Eleven bays o f m eta l -enclos ed s wi tchgear as s ure a h igh leve l o f power continu i ty fo r cr i ti ca l loads a t a la rge com m ercia lins ta l la tion . The s p l i t bus pr im ary s e lective s wi tchgear incorpora te la tes t s o l id s ta te contro ls and features two-way s ource

trans fer wi th a bus -tie s wi tch , provid ing for autom atic trans fer between two u ti l i ty l ines and a s tandby generator .

Retro perspective 30 years ago: Metal-Clad or Metal-EnclosedMV Switchgear? What to choose? | EEP

electrical-engineering-portal.com /retro-perspective-30-years-ago-metal-clad-or-metal-enclosed-mv-switchgear-what- to-choose

Edvard

There are two types of switchgear commonly applied today for switching and protection of in-plant medium voltage(4.16 kV through 34.5 kV) power distribution systems. One is metal-clad switchgear using draw-out air-magneticor vacuum circuit breakers and relays for both load switching and fault protection: the other is metal-enclosedswitchgear using interrupter switches for load switching and power fuses for fault protection.

An understanding of the application and operating philosophies of the two types of gear is necessary to chooseobjectively the gear that will permit the Optimum design for an in-plant system.

The three basic functions of switchgear in an industrial, commercial, or institutional medium-voltage distributionsystem are to:

Distribute and carry load, including permissible overloads, with a minimum of interruptions for scheduledroutine maintenance or for service of the switchgear.

Identify and clear faults quickly enough to minimize damage, while Interrupting the least possible amount ofplant load.

Provide sufficient segmentation of the medium-voltage system so that the extent of circuit outages can helimited during work on cables and loads.

To ensure that each function has been properly considered in relation to both system design and plant operation, anumber of pertinent questions should be explored – including ease of maintenance, number of power interruptions,reclosing, availability of skilled personnel and cost-benefit analysis. The following discussion provides some basicinformation, gathered from the field, for consideration.

How many outages can be permitted for maintenance?

Figure 1 - Segm enting a p lant' s load in to s m al l b locks s upp l ied by m ul tip le rad ia l ci rcu i ts as s ures a h ighleve l o f power continu i ty and operating flexib i l i ty. In -p lant d is tr ibu tion d iagram m ed at le ft s ube jcts a l l loadsto in ter ruptions for fau l t clear ing or m ain tenance on any one load. Segm ented s chem e at r igh t us es s om e

am ount o f cab le to s erve loads and has approxim ate ly s am e s wi tchgear cos t - but provides for betters ervice re l iab i l i ty.

Metal- clad switchgear contains drawout circuit breakers which are removed for required scheduled maintenance;removal of a breaker interrupts its load. Metal- clad switchgear also contains insulated bus which, when testedperiodically, requires a shotdown of the gear.

Metalenclosed switchgear is available with interrupter switches and fuses that require no scheduled maintenance,and the air- insulated bus does not require periodic dielectric testing. Annual maintenance normally consists of littlemore than a visual inspection through the windows of the gear. This switchgear should be seriously considered ifonly infrequent interruptions can be tolerated by plant operations.

How much load will be interrupted for fault protect ion or for maintenance?

Consider maintenance as well as cost when selecting switchgear with fuses and interrupters (metalenclosed) or withbreakers and relays (metal- clad).It is axiomatic to plan system protection so that fault isolation will result in de-energization of only the faulted segment of the system, thus permitting continuous service to other loads.

Additionally, there are many other reasons why portions of the distribution system will be taken out of service – forexample, to add transformers, test cables or even modify circuits to accommodate plant expansions. For theseoccasions. a sufficient number of load switching points should be provided to allow selective switching to minimizethe number of loads interrupted. Circuit breakers are used in applications requiring a very high (above 720 A)continuous current carrying and load- interrupting capability. While this capability may be an advantage in somecases. a higher degree of service continuity can often be achieved with less expensive power fuses by subdividingthe system into a larger number of discrete segments, with the result that load switching or fault interruption on onesegment of the system will affect fewer loads (see Fig. 1).

A high degree ofsegmentation alsoallows the use ofsmaller transformerslocated strategicallythroughout the system,eliminating the need forunnecessarily long,high-ampacitysecondary conductorsrequired where fewer,larger, widelyseparated transformersare used.

Is automaticreclosingnecessary?

Automatic reclosing isneither useful nordesirable on in-plant

Figure 2 - Dis tr ibu tion s ys tem wi th both overhead andins u la ted cab le cons truction. Pro tective device Aclears trans ient fau l ts on overhead s ys tem and

reclos es autom atica l ly, wi th on ly m om entaryin ter ruption o f power to loads . Fau l ts in cab le por tion

of s ys tem are in ter rupted by device B and intrans form er by device C. Perm anent nature o f cab le

and trans form er fau l ts m akes autom atic reclos ingundes i rab le .

desirable on in-plantpower systems consisting of Insulated cables (in conduit or busduct) feeding transformers. Faults on cables and transformers arerare. and those that do occur are not transient: they arepermanent. They result in significant damage. and they are onlyexacerbated – not cleared – by automatic reclosing operations.Metal-enclosed switchgear has achieved widespread use oncable systems because of the simplicity. economy and positiveaction of power fuses in providing protection from permanentfaults.

On the other hand, automatic reclosing can be an advantage onoutdoor, overhead distribution circuits subject to a high incidenceof transient or temporary faults caused by falling tree branches,animal and bird contacts. wind-borne debris, lightning or ice.Overhead circuits are commonly protected by metal- cladswitchgear (with circuit breakers and associated relaying) in asmuch as a short- time interruption of system voltage by opening ofthe switchgear breaker may result in arc extinction, permitting anautomatic reclosing operation to restore service (see Fig. 2).

In deciding whether to utilize automatic reclosing, considerationmust be given to the effect on synchronous motors and largeinduction motors. High inrush current resulting from automaticreclosing may cause severe mechanical damage to the motors.Or, it may result in minor insulation damage which is not apparentat the time, but which will lead to premature failure.

This insulation damage will be accelerated with repeated fastreclosures, as the effects are cumulative. Even manual reclosingmay be undesirable on a cable system: It is often a temptation tohope that the protective device has operated unnecessarily.Rather than take the time to search for the fault. even a trainedperson may, under pressure from production people, reclose inthe hope that the protective device will “hold.”

Since the condition which caused the protective device to operatewill not have been eliminated. reclosing will only reinitiate the fault.This will cause further equipment damage, as well as provide ahazard to personnel in the vicinity. The practice of reclosingbefore locating and correcting the fault is highly questionable.

Is sophist icated relaying required?

Most in-plant system protection needs can be satisfied by anovercurrent protective device – a fuse or a relay and circuitbreaker. With the variety of fuse ratings and time-current curvesavailable either metal-enclosed switchgear with fuses andinterrupter switches or metal- clad switchgear with circuit- breakersand relays may be used.

More complex protection devices respond to conditions otherthan just overcurrent: e.g., reverse-current or reverse-powerrelays, differential relays and overcurrent relays with harmonicrestraint. These devices are a necessity for utility high-voltage or EHV networks, but their desirability can bequestioned for simpler in-plant systems.

Complex relaying introduces the requirement for a much higher level of sophistication in system design andcoordination, as well as relay testing and calibration. Complexity can also invite defeat by operating personnel whoare in a hurry to restore power, and have no time or inclination to review a complex system designed years before.

Is DC control power available?

Metal-enclosed switchgear with fuses and interrupter switches is normally selfcontained with no requirement for anauxiliary power supply. Fault protection is provided by the fuses which use the energy of the fault current to achieveinterruption. Even complex remote controlled or automatic power operation of switches is usually accomplished withac control power from one or more voltage transformers which may also function as voltage sensing devices. Metal-clad switchgear with circuit- breakers and relays usually needs DC control power, and therefore the addition of astationclass battery. Not only does this take considerable space, it often requires more maintenance than theswitchgear itself.

There are many recorded cases of damage to switchgear and plant which could have been avoided if the batterieshad been maintained, if the battery charger hadn’t been turned off-or if fuses had been used initially.

Is single-phasing a problem?

The possibility of single-phasing a load by operation of a fuse need not be an issue in choosing modernmetalenclosed switchgear or metal- clad switchgear. Detectors and relays are available for sensing single-phasing,which could be caused by source- line burndown. broken conductors, singlephase switching, or by blown fuses onthe utility source or on the in-plant feeder.

When the detectors or relays are applied in conjunction with power-operated interrupter switches in metal-enclosedswitchgear (see Fig. 3). the switches are automatically opened if a single-phasing condition occurs. interrupting andisolating all three phases of the load feeder.

Figure-3 - Meta l enclos ed s wi tchgear feeder bay has provis ionsfor pro tection from open-phas e cond i tions . Open-phas e detector( located in low vo l tage com par tm ent in s wi tchgear bay a t lower

r ight) in con juction wi th the power-operated in ter ruper -s wi tchpro tects from a l l open-phas e cond i tions , includ ing s ing le-

phas ingcaus ed by b lown fus es .

Sensing and power operation will raise the cost ofmetal-enclosed switchgear significantly over that formanual gear. But the cost will still be on the order of 35 to40% of that for equivalent metal- clad switchgear withcircuit breakers and relays. And since the addition of thisfeature allows metal-enclosed gear to detect evensource single-phasing. the level of protection is higherthan that normally afforded by metal- clad gear.

Are skilled technicians available?

Any electrical equipment should be operated andmaintained only by qualified persons “having adequateknowledge of the installation, construction. . .(and)operation of the apparatus and the hazards involved”according to the National Electrical Safety Code ANSIC2. The Code, parts of which have been adopted bylocal and state jurisdictional authorities, specifies that“the employer shall inform each employee working on orabout communications equipment or electric- supplyequipment and the associated lines, of the safety rulesgoverning the employee’s conduct while so engaged.”

In addition, such persons shall be “regularly instructed inmethods of first aid and emergency procedures” andhave “an adequate supply of protective devices andequipment.” Users who cannot justify the expense oftraining and equipping employees to specialize in workon electrical equipment should have maintenanceperformed by an electrical contractor skilled in mediumvoltage. Calibration of relays and dielectric testing ofinsulated bus should be performed by a qualified testingorganization.

The choice between metal-enclosed switchgear andmetal- clad switchgear is often made on the basis of theavailability of qualified persons and the willingness ofmanagement to provide funds for maintenance. Metal-enclosed switchgear is available with non-damageable,non-aging, permanently accurate fuses which require nomaintenance and with switches which require noscheduled maintenance or adjustments. A simple visualinspection and occasional exercising are all that is required.

Conversely, the maintenance requirements for circuit breakers, relays and batteries are well established.

Will cable size be based on ampacity?

If cable size is selected solely on the basis of ampacity. the source protective device should be selected to operatefast enough to interrupt maximum available fault current before the insulation suffers thermal damage. In other words,the selection of cable sizes should be based not only on ampacity. but also on the ability to withstand fault currentwhile the source protective device detects and clears a fault.

Fuses clear heavy fault currents In less than .014 sec. It is desirable to protect a cable from damage due to passageof fault current. The choice may well become one of whether to use fuses for protection. or to use metal- cladswitchgear with circuit breakers and relays and specify cables several sizes larger than required by ampacity.

What are the economics?

In light of today’s high cost of money, it is essential to keep capital outlays and operating expenditures to a practicalminimum. Consequently, the economics of switchgear application have become increasingly important in plantdesign. Metal-enclosed switchgear provides protection for an in-plant cable system at a cost of 25 to 40% of metal-clad switchgear.

The high cost of building floor- space may make it desirable to locate the gear out of doors. Both metal- clad andmetal-enclosed gear may be installed outdoors. Metal- clad gear normally requires an additional housing or walk- inshelter so that routine maintenance may be performed during inclement weather. This extra protection is not requiredfor outdoorstyle metal-enclosed gear.

Metal-enclosed switchgear weighs less than metalclad switchgear. thus, it is easier to handle with a minimum ofrigging. Foundation or support channels are not required, permitting it to be located anywhere, even on balconies orrooftops. Only a level floor or pad is required. and room need not be provided to accommodate drawout of circuitbreakers. The time required for the design of an in-plant medium-voltage distribution system is short compared tothe many years it will be in service. Over its life, the switchgear will be called upon to facilitate routine scheduledwork on the power system as well as to limit damage and lost production due to faults. The foregoing fundamentalapplication questions were modeled to assure that the choice of switchgear will take into account how the plant Isoperated and to help develop a switching and protection philosophy.

The user may wish to consider other aspects. As with any engineering decision, it is important that the choice of typeof switchgear be made only after consideration of all relevant factors.

RESOURCE: Electrical Energy Management; June/July 81