Forum Subscribe About Us Magazine Issues Newsletters Advertise Shop

Jun 22, 2011

Tweet COMMENTS 0

Switching Operation Transients Lead to Failures

Case study reveals answers to switching transient problem on plant’s power

factor correction capacitor banks

Larry Ray, Schneider Electric Engineering Services | Electrical Construction andMaintenance

Case study reveals answers to switching transient problem on plant’s power factor correction capacitor

banks

When a metal-casting plant, producing ductile iron

castings for the automotive industry, suffered catastrophic

shutdowns of both of its induction melting furnaces (see

The Heat is On), the consequences were significant. The

furnace failures occurred within three days of each other,

shortly after the plant returned to production after

Christmas shutdown. After investigating the first failure,

plant and electric utility personnel assumed the cause of the

first failure had been found. They resumed the start-up procedure three days later, only to

have an almost identical failure of their backup furnace, causing yet another shutdown.

Plant engineers were concerned that recent changes in the electric utility’s distribution

system might have initiated the failures. However, the utility denied any relationship

between the changes and the failures. At this point, the discussion reached an impasse.

The plant shutdown continued for nearly a week, while repairs were being completed,

jeopardizing the plant’s just-in-time status with its automotive customers.

Upon completion of the repairs, plant personnel had to decide whether to start up the

newly repaired furnaces, thus risking another failure. If production could not start, the

plant faced the expense and delay of shipping patterns to its overseas sister company to

make the necessary castings. The Thursday after the second failure, plant managers

realized that if the facility could not restart by Sunday, the patterns would be shipped

overseas and customers notified of the pending delay. The prospect shook the company all

the way up to its CEO. At this point — with both furnaces shut down and only three days

before the deadline — our team was called in to analyze the situation.

Testing pointed to switching transients associated with the metal casting plant’s 12kV

power factor correction capacitor banks as initiating the failures (click here to see Fig. 1).

These capacitors had been in service for more than a year prior to the failures, but had

been operating in manual mode. Just before the Christmas shutdown, the capacitor bank

control was changed from manual to automatic. This meant that the capacitors were

switching periodically, whereas they had previously remained online all the time. Failures

were linked with transients created by switching operations.

The engineering team recommended the capacitor banks be turned off and locked out.

The plant returned to full production before the Sunday deadline, and the plant engineer

kept the only key to the capacitor lock on the dresser in his home!

Although the immediate crisis had been averted, the full extent of the problem remained

unclear. In addition, the plant would need the capacitors online by summer for voltage

support and power factor correction, so the bank could not remain de-energized

Hot Topics Comments New Topics

Watch Out for that Voltage Drop

Electrical Troubleshooting Quiz - July 21, 2015

Price Versus Cost Versus Value

Tip of the Week: When it Comes to Safety,

Maintenance is Not an Option

Test Your Electrical Symbols IQ

Jo in the Discussion on EC&M Talk

Shocks & Electrocutions at Marinas and Docks

13

la st r eply by kpro in Gen era l Elect rica l T a lk

Discu ssion

Isolated Ground for DC Instruments

la st r eply by jbu t h m a n n in Bon din g &

Grou n din g

HOME > POWER QUALITY > SWITCHING OPERATION TRANSIENTS LEAD TO FAILURES

SHARE

Photo Galleries

Test Your Electrical

Symbols IQ

Are you ready? It’s time

for another challenge on

the symbols front. If you

think you’re an electrical symbols master, then now is

your chance to prove it.

EC&M White Papers

Know Your TCO: A

Look at Medium

NEC Design Ops & Maintenance Contractor Safety Power Quality T raining Basics

Products

REGISTER LOG IN

indefinitely. The team needed to conduct additional analysis that was broad in scope,

including taking a more in-depth look at transients modeling, fuse testing, bus bar finite-

element modeling, harmonics measurements, and simulation.

On-site assessment

Additional on-site testing showed that high-frequency transients were initiated each time

the capacitors switched (Fig. 1). The transients were large enough to cause false turn-on

and possible failure of the induction furnace silicon-controlled rectifiers (SCRs). The

transients did not indicate problems with the vacuum switches; the problems were caused

by the particular configuration of the electrical distribution system. At the end of the on-

site testing, the engineering team formed a theory about the failures:

However, additional engineering analysis, fuse testing, and computer modeling were

required to test each phase of the theory.

Fuse testing at high-power laboratory

Fuses from the metal casting plant were obtained to test their response to high-current

short circuits. The fuses were tested at 39,525 peak amperes at the high-power test lab.

The results clearly eliminated the fast-acting fuses as the cause of the bus failures. Voltage

developed across the fuses was limited to only about 1,500V (click here to see Fig. 2),

much less than the 20,000V required to jump the air gap between bus bars and the

furnace cabinet. This led the engineering team to abandon the initial current chop theory.

Computer modeling of transients

At this point, the engineering team suspected that the plant’s electrical system was

amplifying transients due to its unusual response characteristics. The only way to

investigate this theory, short of turning capacitors on and off with the induction furnaces

energized and at risk of failure, was to model the power system’s response to high-

frequency events. The model included both the plant’s 12kV capacitors and the electric

utility’s four separate 34kV capacitors, assessing the effects of all the possible

combinations of the seven banks.

Modeling showed that capacitor switching transients were severe enough to cause false

turn-on, or even failure, of the furnace SCRs. Further analysis showed that the transients

could be reduced by converting the capacitors to harmonic filters with the addition of line

reactors. In fact, the capacitors had originally been selected to accommodate line reactors

in a harmonic filter arrangement.

Measurements of power system harmonics

The process of converting AC power to DC and back to AC generates harmonic currents.

Power factor correction capacitors do not produce harmonics but can create conditions

that worsen harmonics on the power system. During the on-site assessment, our power

system engineers learned that circuit monitors were located on the capacitor circuits and

were storing harmonic information. This data revealed that voltage distortion at the plant

was slightly higher than the IEEE 519 recommended level of 5%. Although harmonics

had not contributed to the failures, voltage distortion levels were high enough to warrant

further action. Converting the existing banks to harmonic filters solved both the

switching problem and reduced excessive harmonic distortion levels as well.

Computer modeling of bus bar movement

The location of the bus bar failures indicated that current had either jumped the air gap

between the bus bars and cabinet, the bus bar structure had moved during the fault and

touched the cabinet, or the bus bars from different phases had touched during the event.

Fuse testing eliminated the first theory. The bus bars were then modeled to determine if

they moved or touched.

Fault currents flowing through bus bars or wires create large mechanical forces that act

to repel conductors on different phases. The analysis work first required that a finite-

element computer model of the bus bars be created. This model was then analyzed to

JUL 2 1 , 2 01 5

JUL 1 4 , 2 01 5

EC&M Webinars

SPONSORED

ON DEMAND:

Mitigation Methods forArc Flash Hazards -Enhancing Personal

Safety

SPONSORED

ON DEMAND:

Understanding the2015 Edition of NFPA

70E and the Arc FlashHazard

VIEW MORE WEBINARS

EC&M Learning Center

Understanding the 2014 NEC,

Volume 1 (Articles 90 to 480)

This resource has proven itself in the

field and in the exam room. This latest

edition w ill provide you w ith a rock-solid

foundation...

Understanding NEC Requirements

for Grounding vs Bonding

One of the most confusing areas of the

trade continues to be grounding and

bonding, w hich is w hy this book needs

to be in the hands...

Changes to The NEC 2014

Don't let the scale of the code changes intimidate you,

Capacitor switching transients caused false turn-on of induction furnace SCRs and a short

circuit through the converter’s DC bus.

Fast-acting fuses blew to clear the short circuit.

The fuse blowing event “chopped” current in the supplying transformer, creating huge

transient voltages across the bus bars.

Transient voltages were sufficient to jump the air gap between the bus bars and cabinet.

Deficiencies in fuse coordination allowed the resulting short circuit to burn until the bus

bars separated.

VIEW MORE WHITE PAPERS

Voltage VFDs

Learn about the intricacies

involved in calculating TCO

and the elements that

affect it....

More

EC&M TV

BROWSE ALL VIDEOS

New CertificationAnnouncement from

NFPA

...

More

determine the amount of mechanical force generated during various kinds of faults. Once

the mechanical forces were determined, the amount of deflection, or movement, of the

bus bars could be predicted.

Figure 3 (click here to see Fig. 3) shows the results of the modeling. Clearly, mechanical

forces generated by the short circuit current were sufficient to cause bus bars to touch.

Once the bus bars touched, the fault current flowed until bus bars burned in two.

Lessons learned

The final results of the engineering team’s extensive analysis revealed a simple cause of

the failures: Bus bars were inadequately braced to withstand the forces generated during

a short circuit (click here to see Fig. 4). The short circuits resulted from unintended SCR

conduction due to capacitor switching transients, but the short circuits would have

caused little damage if the bus bars had been adequately braced.

Our power system engineers inspected the induction furnace repairs and found that the

furnace manufacturer had installed additional bus bracing after the first failures,

indicating that perhaps the vendor suspected the real problem from the beginning.

In addition to the bus bracing, it was recommended that the bus bars be equipped with

transient voltage surge suppression devices. These devices limit the magnitude of voltage

transients and provide additional protection.

Finally, the power factor correction capacitors are being converted to harmonic filters.

This conversion helps to limit the magnitude of voltage transients, and removes some of

the harmonic currents produced by the furnaces.

The benefits realized from this engineering analysis were significant. First, the cause of

catastrophic failures that had cost $750,000 in repair and restart for the facility were

identified and eliminated. The just-in-time status was preserved, saving millions in

potential lost sales. Not only was the electric utility exonerated, but the plant and utility

gained a better understanding of the intricacies of their shared power system

characteristics.

Ray is the director of Schneider Engineering Services based in Raleigh, N.C. He can be

reached at Larry.ray@schneider-electric.com.

SIDEBAR: The Heat Is On

The furnaces at this plant melt scrap iron, attaining a temperature of 2,600°F. They operate by

inducing currents in the scrap by rapidly varying the magnetic field around the metal. The

highly fluctuating field is produced from a constant frequency AC source through use of large

electronic power converters. These converters change 60-Hz AC voltage and current to DC and

then to 200-Hz AC through silicon-controlled rectifiers (SCRs).

The induction furnaces were served by twin 12kV to 575V transformers. The furnaces

each had an electrical capacity of 8MW. AC power was delivered from transformers to

the SCRs through large copper bus bars, which experienced the catastrophic failures

TwitterFacebookLinkedRSS

this book w ill get you up to speed on the

most essential...

BROWSE ALL TITLES

Connect With Us

Site Features

Author Guidelines

RSS

Sitem ap

Site Archive

Subscribe

View Mobile Site

EC&M Corporate

Privacy Policy

T erm s of Service

About Us

Advertise

Contact Us

Follow Us

Search ecmweb.com

Subscribe to the print magazine

Electrical Wholesaling Electrical Marketing T ransm ission & Distribution World

Copy right © 201 5 Penton

Ecmweb.com

NEC Design Ops & Maintenance Contractor Safety Power Quality T raining Basics Products

Related EC&M Sites

Tweet

shown in the Photo.

The plant operated three 12kV capacitor banks for power factor control. These banks were

turned on or off with vacuum switches. Testing and analysis showed that the vacuum

switches initiated voltage transients each time the capacitors were turned on — and

sometimes when the banks were turned off. Vacuum switches are commonly used on

capacitor banks and other loads that require connection or disconnection at high power

levels.

Please Log In or Register to post comments.

Related Articles

Using Transient Simulation Tools in PQ Investigations

Understanding Transients: A Primer

Tracking Down Transient Voltages

Protecting Communication Networks From Overvoltages

PQ Tip: PWM Drives May Have Bearing on Bearing Failures

SHARE