 75A!

f) Approximate stop to rated speed 5-7 seconds!

If you (or anyone not a PtP) want details, contact me off-line!

Regards, Phil ([email protected])

 

Charz... additional questions related to Harmonic and Ferroresonance potential:

6) How is the 6.6kV transformer earthed?

7) How are 6.6kV system monitoring and protective PTs configured?

Regards, Phil

 

Dear Mr.Phil,

6.The Transformer Neutral is solidly earthed.

7. There is no individual PT for the motor feeder,but there is a bus-section PT which trips only on under/over voltage actuation.

8. Another question, In a harmonic free environment if the system's natural Inductive reactance and capacitive reactance falls in resonance, what will be the impact?

 

Charz... there is a way to "investigate" the transient phenomenon, Ferroresonance! A good first-step is to recognize when L and C parameters are connected in series, or parallel!

First, the prefix "ferro-" refers to phenomenon involving electrical apparatus containing a magnetic-core, hence, transformers, reactors, etc, in parallel, or in series, with components that have capacitance-to-earth like cables, surge-arresters, xfmrs, motors, and generators!

Secondly, the fact that your 6.6kV system is solidly-earthed (unusual, but not unheard of) lessens the likelihood of Ferroresonance but doesn’t necessarily eliminate it! I do have a concern about PTs used for protection or measurement when it is possible they can “saturate” during earth-faults!

Fortunately, you can easily determine the “capacitance” of the distribution system by measuring the capacitive-current in the Xfmr's Neutral-to-earth connection! For determining the saturated reactance of the PT you need only contact the mfg!

Phil