SWITCHGEAR TESTING

4.1       GENERAL

All high- and low-voltage switchgear is subjected to extensive tests by the manufacturer before delivery to the customer.  While it is not the job responsibility of operators and maintenance staff to carry out these tests, it is obviously an advantage to have some knowledge of them.  They are summarised below.

4.2       MANUFACTURERS’ TESTS

Switchgear testing, which is normally taken as meaning circuit-breaker testing, is a very complex subject, of interest chiefly to switchgear designers and manufacturers.  Tests fall into two categories, as follows:

(a)     Routine Tests

These are tests to which each individual circuit-breaker is subjected; they comprise the following:

·         Check of mechanical operation, including measurement of tripping time.

·         Overvoltage withstand: a test of insulation soundness using a d.c. voltage proportional to the rated voltage of the circuit-breaker, usually of one-minute duration.

·         Heat run: a continuous run at full rated current with contacts closed, to check that, when temperature has stabilised, it does not exceed the specified level.

(b)    Type Tests

These are tests made on a prototype circuit-breaker which is representative of others to demonstrate that the design complies with the stringent requirements of the system in which it is to operate; these tests may include the foil owing:

·         Closing and trip tests at 10%, 30%, 60% and 100% of the rated breaking rms symmetrical fault current at not more than 0.15 power factor.  The breaker must not show ‘distress’, and there should not be undue contact burning.

·         A trip test at 100% of the rated breaking peak asymmetrical fault current in one pole and at more than 0.15 power factor.

·         An impulse test to simulate the effect of a lightning strike on the system.  A steep-fronted, high-voltage pulse rising to its maximum in 1.2 ms and falling to 50% in not more than 50 ms, is applied.  The circuit-breaker must not trip or flashover.

Trip and impulse tests are carried out at full voltage and with full fault current.  They call for the use of a large test generator capable of giving out the full rated breaking MVA without significant drop of voltage.  Such test generators are of very special design and are capable of undergoing repeated short-circuits.  There are only a few in the country, located at special Short Circuit Test Stations.  The largest station in the UK is capable of delivering 6 000MVA.  There are also stations on the Continent of which KEMA in Holland is one of the largest.

All the UK stations are administered by the Association of Short-circuit Testing Authorities (ASTA).  On successfully completing a full type test they issue an ‘ASTA Certificate’ of rating, which is complete with oscillograms taken during the test.

Any purchaser of a circuit-breaker of identical design can obtain a copy of its ASTA Certificate.  He would not normally have his own purchased circuit-breakers undergo repeat short-circuit tests.  Only if he had called for any change in the design which might affect the breaker’s performance and so the validity of the certificate would new tests and a new certificate be necessary.  Such special tests would be extremely expensive.

The theory of a.c. circuit interruption is discussed in Chapter 1.  The oscillograms supplied with the ASTA Certificate serve to confirm that the restriking and other waveforms of the circuit-breaker are in accordance with the design.

4.3       USER CHECKS

Operators and maintenance staff, while it is not their responsibility to consider the tests referred to above except as a matter of interest, are required to apply certain routine checks and tests to circuit-breakers and switchboards at the intervals laid down in the appropriate maintenance schedules.  These tests include:

·         Visual examination of the whole switchboard, inside and out, for cleanliness, mechanical damage, corrosion or signs of overheating or leaking where applicable.  Also checking of busbar and copperwork for bolt tightness and cleanliness.

·         Megger testing each part of the switchboard busbar system with all circuit-breakers open.  Tests between each phase and earth (with the other phases earthed), and between phases.

·         Megger testing each circuit-breaker in turn (while isolated) between each phase and earth (with the other two phases earthed), and between phases.

·         Simulation of overcurrent protection on each circuit-breaker by current injection.  This may be of two types - secondary injection’ or ‘primary injection’ (see para. 4.4).

·         Simulation of earth-fault, undervoltage or other protection on each circuit-breaker by manual operation of the protection devices, and checking the alarm indications and that the circuit-breaker trips.

·         Visual and electrical checks of the local trip-and-close battery (where fitted) and its charger(s).

·         Simulation of trip circuit failure and checking correct operation of Trip Circuit Supervision with each mode of failure.

·         Taking an oil sample from each oil circuit-breaker (where applicable) for laboratory testing at specified intervals.

·      Examination of the circuit-breaker contacts after a stated period or a stated number of normal operations, or after a fault clearance.  This includes oil draining where applicable and renewal of oil and of contacts as necessary.

4.4       CURRENT INJECTION TESTING

Current injection tests are made on switchgear to check that the various protective systems operate properly and at the correct preset current levels.  Current injection is of two distinct types, ‘secondary’ and ‘primary’.

4.4.1    Secondary Injection

Secondary injection consists of introducing a variable controlled current from a separate supply source into the circuit which is normally fed by the secondary of each current transformer.  The CT secondary itself is at the same time disconnected and short-circuited.  Most relays have a test block with links which enable this to be done without disturbing the wiring.  Varying the injected current enables the operating settings of the connected relays to be set up or checked and the continuity of the relay circuit to be verified.  Secondary injection does not test the CT itself.

4.4.2    Primary Injection

Primary current injection achieves a similar object but consists of injecting a variable but heavy current into the primary of the current transformer; this avoids disturbing the secondary circuit in any way.  As the primary usually consists of a bare copper conductor bar, connection must be made direct to the bar on both sides of the CT.  In some makes of circuit-breaker heavy lugs are provided on the bars for this purpose.

The heavy current for primary injection is usually obtained from a step-down portable transformer fed from the 240V single-phase station supply.  The transformer is fairly large, about 10kVA, and can supply up to 1 000A of current.  Although primary injection has the advantage that it tests the complete installation, including the CT itself, it is heavy and cumbersome and is consequently less used in the field than secondary injection.

Some current transformers are provided with a separate test winding which is used for primary injection.  In that case, instead of injecting a very large current into the primary bar, a much smaller current can be injected into the test winding, and the CT itself would still be included in the test.  This method, where it can be used, obviates the need for cumbersome primary injection equipment.

4.5       BUSBAR DUCTER TESTS

Continuity resistance of busbar copperwork, especially across joints, needs to be regularly checked to prevent overheating.  It is measured by a specially-sensitive continuity tester called the ‘Ducter’ ohmmeter.  This is a portable device, similar to a megger in appearance, but with an ohmmeter scaled to read down to 1 mW or up to 10 W in six ranges.

This instrument is designed to measure very low values of resistance while a d.c. test current is flowing.  Although principally used for measuring continuity resistance across busbar joints, it can also be used for measuring earth bonding and switch and circuit-breaker contact resistance.

The tester is entirely self-contained and incorporates its own rechargeable battery.  The ohmmeter is scaled 0 - 100 mW, but a 6-way range selector switch extends this up to 10 W.  The meter is of the cross-coil type (similar to that of a megger), which gives a reading independent of the state of the battery voltage.

The instrument has four terminals: C1 and C2 which apply test current through the joint to be tested, and two potential terminals P1 and P2 connected to test prods which are applied either side of the resistance to be tested.  The voltage detected between P1 and P2 is, by Ohm’s law applied to the known test current, proportional to the resistance to be measured.  This voltage is amplified within the tester and applied to one coil of the ohmmeter, the other being fed from the test current.  The meter reads the continuity resistance in microhms direct.

The instrument incorporates a battery tester.  If it indicates a low charge state, the battery must be recharged before use.  A built-in charger enables this to be done from the local a.c. supply voltage.  A completely discharged battery requires 16 hours to recharge.

In some installations the tests across busbar joints and joints on other copperwork such as busbar droppers, cable joints and on the primaries of any wound-type CTs must be carried out with a test current of at least 20A d.c.  The voltage drops so measured are interpreted on a comparative basis, and for identical types of connection or circuit the measured values must not differ by more than 20% from each other.