Transient recovery voltages
Power System Transient Recovery Voltages
When interrupting devices, whether they are high-voltage circuit breakers, disconnectors, or fuses interrupt a current in the power system, the interrupting device experiences a recovery voltage across its terminals. When we bear in mind that during normal system operation the energy stored in the electromagnetic field is equally divided over the electric field and the magnetic field, current interruption causes a transfer of the energy content to the electric field only. This results in a voltage oscillation. The actual waveform of the voltage oscillation is determined by the configuration of the power system. The transient recovery voltage or TRV is present immediately after the interruption of the current. When the TRV oscillation has damped out, the power frequency–recovery voltage is active. The duration of the TRV is in the order of milliseconds, but its rate of rise and its amplitude are of vital importance for a successful operation of the interrupting device.
In the early years of switchgear and fuse design, the TRV was an unknown phenomenon, the recovery voltage was regarded to consist of the power frequency recovery voltage only. The duty on circuit breakers was commonly expressed in terms of the circuit voltage prior to short circuit and the magnitude of the current in the arc. It was, however, experienced that, in practice, other circuit characteristics would affect the duty to an important extend.
Improved measurement equipment, such as the cathode-ray oscillograph and later the cathode-ray oscilloscope, made measurements with a higher time resolution possible and revealed the existence of a high-frequency oscillation immediately after current interruption: the TRV was discovered. This resulted in systemstudies of transmission and distribution networks.
Many investigations were carried out in different countries to determine the TRV across circuit breakers while clearing short-circuits in networks to provide a base for standardisation of the TRV in national rules for type tests on circuit breakers. System studies on transient network analysers brought insight into the frequencies of oscillations and were verified by real tests in the network 
Better understanding of the transient phenomena in the network has led to improved testing practice in the high-power laboratories, more accurate measurement of the current-zero phenomena, and consequently resulted in more reliable switchgear with a higher interrupting capability. The use of as extinguishing medium allowed a leap forward in the short-circuit performance of high-voltage circuit breakers. But it appeared that until that time, unknown transients played an important role. It was not sufficient anymore to simulate the networks with lumped elements, and travelling waves also had to be taken into account. The short-line fault made its appearance . During that period of time, the end of the 1950s, effort was made to represent the TRV oscillations by standardised waveforms to be able to create TRVs by lumped elements within the walls of the high-power laboratory. The four-parameter waveform was proposed by Hochrainer in 1957 , and the possibilities to create the waveform in the high-power laboratory were investigated by Baltensperger . In the early nineteen sixties, several network studies were undertaken in Japan  and Europe  and attempts were made to better-define the analogue modelling of the transient phenomena. The tests for TRVs were specified differently in the various national rules, and Subcommittee 17 on High-Voltage Switchgear of the International Electrotechnical Commission (IEC) asked CIGRE Study Committee on Circuit Breakers (named
Study Committee 3 in those days) to promote new extensive investigations on an international base. A working group, CIGRE working group 3.1, set up for this purpose in 1959, decided to start a complete investigation of TRVs associated with the opening of the first pole of a circuit breaker clearing a three-phase ungrounded fault in some large 245 kV networks. Two of these networks were fully investigated: the Italian network in its 1962 situation and the French network in its 1965 situation. Some 2000 TRV wave shapes associated with short-circuit currents up to 45 kA were collected. Based on this collection of TRVs, a classification of current ratings was proposed 
In publication 56-2 (1971) on high-voltage alternating circuit breakers, IEC recommends characteristic values for simulation of the TRV by the four-parameter method (, , , and ) or by the two-parameter method (and ). The values in the tables were mainly based on studies of the 245 kV systems. For the higher operating voltages up to 765 kV the values were extrapolated because actual data were hardly available at that time. In the meantime, studies were made of TRVs in systems operating at a maximum voltage of 420 kV and above. Another limiting aspect is that only three-phase ungrounded faults at breaker terminals were considered and that only TRVs associated with the opening of the first pole of the breaker were determined. The three-phase ungrounded terminal fault was the base for circuit breaker testing in many national rules as well as in the IEC standard and the evaluation of the TRV for the first clearing pole is less complicated than for other types of faults, such as the single-phase-to-ground fault. The TRV parameters were grouped for certain current values, being 10%, 30%, 60%, and 100% of the maximum short-circuit current rating for the specific voltage level of 245 kV. In the IEC standards, these current values are referred to as duties, and the corresponding TRV parameters are tabled for these duties.
In the light of this, CIGRE Study Committee 13 (Switching equipment) commissioned Working group 13.01 to study the problems of TRVs in extra-high-voltage systems . One of the conclusions was, for example, that the rate of rise of 1 kV/μs, taken as a basis for ungrounded three-phase terminal faults, appeared to be somewhat low and that the stress to be expected was better characterised by a rate of rise of 2 kV/μs and a first-pole-to-clear-factor of 1.3.
It was approved in IEC-SC-17A to revise the TRV tables of the IEC 56-2 and IEC 56-4. New values were proposed based on studies, mentioned above, made by CIGRE Working group 13.01 during the years 1976–1979. Because these studies dealt mainly with rated voltages of 300 kV and above, and questionnaires issued to the utilities gave no indication about serious failures of medium-voltage breakers, IEC-SC-17A decided in 1979 only on the standard TRV values for rated voltages of 100 kV and above. The TRV values for rated voltages below 100 kV remained unchanged. CIGRE SC 13 took the initiative at its meeting in Sydney in 1979 to set up a task force to collect data to establish TRV parameters for medium-voltage circuit breakers. The report of the task force was published in Electra No. 88 .
In June 1981, WG 13-05 was initiated by CIGRE-SC-13 to study the TRV conditions caused by clearing transformer and series reactor–limited faults. The Working Group collected data on transformer natural frequencies and ratings. The report of the working group was published in Electra No. 102 . A combined CIGRE/CIRED working group WG CC-03 undertook from 1994 until 1998 the work of investigating the TRVs in networks till 100 kV. A summary of the report of WG CC-03 is published in Electra No. 181 .
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