Testing of Circuit Breakers.pptx

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 Tafhim Bin Nasir 2

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. Tafhim Bin Nasir 3

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 Tafhim Bin Nasir 4

5. Types of Circuit Breaker Testing There are mainly two types of Circuit Breaker Testing Type Test Routine Test Tafhim Bin Nasir 5

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. Tafhim Bin Nasir 6

7. Type Tests of Circuit Breaker (Cont’d) Mechanical and Environmental tests Mechanical operation test at ambient air temperature Low and High-temperature tests Humidity test Static terminal load test Short circuit tests Making capacity test Breaking capacity test Short time current test Operating duty test Capacitive current switching tests line-charging test cable charging test single capacitor bank test back-to-back capacitor bank test Dielectric tests Power frequency test One minute dry withstand test One minute wet withstand test Impulse voltage dry withstand test According to the IEC-60056 standard, type tests of circuit breakers include the following: Tafhim Bin Nasir 7

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: At manufacturer’s premises Operation tests millivolt Drop test Power frequency voltage test After erection on site Power frequency voltage Tafhim Bin Nasir 8

9. Types of Short-circuit Testing method There are two types of method for Short-Circuit Testing of Circuit Breaker Direct Testing : Used for small capacity circuit breaker Indirect Testing : Used for large capacity circuit breaker Tafhim Bin Nasir 9

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. Tafhim Bin Nasir 10

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. Tafhim Bin Nasir 11 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 3 Ø short circuit power is, P sc = √3×50× ×420× ≈ 3.64 MW This is a huge number. Testing of Transmission level Circuit Breakers is not feasible by using direct testing method.   Tafhim Bin Nasir 12

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: Unit Testing of Circuit Breaker. Synthetic Testing of Circuit Breakers. Tafhim Bin Nasir 13

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. Tafhim Bin Nasir 14

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 Tafhim Bin Nasir 15

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. Tafhim Bin Nasir 16

17. Types of Synthetic Test Circuits There are two types of synthetic test circuits: Parallel Current injection Series Voltage injection Tafhim Bin Nasir 17

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 t , AB and TB open their contacts, and the short-circuit current flows through the arc channels. At t 1 , spark gap is fired and the main capacitor bank C0 discharges through the inductance L 1 and injects a high-frequency current i v in the arc channel of the TB, which adds up to the power frequency current i g . Figure 4: Current and recovery voltage in the parallel current-injection scheme Figure 3: Parallel current-injection circuit Tafhim Bin Nasir 18

19. Parallel current-injection method (Cont’d) At time instant t 2 , 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 t 3 , 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 Tafhim Bin Nasir 19

20. Limitations of Parallel current-injection method During the period of current injection, the voltage on the main capacitor bank C 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. Tafhim Bin Nasir 20

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 t 1 . 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 Tafhim Bin Nasir 21

22. Series Voltage Injection Method (Cont’d) At t 2 , 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 Tafhim Bin Nasir 22

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. Tafhim Bin Nasir 23

24. Reference Van der Sluis , Lou. Transients in power systems. John Wiley & Sons Ltd, 2001. Tafhim Bin Nasir 24

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