Testing & Certification of M V Circuit Breaker

Due to the continuous increase of power handling capacity, increase in network voltages, continuous design improvements and operational requirements of switchgear. In this article, short circuit test duty requirements of 12kV circuit breaker are discussed. By Yugal Agrawal

In Electrical Power network, switchgear equipment plays the role of protection to the connected loads and ensure safe operation of entire electrical system.viz., generator, transmission line and downstream distribution network. Due to the continuous increase of power handling capacity, increase in network voltages, continuous design improvements and operational requirements of switchgear are happening all around the globe. Circuit breaker is the prime component of switchgear ensuring the protection to the electrical network.

Role of CPRI

CPRI is a pioneer testing organisation in India with five decades expertise in the short circuit and dielectric testing, short circuit design data reviews, quality control checks and stage inspection of various power system equipment. Presently, CPRI is expanding its testing activities globally with international institutions such as ASTA Intertek UK, KEMA Netherland and also as a member of STL for testing and certification of various LV and MV Switchgears and Power & Distribution Transformers as per International Standards
Testing laboratories play an important role in determining the capability study as well as development of new technologies. CPRI is continuously engaged in testing of various types of switchgear equipment from last five decades and issuing test certificates and test reports as per national & international standards.

To prove the satisfactory performance of vacuum circuit breakers under different operating conditions, various tests have been carried out as specified in the international and national standards. In the past, large number of circuit breakers has been tested in CPRI, covering different types of interrupters and a large range of short circuit currents.

In CPRI, Bhopal, the medium voltage circuit breakers rated upto 12kV are tested with source power derived from specially made short circuit generators imported from overseas. The circuit breaker has to operate several times switching on and off normal load currents and interrupt huge fault current whenever there is a short circuit happening in the network. Further, they operate under special switching conditions like capacitor switching and out of phase breaking.

Type Tests Certification Procedure

When a circuit breaker has satisfied the specific requirements, the STL member will, upon request, issue one of the following types of certificate:

Type Test Certificate of Complete Type Tests

This certificate provides the verification of the rated characteristics of a circuit-breaker assigned by the manufacturer in accordance with clause 4 of IEC 62271–100. The certificate comprises those tests necessary to show compliance with all type test requirements as detailed in clause 6 of IEC 62271–100.

Type Test Certificate of Dielectric Performance

This certificate provides verification of the dielectric performance of a circuit-breaker in accordance with the requirements of clause 4, item b) of IEC 62271–100. The certificate comprises those tests necessary to prove compliance with the applicable lightning and switching impulse voltage test requirements, the power frequency voltage test requirements (wet and dry) and the radio interference voltage tests, as detailed in sub-clause 6.2 and 6.3 of IEC 62271–100 and when tested in accordance with IEC 60060-1.

Type Test Certificate of Temperature Rise Performance

This certificate provides verification of the temperature-rise limits and measurement of the resistance of the main circuits of a circuit-breaker in accordance with sub-clause 4 d) of IEC 62271–100. The certificate comprises those tests necessary to prove compliance with sub-clause 6.5 of IEC 62271-1 and sub-clauses 6.4 and 6.5 of IEC 62271–100.

Type Test Certificate of Short-Circuit Performance

This certificate provides verification of the short-circuit ratings assigned by the manufacturer in accordance with clause 4, items e) to p) inclusive and also s) of IEC 62271–100. The certificate comprises those tests necessary to show compliance as detailed in sub-clause 6.6 of IEC 62271-1 and sub-clauses 6.6, 6.102 to 6.110 and 6.112 of IEC 62271–100.

Significance of Short- Circuit Test Duties

The different short circuit or terminal fault test duties represent different situation in actual operation and the test stresses chosen accordingly. The basic aim of each test duty is as follows:

Test Duty – T10

Test duty T10 consists of the rated operating sequence at 10 per cent of the rated short-circuit breaking current (Tolerance ±20 per cent) with a d.c. component at contact separation not exceeding 20 per cent and power frequency recovery voltage ±5 per cent of rated voltage.

In the system when breaker is connected to the primary circuit of a transformer and a fault occurs on the secondary side, the fault current is limited to a very low magnitude owing to the impedance of the transformer. Under these conditions, the rate of rise of recovery peak voltage are high. In order to stress the breaker for maximum TRV the test current should be kept symmetrical, hence the influence of dc component on the stress is negligible. The rate of rise of TRV is 1.92kV/µSec with 25kVpeak @ 12kV(specified in IEC62271-100).

Test Duty – T30

Test duty T30 consists of the rated operating sequence at 30 per cent of the rated short-circuit breaking current (Tolerance ±20 per cent) with a dc component at contact separation not exceeding 20 per cent and power frequency recovery voltage ±5 per cent of rated voltage.

In the interconnected system invariably, the load is fed by several sources in parallel. In the event of fault occurring with only the local source connected and others disconnected, the stresses appearing represent the situation of this duty. It may then have dc component much higher than the maximum possible at rated short circuit breaking current. The test duty should, therefore, verify the ability of a breaker to cope with high dc component.

Test Duty –T60

Test duty T60 consists of the rated operating sequence at 60 per cent of the rated short-circuit breaking current (Tolerance ±10 per cent) with a dc component at contact separation not exceeding 20 per cent and power frequency recovery voltage ±5 per cent of rated voltage.

This test duty may be regarded to prove a somewhat higher rate of rise of TRV at reduced short circuit current compared to 100 per cent short circuit current. The choice of symmetrical current seems logical as the objective is to verify the higher TRV rate of rise capability at reduced short circuit current. The rate of rise of TRV is 0.81kV/µSec with 22.0kVpeak @ 12kV (specified in IEC 62271-100).

Test Duty – T100s

Test duty T100s consists of the rated operating sequence at 100 per cent of the rated short-circuit breaking current (Tolerance +5 per cent) with the percentage of the dc component at contact separation shall not exceed 20 per cent of the ac component.
An important message in the test is related to the implementation of recovery voltage over the arc.

i. The ability to interrupt rated symmetrical short circuit breaking current at full TRV,
ii. Reclosing capability and
iii. Ability to perform the rated operating cycle. A long tripping delay in the CO operations is maintained to allow the dc component to decay giving an essentially symmetrical current, hence to achieve full TRV (The rate of rise of TRV is 0.34kV/µSec with 20.6kVpeak @12kV as specified in IEC 62271-100).

The characterisation is slightly easy, as current zeros are showing every 60 electrical degree. The imposing time for recovery voltage has been defined by IEC standard.

Mostly testing stations have an inherently higher X/R ratio resulting in higher peak/rms factor. Hence, there is a provision in the standard to split the 100 per cent symmetrical duty into making and breaking capacity tests respectively to keep the making current within the safe limits of the breaker. The making capacity tests (test duty T100s(a)) is duty with rated operating cycle and short circuit current slightly lower than the rated short circuit breaking current in order to achieve the required making current. However, by adding power resistors in the test circuit the splitting of TD-T100s(a) can be avoided. The breaking capacity test (test duty T100s(b)) consists of only break shots at 100 per cent rated short circuit current.

There are three specific operating sequences namely:

a O–3min–CO–3min–CO
b CO–15sec–CO (Auto Reclose Duty)
c O–0.3sec–CO–3min–CO (Rapid Auto Reclose Duty)

The objective of keeping 3-minute interval is to allow the operating personnel to accept the fault and to reclose the circuit manually after taking appropriate decision. The 0.3 second interval presumes the nature of fault to be transient which may die down within this time span of 0.3 second and is termed as the rapid auto reclose duty. The 15 second interval depictsan alternatives operating sequence for auto reclose duty. The choice may depend upon the type of application.

Test Duty – T100a
(100 per cent Asymmetrical current test)

This duty seems to be the most onerous duty for many circuit breakers to prove the capability in following respects:

i. Interrupting capability after maximum arc energy condition
ii. Sufficient operating energy.

Test-duty T100a is only applicable when the minimum opening time top of the circuit-breaker, as stated by the manufacturer, plus the relay time is such that the dc component at the instant of contact separation is greater than 20 per cent.

Single-phase and double-earth fault tests

Circuit-breakers shall be capable of clearing single-phase short-circuit currents which may occur in two different cases:

• In effectively earthed neutral systems in case of single-phase faults or,
• In non-effectively earthed neutral systems in case of double earth faults, i.e. earth faultson two different phases, one of which occurs on one side of the circuit-breaker and theother one on the other side.

Depending on the neutral earthing condition of the system in which the circuit-breaker is intended to be used. This is applicable only to three pole breakers intended for use on an effectively earthed system, with the three poles coupled mechanically and fitted with a common opening release. The tests are intended to show the operations of the circuit breakers, not adversely affected by the unbalanced forces produced. Tests on breakers with operating rods eccentric to the mechanism are conducted on the pole which is farthest from the operating rod, which gives the maximum unbalanced forces on the inter-pole coupling mechanism.

Conclusion

When the circuit breaker is interrupting short circuit currents, a high magnitude and fast rising transient recovery voltage (TRV) appears on the circuit breaker terminals which stresses the insulation medium of circuit breakers. In the testing laboratory, the source has to supply high fault current and fast rising TRVs to evaluate the circuit breaker performance. The guidelines for setting the fault current magnitude and Transient Recovery Voltage parameters are given in IEC standard 62271-100. These parameters represent the most onerous system conditions.

The medium voltage switchgear manufacturer in and around central part of our country and other places utilise CPRI, Bhopal laboratory for circuit breaker certification and development. This facility is a boon for developing not only breakers and other switchgear equipment like fuses, disconnectors, earth switches and lightning arresters etc. The STDS, Station-1 is serving our nation for development of switchgear for past 57 years.