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CONTENTS
 INTRODUCTION 01
 ESSENTIAL QUALITIES OF A HIGH VOLTAGE 02
CIRCUIT BREAKER
 HISTORY 03
 THE NEED FOR TESTING 04
 TESTING OF CIRCUIT BREAKERS 05
 CIRCUIT BREAKER SWITCHING AND ARC MODELING 07
 DIGITAL TESTING 08
 MEASUREMENT AND DATA ANALYSIS 09
 ARC-CIRCUIT INTERACTION SOFTWARE 13
 APPLICATIONS OF DIGITAL TESTING 15
 INFLUENCE OF PARALLEL CAPACITANCE 15
 CRITICAL LINE LENGTH DETERMINATION 16
 ADVANTAGES & DISADVANTAGES 18
 CHALLENGES 18
 CONCLUSION 19
 REFERENCES 20
2
ABSTRACT
The functionality of high voltage circuit breaker is tested in high power
laboratories. With the advancement of power system need for high voltage circuit breaker
has increased a lot, as they play an important role in transmission and distribution. Among
the tests, Routine tests are performed on every piece of circuit breaker in the premises of
the manufacturer. where Type tests are performed in a high voltage laboratory. The test is
performed on sample pieces of circuit breaker of each type to confirm their characteristics
and rated capacities according to their design. Digital testing of high voltage circuit
breaker employs a software product for testing of a breaker model, once its characteristic
fingerprints are obtained from the standard tests. High-resolution measurements of current
and voltage are done in the critical period. The arc voltage is measured with standard
broadband RCR type voltage divider and current with special Rogows.
3
INTRODUCTION
With the advancement of power system, the lines and other equipment operate at
very high voltages and carry large currents. High-voltage circuit breakers play an
important role in transmission and distribution systems. A circuit breaker can make or
break a circuit, either manually or automatically under all conditions viz. no-load, full-load
and short-circuit conditions. The American National Standard Institute (ANSI) defines
circuit breaker as: “A mechanical switching device capable of making, carrying and
breaking currents under normal circuit conditions and also making, carrying for a specified
time, and breaking currents under specified abnormal circuit conditions such as those of
short circuit”. A circuit breaker is usually intended to operate infrequently, although some
types are suitable for frequent operation.
4
ESSENTIAL QUALITIES OF HV CIRCUIT BREAKER
High-voltage circuit breaker play an important role in transmission and
distribution systems. They must clear faults and isolate faulted sections rapidly and
reliably. In-short they must possess the following qualities.

In closed position they are good conductors.



In open position they are excellent insulators.



They can close a shorted circuit quickly and safely without unacceptable contact
erosion.



They can interrupt a rated short-circuit current or lower current quickly without
generating an abnormal voltage.

The only physical mechanism that can change in a short period of time from a
conducting to insulating state at a certain voltage is the arc.
5
HISTORY
The first circuit breaker was developed by J.N. Kelman in 1901. It was the
predecessor of the oil circuit breaker and capable of interrupting a short circuit current of
200 to 300 Ampere in a 40KV system. The circuit breaker was made up of two wooden
barrels containing a mixture of oil and water in which the contacts were immersed. Since
then the circuit breaker design has undergone a remarkable development. Now a days one
pole of circuit breaker is capable of interrupting 63 KA in a 550 KV network with SF6 gas
as the arc quenching medium.
6
THE NEED FOR TESTING
Almost all people have experienced the effects of protective devices operating
properly. When an overload or a short circuit occurs in the home, the usual result is a
blown fuse or a tripped circuit breaker. Fortunately few have the misfortune to see the
results of a defective device, which may include burned wiring, fires, explosions, and
electrical shock.
It is often assumed that the fuses and circuit breakers in the home or industry are
infallible, and will operate safely when called upon to do so ten, twenty, or more years
after their installation. In the case of fuses, this may be a safe assumption, because a
defective fuse usually blows too quickly, causing premature opening of the circuit, and
forcing replacement of the faulty component. Circuit breakers, however, are mechanical
devices, which are subject to deterioration due to wear, corrosion and environmental
contamination, any of which could cause the device to remain closed during a fault
condition. At the very least, the specified time delay may have shifted so much that proper
protection is no longer afforded to devices on the circuit, or improper coordination causes
a main circuit breaker or fuse to open in an inconvenient location.
7
TESTING OF CIRCUIT BREAKERS
The design of circuit breaker is not only a science but also an art. Because of the
complex phenomena involved, circuit breaker prototypes have to be verified by practical
tests in the laboratory. There are two types tests of circuit breakers, namely Routine tests
and Type tests. Routine test are performed on every piece of circuit breaker in the premises
of the manufacturer. The purpose of the routine test is to confirm the proper functioning of
a circuit breaker. Type tests are performed in a high voltage laboratory; such tests are
performed on sample pieces of circuit breaker of each type to confirm their characteristics
and rated capacities according to their design. These tests are not performed on every piece
of the circuit breaker. All routine and type tests are performed according to Indian
Standard (IS) codes, or International Electromechanical Commission (IEC) codes or
British Standard (BS) codes.
In High-power laboratories the ability of the circuit breakers to interrupt the
circuit currents is verified in test circuits which is infact the lumped element representation
of the power system. These test circuits must produce the corrected forms of the short
circuit current as well as the final voltage that strikes the circuit breaker immediately after
the breaker has interrupted the test current. The forms of voltage and current to which the
test object must be subjected are laid down in ANSI and International Electromechanical
commission (IEC) standards. These standardized waveform represent 90% of the possible
conditions in the real system.
8
9
CIRCUIT BREAKER SWITCHING AND ARC MODELLING
The switching action, the basic function of the circuit breaker refers to the
change from conductor to insulator at a certain voltage. Before interruption, the short
circuit flows through the circuit breaker arc channel. Because of non zero resistance of the
channel , the short circuit current causes a voltage across the contacts of the circuit
breaker; the arc voltage. The arc behaves as a non-linear resistance. Thus both are voltage
and arc current cross the zero value at the same time instant. If the arc is cooled at the time
current goes through zero the circuit breaker interrupts the current because the electrical
power input is zero. During current interruption, the arc resistance increases practically
from zero to almost infinity in microseconds. Immediately after current interruption, the
transient recovery voltage builds up across the circuit breaker .As the gas mixture in the
interelectrode space does not change to a completely insulating state instantaneously, the
arc resistance is finite at that time and a small current can flow ;the post –arc current.
Black box arc models are mathematical description of the electrical properties of
the arc .This type of model does not simulate the complicated physical processes inside the
circuit breaker but describes the electrical properties of the circuit breaker. Measured
voltage and current traces are used to extract the parameters for the differential equations
describing the nonlinear resistance of the electrical arc for that specific measurement.
10
DIGITAL TESTING
The functionality of high-voltage circuit breakers is tested in high-power
laboratories. Due to the necessary power and the physical size of the equipment , testing is
rather expensive and time consuming. In order to obtain as much information as possible
about the degradation and operating of the circuit breaker from cost intensive tests, a
project started with the following parties.
1. KEMA high power laboratories The Netherlands.
2. Delft University of technology, The Netherlands.
3. Siemens AG, Germany
4. RWE Energy ,Germany
The project is aimed at developing digital testing of high-voltage circuit
breakers. ie., a software product for testing a model of such a device , once its
characteristic fingerprints are obtained from refined measurements during standard tests.
Digital offers a wide range of new possibilities for users, manufactures, standardizing
bodies and test laboratories for fine tuning circuit breaker abilities in relation with standard
and real power systems. The steps followed so far to enable digital testing are described in
the following section.
11
MEASUREMENT AND DATA ANALYSIS
High resolution measurements of current and voltage in the critical period
around short circuit current zero must supply the necessary parameters characterizing the
breaker’s behaviour. A tailor-made high-frequency measuring system was realized for this
purpose. This system consists a number of battery powered, single channel 40MHz 12bit
AD converters each storing data temporarily in on board local RAM (256k samples each).
The concept of on-site data storage is necessary for reaching a maximum overall system
bandwidth. Cables to the current and voltage sensors can thus be kept very short and the
system can operate on floating potential. The arc voltage is measured with standard broad
band RCR type voltage dividers. Current is measured with a special Rogowski coil. After
the remote RAM is filled, data is transmitted serially through optical fibres to the
processing unit in the command centre. The greatest challenge with respect to developing
the equipment in this application design lies in the electromagnetic compatibility, since the
microelectronics has to function in an extremely hostile environment of intense Electro
Magnetic fields of various origin.
The system relies heavily on digital signal processing methods for
reconstructing the actual voltage and current signals from the raw sensor output. On the
one hand, this has to do with the specific frequency sensors and on the other hand, with
corrections needed for the reproducible induced voltage and capacitive current that distort
the measured signals. Tests in various laboratories have proven that the system can
measure post-arc current as small as 50 mA, microseconds after the interruption of many
tens of kA.
12
Figure 1. Voltage and current measurement data
Data analysis software has been produced to carry out the signal reconstruction
practically on line during the tests (Figure-1) and to evaluate the performance of the test
object. Even the newest professional multipurpose mathematical or laboratory software is
not competitive to this custom-made software considering flexibility and speed in
visualizing and data processing of practically unlimited amount of measured data in a user
friendly way.
After an extensive series of the most critical fault interruption duty for circuit
breakers, a test data-base from various types of commercially available circuit breakers
was set up. With this experimental material, an empirical arc model based on classical arc
models was validated that gave very good coverage of the observed processes. From the
total number (more than 250) interruption attempts, the result of the attempt i.e., failure or
13
success was predicted correctly in more than 90% of the cases by evaluating the
characteristics of the arc behaviour with model.
Figure 2 Parameter extraction software
The model has a set of three parameters, which are extracted automatically
during the evaluation of each test. See figure 2. Automated analysis of the collection of all
the parameter sets (in other. words the breaker’s finger prints) obtained from a whole
series of tests makes it possible to evaluate various physical quantities as a function of test
conditions.
The aim of using this method is to quantify the breaker performance (the margin
of interruption M), indicating how successful the breaker passed the test (M>O) or how far
it is off from passing it (M<O).
14
An example is given in Figure 3 where the degradation of the three breaker poles
(A,B and C) is presented during a sequence of successive tests. It can be seen clearly that
the margin of the breaker decreases with every test. The rate of margin decay (among
others) is a measure of the endurance of the breaker with respect to this type of tests.
Figure 3. Degradation of the circuit breaker poles
15
ARC-CIRCUIT INTERACTION SOFTWARE
At the final stage of the realization of digital testing, measured arc model
parameters will be used as input for the arc model. Of course, this arc model behaves as a
non-linear element in the electrical circuit and must therefore be analyzed with a dedicated
computer program. The analysis of arc-circuit interaction involving non linear elements in
relation to stiff differential equations makes it necessary to perform the calculations with a
variable step size and adjustable accuracy of the computed currents, voltages and
conductances. Because they have fixed step-size solvers, EMTP and comparable programs
are less suitable for this purpose and therefore a new approach, the integration of
differential algebraic equations (DAE) by means of the backward differentiation formula
(BDF) method has been chosen in developing a new software package for electrical
transients computation. This new transient program, X Trans has been developed at the
Delft University of Technology especially for arc-circuit interaction studies. The program
runs on a PC with the MS-Windows operating system and works fully graphical as shown
in figure 4. The program is in use at several high-power and high-voltage laboratories in
the world.
16
Figure 4.XTrans transient program
The program makes use of libraries that contain information about the behaviour
of element models. The program structure is depicted in figure 5. The structure has been
realized with object oriented programming. The compiled code of the element models is
placed in dynamic link libraries (DLL). The models are therefore separate from the main
program, which makes it easy to create new models and use them in the main program.
17
Figure 5. Structure of the XTrans transient program
APPLICATIONS OF DIGITAL TESTING
1. INFLUENCE OF PARALLEL CAPACITANCE
Powerful possibilities with digital testing are created when the arc model,
validated as described in the section on Measurements and Data Analysis is coupled with a
circuit analysis package. Then the performance of a circuit breaker, the fingerprints of
which are obtained from real tests can be estimated in circuits other than the test circuit.
For example, the influence of various standard substation components on the
breaker’s capabilities can be estimated through digital testing.
Here the influence of a parallel capacitance is calculated (for example, the
parasitic capacitance of a current transformer, CT) in the substation.
In Table 1, the performance of a short-line fault interruption is compared in the
presence of two types of CTs; CT1 having 200 pF of parasitic capacitance and CT2 having
400 pF. These CTs can be located near the circuit breaker or remote to the breaker (the
latter implying an additional 50 microhenry of bus bar between CT and breaker). As a
reference the case without CT has a performance of 1.0.
18
Table 1 shows that the difference between the two types of CTs is rather small
when compared to gain obtained by the CT that was installed to the breaker as closely as
possible.
Table 1. Influence of a current transformer
2. CRITICAL LINE LENGTH DETERMINATION
One of the most severe currents for a circuit breaker to interrupt is the short line
fault (SLF). In the case of a short-line fault, the short circuit point is on a high voltage
transmission line a few kilometers away from the breaker terminals. After current
interruption a very steep triangular-shaped waveform (with a rate of rise of 5-10
KV/microsecond) stresses the extinguishing medium between the contacts. The percentage
SLF indicates to what extent, the short circuit current is reduced by the transmission line.
Eg. a short circuit current of 40KA is reduced to 36KA in case of a 90% SLF. In the IEC
standard, 75% and 90% SLF tests are prescribed.
Figure 6. A direct SLF test circuit in the XTrans program
19
As an example of digital testing, the critical line length, the short line fault
percentage that stresses the circuit breaker most, will be determined for a 145KV, 31.5KA
SF6 circuit breaker. A direct SLF test circuit is shown in Figure 6. Three different
indicators active at different time intervals (before current zero, at current zero and after
current zero) are used to quantify the stress on circuit breaker model.

Before current zero:- The time before current zero where the arc resistance equals
the surge impedance of the transmission line t (R=Z) The closer the value is to
current zero, the more severe the breaker is stressed by the test circuit.



At current zero:- The arc resistance is R0. The lower the arc resistance value at the
current zero crossing, the stronger the breaker is stressed by the test circuit.



After current zero:- The post arc energy is Epa. This value is the integral of the
multiplication of the small post-arc current and the recovery voltage. It is clear that
only for successful interruptions an Epa value can be calculated. The higher the
Epa value is, the more severe the breaker is stressed by the test circuit.

The actual computation is based on 75 current zero recordings of the circuit
breaker of which the circuit breaker model parameters have been determined. For each set
of parameters, the stresses at various short line fault percentages is computed. At last the
overall stress is visualized which is shown in Figure 7.
Figure 7. Critical line length determination by means of digital testing
All three indicators show that the circuit breaker model is stressed most severely
at a 93% SLF, whereas a 90% SLF is prescribed in the IEC standard. This shows that
20
digital testing can be applied to use the information obtained from laboratory tests for the
development of future standards.
ADVANTAGES OF DIGITAL TESTING

Evaluation of relevance of future standards with respect to real power systems.



Evaluation of the relevance of future standards for different circuit breaker
technologies and extinguishing media.


Estimation of circuit breaker’s interrupting limit.



Reduction of full scale testing in high-power laboratories.



Identification of network topologies that can pose special difficulties to a circuit
breaker.


Acceleration at development of new circuit breaker design.



Monitoring the aging process of circuit breaker in service.



Expansion of services for high-power laboratories

DISADVANTAGE OF DIGITAL TESTING

Testing in costlier

CHALLENGES

Micro electronic instruments for sensing and measuring the parameters are kept in
intense magnetic fields.

CONCLUSION
Digital testing gives precise information about the breaker, as obtained from
laboratory tests. This is useful for the development of future standards. Powerful
21
possibilities with digital testing are created when arc model and data analysis is coupled
with a circuit analysis package. The performance of a circuit breaker whose finger prints
are obtained from real tests can be estimated in other circuits also.
22
REFERENCES
1. “Digital testing of high voltage circuit breakers” by Pieter Schavemaker, Lou
vandersluis, IEEE Computer Applications in Power, April 2000.
2. “Evaluation of H.V. Circuit breaker performance with a new arc model” by
R.P.P. Smeets, V. Kertesz, lEE Proceedings on generation Transmission and
Distribution.
3. Circuit breaker testing technology by Paul E. Schoen, President PS Technology,
Inc. March 26, 1997.
23

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DIGITAL TESTING OF HIGH VOLTAGE CIRCUIT BREAKER

  • 1. 1 CONTENTS  INTRODUCTION 01  ESSENTIAL QUALITIES OF A HIGH VOLTAGE 02 CIRCUIT BREAKER  HISTORY 03  THE NEED FOR TESTING 04  TESTING OF CIRCUIT BREAKERS 05  CIRCUIT BREAKER SWITCHING AND ARC MODELING 07  DIGITAL TESTING 08  MEASUREMENT AND DATA ANALYSIS 09  ARC-CIRCUIT INTERACTION SOFTWARE 13  APPLICATIONS OF DIGITAL TESTING 15  INFLUENCE OF PARALLEL CAPACITANCE 15  CRITICAL LINE LENGTH DETERMINATION 16  ADVANTAGES & DISADVANTAGES 18  CHALLENGES 18  CONCLUSION 19  REFERENCES 20
  • 2. 2 ABSTRACT The functionality of high voltage circuit breaker is tested in high power laboratories. With the advancement of power system need for high voltage circuit breaker has increased a lot, as they play an important role in transmission and distribution. Among the tests, Routine tests are performed on every piece of circuit breaker in the premises of the manufacturer. where Type tests are performed in a high voltage laboratory. The test is performed on sample pieces of circuit breaker of each type to confirm their characteristics and rated capacities according to their design. Digital testing of high voltage circuit breaker employs a software product for testing of a breaker model, once its characteristic fingerprints are obtained from the standard tests. High-resolution measurements of current and voltage are done in the critical period. The arc voltage is measured with standard broadband RCR type voltage divider and current with special Rogows.
  • 3. 3 INTRODUCTION With the advancement of power system, the lines and other equipment operate at very high voltages and carry large currents. High-voltage circuit breakers play an important role in transmission and distribution systems. A circuit breaker can make or break a circuit, either manually or automatically under all conditions viz. no-load, full-load and short-circuit conditions. The American National Standard Institute (ANSI) defines circuit breaker as: “A mechanical switching device capable of making, carrying and breaking currents under normal circuit conditions and also making, carrying for a specified time, and breaking currents under specified abnormal circuit conditions such as those of short circuit”. A circuit breaker is usually intended to operate infrequently, although some types are suitable for frequent operation.
  • 4. 4 ESSENTIAL QUALITIES OF HV CIRCUIT BREAKER High-voltage circuit breaker play an important role in transmission and distribution systems. They must clear faults and isolate faulted sections rapidly and reliably. In-short they must possess the following qualities.  In closed position they are good conductors.    In open position they are excellent insulators.    They can close a shorted circuit quickly and safely without unacceptable contact erosion.    They can interrupt a rated short-circuit current or lower current quickly without generating an abnormal voltage.  The only physical mechanism that can change in a short period of time from a conducting to insulating state at a certain voltage is the arc.
  • 5. 5 HISTORY The first circuit breaker was developed by J.N. Kelman in 1901. It was the predecessor of the oil circuit breaker and capable of interrupting a short circuit current of 200 to 300 Ampere in a 40KV system. The circuit breaker was made up of two wooden barrels containing a mixture of oil and water in which the contacts were immersed. Since then the circuit breaker design has undergone a remarkable development. Now a days one pole of circuit breaker is capable of interrupting 63 KA in a 550 KV network with SF6 gas as the arc quenching medium.
  • 6. 6 THE NEED FOR TESTING Almost all people have experienced the effects of protective devices operating properly. When an overload or a short circuit occurs in the home, the usual result is a blown fuse or a tripped circuit breaker. Fortunately few have the misfortune to see the results of a defective device, which may include burned wiring, fires, explosions, and electrical shock. It is often assumed that the fuses and circuit breakers in the home or industry are infallible, and will operate safely when called upon to do so ten, twenty, or more years after their installation. In the case of fuses, this may be a safe assumption, because a defective fuse usually blows too quickly, causing premature opening of the circuit, and forcing replacement of the faulty component. Circuit breakers, however, are mechanical devices, which are subject to deterioration due to wear, corrosion and environmental contamination, any of which could cause the device to remain closed during a fault condition. At the very least, the specified time delay may have shifted so much that proper protection is no longer afforded to devices on the circuit, or improper coordination causes a main circuit breaker or fuse to open in an inconvenient location.
  • 7. 7 TESTING OF CIRCUIT BREAKERS The design of circuit breaker is not only a science but also an art. Because of the complex phenomena involved, circuit breaker prototypes have to be verified by practical tests in the laboratory. There are two types tests of circuit breakers, namely Routine tests and Type tests. Routine test are performed on every piece of circuit breaker in the premises of the manufacturer. The purpose of the routine test is to confirm the proper functioning of a circuit breaker. Type tests are performed in a high voltage laboratory; such tests are performed on sample pieces of circuit breaker of each type to confirm their characteristics and rated capacities according to their design. These tests are not performed on every piece of the circuit breaker. All routine and type tests are performed according to Indian Standard (IS) codes, or International Electromechanical Commission (IEC) codes or British Standard (BS) codes. In High-power laboratories the ability of the circuit breakers to interrupt the circuit currents is verified in test circuits which is infact the lumped element representation of the power system. These test circuits must produce the corrected forms of the short circuit current as well as the final voltage that strikes the circuit breaker immediately after the breaker has interrupted the test current. The forms of voltage and current to which the test object must be subjected are laid down in ANSI and International Electromechanical commission (IEC) standards. These standardized waveform represent 90% of the possible conditions in the real system.
  • 8. 8
  • 9. 9 CIRCUIT BREAKER SWITCHING AND ARC MODELLING The switching action, the basic function of the circuit breaker refers to the change from conductor to insulator at a certain voltage. Before interruption, the short circuit flows through the circuit breaker arc channel. Because of non zero resistance of the channel , the short circuit current causes a voltage across the contacts of the circuit breaker; the arc voltage. The arc behaves as a non-linear resistance. Thus both are voltage and arc current cross the zero value at the same time instant. If the arc is cooled at the time current goes through zero the circuit breaker interrupts the current because the electrical power input is zero. During current interruption, the arc resistance increases practically from zero to almost infinity in microseconds. Immediately after current interruption, the transient recovery voltage builds up across the circuit breaker .As the gas mixture in the interelectrode space does not change to a completely insulating state instantaneously, the arc resistance is finite at that time and a small current can flow ;the post –arc current. Black box arc models are mathematical description of the electrical properties of the arc .This type of model does not simulate the complicated physical processes inside the circuit breaker but describes the electrical properties of the circuit breaker. Measured voltage and current traces are used to extract the parameters for the differential equations describing the nonlinear resistance of the electrical arc for that specific measurement.
  • 10. 10 DIGITAL TESTING The functionality of high-voltage circuit breakers is tested in high-power laboratories. Due to the necessary power and the physical size of the equipment , testing is rather expensive and time consuming. In order to obtain as much information as possible about the degradation and operating of the circuit breaker from cost intensive tests, a project started with the following parties. 1. KEMA high power laboratories The Netherlands. 2. Delft University of technology, The Netherlands. 3. Siemens AG, Germany 4. RWE Energy ,Germany The project is aimed at developing digital testing of high-voltage circuit breakers. ie., a software product for testing a model of such a device , once its characteristic fingerprints are obtained from refined measurements during standard tests. Digital offers a wide range of new possibilities for users, manufactures, standardizing bodies and test laboratories for fine tuning circuit breaker abilities in relation with standard and real power systems. The steps followed so far to enable digital testing are described in the following section.
  • 11. 11 MEASUREMENT AND DATA ANALYSIS High resolution measurements of current and voltage in the critical period around short circuit current zero must supply the necessary parameters characterizing the breaker’s behaviour. A tailor-made high-frequency measuring system was realized for this purpose. This system consists a number of battery powered, single channel 40MHz 12bit AD converters each storing data temporarily in on board local RAM (256k samples each). The concept of on-site data storage is necessary for reaching a maximum overall system bandwidth. Cables to the current and voltage sensors can thus be kept very short and the system can operate on floating potential. The arc voltage is measured with standard broad band RCR type voltage dividers. Current is measured with a special Rogowski coil. After the remote RAM is filled, data is transmitted serially through optical fibres to the processing unit in the command centre. The greatest challenge with respect to developing the equipment in this application design lies in the electromagnetic compatibility, since the microelectronics has to function in an extremely hostile environment of intense Electro Magnetic fields of various origin. The system relies heavily on digital signal processing methods for reconstructing the actual voltage and current signals from the raw sensor output. On the one hand, this has to do with the specific frequency sensors and on the other hand, with corrections needed for the reproducible induced voltage and capacitive current that distort the measured signals. Tests in various laboratories have proven that the system can measure post-arc current as small as 50 mA, microseconds after the interruption of many tens of kA.
  • 12. 12 Figure 1. Voltage and current measurement data Data analysis software has been produced to carry out the signal reconstruction practically on line during the tests (Figure-1) and to evaluate the performance of the test object. Even the newest professional multipurpose mathematical or laboratory software is not competitive to this custom-made software considering flexibility and speed in visualizing and data processing of practically unlimited amount of measured data in a user friendly way. After an extensive series of the most critical fault interruption duty for circuit breakers, a test data-base from various types of commercially available circuit breakers was set up. With this experimental material, an empirical arc model based on classical arc models was validated that gave very good coverage of the observed processes. From the total number (more than 250) interruption attempts, the result of the attempt i.e., failure or
  • 13. 13 success was predicted correctly in more than 90% of the cases by evaluating the characteristics of the arc behaviour with model. Figure 2 Parameter extraction software The model has a set of three parameters, which are extracted automatically during the evaluation of each test. See figure 2. Automated analysis of the collection of all the parameter sets (in other. words the breaker’s finger prints) obtained from a whole series of tests makes it possible to evaluate various physical quantities as a function of test conditions. The aim of using this method is to quantify the breaker performance (the margin of interruption M), indicating how successful the breaker passed the test (M>O) or how far it is off from passing it (M<O).
  • 14. 14 An example is given in Figure 3 where the degradation of the three breaker poles (A,B and C) is presented during a sequence of successive tests. It can be seen clearly that the margin of the breaker decreases with every test. The rate of margin decay (among others) is a measure of the endurance of the breaker with respect to this type of tests. Figure 3. Degradation of the circuit breaker poles
  • 15. 15 ARC-CIRCUIT INTERACTION SOFTWARE At the final stage of the realization of digital testing, measured arc model parameters will be used as input for the arc model. Of course, this arc model behaves as a non-linear element in the electrical circuit and must therefore be analyzed with a dedicated computer program. The analysis of arc-circuit interaction involving non linear elements in relation to stiff differential equations makes it necessary to perform the calculations with a variable step size and adjustable accuracy of the computed currents, voltages and conductances. Because they have fixed step-size solvers, EMTP and comparable programs are less suitable for this purpose and therefore a new approach, the integration of differential algebraic equations (DAE) by means of the backward differentiation formula (BDF) method has been chosen in developing a new software package for electrical transients computation. This new transient program, X Trans has been developed at the Delft University of Technology especially for arc-circuit interaction studies. The program runs on a PC with the MS-Windows operating system and works fully graphical as shown in figure 4. The program is in use at several high-power and high-voltage laboratories in the world.
  • 16. 16 Figure 4.XTrans transient program The program makes use of libraries that contain information about the behaviour of element models. The program structure is depicted in figure 5. The structure has been realized with object oriented programming. The compiled code of the element models is placed in dynamic link libraries (DLL). The models are therefore separate from the main program, which makes it easy to create new models and use them in the main program.
  • 17. 17 Figure 5. Structure of the XTrans transient program APPLICATIONS OF DIGITAL TESTING 1. INFLUENCE OF PARALLEL CAPACITANCE Powerful possibilities with digital testing are created when the arc model, validated as described in the section on Measurements and Data Analysis is coupled with a circuit analysis package. Then the performance of a circuit breaker, the fingerprints of which are obtained from real tests can be estimated in circuits other than the test circuit. For example, the influence of various standard substation components on the breaker’s capabilities can be estimated through digital testing. Here the influence of a parallel capacitance is calculated (for example, the parasitic capacitance of a current transformer, CT) in the substation. In Table 1, the performance of a short-line fault interruption is compared in the presence of two types of CTs; CT1 having 200 pF of parasitic capacitance and CT2 having 400 pF. These CTs can be located near the circuit breaker or remote to the breaker (the latter implying an additional 50 microhenry of bus bar between CT and breaker). As a reference the case without CT has a performance of 1.0.
  • 18. 18 Table 1 shows that the difference between the two types of CTs is rather small when compared to gain obtained by the CT that was installed to the breaker as closely as possible. Table 1. Influence of a current transformer 2. CRITICAL LINE LENGTH DETERMINATION One of the most severe currents for a circuit breaker to interrupt is the short line fault (SLF). In the case of a short-line fault, the short circuit point is on a high voltage transmission line a few kilometers away from the breaker terminals. After current interruption a very steep triangular-shaped waveform (with a rate of rise of 5-10 KV/microsecond) stresses the extinguishing medium between the contacts. The percentage SLF indicates to what extent, the short circuit current is reduced by the transmission line. Eg. a short circuit current of 40KA is reduced to 36KA in case of a 90% SLF. In the IEC standard, 75% and 90% SLF tests are prescribed. Figure 6. A direct SLF test circuit in the XTrans program
  • 19. 19 As an example of digital testing, the critical line length, the short line fault percentage that stresses the circuit breaker most, will be determined for a 145KV, 31.5KA SF6 circuit breaker. A direct SLF test circuit is shown in Figure 6. Three different indicators active at different time intervals (before current zero, at current zero and after current zero) are used to quantify the stress on circuit breaker model.  Before current zero:- The time before current zero where the arc resistance equals the surge impedance of the transmission line t (R=Z) The closer the value is to current zero, the more severe the breaker is stressed by the test circuit.    At current zero:- The arc resistance is R0. The lower the arc resistance value at the current zero crossing, the stronger the breaker is stressed by the test circuit.    After current zero:- The post arc energy is Epa. This value is the integral of the multiplication of the small post-arc current and the recovery voltage. It is clear that only for successful interruptions an Epa value can be calculated. The higher the Epa value is, the more severe the breaker is stressed by the test circuit.  The actual computation is based on 75 current zero recordings of the circuit breaker of which the circuit breaker model parameters have been determined. For each set of parameters, the stresses at various short line fault percentages is computed. At last the overall stress is visualized which is shown in Figure 7. Figure 7. Critical line length determination by means of digital testing All three indicators show that the circuit breaker model is stressed most severely at a 93% SLF, whereas a 90% SLF is prescribed in the IEC standard. This shows that
  • 20. 20 digital testing can be applied to use the information obtained from laboratory tests for the development of future standards. ADVANTAGES OF DIGITAL TESTING  Evaluation of relevance of future standards with respect to real power systems.    Evaluation of the relevance of future standards for different circuit breaker technologies and extinguishing media.   Estimation of circuit breaker’s interrupting limit.    Reduction of full scale testing in high-power laboratories.    Identification of network topologies that can pose special difficulties to a circuit breaker.   Acceleration at development of new circuit breaker design.    Monitoring the aging process of circuit breaker in service.    Expansion of services for high-power laboratories  DISADVANTAGE OF DIGITAL TESTING  Testing in costlier  CHALLENGES  Micro electronic instruments for sensing and measuring the parameters are kept in intense magnetic fields.  CONCLUSION Digital testing gives precise information about the breaker, as obtained from laboratory tests. This is useful for the development of future standards. Powerful
  • 21. 21 possibilities with digital testing are created when arc model and data analysis is coupled with a circuit analysis package. The performance of a circuit breaker whose finger prints are obtained from real tests can be estimated in other circuits also.
  • 22. 22 REFERENCES 1. “Digital testing of high voltage circuit breakers” by Pieter Schavemaker, Lou vandersluis, IEEE Computer Applications in Power, April 2000. 2. “Evaluation of H.V. Circuit breaker performance with a new arc model” by R.P.P. Smeets, V. Kertesz, lEE Proceedings on generation Transmission and Distribution. 3. Circuit breaker testing technology by Paul E. Schoen, President PS Technology, Inc. March 26, 1997.
  • 23. 23