Powell Technical Briefs - Combined 1-106

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01.4TB.001

Fast Bus Transfer – Revised

January 22, 2013

Fast bus transfer is normally used for transferring a bus supplying motors to an emergency power source on failure of the normal source of power. It is essential that this transfer be accomplished with a minimum of "dead time" to prevent loss of critical motors or damage to the motors on re-energization.

Two schemes of operation are used for fast transfer. In the first scheme, the trip signal to the opening breaker and the close signal to the closing breaker are given simultaneously. This is called a simultaneous fast bus transfer and the dead time will typically be 1-3 cycles. However, there is a possibility of overlap between the two sources, which may lead to the incoming breaker closing into a fault. This can be prevented by adding a few milliseconds of time delay to the closing signal. In the second scheme, the closing signal of the second breaker is initiated by a "b" contact of the opening breaker. This may be either standard "b" contact or a fast "b" contact. This is called a sequential fast bus transfer and the dead time will typically be 5-7 cycles. Both the schemes require a high speed sync check relay between the alternate source and the motor bus for phase angle measurement. Make sure that the V/Hz value does not exceed 1.33 p.u across the alternate source and the motor bus before closing the alternate source breaker.

We have performed the timing tests on the PowlVac® vacuum circuit breaker to determine fast transfer dead times. The following table lists the dead times for simultaneous and sequential fast bus transfer schemes.

Source of Closing Signa l Dead Time, ms

No Arcing With Arcing Simultaneous Close and Trip

Signals 7.0 - 17.0 (1.0)* - 9.0

Trip Then Close, Using Fast "b"

Contact 53.0 - 63.0 45.0 - 55.0

Trip Then Close, Using Standard

"b" Contact 57.5 - 67.5 49.5 - 59.5 *Possible overlap

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01.4TB.002 01.4TB.002

Closing and Latching Capability of Medium

Closing and Latching Capability of Medium Voltage Power

Voltage Power

Circuit Breakers

May 18, 1990 Superseded by PTB #107 (November 15, 2012)

ANSI Standard C37.06-1987, American National Standard for Switchgear - AC High Voltage Circuit Breakers Rated on a Symmetrical Current Basis - Preferred Ratings and Related Required Capabilities, includes a column in Table 1 headed Closing and Latching Capability. In older editions of this standard, the current value in this column was given in rms kiloamperes, and was determined by multiplying the maximum symmetrical interrupting capability by 1.6. In the 1987 edition, this current is expressed in crest kiloamperes, and the value is determined by multiplying the maximum symmetrical interrupting capability by 2.7.

Other standards had previously required the closing and latching current to have a crest value of 2.7 times the maximum symmetrical interrupting current, so the performance required of the circuit breaker has not really changed. Only the method of stating the requirement has changed. This change was made to bring the ANSI standard in line with the IEC standard, which also expresses closing and latching capability in crest amperes.

Since many specification writers will be using older standards, or copying older specifications, we will probably see both methods of specifying closing and latching current used in specifications for many years. The following table gives both sets of values.

Rated Maximum Voltage kV, rms Rated Short Circuit Current kA, rms Nominal MVA

Closing and Latching Capa bili ty per ANSI C37.06

1979 Editi on k A, rms 1987 Edit ion kA, Crest

4.76 29 250 58 97 4.76 41 350 78 132 8.25 33 500 66 111 15.0 18 500 37 62 15.0 28 750 58 97 15.0 37 1000 77 130

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01.4TB.002 01.4TB.002

Closing and Latching Capability of Medium

Closing and Latching Capability of Medium Voltage Power

Voltage Power

Circuit Breakers

page 2 page 2

If the specified value of closing and latching current matches a value from either edition of the standard, we can assume that a standard breaker is desired. If there is any possibility of confusion, the specifier should be contacted to determine which basis is being used to specify the close and latch rating.

Baldwin Bridger, P.E. Technical Director

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01.4TB.003 01.4TB.003

Capacitance Current Switching Capability of PowlVac

® ®

Circuit Breakers

June 7, 1990 Superseded by PTB #108 (November 15, 2012)

We have recently had capacitance current switching tests performed on our "Dash 3" PowlVac®_circuit

breakers, using GE interrupters. The results of these tests showed that these breakers are qualified as definite purpose circuit breakers, in accordance with ANSI Standard C37.06-1987, Table 1A, for both isolated and back-to-back switching of capacitors.

Table 1 lists the maximum rating of capacitor bank that can be switched by each rating of circuit breaker when applied in accordance with ANSI/IEEE Standard C37.012-1979. The values in the table were calculated using a total current multiplier of 1.25 for ungrounded capacitor banks and 1.35 for grounded banks. These multipliers include allowances for higher than normal voltage, capacitor tolerance, and harmonic components in the current. See ANSI/IEEE C37.012-4.7.1. When PowlVac®_{circuit breakers}

are used in a back-to-back switching situation, inrush currents and frequencies must be limited to the values given in Table 1A of ANSI C37.06-1987. This may require the addition of reactance between the two capacitor banks.

Table 1: Ca pacitor Bank Switc hing Capability of " Dash 3" PowlVac ®_{Circuit Breake rs}

Circuit Breaker Type and Rating

System Voltage kV

Maximum Nameplate Rating of Capacitor Bank, MVAR

Ungrounded Bank Grounded Bank 1200A

Breaker Breaker 2000A Breaker 1200A Breaker 2000A 05PV0250 4.76kV 250MVA 2.4 2.09 3.33 1.94 3.08 4.16 3.63 5.76 3.36 5.34 4.76 4.15 6.60 3.85 6.11 15PV0500 15.0kV 500MVA 11.5 10.04 15.93 9.30 14.75 12.47 10.88 17.28 10.08 16.00 13.2 11.52 18.29 10.67 16.94 13.8 12.05 19.12 11.15 17.71 14.4 12.57 19.95 11.64 18.48

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01.4TB.003 01.4TB.003

Capacitance Current Switching Capability of PowlVac

® ®

Circuit Breakers

page 2 page 2

Note: This table does not apply to PowlVac®_{circuit breakers using Mitsubishi interrupters. We have not}

tested those breakers for capacitance current switching capability, but we do have some data from Mitsubishi that allows us to apply them. Such applications should be referred to me for checking.

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01.4TB.004 01.4TB.004

Umbilical Cord Used on PowlVac

® ®

Circuit Breakers

July 28, 1990

Occasionally, customers or prospective customers question our use of a manually-operated control disconnect ("umbilical cord") on our PowlVac®

circuit breakers. Some of the questions asked, and our answers to them, are:

Q. Why doe s Powe ll use a n umbili cal cord for its cont rol dis connect?

A. The use of the umbilical cord is part of our user-friendly design, which locates all circuit breaker control accessories in the front of the cell. In addition to the control disconnect, these devices include the mechanism-operated cell switch (MOC) and the truck-operated cell switch (TOC). In our PowlVac®

design, these devices are located where they may be observed by an operator inserting or removing the circuit breaker, allowing the operator to check alignment and operation when the circuit breaker is installed. These devices are also available for servicing without removing the circuit breaker from the cell. Q. Is this d esign safe?

A. Yes. The umbilical cord's plug mechanism is mechanically interlocked with the circuit breaker to insure safe operation. Interlocks provided include:

• The circuit breaker cannot be inserted into the cell without plugging in the umbilical cord. • Once the circuit breaker racking mechanism has been operated to start the circuit breaker

insertion process, the plug cannot be removed. It is therefore not possible to disconnect the control circuits of a circuit breaker that is in service.

• Unplugging the umbilical cord trips the circuit breaker if it is closed and discharges the closing

spring if it is charged. Since the plug must be removed in order to remove the circuit breaker from its cell, these interlocks insure that the circuit breaker is open and all energy storage springs are discharged when the circuit breaker is taken out of the cell.

Q. Why does P owell differ fr om all other manufacturers in the method of di sconnecting t he control connections to the circuit breaker?

A. Powell does not differ from "all other manufacturers". While the umbilical cord design has not been used frequently in the United States, other American manufacturers have used it. It is also commonly used in Europe. We chose to use this design because we think it offers superior performance in total.

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01.4TB.004 01.4TB.004

Umbilical Cord Used on PowlVac

® ®

Circuit Breakers

page 2 page 2

Q. Does the umb ilic al cord design meet ANS I standa rds?

A. Yes. This design, including required interlocking, is covered in detail in ANSI/IEEE Standard C37.20.2-6.2.7. The PowlVac®

circuit breaker meets these requirements.

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01.4TB.005 01.4TB.005

Comparison of Porcelain &

Comparison of Porcelain & Cycloaliphatic Epoxy Insulation

Cycloaliphatic Epoxy Insulation

July 29, 1990 PowlVac®

vacuum circuit breakers and metal-clad switchgear use a primary insulation system of cycloaliphatic epoxy. This insulation has given excellent results in the eight years since we first introduced PowlVac®

, but we still have customers who request porcelain.

Powell is far from alone in using cycloaliphatic epoxy insulation. The material has been in common use in Europe for a generation, and other U. S. users include Westinghouse, S&C and Square D. It is especially interesting to see the first two of these companies using cycloaliphatic epoxy. A few years ago, both were strong proponents of porcelain insulation.

Although there are many formulations of cycloaliphatic epoxy and a number of varieties of porcelain, each of which has its own specific qualities and parameters, there are a number of general comparisons which can be made.

First, in the physical area, the following relationships are typical:

• Cycloaliphatic epoxy ("cyclo") weighs less than 70% of porcelain's weight. • The thermal coefficient of expansion of cyclo is 1/20th that of porcelain. • The tensile strength of cyclo is about 11 times that of glazed porcelain. • The compression strength of cyclo is 4 to 6 times that of glazed porcelain. • The flexural strength of cyclo is 16 to 18 times that of glazed porcelain. • The Izod impact strength, unnotched, is about the same as glazed porcelain. • Dimensional and shape control is much easier in cyclos than in porcelain. • While the repairability of cyclos is limited, porcelain is unrepairable.

In the electrical area, you will find:

• The dielectric constant of cyclo is only about two-thirds that of porcelain.

• The temperature class of porcelain is much higher than that of cyclo, but cyclo mixtures with

temperature classes of 105 C or 130 C are readily available.

• The track resistance of cyclo is slightly less than that of porcelain.

• The water absorption of cyclo is slightly greater than that of porcelain, but is still in the range of

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01.4TB.005 01.4TB.005

Comparison of Porcelain &

Comparison of Porcelain & Cycloaliphatic Epoxy Insulation

Cycloaliphatic Epoxy Insulation

page 2 page 2

Finally, cyclo exhibits excellent resistance to common industrial chemicals, is readily washable, and has excellent erosion resistance and weathering properties.

In summary, we believe that the excellent physical properties of cyclo make it the insulating material of choice in spite of some small sacrifice in electrical properties. This is especially true for applications requiring great strength under severe dynamic loading, such as support insulators in circuit breakers and switchgear.

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01.4TB.006 01.4TB.006

Effect of Solar Radiation on Outdoor Metal-Enclosed

Switchgear

July 30, 1990

From time to time we get questions about the rating of outdoor metal-enclosed switchgear which is exposed to solar radiation. It is fairly obvious to anyone who thinks about it that switchgear sitting out in the sun gets hotter than switchgear sitting in the same ambient air temperature inside a building where it has no solar exposure. How should we handle this extra heat?

Metal-enclosed switchgear built to ANSI standards, as is all Powell switchgear, is rated in accordance with the usual service conditions set forth in those standards. All four of the ANSI product standards we commonly use (C37.20.1 for low voltage switchgear, C37.20.2 for metal-clad switchgear, C37.20.3 for interrupter switchgear, and C37.23 for bus duct) include as one of the usual service conditions that the effect of solar radiation is not significant. Thus, all testing and rating of switchgear ignores the effect of solar radiation.

When switchgear is installed in a location where solar radiation is significant, there is another ANSI standard to give guidance in properly applying the switchgear. ANSI/IEEE C37.24-1986, IEEE Guide for Evaluating the Effect of Solar Radiation on Outdoor Metal-Enclosed Switchgear , gives the information necessary to allow calculating the derating of the continuous current capability of switchgear exposed to the sun. This standard is site-specific; the derating depends on the location of the switchgear installation. As a switchgear manufacturer, we assume that our customers specify switchgear ratings in accordance

with the usual service conditions given in the product standards. We further assume that the specifier will do the necessary evaluation and either limit his loads or upgrade his ratings to take care of any solar radiation derating that is needed. If requested, we will be glad to discuss this derating with our customers, and to assist them with the calculations if necessary, but we should not be expected to automatically quote a 2000A circuit breaker where a 1200A circuit breaker is specified, just because the installation is outdoors in Yuma, Arizona.

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01.4TB.007 01.4TB.007

Seismic Testing of PowlVac

® ®

Switchgear

September 29, 1990

We often see specifications that call for switchgear "to be suitable for use in seismic zone X", where X may be any number from 0 to 4, depending on the location of the final installation of the switchgear. Unfortunately there is no ANSI standard that defines "suitable for use in seismic zone X". Seismic requirements for nuclear generating station equipment, which do exist in standards, are not stated in terms of seismic zones, but are site specific.

ANSI Standard A58.1-1982,Minimum Design Loads for Buildings and Other Structures, gives some guidance for the seismic loading that various items must withstand, using the basic formula:

where is the lateral force to be designed for,

is the seismic zone coefficient, which varies from 0.125 for Zone 0 to 1 for Zone 4,

is the occupancy factor, which varies from 1 for Category I to 1.5 for Category III,

is the horizontal force factor, which is 0.3 for all machinery in a building, and is the weight of the equipment.

From basic mechanics, Force = Mass x Acceleration. In the above formula, Fp is a force. Wp is a weight,

which is the product of a mass and the acceleration of gravity, or g. It follows that the product of Z, I and Cp is a dimensionless coefficient for g. For a worst case situation, where the switchgear is installed in a

critical occupancy in Zone 4, the value of this coefficient is 1 x 1.5 x 0.3, or 0.45. Since seismic testing is performed in terms of acceleration rather than force applied, the test level for a worst case installation should be 0.45 g.

The other aspect of suitability is the performance of the equipment under the specified conditions. Here, we have absolutely no guidance from ANSI standards. Based on past experience and input from various users, Powell has decided that the following are reasonable criteria for suitability:

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01.4TB.007 01.4TB.007

Seismic Testing of PowlVac

® ®

Switchgear

page 2

1) There shall be no structural damage that prevents normal operation of the equipment after the event. 2) No doors or covers shall open during the event.

3) The circuit breakers shall not open or close during the event except on command. 4) The circuit breakers shall not move from the fully connected position during the event.

5) After the event, it shall be possible to open and close the circuit breakers and rack them into and out of the connected position.

6) Primary and control fuses shall remain in their fuse clips. 7) Transformer rollout drawers shall not come open during the event.

8) After the event, primary circuits shall withstand a 27 kV power frequency withstand test (hipot). The value of 27 kV is chosen because it is the power frequency withstand voltage specified for field testing of 15 kV metal-clad switchgear.

About four years ago, Powell had samples of PowlVac®

metal-clad switchgear tested for the ability to withstand Zone 4 seismic forces. These samples were single-unit equipments, to give the narrowest structure possible, and had the heaviest circuit breakers installed in the highest positions in which they are ever used. They were therefore worst-case seismic samples.

Based on the requirements of ANSI A58.1-1982, we chose to use 0.45 g as the zero period acceleration (ZPA) value for these tests. The seismic experts at Southwest Research Institute in San Antonio took this value and developed a required response spectrum (RRS) that peaked at about 1.8 g at 3.5 Hz for vertical acceleration and about 1.9 g at 2.5 Hz for horizontal acceleration, with a minimum value of 0.45 g (the ZPA) at frequencies above 32-33 Hz. Full seismic tests were done by Southwest Research Institute at these values of acceleration.

The eight criteria listed on the previous page were used to judge the performance of the equipment under seismic test. In addition, the circuit breakers were successfully closed and tripped on command during the seismic test. Except for a minor problem with the transformer rollout drawer, the equipment performed as required. The rollout drawer fastening system was reinforced, and the equipment performed successfully on retest.

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01.4TB.007 01.4TB.007

Seismic Testing of PowlVac

® ®

Switchgear

page 3

Based on these tests, standard PowlVac® metal-clad switchgear is suitable for use in seismic zones 0, 1 and 2. With the addition of holding clips at the transformer rollout drawers, PowlVac®

is suitable for use in zones 3 and 4.

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01.4TB.008 01.4TB.008

Preventing Voltage Feedback in Synchronizing Circuits

October 22, 1990

Many synchronizing schemes use two lamps in series, connected from the incoming voltage source to the running voltage source. This "dark lamp" synchronizing indication can be used by an operator to supplement the meter and synchroscope readings to insure synchronism before closing the incoming circuit breaker.

This scheme, however, can allow energizing of a supposedly dead bus if the synchronizing switch is accidentally left in the "ON" position. The two lamps will be in series with the secondary of the bus voltage transformer, and this circuit will be connected across the energized incoming voltage transformer secondary. The portion of this voltage which appears across the bus voltage transformer will be stepped up by the ratio of the bus voltage transformer, and this higher voltage will be applied to the switchgear bus.

To prevent this voltage feedback, a dead bus relay (27B) should be connected in the circuit as shown in the figure below. For simple synchronizing schemes, where one or more generators are manually synchronized to a common bus, this circuit with its one27B relay is satisfactory. For more complex schemes, involving automatic synchronizing, machine-to-machine synchronizing, or synchronizing to a utility source, a more complex circuit may be necessary to insure that no voltage feedback circuits exist. All synchronizing circuits should be reviewed carefully to prevent voltage feedback through the

synchronizing lamps.

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01.4TB.009 01.4TB.009

Fuses for Use in DC C

Fuses for Use in DC Control Circuits

ontrol Circuits

January 9, 1991 See revised PTB #105 (September 14, 2012)

The majority of control circuits in metal-enclosed switchgear, particularly in metal-clad switchgear, are supplied from a dc power source. For nearly half a century Powell and other switchgear manufacturers have used 250-volt cartridge fuses (so-called "Code fuses") to protect these control circuits. Typical fuse types are Bussmann Type NON and Shawmut Type OT. The application of these fuses to this type of circuit has been generally successful and has been generally accepted by our customers.

From time to time, however, someone raises the question of the dc rating of these fuses. Bussmann advises me that the Type NON has been tested successfully for 10 kA interrupting capability at 250 V dc, which is the rating commonly ascribed to these fuses. Based on this test data, we can safely apply these fuses to dc control circuits where the short circuit level of the control circuit is 10 kA or less. The typical control battery used for switchgear can deliver a short circuit current of about 10 times its one-minute discharge rating, so it would be a very unusual dc control circuit that had a short circuit capability in excess of 10 kA.

Another question sometimes raised is whether or not these fuses are UL listed for dc applications. The answer is no. If a fuse with a UL listing for dc use is required, we should use either Fusetron Type FRN-R or Low-Peak Type LPN-RK. These fuses are dual-element time delay types which may be used in the same fuse blocks used for Type NON fuses.

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01.4TB.010 01.4TB.010

Transient Recovery Voltage (TRV) Values for Testing

PowlVac

® ®

Circuit Breakers

January 10, 1991 Superseded by PTB #104 (September 14, 2012)

The interrupting performance of any circuit interrupter is affected by the transient recovery voltage appearing across the first pole to interrupt. Both the absolute value of this voltage and its rate of rise are important in determining the interrupter's ability to meet its interrupting rating. The required values of transient recovery voltage are included in ANSI/IEEE C37.06-1987, along with the other ratings of circuit breakers.

The conventional way of specifying the rate of rise of the transient recovery voltage is to specify the peak value (E2) and the time required to reach that peak (T2). The rate of rise is then determined by dividing E2 by T2. The nominal values are those for a full rated short circuit interruption. For lower currents, both higher peaks and faster times are specified. Table 6 of ANSI/IEEE C37.06-1987 lists the multiplying factors to be applied to E2 and T2 for interrupting currents below the full rating of a circuit breaker. Table 1 of ANSI/IEEE C37.06-1987, which gives the preferred ratings of indoor oilless circuit breakers, such as PowlVac® breakers, calls for E2 to be 1.88 times the breaker's rated maximum voltage for tests at 100% of the circuit breaker's interrupting rating. Unfortunately, values of T2 are not standardized, leaving the manufacturer with no guidance on this subject. In order to assign some reasonable value to T2, Powell decided to use the rate-of-rise values given in Table IIA of IEC Standard 56, interpolating between the listed values to match the ANSI voltage ratings, and multiplying the rate-of-rise values by E2 to obtain T2. The values obtained by this method were used in the testing of PowlVac® circuit breakers, and are given in the table below.

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01.4TB.010 01.4TB.010

Transient Recovery Voltage (TRV) Values for Testing

PowlVac

® ®

Circuit Breakers

page 2 page 2

PowlVac® Transient Recovery Voltage Test Values

Current % of Interrupter Rating

Transient Recovery Voltage

Rated Maximu m Volt age = 15 kV Rated Maximum Voltage = 4.76 kV

7 to 13 33.00 29 1137 10.47 19.8 529 20 to 30 31.86 29 1098 10.11 19.8 510 40 to 60 30.17 49 615 9.58 33.1 289 100 28.20 73.6 383 8.95 49.4 181

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01.4TB.011 01.4TB.011

Consequences of Vacuum Interrupter Failure

March 1, 1991

Users and prospective users of vacuum circuit breakers frequently ask us what happens if a vacuum interrupter fails to interrupt. The short answer to this question is that the interrupter is usually destroyed and must be replaced. However, this short answer needs some additional comment to be really informative.

First, failure of a properly applied vacuum interrupter to interrupt a fault current within its rating is a very rare event. In the 8 years that we have been building PowlVac®_{vacuum circuit breakers, we have} manufactured over 3200 breakers. Assuming an average of two years in service for these breakers, we have a history of nearly 20,000 interrupter-years of service. We have never heard of a failure to interrupt by any of these circuit breakers. We are proud of this history, but, based on industry statistics, we are not surprised by it.

Second, even if an interrupter does fail, the consequences are not the disastrous burn down that some people imagine. During some recent design tests of a prototype of a new version of the PowlVac® breaker, we drove an interrupter far past its rated contact life span and had a failure. Photo 1 shows the failed interrupter. When failure occurred, the internal shield was burned through and the ceramic envelope, exposed directly to the arc, broke apart. The arc continued for several cycles, until the circuit was opened by a backup circuit breaker. Aside from the failed interrupter, the only damage to the circuit breaker was a small area of smoke and burn discoloration on the nearby insulating material. Photo 2 shows this area, which was about 6 inches square. Five minutes with an industrial cleaner and a couple of paper towels removed all but about one square inch of this discoloration. The remaining area seemed to be singed, but there was no detectable erosion of the surface of the insulating material. Had this breaker been in service, it could have been returned to service immediately after replacing the interrupter.

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01.4TB.011 01.4TB.011

Consequences of Vacuum Interrupter Failure

page 2 page 2

Summing up, interrupter failures are rare, and when they do happen, most are not a major disaster.

Photo 1 Failed Vacuum Interrupter

Photo 2 Discolored Insulation at

Failur e Location

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01.4TB.012 01.4TB.012

Continuous Current Carrying Capability of Low Voltage

Circuit Breakers

March 4, 1991

Various types of low voltage circuit breakers have differing continuous duty capabilities. Some are rated to carry 100 percent of their trip rating continuously, while others are rated to carry only 80 percent of their trip rating continuously. It is important that we understand the difference and apply these breakers properly.

The general run of molded case circuit breakers in frame sizes of 400 A and below are rated to carry only 80 percent of their rated trip current on a continuous basis. Particularly when these breakers are mounted close to each other in a panelboard, the extra heat generated by carrying 100 percent of the trip rating will both lead to false tripping and cause long-term degradation of the insulating material of which these breakers are made.

On the other hand, all low voltage power circuit breakers and the general run of insulated case circuit breakers are capable of carrying 100 percent of their trip rating on a continuous basis.

Some confusion can arise when using large molded case circuit breakers, in frame sizes of 600 A and above. These breakers may be rated either 80 percent or 100 percent, depending on the model and the manufacturer. As you would expect, the 100% breaker costs considerably more than the 80% breaker. Some models have both 80% and 100% ratings available. The 100% rated breaker may require a larger enclosure and/or more ventilation than the 80% rated breaker of the same model.

Please observe the following application rules:

1) Apply MCCB's in 400 A frame size and smaller based on continuous loads of not more than 80% of the circuit breaker's trip rating. If trip ratings are selected by our customer, assume that they are based on the 80% load requirement.

2) Apply insulated case breakers and low voltage power circuit breakers based on continuous loads of not more than 100% of the breaker's trip rating. If trip ratings are selected by our customer, assume that they are based on the 100% load requirement., Be sure that the insulated case breakers selected are 100% rated.

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01.4TB.012 01.4TB.012

Continuous Current Carrying Capability of Low Voltage

Circuit Breakers

page 2 page 2

3) Apply large molded case circuit breakers based on either the 80% or the 100% rating, making sure that the breaker selected fits the application, and that adequate space and ventilation is provided for the breaker chosen. If trip ratings are selected by our customer, be sure that you understand which basis was used for selection.

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01.4TB.013 01.4TB.013

Future Use of Space in

Future Use of Space in Powell Equipment

Powell Equipment

March 27, 1991

Powell's switchgear and motor control equipments frequently include space which is not used by active switching devices, but is available for future use. This space varies in the amount of equipment present, and is called by many different names. Some of the terms used include space, future, future space, equipped space, space only, spare, and blank. Unfortunately, there are no industry standards defining these terms and their use varies widely throughout the industry, so there is often confusion between specifier and manufacturer or between engineering and shop personnel about what is desired on a particular job.

In order to minimize the confusion, we have adopted the following terms and descriptions in Powell for internal use:

Spare - A complete, ready-to-operate unit, including the drawout switching device (circuit breaker or motor starter) and all required secondary devices, fully wired. A spare differs from an active unit only in that the spare has no assigned function in the power system.

Fully Equipped Space - A spare without the drawout switching device. Includes all required secondary devices and wiring, a finished unit door, primary buswork and disconnecting devices, and all cell parts required for inserting the drawout switching device.

Equipped Space - Includes a door with cutouts for primary switching devices but not for secondary and control devices, primary disconnecting devices and riser bus connecting them to the main bus, and all cell parts required for inserting the drawout switching device. No primary or secondary devices are included, and wiring is minimal.

Blank Space - A blank door, no primary or secondary devices, buswork, wiring, or cell parts required for inserting the drawout switching device. Steelwork should be done so that the blank space can be equipped in the field with little or no cutting or welding.

Blank - An area that can never be used for a primary switching device. This area is made unusable by thermal limitations of the equipment, inability to bus to the area or to maintain proper isolation of bus or outgoing leads, or some similar problem.

Related to these definitions but somewhat different is Mounting and Wiring for a future device or a device to be field installed by the user. Mounting and wiring may be furnished in any of the above units or in an active unit. Mounting and wiring includes the necessary space, physical supports, and primary and secondary connections to allow easy installation of the future device. This may include temporary primary and/or secondary connections or jumpers to allow use of the circuit pending the addition of the future device.

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01.4TB.013 01.4TB.013

Future Use of Space in

Future Use of Space in Powell Equipment

Powell Equipment

page 2 page 2

Where any of these conditions leave openings in the front door or in isolation barriers required by standards, the opening must be covered by a temporary cover plate.

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01.4TB.014 01.4TB.014

Autotransformer Starting of Motors

April 1, 1991

One of our customers recently experienced failures of two autotransformers used in medium voltage motor starters. The circuit used was the familiar 3-contactor, 2-coil Korndorfer circuit, which has been used for many years and appears in textbooks and handbooks on motor control. The primary circuit is shown below:

An investigation of the failed autotransformers by their manufacturer showed that the failure had been a surface flashover from the line end of the winding either to another tap of the winding or to a ground point. There was no damage to the winding or the core, and the autotransformers could be easily repaired and put back into service.

We consulted with both the autotransformer manufacturer and the manufacturer of the contactors used in the starter, and found that there had been previous experiences of this problem. The flashovers occurred because system transients generated during the starting sequence caused an excessive voltage to appear on the line end of the autotransformer winding. Upon analysis, we found several conditions that contributed to this problem:

• The starter was located at the end of a rather weak supply line.

• During the starting sequence, the user switched in a rather large capacitor bank to minimize the

line voltage drop. This bank was switched off automatically, during the starting sequence, when the voltage recovered to a fixed point.

• The autotransformer was set on the 80% tap.

• We are uncertain of the setting of the timer used to transfer from the starting connection to the

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01.4TB.014 01.4TB.014

Autotransformer Starting of Motors

page 2 page 2

Although the contactors used in this particular installation were vacuum contactors, the manufacturer informs us that similar problems have been encountered with both air and vacuum contactors. The type of contactor used doesn't seem to be a factor in the occurrence of the problem.

Further discussions with our suppliers led to several suggestions to minimize the occurrence of this problem:

• Insulate the transformer connection points, both the taps that are used and the unused taps. This

should be done on all future starters of this type.

• Use a lower voltage tap on the autotransformer, such as 65% or 50%, if the motor will accelerate

successfully on these taps.

• For induction motors, be sure that the timer that transfers to the running connection is set at a

long enough time so that the motor is fully accelerated before changing to the running connection.

• Add an instantaneous current relay to the circuit, set to pick up at about 5 A and drop out just

below that current. This relay will pick up when the motor is started and drop out when it reaches full speed. Connect the coil of this relay in any phase CT. Use the contact of this relay to bypass the timing relay contact, insuring that the motor has fully accelerated before the starter is transferred to the running connection. See the control circuit below. In the future, please include this relay in all starters of this type.

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01.4TB.014 01.4TB.014

Autotransformer Starting of Motors

page 3 page 3

• In extreme cases, it may be necessary to connect intermediate class surge arresters to the line

taps of the two autotransformer coils.

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01.4TB.015 01.4TB.015

Directional Overcurrent and Directional Power Relays

May 24, 1991

From time to time we experience some confusion about the difference between directional overcurrent relays, ANSI device67, and directional power relays, ANSI device32. Although there are some similarities between these two types of relays, they are really very different in both construction and application.

Directional overcurrent relays (67) respond to excessive current flow in a particular direction in the power system. The relay typically consists of two elements. One is a directional element, which determines the direction of current flow with respect to a voltage reference. When this current flow is in the

predetermined trip direction, this directional element enables ("turns on") the other element, which is a standard overcurrent relay, complete with taps and time dial, as found on a normal non-directional overcurrent relay. Because these relays are designed to operate on fault currents, the directional unit is made so that it operates best on a highly lagging current, which is typical of faults in power systems. Directional overcurrent relays are normally used on incoming line circuit breakers on buses which have two or more sources. They are connected to trip an incoming line breaker for fault current flow back into the source, so that a fault on one source is not fed by the other sources. In complex distribution or sub-transmission networks, these relays may be used to improve coordination of the system.

Directional power relays (32) measure real power , so they operate best at a high power factor. Various degrees of sensitivity and speed of operation are available in various models of directional power relays. There are three typical uses of these relays:

• Connected to measure power flow into a generator, the relay will operate to trip the generator

breaker if the generator begins to draw power from the system and act as a motor. This is usually due to loss of prime mover power.

• Connected to measure power flow into a transformer from the secondary side, a very sensitive

directional power relay can measure core loss power input to the transformer, detecting loss of the primary source to the transformer. The transformer can then be disconnected from the system.

• A directional power relay can be used to limit power flow in a circuit. The relay may trip a breaker

or initiate control action to change the system configuration. By using quadrature potential connections or a phase shifting transformer, these relays can be made to measure vars

. A typical use would be to limit the real or reactive power drawn from a utility source to a contractual level.

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01.4TB.015 01.4TB.015

Directional Overcurrent and Directional Power Relays

page 2 page 2

Neither the functions (67 and32) nor the actual relays are interchangeable. Be sure to use the function and the hardware which fit the application.

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01.4TB.016 01.4TB.016

Preventing Condensation in Medium Voltage Motors

June 12, 1991

Condensation or other accumulation of moisture can be very damaging to the windings and mechanical parts of a motor, especially a medium voltage motor. This is not usually a problem for a motor that is running, as the windings generate enough heat to prevent condensation. When the motor is stopped, however, supplementary heat is often required to keep the motor dry.

One way of providing the required heat is to install heaters in the motor. Another way is to energize the motor windings from a low voltage source. The one-line diagram below shows the connections for this method of heating the windings. This method may be preferable to the use of heaters, as it actually heats the windings instead of relying on the transmission of heat from a separate heater.

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01.4TB.016 01.4TB.016

Preventing Condensation in Medium Voltage Motors

page 2 page 2

• The heating contactor must be a full line voltage contactor, as the motor winding side of this

contactor is energized at line voltage when the motor is running.

• The running contactor and the heating contactor must be mechanically and electrically interlocked

so that only one of them can be closed at any time.

• There needs to be a time delay between the opening of the running contactor and the closing of

the heating contactor, to allow the residual voltage on the motor to decay before the motor windings are connected to the low voltage source. Since it is not critical to apply the heating circuit immediately, it is recommended that this time delay be in the order of 2 to 5 minutes.

• Tests show that there is an open circuit time of approximately 75-80 milliseconds when the

running contactor is picked up by a "b" contact of the heating contactor. The user should consider whether this is an adequate time period to prevent unwanted system problems. If not, a time delay of a few seconds can be inserted in the pickup circuit of the running contactor to be sure that the heating contactor has cleared before the motor is energized by the operating voltage.

• The voltage applied to the motor windings must be carefully selected to produce the proper

heating. This value must be selected by the user, based on input from the motor manufacturer.

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01.4TB.017 01.4TB.017

Ground Lead Disconnectors on

Ground Lead Disconnectors on Distribution-Class Surge

Distribution-Class Surge

Arresters

July 18, 1991

Many current models of zinc oxide distribution or riser pole arresters come equipped with ground lead disconnectors. This is a device which is mounted on the ground end of the arrester and which looks about like a small hockey puck. The enclosure is black, blue or green plastic, a couple of inches in diameter and an inch or so tall.

The normal failure mode of these arresters is a short circuit to ground, causing ground fault current to flow. This current will cause the arrester body to fail if it is not stopped quickly. The first function of the ground lead disconnector is to disconnect the ground lead of the surge arrester in case of an internal failure of the arrester, preventing explosive failure of the arrester body. The ground lead disconnector contains a cartridge in series with a gap. The gap is shunted by a resistor. As the current rises, the voltage across the gap increases until the gap flashes over, creating an arc which ignites the cartridge, blowing the ground lead free.

The ground lead disconnector is not a fault current interrupter. The arc drawn by the ground lead as it separates from the body of the arrester may or may not go out on its own. If it does not go out, a circuit breaker, recloser or fuse must operate to extinguish the arc. The ground lead disconnector is expected to create a gap which will not reignite when power is reapplied to the circuit, but the gap which will be created is a function of the length and flexibility of the ground lead.

The second function of the ground lead is to give a visible indication of arrester failure for arresters mounted on overhead distribution lines. If a lineman sees an arrester with its ground lead hanging in midair, he knows that he has a failure which must be replaced.

These explosive ground lead disconnectorsare not suit able for use in metal-enclosed equipme nt.

We do not want the explosion and subsequent uncontrolled arc inside equipment, where the clearances are not nearly as great as on overhead lines, and where secondary damage from the arc is much more likely to occur. The visible indication function of the disconnector is useless if the device is mounted within an enclosed equipment.

All surge arresters used in Powell's equipments should be of the type without ground lead disconnectors. If a user requests that we include a surge arrester with a ground lead disconnector, we should offer an equivalent model without the disconnector.

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01.4TB.018 01.4TB.018

Operating Times of PowlVac

® ®

Circuit Breakers

July 19, 1991

We are frequently asked about the actual operating times of PowlVac®_{circuit breakers. The following}

values may be used in application studies for these circuit breakers. Closing Time

For all current production models of PowlVac®_{circuit breakers, the time from energizing the closing coil}

with rated control voltage until the primary contacts touch is 80 milliseconds or less. Typical values are in the 44 to 45 millisecond range.

Opening Time

Opening times vary with the model of PowlVac®

breaker, as shown in the following table. All times are from energizing of the trip coil with rated control voltage until the primary contacts part.

Breaker Model " Dash 2" " Dash 3" Vacuum Interrupter Mitsubishi General Electric Opening Time, mill iseconds

Design Limits Typical Test Values

25-35 26 or 27

40-50 48 or 49 "S" (asymmetry) Factor 1.2 1.1

All of these breakers are rated 5 cycles interrupting time in accordance with the preferred ratings found in Table 1 of ANSI C37.06-1987, even though they may be faster. The "Dash 2" breaker, in particular, is very nearly a 3 cycle breaker.

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01.4TB.019 01.4TB.019

Use of PowlVac

® ®

Circuit Breakers for

Circuit Breakers for Continuous Currents

Continuous Currents

Above 3000 Amperes

August 26, 1991

In accordance with ANSI/IEEE Standard C37.06, the highest continuous current rating of our standard line of PowlVac®

circuit breakers is 3000 A. For systems that require continuous current ratings above 3000 A, we can offer two possible solutions.

First, we can offer our standard 3000 A circuit breaker with cooling fans. We have a design that has been successfully tested at 3750 A, and the results of that test indicate that the fan-cooled breaker may be applied at 4000 A without overheating. This design requires a unit somewhat wider than the standard 36-inch switchgear unit to include the necessary air ducts. The standard fan control equipment includes a current-actuated control to start the fans at about 2500 A and an alarm circuit which uses air flow switches to detect and alarm loss of cooling air at currents above this level. A completely redundant second set of fans can be furnished if desired. Fan cooling is our preferred method of obtaining higher continuous current ratings.

A second method of providing for high continuous currents is to parallel two circuit breakers. Using this approach, we can provide for continuous currents of about 3500 A by paralleling two 2000 A breakers and about 5000 A by paralleling two 3000 A breakers. When breakers are paralleled, the interrupting rating is neither increased nor decreased. Precise timing in closing or opening the two paralleled breakers is not critical, as whichever breaker closes first can carry the continuous current for the few milliseconds until the second breaker closes, and the last breaker to open has the capability of interrupting the full fault current. Paralleling of breakers does require special circuitry to balance the currents between the two breakers and individual overcurrent protection for each breaker as well as combined overcurrent protection for the entire circuit. Main bus construction must also be very carefully balanced to insure equal impedance in both legs of the circuit. Parallel breakers should only be used for a user who refuses to use fan cooled circuit breakers.

Regardless of which breaker uprating method is used, special attention must be given to the design of any portions of the switchgear bus which are rated over 3000 A. If the main bus exceeds 3000 A, standard PowlVac® bus cannot be used, and the required special bus design limits the switchgear to one-high construction.

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01.4TB.020 01.4TB.020

Application of Dummy Circuit Breakers in Metal-Clad

Switchgear

August 27, 1991

Dummy circuit breakers are used in metal-clad switchgear to provide a method of disconnecting and isolating a circuit or circuits without using a circuit breaker. A common use of a dummy circuit breaker is as a temporary connection in a switchgear cell where a circuit breaker will be installed as part of a planned future expansion. Another use might be to isolate one end of a tie bus or cable from a switchgear bus.

Because a dummy circuit breaker is really a set of three jumper bars mounted on a breaker carriage, it has absolutely no current interrupting rating. If an attempt is made to withdraw the dummy circuit breaker with current flowing, arcing will occur at the primary disconnect fingers. This may result in operator injury, equipment damage, or both. Therefore, dummy circuit breakers normally are interlocked with other switching devices so that the dummy cannot be withdrawn until the other devices are opened, insuring that no current is flowing in the dummy.

A particular application that can be troublesome is isolating a tie cable that has been opened by a circuit breaker at the other end. If the cable is still attached to an energized bus through the dummy breaker, cable charging current will flow through the dummy. It only takes a few hundred feet of 15 kV cable to draw a charging current of as much as half an amp. This highly capacitive current is difficult to interrupt. It is recommended that the interlocking for any circuit involving power cable and a dummy circuit breaker be arranged so that the cable is completely deenergized before the dummy circuit breaker is removed to isolate the cable.

Deenergizing the unloaded bus of a lineup of metal-clad switchgear by withdrawing a dummy circuit breaker is an acceptable application. The limited length and very low capacitance of a switchgear bus structure keeps the charging current low enough to be successfully interrupted by withdrawing a dummy circuit breaker.

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01.4TB.021 01.4TB.021

Switching Capability of Rollout or Tiltout Carriages

December 3, 1991

We are often asked about the switching capability of the rollout or tiltout carriages used in medium voltage switchgear to mount voltage transformers, small control power transformers, and fuses for larger control power transformers. This question usually takes the form "How large a CPT can you handle with fuses mounted in a rollout or tiltout?"

There is no industry standard to measure this switching capability, and no test data is available to certify this performance. The switching capability will vary with the details of the design, and to some extent will depend on the operator, since the speed of opening a rollout or tiltout depends on the individual opening the device.

Within these restraints, however, our experience with 5 kV and 15 kV equipments over the years has led us to adopt the following limits:

• Voltage transformers: A set of three wye connected VT's or two open delta connected VT's can

be switched with a rollout or tiltout without any interlocking of the secondary circuit.

• Control power transformers: A CPT up to 50 kVA single phase or 75 kVA three phase can be

switched with a rollout or tiltout provided the carriage is interlocked so that the CPT must be unloaded before opening the primary device. The CPT may be mounted on the rollout or tiltout, or the rollout or tiltout may contain only the fuses for a stationary mounted CPT. Larger CPT's must be switched with some other mechanism, such as a load break disconnect switch.

• Capacitors: Rollouts or tiltouts must not be used to switch capacitors.

Any other application should be reviewed by Powell's engineering department.

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01.4TB.022 01.4TB.022

Short Circuit Currents – Crest, rms Symmetrical and rms

Asymmetrical

December 4, 1991

The figure below shows a typical short circuit current wave form and defines the various component parts of this wave. At the moment of initiation of a short circuit the ac current wave, which is normally

symmetrical about the zero axis BX is offset by some value, creating a waveform which is symmetrical about another axis, CC'. The degree of asymmetry is a function of several variables, including the parameters of the power system up to the point of the short circuit and the point on the ac wave at which the short circuit was initiated. In a 3-phase circuit, there is usually one phase which is offset significantly more than the other two phases.

It is convenient to analyze this asymmetrical waveform as consisting of a symmetrical ac wave

superimposed on a dc current. CC' represents the dc current, and the value of that current at any instant is represented by the ordinate of CC'. The dc component of the current normally decays rapidly, and reaches an insignificant value within 0.1 s in most power systems. The rate of decay is a function of the system parameters. When the initial value of the dc current is equal to the initial peak value of the ac current, the resulting waveform is said to be fully offset, or to have a 100% dc component. It is possible, in some power systems, to have an offset in excess of 100%, which may result in a waveform that has no current zeros for one or more cycles of the ac power frequency.

The ac component of the short circuit current will also decay, at a rate dependant on the system parameters. In general, the closer the fault is to generators or other large rotating machinery, the faster the decay will be.

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01.4TB.022 01.4TB.022

Short Circuit Currents – Crest, rms Symmetrical and rms

Asymmetrical

page 2 page 2

In the figure, IMC is the crest, or peak, value of the short circuit current. It is the maximum instantaneous

current in the major loop of the first cycle of short circuit current.

The rms symmetrical value of the short circuit current at any instant, such as EE', is the rms value of the ac portion of the current wave. Its value is equal to , and it is shown graphically by the distance from CC' to DD'. The rms asymmetrical value of the short circuit current is the rms value of the combined ac and dc waves, and it is calculated by the formula:

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01.4TB.023 01.4TB.023

Using Design Tests to Qualify

Using Design Tests to Qualify Several Ratings of Equipment

Several Ratings of Equipment

December 5, 1991

The many variations in construction and ratings encountered in the typical switchgear or motor control product line make the planning of design and conformance test programs quite complex at times. Of course, it is possible to run every test on every possible rating of equipment but such an extensive program is very expensive and is seldom required to fully document the performance of a product line. The ANSI standards for switchgear recognize this complexity and provide for the qualification of a piece of equipment for all lower ratings provided test results show it to be qualified for the highest rating for which it is used. Some of the conformance test standards in the ANSI C37.50 series discuss the principles of testing to qualify multiple ratings. These standards also give guidance in the grouping of equipment ratings for testing.

A typical example of qualifying multiple ratings by a single test is the bus structure used in PowlVac® metal-clad switchgear. This bus structure is the same for all voltage and short circuit ratings, varying only for continuous current ratings. To demonstrate the momentary and short-time current ratings of this bus structure, tests are performed on the bus with the lowest continuous current rating, 1200 A, which uses the smallest, weakest bars of any continuous current rating of PowlVac®_{bus. The tests are performed at}

the maximum momentary current, 132 kA crest, and the maximum short-time current, 49 kA rms, required for any rating of PowlVac®_{switchgear. It is fairly obvious that passing these tests qualifies the}

1200 A bus for this rating and for all lower momentary and short-time current ratings. What may not be quite so obvious is that successful tests on the 1200 A bus also qualify higher continuous current ratings, such as 2000 A and 3000 A. These higher bus ratings are covered because they use larger bus bars, which are mechanically stronger and which have greater thermal capacity than the bus bars used in the 1200 A bus.

The grouping of ratings and the selection of which rating to test requires a thorough knowledge not only of the standards but also of the particular product line being tested. The grouping of ratings may differ for different tests. It also may differ for different products, or different manufacturers offerings in the same product line. The example given in the previous paragraph is true for PowlVac®_{switchgear, but may not}

necessarily be true for other manufacturers' similar products.

Although Powell and many other manufacturers have used these principles in performing their design tests for many years, not everyone in the industry understands the concept. To aid in this understanding, all future Powell test reports will document the additional ratings covered by any test.

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01.4TB.024 01.4TB.024

Sizing Bus Bars in Switchgear and Motor Control

February 7, 1992

We occasionally get questions about how we select the size of bus bar for various continuous current ratings in Powell equipments. The answer is that we use temperature rise as the basic criterion. All of the ANSI, IEEE and NEMA standards for switchgear and motor control have requirements for the maximum

operating temperature of various parts of the equipment. For bus bars, the requirement is generally for a temperature rise of no more than 65°C, although this may vary for different classes of equipment. These requirements are designed to prevent overheating the insulation supporting and enclosing the bus bars, since excessive temperature shortens the life of the insulation.

A number of factors affect the temperature rise of bus bars. Some of the major ones are:

• Size and material (copper or aluminum) of the bus bar.

• Whether the bar is insulated. Surprisingly, a bus bar covered with insulation generally runs cooler

than an equivalent bare bus bar, because the usually darker color of the insulating material is a better radiator of heat than the shiny surface of a bare bus bar.

• Size and material (magnetic or non-magnetic) of the enclosure around the bus. • Flow of ventilating air past the bus bars or the bus enclosure.

• Proximity of other conductors and other heat-producing devices.

The complex interaction of these and other factors makes it nearly impossible to calculate temperature rise, and leads to the requirement in all applicable standards for continuous current tests to determine the temperature rise of a bus design.

Specifications will sometimes call for bus sized by current density, a favorite requirement being 1000 A per square inch for copper bus. This may be a good way to choose bus sizes for the mythical "single conductor in free air", but it isn't a satisfactory way to design buswork in practical equipments. Consider the following chart, based on bus sizes used in our PowlVac®_{metal-clad switchgear:}

Switchgea r Bus Rating 1200 A 2000 A 3000 A Number of bus bars per phase 1 1 2 Size of bus bar, inches 1/4 x 4 1/2 x 6 1/2 x 6 Cross section area of bus, square inches 1 3 6 Current density, amps per square inch 1200 667 500 Maximum temperature rise, from test data 60°C 59.7°C 59.5°C

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01.4TB.024 01.4TB.024

Sizing Bus Bars in Switchgear and Motor Control

page 2 page 2

The last line of the chart shows that the temperature rises of the three bus ratings are almost identical in spite of the 2.4:1 ratio of the current densities.

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01.4TB.025 01.4TB.025

Application of Metal-Enclosed Switchgear at High Altitude

February 11, 1992

Both low- and medium-voltage metal-enclosed switchgear and the circuit breakers used in these equipments depend on air for both cooling and insulation. At high altitudes, the less dense air is less efficient both as in insulator and as a heat transfer medium. Because of this, the ANSI standards require derating when these equipments are used at high altitudes. The following tables show the altitude correction factors taken from the ANSI standards.

Low Voltage Switchgear and Breakers Alti tu de (ft)* Voltage Current

6600 (2000 m) (and below) 1.00 1.00 8500 (2600 m) 0.95 0.99 13,000 (3900 m) 0.80 0.96 Medium Voltage Switchgear and Breakers Alti tu de (ft)* Voltage Current

3300 (1000 m) (and below) 1.00 1.00 5000 (1500 m) 0.95 0.99 10,000 (3000 m) 0.80 0.96 * Intermediate values may be obtained by interpolation.

You will notice that there are different altitudes given for low voltage and medium voltage. I have never been able to get a reasonable answer as to why this is true, and I understand that the committee responsible for the standards is reviewing these values with the idea of reconciling them.

In all cases, the current correction factor is applied to the continuous current rating of the switchgear and the circuit breakers. This does not usually present a problem, as we seldom design a system with load currents over 95% of the equipment rating. The current derating does not apply to interrupting current or any of the other high-current ratings of the breakers.

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01.4TB.025 01.4TB.025

Application of Metal-Enclosed Switchgear at High Altitude

page 2 page 2

For low voltage equipments, the voltage correction factor applies to the low frequency withstand (hipot) rating of both the breaker and the equipment. It also applies to the rated maximum voltage of the circuit breaker. When derating the rated maximum voltage, the short circuit rating of the circuit breaker cannot exceed the rating at the voltage before derating. For instance, if a breaker is used on a 480 V system, as most of those in Powell equipment are, with a 0.95 rating factor the short circuit rating at 480 V may be used, since the rated maximum voltage for that system nominal voltage is 508 V, and 0.95 x 508 is 482.6 V, slightly above the 480 V service voltage. However, if this same system required a 0.80 rating factor, the breaker short circuit rating at 600 V must be used, since 0.80 x 508 is only 406 V, less than the service voltage, but 0.80 x 635 is 508 V, comfortably above the service voltage.

For medium voltage equipments, the voltage correction factor applies to the low frequency withstand (hipot) rating and the impulse withstand (BIL) rating of both the breaker and the equipment. It also applies to the rated maximum voltage of the circuit breakerunless a sealed interrupter, such as a vacuum interrupter, is used. The use of surge arresters to protect the equipment should be considered for all such high altitude installations.

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01.4TB.026 01.4TB.026

Voltage Ratings of Surge Arresters

April 13, 1992

Surge arresters (formerly known as lightning arresters) are applied to electrical power distribution systems to protect the equipment and the circuits from damaging overvoltages caused by lightning or other surges. It is important that surge arresters of the correct voltage rating be used. The proper voltage rating depends on the system line-to-line voltage, the method of system grounding, and the type of surge arrester used.

Older designs of surge arresters generally consist of silicon carbide resistor blocks in series with air gaps. These arresters carry no current in the normal state. Each arrester model has a single voltage rating. For solidly (effectively) grounded systems, the next higher arrester rating above the system line-to-neutral voltage is used. For resistance grounded or ungrounded systems, a ground fault on one phase can raise the other two phases to line-to-line voltage above ground, so the next higher rating above the system line-to-line voltage is used. Except for a few special conditions, application seems quite simple. About a decade ago, the metal oxide surge arrester was introduced to the industry. It consists of a

number of blocks of a variable resistance material, usually zinc oxide, with no gaps. It does carry some slight current at all times. It has many advantages as a surge protector, but it is somewhat more complicated to apply correctly. Instead of one voltage rating, it has three: a nominal voltage, a maximum continuous operating voltage, and a one-second temporary overvoltage capability. Although there is a slight variation with the nominal rating, the maximum continuous operating voltage is about 85% of the nominal rating and the one-second temporary overvoltage capability is about 120% of the nominal rating. For times other than one second, the temporary overvoltage capability is established by curves supplied by the surge arrester vendor. Care must be taken to avoid overstressing the arrester.

As an example, let's consider a 13.8 kV system. For a solidly grounded system, the continuous operating_{voltage is 13,800 divided by the square root of 3, or 7970 V. This is above the MCOV of 7,650 V for an} arrester rated 9 kV. Depending on the value and expected duration of system overvoltages, it may be necessary to use a 10 kV arrester with an MCOV of 8.4 kV or a 12 kV arrester with an MCOV of 10.2 kV. For an ungrounded 13.8 kV system, the 12.7 kV MCOV of a 15 kV arrester is not adequate. It is necessary to use an 18 kV arrester with an MCOV of 15.3 kV. Finally, for a resistance-grounded 13.8 kV system, the choice will be between arresters rated 12 kV, 15 kV and 18 kV, depending on the time needed to relay ground faults off the system.

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01.4TB.027 01.4TB.027

Testing of Switchgear and Motor

Testing of Switchgear and Motor Control Equipment

Control Equipment

April 14, 1992

Although each particular product line is governed by its own industry standards, switchgear and motor control equipment of the types built by Powell are generally subject to three major categories of tests. As defined in ANSI/IEEE C37.20.2-1987 for Metal-Clad and Station-Type Cubicle Switchgear, these categories are:

Design Tests: Tests made by the manufacturer to determine the adequacy of the design of a particular type, style or model of equipment or its component parts to meet its assigned ratings and to operate satisfactorily under normal service conditions or under special service conditions if specified, and may be used to demonstrate compliance with the applicable standards of the industry.

Production Tests: Tests made for quality control by the manufacturer on every device or on representative samples, or on parts, or materials required to verify during production that the product meets the design specifications and applicable standards.

Conformance Tests: Conformance tests demonstrate compliance with the applicable standards. The test specimen is normally subjected to all planned production tests prior to the initiation of the conformance test program.

Typical design tests for equipment and circuit breakers will include continuous current (heat runs), momentary and short time current, low-frequency withstand (hipot), impulse withstand (BIL) for medium-voltage equipment, and mechanical tests to demonstrate the effectiveness of interlocks. In addition, circuit breakers are subjected to a series of interrupting tests to demonstrate their ability to interrupt currents of various magnitudes, operational life tests, and several types of timing tests. Many of these tests are somewhat destructive, and therefore they are run on manufacturer's prototypes, not on production equipment which is supplied to customers.

Conformance tests generally include certain of the design tests, chosen to demonstrate compliance with the standards. These tests are frequently used for third-party certification of a design.

Production tests include hipot to demonstrate insulation integrity and mechanical and control circuit tests to demonstrate proper operation. In addition, circuit breakers receive timing tests to show proper closing and opening speed. Records of these tests, which Powell furnishes to customers on request, can be used as baseline data for future maintenance programs.

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01.4TB.027 01.4TB.027

Testing of Switchgear and Motor

Testing of Switchgear and Motor Control Equipment

Control Equipment

page 2 page 2

Each type of test, and each test within a given type, has a particular part to play in the overall process of producing quality equipment properly rated for a user's needs. No single test demonstrates the proper design and operation of switchgear or motor control equipment. It takes a combination of tests to do the job properly.