6671501 selection of 400kv circuit breakers and isolators

8/7/2019 6671501 Selection of 400kV Circuit Breakers and Isolators

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Selection of 400 kV Circuit Breakers and Isolators

MATA PRASAD S.C.AGARWAL R.K.JAIN

Formerly of U.P. State Electricity Board, Gokhale Marg, Lucknow

SYNOPSIS

The equipments used in the substation under general category of switchgear are circuit

breakers and isolators. For an EHV substation, the judicious selection of various parameters will

reflect heavily on the performance and reliability of the plant.

The interrupting and load carrying capability of circuit breakers with special reference of

long time application and desire to limit the switching surges to a value around 2.0 p.u. havebeen considered.

The selection of isolators play an important role influencing the outlook of the switchyard.

The use of pantograph isolators has been made for simplicity, better performance and trouble-free maintenance.

1. Introduction

1.1 Two main functions being performed by switchgear in any power system are:

(a) To permit plant and transmission lines to be conveniently put into and taken out of

service.

(b) Under faulty conditions to enable the same plant and lines to be rapidly and safely

isolated automatically from the unfaulted system.

1.2 The above vigorous duties and important functions performed by the circuit breakers andisolators in a network at 400 kV and above require a high degree of reliability. On such

equipments depend the proper operation of the protected parts of EHV installations.

2. Factors Governing the Selection of Circuit Breakers

2.1 The selection of the type of circuit breakers is governed mainly by the following importantfactors:

()i Use of pre-insertion resistors to control the switching surge over-voltage ;

()ii Requirement of inherent restrike-free operation under all conditions ;

()iii Consistent characteristics ;


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()iv Simple and reliable mechanism ;

()v Operating speed ;

()vi Ease in maintenance ;

()vii Reliability and life of plant in view of future developments.

2.2.1Use of Pre insertion Resistors:

The importance of pre-insertion resistor in reducing over-voltages is of extreme significance and

is well-known. It must, therefore, have the greatest reliability so far as switching-in action isconcerned. Failure of this item will mean outage of breaker besides the possible damage which

might result due to over voltages. The sequence of operation, control of insertion time and

synchronizing the complete action are matters of great precision.

2.2.2Requirement of inherent Re-strike free operation under all conditions:

The principal requirement of a circuit breaker for 400kV system is its inherent ability to interrupt

the charging current of the line.

The world practice is towards such design whose characteristics are well defined. To this

category comes the SF6 breaker whose performance practically for all types of duties out class airblast. The natural choice is SF6 breakers.

2.2.4Simple and reliable mechanism:

The performance of circuit breaker depends quite a lot upon the operating mechanism to close

and open the contacts.

2.2.5Operating Speed:

Not only from the considerations of transient stability but also because of less system

disturbances, less damage to the plant involved in a fault and less wear and tear on the breakercontacts, shorter total break time has been preferred. The high speed breakers have assumedspecial importance both in the weak system having remotely situated power station loosely

connected and in the system stiffly connected with strong sources.

It is felt that reliability and high speed operation must go together. Presently two cycle breakers

do not involve any additional mechanism and are proved very reliable for preference.

To attain a time of two cycles in air blast breakers without affecting the reliability is very easy

whereas high energy and high inertia linkages of minimum oil breakers would impose a servere

limitation on it. Thus only air blast circuit breakers remain in the field. This is one of the reasons

why air blast breakers have been insisted upon.

The other importance aspect is of pole span. The influence of non-synchronous opening and

closing of three poles when higher than five millisecond is quite significant and is consideredimportant from over-voltages due to energisation and reenergisation. With increase in pole span

the over voltages are higher but after a certain limit there is no noticeable increase. This occurs

when all the unfavourable closing moments are with in the pole span. With closing resistors itmust, however, be ensured that pre-insertion time is more than pole span of the breakers.


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2.2.6Maintenance problem:

It is a recognized fact that air blast breaker in particular requires much less maintenance. Areview of CEGB practice would reveal that time schedule of major overhaul or inspection of air

blast breaker is much less than that of oil breakers. The principal components such as breaking

chamber and control mechanism practically require very little attention. The main attention isrequired on compressed air plant. In order to have smooth working, rule three compressor sets

each of 100 percent rating have been specified. It has been the experience in our board and also

abroad that most of the equipment induced major failures has been caused by the mal functioningof very minor components owning to defects which originated during equipment design and

modification stages. The second factor contributing to trouble has been the installation problem.

The lack of good workmanship and skill causes many inherent failures.

2.2.7Reliability and life of plant in view of future development:

The reliable aspect of any equipment having moving components are quite complex. For EHV

and UHV range it appears that SF6 breakers hold the field. The development of breaker nozzle

has reached to such a degree of perfection in respect of nozzle efficiency and aerodynamics thatat 420 kV four breaks/ phase and at 800 kV 8 breaks/ phase are already available. With the

development of double chamber assemblies a few manufacturers have been able to reducenumber of vertical columns affects very appreciably the reliability index in terms of components

failure and flash-over across leakage path. Similarly low energy, low inertia linkages require less

maintenance and are more reliable.

It is also appreciable that important part like breaker chamber if opened too frequently may cause

trouble due to frequent disassembly and reassembly. In terms of chamber maintenance the air

blast chamber stands at No. 2; No. 1 place being occupied by SF6.

3. Factor Governing the Selection of Isolators

3.1 The layout of the 400 kV substation utilised double main and transfer bus system with

strung main, transfer and jack buses. Pipe connectors have been adopted only for

connections between isolators, circuit breakers and other equipments such as instrumenttransformers, lightning arrestors, power transformers, etc.

3.2 The pantograph type vertical break isolators have been selected mainly due to following

considerations.

3.2.1 Better Open Gap Coordination :

The only purpose for purchasing and installing an isolator is to provide a visible open gap in thecircuit. On this open gap the mens lives may depend. To achieve this it is fitting and proper that

the isolator should be selected in such a way that any surge, either a lightning or switching will

be directed to ground rather than flash across the open gap.


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(1) For EHV system it is well provide that

(a) In EHV system using reduced insulation levels it is the switching surge which dictatesthe insulation requirements rather than impulse wave.

(b) Switching surges behave in a more strange manner as compared to familiar impulsesurges.

(c) The range in kV from 100 percent withstand to 100 percent flashover is much largerfor switching surges than for impulse.

(d) 250 75/3500 500 micro second switching surge waves have been found to

produce the lowest break-down voltage on porcelain and air insulation system.

(2) Keeping above in view it is desired that the open gap flashover should not exceed 10

percent at a voltage which will produce a 90 percent probability of flashover (10 percent

probable withstand) on the insulators.

(3) For the voltage of which produces 90 percent probability of flashover on a 1800 BILinsulator column is 1450 kV where as the same 1450 kV voltage will produce a

probability of flashover of less than 10 percent for a 4267 mm (168 in.) single break

isolator whereas for a double break isolator a gap of more than 5080 mm (200 ) isrequired. This indicates that a fairly large gap is needed for a double isolator requiring

inordinate long blades. Hence the overall switch length must be at least several hundred

millimeters longer that the single break switches offering the same open gap insulationlevel. The open gap coordination is not at all a problem in vertical reach pantograph

type of isolator.

3.2.2Cost of Structures:

Due to large open gap requirements in double break isolators over single break isolators the size

of the double break isolators is much larger causing increased cost of structures. Whereas in case

of pantograph type isolators only one insulator has to be supported causing still reduction in cost.

3.2.3Phase to Phase clearance requirements:

Due to larger open gap requirements the phase spacing of the double break isolators at 400kVmust be at least 1525mm (5ft) greater than that of the vertical break isolators for the same phase

to phase switching surge with stand values.

3.2.4Requirement of fourth insulator:

For full blade control the double break isolators require four insulators per pole instead of three.

This increases the isolator weight, installation and maintenance cost by 333

1percent.

3.2.5Reduced cost of insulators of Pantograph type isolators:

In case of horizontal single break isolator the cost of insulator is more due to the fact that long

blade movement requires larger cantilever strength. Whereas in pantograph type vertical break


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isolator the requirement of cantilever strength is negligible causing less cost involvement

towards insulators hence reduction in overall cost of the isolator.

3.2.6Clarity and lower bay size requirements:

For clarity and providing adequate clearances pantograph type bus isolators have been preferred

over the horizontal break and horizontal travel isolators to avoid direct taps from the bus bars toisolators which are unavoidable for the later version. The wind may give rise to large swings of

the tap connection hence larger bay widths may be required for the horizontal break isolators.

Hence the pantograph type bus isolators are must suited.

4. Conclusion

4.1 In view of the pre insertion resistors, inherent restrike free interruption, consistentcharacteristics, simple and reliable mechanism, fast operating speed, ease in maintenance, for

our 400kV system the SF6 circuit breaker is a best suited choice.

4.2 Considering the best open gap coordination, i.e. free from phase to phase sparkover, lowercost of mounting structure, lower phase to phase clearance requirements, i.e., lower bay

width, lower cost of insulators, neat and clear lay out, the best choice could be of pantographtype bus isolator over the other.

6671501 selection of 400kv circuit breakers and isolators

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