INTRODUCTIONWith the advancement of power systems, the lines and other equipment operate at very high voltages and currents. High voltage circuit breakers play an important role in transmission and distribution systems. A circuit brraker can either make or break a circuit, either manually or automatically under all conditions viz vo load, full load and short circuit conditions. The American national standard institute (ANSI) defines circuit breaker as ³ a mechanical switching device capable of making , carrying and breaking cureents under specified normal conditions such as those of short circuit ³. A ciruit breaker is usually intended to operate infrequently, although some types are suitable for frequent operation. ESSENTIAL QUALITIES OF HV CIRCUIT BREAKER High voltage circuit breakers play an important role in transmission and distribution Systems. They must clear faults and isolate faulty sections rapidly and reliably. In short , they must possess the following properties. y y y In closed position they are good conductors . In open position they are excellent insulators. They can close a short circuit quickly and safely without unacceptable contact erosion. y They can interrupt a 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 state to insulating state at a certain voltage is the arc. HISTORY The first circuit breaker was developed by J.N. kelman in 1901. It was the Predessor of the oil circuit breaker and capable of interrupting short circuit Currents of 200 to 300 Ampere in a 400 KV system. The circuit breaker was made of two wooden barrels containing a mixture of oil and water in which the contacts were immersed. Since 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 SF 6 gas as the arc quenching medium. THE NEED FOR TESTING Almost all people have experienced the effects of protective devices operating frequently. When an overload or short circuit occurs in the home, the 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 winding, fires , explosions , and electrical shock. It is often assumed that the fuses and the circuit breaker in the home or industry are infallible, and will operate safely when called upon to do so ten, twenty or more years after installation. In the cases 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 , are however, mechanical devices, are subjected to deterioration due to wear , corrosion and environmental contamination , any of which could cause the device remain closed during a faulty condition . At the very least , the specified time delay may have shifted so much that proper direction is no longer afforded to devices on the circuit , or improper co-ordination causes a main circuit breaker or fuse to open in an inconvenient location. TESTING OF CIRCUIT BREAKER The design of circuit breaker is not only an science but an art. Because of the Complex phenomena involved , circuit breakers need to be verified by practical tests in the laboratory. There are two types of tests of circuit breakers , namely routine and type tests. Routine tests are performed on every piece of circuit breaker in the premises of the manufacturer. The purpose of the routine test is to perform proper functioning of the circuit breaker . These types of tests are performed in a high voltage laboratory, such types of tests are performed on sample pieces of circuit breakers of each type to conform 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 Electomechanical commission codes (IES) 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 currents 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 Waveforms represent 90% of the possible conditions in the real time system . 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 short circuit 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 arc 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 partially from zero to infinity in milli seconds . Immediately after current interruption, the transient recovery voltage builds across the circuit breaker . As the gas mixture in the interelctrode 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 stimulate the complicated physical Process 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 non linear resistance of the electrical arc for that Specific measurement. DIGITAL TESTING The functionality of high voltage circuit breakers is tested in high power laboratories . Due to 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 breakers 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 i.e., a software product for testing a model of such a device , once its characteristic finger prints are obtained from refined measurements during standard tests. Digital offers a wide range of new possibilities for users , manufacturers , standardizing bodies and test laboratories for fine tuning circuit braker abilities in realization with standard and real power systems . The steps followed so far to enable digital testing are described in the following section . 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 breakers behavior . A tailor made high frequency measuring system was realized for this purpose . This system consists a number of battery powered , single channel 40 MHZ 12 bit AD Converters each storing data temporarily in on board local RAM ( 256 k 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 potenial . 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 challemge 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 electromagnetic 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 other hand , this has to do with suitable frequency sensors and on the other hand with corrections needed for the reproducible induced voltage and capacitive current that distort the measured signal . 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 measured data in a user friendly way . After an extensive series of the most critical fault interruption duty for circuit brakers , a test data base from various types of commercially available circuit breakers was set up . It is observed from total number of ( more than 250) interruption attempts , the result of the attempt failure or success was predicted correctly in more than 90% of cases by evaluating the characteristics of arc behaviour with model . 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 , obtained from the whole series of tests makes it possible to evaluate various physical quantities as a function of test conditions .The main aim is to quantify the breaker performance (the margin of interruption M , ) indicating how successful the breaker passed the test ( M>O) or how it is off from passing it ( M<O) An example is given in figure 3 where the degradation of three poles ( A , B and C) is presented during a succesive steps . It can be seen clearly that the margin of breaker decreases with every test . the rate of margin decay is a measure of the endurance of the breaker with these type of tests. ARC-CIRCUIT INTERACTION SOFTWARE At the final stage of realistation 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 analysed with a dedicated computer program . The analysis of arc ± current 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 size solvers , EMTP and comparable programs are less suitable for this purpose , and therefore a new approach , the integration of the Differential algebric equations (DAE) by means of the backward differential formula (BDF) has been choosen in developing new software for electrical transients computation . This new transient program , X-Trans has been developed at the Delft university of technologies , especially for arc-circuit interaction studies . The program runs on a pc with the MS Windows operating system and fully graphically as shown in the figure 4. The program is in use at high voltage and high power laboratories in the world . 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 realised with object orient programing. The compiled code of 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 main program . APPLICATIONS OF DIGITAL TESTING 1. INFLUENCE OF PARALLEL CAPACITANCE : Powerful possibilities with digital testing are created with the arc model, validated as described in sections on Measurements and Data Analysis is coupled with a circuit analysis package .Then the performance of a circuit breaker , the finger prints of which are obtained from real tests can be estimated in circuits other than test circuit. For example, the influence of various standard substation components on the breakers capabilities can be estimated by digital testing . Here the influence of 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 presence of two types of CTs ; CT 1 having 200 pf of parasitic capacitance and CT 2 having 400 pf . These CTs are located near the circuit breaker or remote to the breaker . Table 1 shows the difference between 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 . 2. CRITICAL LINE LENGTH DETRMINATION 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 points is on high voltage Transmission line a few kilometers away from the breaker terminals . Ater current interruption a very steep triangle shaped waveform stresses the extinguishing medium between the contacts . The percentage SLF indicates to what extent the current can be reduced by the transmission line . As an example of digital testing , the critical line length , the short line fault percentage that stresses the circuit breaker most , will be determined by 145KV , 31.5 KA SF 6 circuit breaker. A direct SLF test circuit is shown in figure 6 . three differeent indicators active at different time intervals are used to quantify the stress on circuit breaker model . y Before current zero: The time before current zero where the arc resistance equals the surge impedance of the transmission line (R=Z) . The closer is the value to current zero , the more severe the breaker is stressed by test circuit . y At current zero: The arc resistance is R0. The lower the arc resistance value at the current zero crossing , the stronger is the breaker stressed by the test circuit . y 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 succesful 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 parameters are determined. The stresses at various short line percentages is computed . The overall stresses is shown below All three indicators show that the circuit breaker is stressed most severely at 93% SLF, whereas a 90% SLF is prescribed in the IEC standard . This shows that digital testing can be applied to use the information obtained from laboratory tests for the development of future standards . ADVANTAGES OF DIGITAL TESTING y y 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 . y y y Estimation of circuit breakers interrupting limit . Reduction of full scale testing in high-power laboratories . Identification of network topologies that can pose special difficulties to a circuit Breaker. y y y Acceleration at development of new circuit breaker design . Monitornig the aging process of circuit breaker in service Expansion of services for high-power laboratories DISADVANTAGE OF DIGITAL TESTING y Testing is costlier CHALLENGES Micro electronic instruments for sensing and measuring the parameters are kept in intense magnetic fields. y CONCLUSION Digital testing gives precise information about the breaker , as obtained from laboratory tests. This is useful for the development of future standards . powerful 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 . REFERENCES 1. Digital testing of high voltage circuit breakers by Pieter Schavemaker, Lou Vandersluis, IEEE Computer Applications in Power Systems . 2. Evaluation of HV circuit breakers performance with a new arc model by R.P.P. Smeets , V.Kertesz , IEEE Preceedinds on generation transmission and distribution . 3. Circuit breaker testing technology by Paul E. Schoen , President PS technology , INC . March 26, 1997.