MV Motor Surge Pak 1

[joan271273]

In certain instances MV motors are fitted with SURGE pak ( Surge arrester and capacitor as a single module ). These devices serve to reduce the magnitues of surges applied to motor windings in particular due to switching operations. Can someone advise how are these devices sized.

 

[jbartos]

Suggestions:
1. Try IEEE Std 1036-1992
IEEE Guide for Application of Shunt Power Capacitors
Abstract: Guidelines for the application, protection, and ratings of equipment for the safe and reliable utilization of shunt power capacitors are provided. This guide applies to the use of 50 and 60 Hz shunt power capacitors rated 2400Vac and above, and assemblies of capacitors. Aplications that range from simple unit utilization to complex bank situations are covered.
2. Revisit your previous posting and provided references for your requested information (attached below):
======================================================
joan271273 (Electrical) Oct 31, 2000
MV ( 4.16kV and above ) motors in some events require the use of the so called "surge packs (arrester-capacitor)" between the neutral of the motor and ground.Can someone indicate a procedure to:
1.- Determine their need or not.
2.- Sizing of the device.
3.- Installation information.
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jbartos (Electrical) Nov 1, 2000
Suggestion: Excellent reading about your posting appears to be in:
References: 1. Smeaton R. W. "Switchgear and Control Handbook," Second Edition, McGraw-Hill Book Co., pages 3-65 to 3-69
2. Smeaton R. W. "Motor Application and Maintenance Handbook," McGraw-Hill Book Co., 1986

 

[jack6238]

For joan271273. Some excellent references for your use. I have found these to be the best of the many that are available.
-Lightening Protection For Electrical Equipment by Edward Beck, Manager, Lightning Arrester Engineering Section, Westinghouse Electric Corporation, McGraw-Hill, 1954
-Electrical Transients in Power Systems by Allan Greenwood, Consulting Engineer Power Transmission Division, General Electric Company, Wiley-Interscience, 1971
-Polyphase Induction Motors, Analysis, Design, and Application by Paul L. Cochran, General Electric Company, Marcel Dekker, 1989

The recommendations for lightening arrester and capacitors are as follows:

CAPACITORS-Located at the machine terminals

Motor Bus Voltage Capacitors in Micro-farads per
in KV Phase

2.4 .5
4.16 .5
4.8 .5
6.9 .5
11.5 .25
13.8 .25


Arresters-Located at the machine terminals

Motor Bus Voltage Station Type Arrester Rating
in KV in KV

2.4 3.0
4.16 4.5
4.8 6.0
6.9 7.5
11.5 12.0
13.8 15.0


A few words concerning the need and purpose of the surge module. The motor insulation system is able to withstand short time overvoltages to a greater degree than it is able to withstand long-time overvoltages. However repeated short-time overvoltages, even of very short duration, have a cummulative effect in weakening the insulation system and thus can result in insulation system failure. The insulation system of interest is the turn-turn insulation and the ground wall insulation in the stator slots.

An incoming transient wave, lightning stroke or a switching surge, has a relatively steep slope and rises from zero voltage to the peak voltage in a relatively short period of time. This steep front voltage wave can be impressed across a small amount of the motor coil and subjects the insulation system to tremendous voltage stresses. For example the typical lightninig surge will rise from zero voltage to peak voltage in apprximately 1.5 micro-seconds. This means that one part of the coil will experience zero volts while the insulation immediately behind it can experience the full peak voltage of the incoming transient . The time for the voltage to reach its peak and the speed at which the voltage is propragated through the winding is dependent on the combined lumped capacitance and inductance of the specific system. The purpose of the capacitor is to cause the rise time of the incoming surge to be modified from a rise time of say 1.5 micro-seconds to a rise time of around 10 micro-seconds or more. By increasing the rise time of the transient voltage, a smaller voltage gradient is impressed across the motor insulation system. The purpose of the arrester is to limit the magnitude of the incoming voltage to a value which the insulation system can withstand.

The system is most easily modeled by having the system inductance in series with a lumped capacitance. A transient voltage is then impressed in this circuit and the voltage rise time and speed of propagation can be calculated from formulas. The speed of the wave is given by
v(velocity in meters/sec)=(1/LC)^.5 and the time to reach the peak voltage is T(sec)=(3.14159/2)*(LC)^.5.

When the motor surge impedance is added to this model it is in parallel with the lumped capacitance and the surge impedance is represented by Z= (L/C)^.5. Surge impedance of machines can be obtained from the manufacturer and my motor data for 2300 volt motors shows values from 100-1000 ohms for machines ranging from 250 HP to 3000 HP at speeds from 3600 RPM to 900 RPM. The smaller high speed machines having the higher surge impedance and the larger machines having smaller surge impedance values. I have in front of me the surge impedance of a 300 HP, 2300 volt, 1800 RPM which is listed as 575 ohms and a 2000HP, 2300 volt, 900 RPM is shown as 105 ohms.

If these motors are cable supplied from transformers and not subjected to direct strokes to the motor supply circuit, then the incoming lightning or switching surge must pass through the transformer inductance. The transformer inductance can be obtained by seperating the transformer impedance into the X and R values and calculating the inductance in millihenrys from the transformer X value.(L=Xl/377)

I have modeled many machines cable connected from transformers and found that the combination of the transformer inductance and lumped capacitance of the cable circuit have been more than adequate in providing the sloping required for the incoming transient voltage. Thus, I have not installed surge protection for these types of installations for induction motors and have had no problems. These motors have been supplied with both the air magnetic and vacuum type contactors and/or breakers and I know of no problems in more than 30 years. The surge pack takes up space and the motor terminal box can become a nightmare in the installation/removal of the individual motors. Having said this, I always install surge protection equipment for generators operating at 6900 volts and above.The cost and importance of the individual rotating machine to be protected is always the determining factor in application of this protection. Also the terminal box of this equipment is normally designed to accomodate this surge protective equipment.

The book by Beck has an excellent chapter on protecting rotating machines and there is an example of the calculations in Chapter 14, pages 226-231.

A couple of final thoughts on this subject. The insulation system of the machine fails when the peak voltage of the ground insulation system in the slots is exposed to short time values which exceed the rating of the insulation system. Repeated short time transients will weaken the system and may cause premature insulation failure due to electrical fatigue and a result in a machine ground fault. The more serious concern is exceeding the turn to turn insulation system rating. The maximum voltage across the turn-turn insulation system is usually at the terminals of the machine and not within the windings. I should also mention that the speed of propagation of the traveling wave is significantly different for the winding which is located within the slot than the portion which is located in the end turn portion. Speeds within the slot are around 15-20 meters/micro-second and the speed in the end turns is around 150-200 meters/micro-second. This is due to the fact that the capacitance and inductance for the portion of the winding within the slot is much greater than the winding in the end turns.
Hope this helps.

 

[joan271273]

Thanks for the excellent response. Is Ref #1 a book published by Westinghouse directly or via someone like John Wiley.

The surge packs that I have seeing installed were mostly in short cable runs fed via 6.9kV Vacuum contactors and yes they did complicate the MV cable terminations.One thing that I forgot to ask usually the manufacturer's start up rep's have indicated that NO Field test is required for these units prior to start up. What is your experience.

 

[jack6238]

There a couple of things I check prior to energizing this equipment. My main concern is checking for shipping damage. I have not experienced any capacitor problems but I have found damaged(hairline cracks) lightning arresters that failed when energized.Also had one unit explode during normal operation and I suspect it suffered some internal damage. If you have a dc high potential test set you can give them a test at the MCOV *(2)^.5 where MCOV is the maximum continuous operating voltage. All lightning arresters will have a small milliamp leakage current at the rated system voltage. There are some ac tests you can make on the capacitors to check the capacitance value. I normally only make this test on power factor correction units and rearrange the cans to ensure the bank is as balanced as possible. Of course the standard check for termination tighteness and proper ground connection is a must.
Since my last post I reviewed some calculations I made several years ago for a 10 MVA transformer, 34.5KV-2.4 KV, Z=6.31% and X/R=18.28, supplying a lineup of 5 motors ranging in HP from 2250 HP to 800 HP all cable supplied at 100-200 ft. The surge impedance of the motors,obtained from the manufacturer, was:
2250 HP, 900 RPM, Z= 72 ohms
2000 HP, 900 RPM, Z= 105 ohms
800 HP, 1200 RPM Z= 191 ohms

There was metal oxide,station type, 30 KV lightning arrester installed on the 34.5 Kv primary of the transformer. I modeled this system and the maximum peak voltage at the motor terminals for a 40 KA discharge through the arrester was 3.8 KV. When you have multiple motors on a bus, the surge impedance of the group is the parallel value of the surge impedances. The rise time of the voltage waveform modified by the transformer inductance, cable capacitance and motor surge impedance was 15 microseconds.
For the motor BIL level I used (2*rated volts + 1000)*1.414 *1.25. which is nearly 10 KV. Some machines may have BIL values which are 1.4 rather than the 1.25 multiplier use above.
The Beck book mentioned in my previous post was published by McGraw-Hill in 1954. If you have trouble finding it, a lot of the same information was included in a 12 page Westinghouse Application Data 38-300, dated August 1956. Let me know if you need this publication and I will fax it. Also let me know if you want the publication on the Field Testing of Capacitors-it's by GE.
If you have all of the system data, it would be an interesting exercise to check the need for the surge pack module. Here is what you will need.
-Transformer impedance and X/R ratio. You can calculate the X & R values from the impedance and the transformer full load losses.
-Cable size and length and the insulation thickness.
-Diameter of the conductors.
-Dielectric constant of the cables
-Surge impedance of the motors.
You will not need the capacitance of the motor because the combination of the motor inductance and motor capacitance acts like a surge impedance to the type of voltage wave fronts being investigated. Also it is not necessary to be overly concerned with switch surges since they have a longer wavefront than the lightning surges. I usually model switching surges with a 8 microsecond wave front.

Here's another interesting test I make on 480, 2400 and 4160 volt systems at startup and that is measure the system charging current. Actually it is pretty easy and just confirms some of the data I've been collecting on charging currents. That's another topic.Let me know soon on those publications, I will be gone for the Holidays.

 

[joan271273]

I would be very grateful if you could fax that type of information to me. My fax # is (408) 524-7714.

Thanks for the reply as the procedure posted make the determination a tad simpler.

 

[jack6238]

joan271273
I will send the data tomorrow or Fri. at the latest.
Good luck

 

[jack6238]

joan271273
I attempted to send 30 pages this morning to the fax number you mentioned. Attempt at 10:30 my time was no answer on the machine and at 11:30 and 12:15 were busy signals. I'll try again tomorrow.

 

[joan271273]

Sorry Jack, fax was disconnected. You can try later today or tomorrow . I appreciate the Help.

 

[joan271273]

Thanks for the FAX jack6238 really good information, how can I can relate in the example presented by you if I am dealing with switching surges. For example let's assume old vacuum breakers. In upgrades to existing plants you are usually including new motor loads that have to be connected to existing "spare" cubicles that use the old Westinghouse DL VCB or German AEG switchgear.

By the way I located the book via the ABE book exchange.I also found the Old A.C Wright book on Current Transformers.