TESTING OF CIRCUIT BREAKERS

A.K.TripathyFormer DG CPRI, Advisor Research

Silicon Institute of Technology

Testing is done to evaluate the Performance of the equipment

• TESTING REQUIREMENTS OF CIRCUIT BREAKERS• • The performance of a High Voltage Circuit Breaker is evaluated in terms of

its capability to –• Carry rated current without temperature going beyond limits • Carry the short circuit currents• Interruption of the circuits involving Heavy fault currents due to short circuits• Short line faults or kilometric faults• Line currents under out of phase conditions.• Closing on faulted circuits• Switching of capacitive currents: and • Switching of magnetizing and small inductive circuit currents

Testing is madatory

• What Tests..?

• Routine tests• Type tests• Special tests• Acceptance tests

Routine Tests

• • 3.1.1 Measurement of no load timing:• • No load opening time, closing time, contact travel

characteristics like velocity are measured for the circuit breaker. They are very essential parameters in determining the condition of the contacts, operating mechanism of the circuit breaker

Contact travel

• Significance of contact travel and velocity measurement:

• This could easily be considered as the most important function being monitored. It provides dynamic information about the operating components of the circuit breaker as a whole including not only mechanical links but also the interrupter contacts. The information that can be extracted from these measurements is always extremely valuable for judging the overall status of the circuit breaker and this is one of the simplest activities to monitor.

Signature comparison

• From the measured travel characteristics by comparing the new data to a base line signature for the specific circuit breaker it should be possible to infer not only deterioration of linkages, but increased friction that could mean lack of proper lubrication and or deterioration of bearings

Type Tests• TYPE TESTS• The various type tests as per the latest standard 62271-100(2003) are listed as follows• Dielectric tests• RIV tests• Measurement of resistance of the main circuit• Temperature rise tests• Short time withstand and peak withstand current test• Electromagnetic compatibility tests• Mechanical operation tests• Short circuit test duties for terminal faults• Short line fault test• Out of phase making and breaking tests• Capacitive current switching tests• Small inductive current switching is covered under IEC: 61233(1994)

Strength to withstand over voltage• 1 DIELECTRIC TESTS:• Dielectric tests mainly comprise of lightning impulse test and one minute

power frequency high voltage withstand test. Both these tests are carried out as per the guidelines given in IEC-62271-100 and IEC: 60694.These tests are conducted to verify the overall dielectric integrity of breaker. Switching impulse tests, artificial pollution tests and partial discharge tests are however optional tests.

• Power frequency tests : Conducted on a clean new circuit breaker, the test voltage varies with circuit breaker rated voltage. The test voltage with a frequency between 15-100 Hz is applied as follows: (i) between poles with circuit breaker closed, (ii) between poles and earth with circuit breaker open, and (iii) across terminals with circuit breaker open.

• The voltage is gradually increased and maintained at test value for 1 minute.• For outdoor circuit dry and wet tests are conducted.

Against impulse

• 2 IMPULSE TESTS• In this test impulse voltage of standard wave

shape of 1.2/50 microseconds and magnitude is applied to the breaker.

Temperature Rise

• TEMPERATURE RISE TEST• Verification of the temperature rise limits is by a type test

that is mandatory for each design of circuit breaker. The tests should be made on an arrangement of the breaker, representative of its type, carrying a current not less than it s rated normal current, and without exceeding the limits of temperature rise specified.

• This test is carried out to verify the rated current carrying capability of the breaker continuously without giving rise to abnormal temperature rise, by simulating test conditions close to service conditions

Contact Resistance• MEASUREMENT OF RESISTANCE OF THE MAIN CIRCUIT

• • Important in deciding the contact condition of the breaker. The

resistance across the closed contacts before and after temperature rise test and short circuit tests shall be recorded as reference value for routine production and testing.

• Circuit breaker contacts must first be able to carry their assigned continuous current rating, without overheating, or deteriorating and must do so within reasonable limits or power consumption. In addition, during short circuit conditions, they must be able to carry large currents for some specified periods of time, and again they must do so without deteriorating or arcing.

Mechanical Operation

• MECHANICAL OPERATION TEST• The performance of the circuit breaker is

undoubtedly entirely dependent on the correct behaviour of the mechanical components. In order to have electrical operations in life it is important to have long mechanical no-load life. As per the new standard the breakers are classified as class M1 suitable for performing 2000 no-load mechanical operations and class M2 suitable for performing 10000 no-load mechanical operations

Gas Density Supervision• SF6 tightness and gas density supervision of high voltage circuit • breakers: • • . The gas density is checked on the circuit breakers with a density monitor.

Different events can lead to density variation detected by density monitor • • SF6 leakage itself • SF6 adsorption by filtering products at low temperature,• Rapid and wide variations of temperature • • Only the first event corresponds to an actual failure. The second and third

correspond to physical modifications inside the breaker but they do not have to lead to a signal from density monitor.

Leakage of Gas

• The possible origins and causes are various for the SF6 leakage, for example

• • - Corrosion near a seal • - Damage of a seal, • - Impurity under a seal, • - Porosity of metal component (casting, brazing), • - Break or crack of the enclosure (porcelain), •

Is leakage a failure?

• It must be noticed that the false SF6 leakage can exist; it is due to physical event like important temperature variations, due to failure of density monitor, also due to incorrect refilling after maintenance. Moreover every equipment has an accepted failure rate which allows a minimum interval to be determined during which replenishment will not be required (typically 10-20 years). If the leakage exceeds this accepted rate, resulting in a need for more frequent replenishment, then this would be considered as a failure.

•

Failure modes• The main cause of the major failures and minor failures is the leakage of

SF6, oil or air.• In Hydraulic operating mechanism oil leakage is the major problem and in

pneumatic Operating mechanism air leakage is the major problem.• The major causes for the SF6 gas leakages are defective leap seal, poor

gaskets, defective gas density monitor etc. Most of these failures occur due to manufacturing problems as it can be verified from the experiences of the utilities.

• The oil leakages in hydraulic operating mechanism are from pipings, rusting of pipes, sludge formation, problems due to air bubbles in oil, tripping of hydraulic pumps etc. The air leakages are due to the defects in air pressure valves etc.

• Most of these defects are due to manufacturing problems. Poor maintenance schedule is also responsible for most of these failures

Current Tests

• SHORT TIME WITHSTAND AND PEAK WITHSTAND CURRENT TEST.

• This test introduces mechanical forces in the breaker particularly during the initial peak current. The contacts tend to open if the contact pressure is not adequate. The duration of test is generally one second and in some special cases it is three seconds

•

Short ckt. tests• SHORT CIRCUIT TEST DUTIES FOR TERMINAL FAULTS.]•

Methods of Testing : • Broadly two methods of testing are prescribed: • Direct Testing - Using one source for current and recovery

voltage. • Synthetic Testing - Using separate sources for current and

recovery voltage.

Three Phase Testing

Complete 3 Phase Testing: • It is obvious that 3-phase test is the best as the

circuit breaker is properly stressed and not over stressed as is often the case in single phase testing. However due to limitations of the testing stations, IEC allows single pole testing as well as unit testing provided certain conditions are met.

•

Single pole Testing

• Single Pole Testing:• • According to this method, a single pole of a 3 pole CB is

tested by single phase method applying to the pole the same current and the same power frequency voltage which would be impressed upon the most highly stressed pole during 3 phase making and breaking by the complete 3 pole CB under corresponding conditions. Here the power requirement is less, circuitry is less complicated, the time required for test set up and reconditioning are less

Unit Testing

Unit Testing : • When a testing laboratory is not equipped to test

even on complete single pole of a circuit breaker, tests are carried out on one or more Units of a pole. This is known as Unit Testing. Hence the power requirement is further reduced. However it is essential that the units are identical with proper voltage distribution between the units.

Short Circuit Test Procedure– Basic Short Circuit Test Procedure:

• • Requirement of Standards for Transient Recovery Voltage

(TRV) and Power Frequency Recovery Voltage: The system or circuit TRV is modified by the circuit breaker due to the arc characteristics of the interrupting media and by the use of opening resistors, grading capacitors, surge capacitors etc,. The unmodified TRV referred to as inherent TRV is taken as a standard for testing of the circuit breaker. The inherent TRV is a1so known as Prospective IRV. In laboratories the TRV is set using lumped R, L, C elements.

TRV 2 and 4 parameter

• •  

• The inherent or prospective TRV envelope of the test circuit

should lie above the specified TRV envelope and should not

cross the envelope at any point (as shown in Figure – 3.2.7.5).

During a short circuit test it is desirable to record the actual test

TRV. •  

•  • Figure - 3.2.7.5 : Prospective TRV envelope of the test

circuit and the specified TRV envelope,

Power Frequency test voltage

• The Power Frequency Recovery Voltage shall

not be less than 95% of the specified value and shall be maintained for at least 0.1 second.

Short circuit Test duties– BASIC SHORT CIRCUIT TEST DUTIES

• TEST DUTY- T10• This duty demonstrate the capability of the breaker when it is

connected on the primary circuit of a transformer and fault occurs on secondary side. The fault current is limited by high impedance of the transformer and hence the current for this test is 10% of the rated short circuit current. Owing to high inductance of the transformer the rate of rise of recovery voltage is very high, which is generally depends on the natural frequency of the transformer. The dielectric failures during recovery period are very common while carrying out this duty.

T30 Test duty• TEST DUTY-T30• In an interconnected system only one source out of bunch feeding

the fault represents this duty. The current in this duty is 30% of rated short circuit current and TRV conditions are same as that of T-10 duty.

• • TEST DUTY –T60• This duty is specified to prove the breaker capability to withstand

the moderate thermal as well as dielectric stresses. Hence the short circuit current is increased to 60% of its rated value and TRV stresses are also reduced compared to the T-10 and T-30 duties.

•

T100 test duty• TEST DUTY –T100s • This duty demonstrates the capability of breaker to re-close on

fault following interruption of fault current and ability to complete the rated operating sequence assigned. The sequence may be O-3min-CO-3min-CO

• or• O-0.3sec.-CO-3min-CO• Or • CO-15sec.-CO• The interruption current will be equal to its rated short circuit

breaking current. The TRV is full TRV specified in the standard.•

T100a duty• TEST DUTY –T100a• This duty verifies the interrupting capability after maximum arcing

conditions and verifies sufficient operating energy. Maximum arcing occurs due to the fact that the breaker clears the current with offset due to the dc component. In the event of fault the contact separation may take place at a moment close to zero in the minor loop so that final extinction takes place only after arcing during full subsequent major loop. The arc energy seen by the breaker will be larger than compared to the case where extinction takes place only after arcing during minor loop. The duration of major loop for different time constant of the system is specified in the standard.

•

DOUBLE EARTH FAULT AND SINGLE PHASE EARTH FAULT:

• Double earth fault and single phase test are applicable for 3-phase breakers intended for use in isolated neutral and solidly earthed neutral system respectively, with three poles coupled mechanically and fitted with common operating release. These tests are conducted on any one of the three poles, farthest from the symmetrical mechanism to verify the operation of the breaker not adversely affected by the unbalanced forces produced.

SHORT LINE FAULT TEST:

• . By experience and computations it has been found that when a fault occurs on a transmission line a short distance from the C.B. terminals, an onerous fault condition results with a TRV of extremely high RRRV. This is due to the travelling wave reflected between the C.B. and the fault location. The peak value and the frequency of oscillation of the saw tooth transient are directly and inversely proportional respectively to the distance of the fault from the C.B. terminal. There is a critical length of the line at which the breaker is most severely stressed.

Test as per IEC

• IEC recommends these tests to 3 phase CB's designed for direct connection to overhead transmission lines and having rated voltage of 52 kV and above and a rated short circuit breaking current exceeding 12.5kA. For rating and testing purposes, it has been agreed internationally to consider only a short line fault occuring from phase to earth in an earthed neutral system, the severity of this being sufficient to cover other cases.

•

Short line fault:Contd

• Two test duties L90 and L75 each consisting of the rated operating sequence confined to opening operations only at 90% and 75% of the rated S.C. breaking current and appropriate respective TRV, % dc component at contact separation shall not exceed 20%.

•

• This test is applicable for breakers above 52 kV class and duty seems to be more onerous .This duty represents the situation where the fault occurs close to the line side terminal of the breaker ( a range of about 1 to 8km). The line side recovery voltage after current interruption, which is a saw tooth wave resulting from series of traveling waves reflected back and forth along the line between the circuit breaker and the fault point, causes a sharp increase in rate of rise of recovery voltage in the initial stages represented as ITRV .

OUT OF PHASE SWITCHING DUTY

• :• Out of phase conditions occur in the

interconnected systems due to overloading, short circuits or synchronizing faults. The coupling breaker in this type of system has to clear fault currents between two sections. To verify this out of phase switching duty is performed at higher recovery voltage, the magnitude of which depends on method of earthing of the system

CAPACITIVE CURRENT SWITCHING TESTS:

•

• .• Switching ON of capacitor bank results in inrush current, the

natural frequency of which depends up on the inductance limiting this current and capacitance of the capacitor bank. Switching OFF of capacitor current may results in restrikes after opening due to current chopping phenomena. As per the existing standard 62271-100(2003) the HT breakers are classified based on their numerical probability of restriking during their entire specified duty. Class C1 breakers exhibit low probability of restrikes and Class C2 exhibit very low probability of restrikes.

LOW INDUCTIVE CURRENT SWITCHING TEST

• .• The interruption of low inductive currents involves switching ON and

OFF of transformer no-load currents, induction motor switching and shunt reactor switching.

• Switching ON of induction motor at standstill and shunt reactors will produce traveling waves which will be reflected back from the load (induction motor or shunt reactor) due to high surge impedance and appear as high voltage spikes, the magnitude of which depends upon the ratio of surge impedance of the load to that of the cable or line.

• Over voltages are inevitable due to current chopping while switching OFF these low inductive currents. The magnitude of these over voltage depends on voltage before chopping, chopping current and surge impedance of the load and the system.