Testing of Circuit Breakers Presented by Tafhim Bin Nasir

1. Testing of Circuit Breakers
Presented by
Tafhim Bin Nasir

2. Outline
• Introduction
• Circuit Breaker Testing
• Type Tests of Circuit Breaker
• Types of Circuit Breaker Testing
• Synthetic Test Circuits
• Types of Synthetic Test Circuits
• Parallel current-injection method
• Series Voltage Injection Method
• Advantages of Synthetic Testing
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3. Introduction
• Circuit breakers(CB) have been an indispensable part of power systems because they
act as protective devices to switch off short-circuit currents and are used to isolate
faulted sections of the network.
• Circuit breaker development relies heavily on testing.
• CB Testing is done to evaluate the capability of power switching components for
interrupting fault currents in power networks.
• There are several methods for CB testing. In this presentation we are going to learn
about the Synthetic Testing of CB.
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4. Circuit Breaker Testing
Circuit breakers perform a vital role in protecting expensive
equipment from damage through faults i.e. connecting and
disconnecting the electrical power in a reliable way; this requires
proving their reliability with on field tests during installation and
with regular maintenance tests during its lifetime to prevent costly
failures and problems that could even compromising the safety of
the substation.
Circuit Breaker Testing (also known as 'Trip Profiling') is used to
test both the performance of individual switching mechanisms and
the timing of the overall tripping system. Figure 1: Typical Circuit Breaker Testing
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5. Types of Circuit Breaker Testing
There are mainly two types of Circuit Breaker Testing
• Type Test
• Routine Test
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6. Type Tests of Circuit Breaker
• Type tests are not performed on every piece of circuit breaker but on sample pieces
of each type to confirm their characteristics and rated capacities according to their
design.
• The purpose of type tests is to prove design features and quality of circuit breaker.
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7. Type Tests of Circuit Breaker (Cont’d)
Mechanical and Environmental tests
I. Mechanical operation test at ambient air
temperature
II. Low and High-temperature tests
III. Humidity test
IV. Static terminal load test
• Short circuit tests
I. Making capacity test
II. Breaking capacity test
III. Short time current test
IV. Operating duty test
• Capacitive current switching tests
I. line-charging test
II. cable charging test
III. single capacitor bank test
IV. back-to-back capacitor bank test
• Dielectric tests
I. Power frequency test
a) One minute dry withstand test
b) One minute wet withstand test
II. Impulse voltage dry withstand test
According to the IEC-60056 standard, type tests of circuit breakers include the following:
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8. Routine Tests of Circuit Breaker
• Routine tests are performed on every piece of circuit breaker in the premises of the
manufacturer.
• The purpose of this tests is to confirm proper functioning of the circuit breaker.
• Routine tests are performed before circuit breaker dispatch to ensure the product.
• This gives result about quality defects in materials and construction of circuit
breaker.
• Routine tests are classified as:
a. At manufacturer’s premises
I. Operation tests
II. millivolt Drop test
III. Power frequency voltage test
b. After erection on site
I. Power frequency voltage
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9. Types of Short-circuit Testing method
There are two types of method for Short-Circuit Testing of Circuit Breaker
I. Direct Testing: Used for small capacity circuit breaker
II. Indirect Testing: Used for large capacity circuit breaker
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10. Direct Testing
• Test in which the applied voltage, current
and the transient and power-frequency
recovery voltages are all obtained from a
circuit having a single-power source,
which may be a power system or special
alternators as used in short-circuit testing
stations or a combination of both.
In the circuits,
• Zs is the series impedance used to adjust
the short circuit current level, which is
usually a current limiting inductor.
• Zp is the parallel impedance to test
objects. Usually, Zp is the series
combination of a capacitance and
resistance. It is inserted to shape the
applied TRV after interruption of current.
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11. Process flow of Direct Test Method:
• It is started by closing the making switch of the Test circuit and then opening of the Test object as
current flows.
• A backup breaker is normally placed in series to ensure reliable current interruption in case of any
failure of the test object.
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Challenges in Direct Testing Method:
• The main challenge of direct testing method switchgear is the rating of power source.
• As both short circuit current and necessary TRV are supplied by one single power source, it has to be very
powerful.

12. Power Calculation of Direct Testing Method:
If Current interruption capability of a 420KV Circuit Breaker is 50KA,
The 3Ø short circuit power is, Psc= √3×50×103
×420×103
≈ 3.64 MW
This is a huge number. Testing of Transmission level Circuit Breakers is not feasible by using direct testing
method.
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13. Indirect Testing of Circuit Breaker
• With the increase in the breaking capacity of the HV circuit breakers, it has become
uneconomical and unpracticable to increase the short-circuit capacity of testing station.
It is consequently necessary to utilize some form of Indirect Testing of Circuit Breaker.
• The important methods of Indirect Testing of Circuit Breaker are:
I. Unit Testing of Circuit Breaker.
II. Synthetic Testing of Circuit Breakers.
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14. Unit Testing of Circuit Breaker
• It is a common practice to design breakers with a number of rupturing units in series,
each having a capacity within the available testing power. A test on one unit can be
accepted as proof for all units. Such tests are known as Unit tests.
• Up to short-circuit levels of some 35,000 MVA this method has proved satisfactory
as this design limitation has not seriously affected the cost of the circuit breakers
• But recently the rupturing capacities of the modern circuit breakers have exceeded
this limit and is of the order of 50,000 MVA which calls for units of higher rupturing
capacities for greater economy.
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15. Synthetic Testing
• In the fault-interrupting process of a circuit breaker in the
actual network, two distinct intervals can be distinguished.
• The circuit breaker is in a closed position when the fault
occurs.
• The short-circuit is detected by a current transformer, the
protection delivers a tripping signal to the breaker, and the
mechanism of the breaker moves the breaker contacts apart.
• When the contacts part, an arc is initiated between the arcing
contacts, and the interrupting chamber is designed such that at
the first or second current zero crossing, the short-circuit
current is interrupted and the breaker is stressed by the
transient recovery voltage (TRV) and the power frequency–
recovery voltage.
• Synthetic testing methods are based on the fact that during the
interrupting process, the circuit breaker is stressed by high
current and by high voltage at different time periods.
Figure 2b: Short-circuit current, arcing voltage, and recovery
voltage for a circuit breaker clearing a system fault.
Figure 2a: Synthetic Test Circuit
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16. Synthetic Test Circuits (Cont’d)
• This gives the possibility of using two separate sources of energy: one source supplying the short-
circuit current during the arcing period and another source supplying the transient recovery voltage and
the power frequency–recovery voltage.
• The overlap of the current and voltage source takes place during the so-called interaction interval
around the current zero, where the current interruption takes place.
• Two different synthetic test methods, current injection and voltage injection, are employed as synthetic
testing technique. The parallel current-injection circuit and the series voltage injection test circuit are
widely accepted for the testing of high-voltage circuit breakers.
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17. Types of Synthetic Test Circuits
There are two types of synthetic test circuits:
I. Parallel Current injection
II. Series Voltage injection
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18. Parallel current-injection method
• Also known as Weil-Dobke Circuit.
• Short-circuit current is supplied by the short-circuit generators,
which are used as the current source.
• Before the test, both the auxiliary breaker (AB) and the test breaker
(TB) are in a closed position.
• At t0, AB and TB open their contacts, and the short-circuit current
flows through the arc channels.
• At t1, spark gap is fired and the main capacitor bank C0 discharges
through the inductance L1 and injects a high-frequency current iv in
the arc channel of the TB, which adds up to the power frequency
current ig.
Figure 4: Current and recovery voltage in the parallel
current-injection scheme
Figure 3: Parallel current-injection circuit
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19. Parallel current-injection method (Cont’d)
• At time instant t2, the power frequency short-circuit current ig
reaches zero. Because the driving voltage of the generator is
rather low, it is relatively easy for the AB to interrupt ig, and the
AB separates the high-current circuit from the high-voltage
circuit.
• At t3, TB interrupts the injected current and is stressed by the
transient recovery voltage resulting from the voltage, oscillating
in the voltage circuit across the test breaker. The spark gap is still
conducting during this time period.
Figure 4: Current and recovery voltage in the parallel
current-injection scheme
Figure 3: Parallel current-injection circuit
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20. Limitations of Parallel current-injection method
• During the period of current injection, the voltage on the main capacitor bank C0 reverses in polarity.
• After interruption of the injected current by the breaker under test, the injection circuit produces the
transient recovery voltage.
• By the time the TRV oscillations have damped out, the remaining recovery voltage is a DC voltage, and
this puts a higher stress on the breaker than the power frequency AC–recovery voltage from the direct
test circuit.
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21. Series Voltage Injection Method
• The initial part of the TRV comes from the current source circuit
and, after current zero, the voltage source circuit is injected,
supplying the rest of the TRV.
• Before the test, both the AB and the TB are in a closed position.
• The generator supplies the short-circuit current and the post-arc
current for the TB.
• During the whole interruption interval when the arc voltage
changes significantly, the arcs in the TB and in the AB are
connected in series.
• The AB and TB clear almost simultaneously at time instant t1.
• When TB and AB have interrupted the short-circuit current, the
recovery voltage of the current source is brought to TB via the
parallel capacitor.
Figure 6: The current and recovery voltage of the
series voltage injection test circuit
Figure 5:The Series Voltage Injection synthetic test
circuit
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22. Series Voltage Injection Method (Cont’d)
• At t2, the spark gap is fired, and the voltage oscillation of the
injection circuit is added to the recovery voltage of the current
source. The resulting TRV is supplied by the voltage circuit
only.
• During the interaction interval, the circuit parameters of the
voltage-injection circuit differ from the circuit parameters of the
direct test circuit. This is not the case for the current-injection
circuit, and it is for this reason that the current-injection method
is used when the direct test circuit reaches its testing limits.
Figure 6: The current and recovery voltage of the
series voltage injection test circuit
Figure 5:The Series Voltage Injection synthetic test
circuit
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23. Advantages of Synthetic Testing
• The circuit breaker can be tested for desired TRV and RRRV.
• The short circuit generator has to supply currents at less voltage.
• The flexibility can be obtained by independent control of test current and test voltage.
• A circuit breaker having capacity five times that of the plant can be tested.
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24. Reference
• Van der Sluis, Lou. Transients in power systems. John Wiley & Sons Ltd, 2001.
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25. THANK
YOU