SHORT CIRCUITS IN 3kV DC SYSTEM: STRESSES ON VEHICLES AND INFRASTRUCTURE ELECTRIC EQUIPMENT

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1 SHORT CIRCUITS IN 3kV DC SYSTEM: STRESSES ON VEHICLES AND INFRASTRUCTURE ELECTRIC EQUIPMENT 1 Gabriele Alessandro ANTONACCI, 2 Paolo MEIATTINI 1 Trenitalia S.p.A., Unità Tecnologie Materiale Rotabile, V.le S. Lavagnini, 58 - Firenze Italy 2 Rete Ferroviaria Italiana, Direzione Compartimentale Infrastruttura, V. Salvagnoli, 1 Firenze Italy 1. GENERAL The separation between line and rolling stock management, European interoperability specifications and technological development need in general an improving in electrical systems simulation capability and testing procedures, to make correct evaluations of stresses on vehicles and infrastructure equipment. In particular short circuit is a typical system phenomenon, in which various elements have to be taken into consideration: for example vehicle position and configuration, feeding system parameters, components specifications. Various stresses are caused by a short circuit: a concise list is proposed below. - Energetic and thermal stresses: the energies associated with the overcurrents give rise to heating phenomena in the conductors and in various components. - Dielectric stresses: the opening of the circuit breakers involves voltage fluctuations that are both concomitant with (arc voltages), and subsequent to (restoring voltages) the operability of the switches themselves. - Overpressures in the high voltage cabins: the opening of air high-speed circuit breakers may cause a pressure wave, the effects of which on the HV cabin structure must be verified. - Electromagnetic stresses: peak currents create electrodynamic stresses in the conductors and insulators. - EMC: voltage and current fluctuations with high values of the relative derivative exalt the coupling phenomena between the traction system and other systems, with possible disturbances in the control and drive circuits. - Step and contact voltages: the earthing system must be sized according to the fault currents; the step and contact voltages associated with these must remain below the established limits, in terms of both value and duration. 2. MEASUREMENTS Various testing results related to real short circuits in substations and rolling stock are described. The first part will be an analysis about co-ordination between substation and rolling stock circuit breakers, taking into account energetic and dielectric parameters. For the relative testing, three 5.4 MW units available in the E lectric Sub-Station were used, with the system disconnected from the contact lines of the trains. Train pantograph was connected downstream from a n ESS high-speed circuit breaker ; a contactor was assembled onboard the train designed to establish the short circuit, as well as a LEM for measuring the current. Wcrr pag. 1

All the measuring channels were interfaced with digital instrumentation by optical fibre channels and a wide

2 It must be pointed out that the test reproduced the conditions existing during operation, with the two switches of the ESS and the train opening simultaneously. All the measuring channels were interfaced with digital instrumentation by optical fibre channels and a wide interconnecting band. The test called for a specific safety standard with special attention paid to the test system and the measuring instruments. Figure 1.1 Figure 1.2 Wcrr pag. 2

3 Figures 1.1 and 1.2 illustrate the following diagrams pertaining to the short circuit test conducted: - current of the short circuit section; - voltage upstream from the DC circuit breaker of the train; - voltage upstream from the ESS high-speed circuit breaker; - power coming into the short circuit section; - Joule i 2 t integral; - energy coming into the short circuit section. Several elements were collected during this test aimed at determining the energy distribution between the two DC circuit breakers in series. In fig. 1.3 the voltages and energies relating to the two high-speed circuit breakers are compared. This diagram indicates how the circuit breaker on the train bears the greatest stresses from a thermal point of view, even though it must be pointed out that the two breakers were of different types, with a shorter mechanical reaction and calibration time in the train circuit breaker than in the substation one. Figure 1.3 From a dielectric point of view (and therefore, relating to the stresses concomitant with the opening) the greatest stresses seem to be borne by the ESS. In particular, a high overvoltage can be noted during the restoring phase on the ESS bus-bars: this stress is applied to the positive pole of the ESS circuit breaker and the filter capacitors, and to the negative pole of the filter inductances. Also to be noted in the field of dielectric phenomena a sudden overvoltage in the conductor section between the two high-speed circuit breakers at about 60 ms from the moment of beginning. This phenomenon could be attributed to a conducting section that has remained charged during the overvoltage wave and discharges at the moment in which the power difference between this and the connecting circuit section reaches a sufficient value. Wcrr pag. 3

4 The second part describes a long short circuit investigative test on 3 kvdc system opened by 130 kvac circuit breakers. The tests were conducted as surveys on an experimental spark extinguishing device as an alternative to asbestos, designed for an old type of high-speed circuit breaker, in order to verify its use limits. In the tests in question two of the three 5.4 MW units available in the ESS were used, and the system was disconnected from the contact lines of the trains. The high-speed circuit breaker being tested was connected to the 3kVdc output bus -bar of the conversion units. Downstream from this circuit breaker the circuit continued with a contactor designed to establish the short circuit, a rheostat for conducting preliminary tests with reduced currents and a LEM for measuring the current. Subsequently the circuit was connected to the negative bus-bar of the ESS. All the measuring channels were interfaced with digital instrumentation with optical fibre channels and a wide interconnecting band. Moreover, a camera connected to a video-recorder and monitor was also positioned in the area near the test circuit breaker. The diagrams in figures 2.4 and 2.5 and the photo in fig. 2.6 (obtained from the videorecordings made) relate to the final test on the spark extinguishing device concerned without any limiting resistances, which caused it to yield. Figure 2.4 Wcrr pag. 4

5 Figure 2.5 Figure 2.6 Wcrr pag. 5

Parameters deriv ing from these diagrams are connected to the opening of a short circuit made by 130 kvac protection devices: however it is necessary to keep in mind the fact that the connected units

6 Parameters deriv ing from these diagrams are connected to the opening of a short circuit made by 130 kvac protection devices: however it is necessary to keep in mind the fact that the connected units were only two. Moreover the fault presents a certain impedance limiting the energy involved. 3. SIMULATION A simulation code able to make a simulation of 3 kvdc electric feeding system during short circuits has been developed. Input variables are system configuration, substation parameters, geometric line conditions, circuit breakers or fuse parameters, and overvoltages arrester characteristics. Output variables are system voltages and currents and their derivatives and energetic parameters related to phenomena. This elaboration model has a good correspondence with measurement results. In fig. 1.7 a parameter evaluation near substation is presented. Icc1 is short circuit prospective current at 3 x 5.4 MW 3 kv dc substation terminals inside a traction vehicle; Icc2 is short circuit prospective current at 3 x 5.4 MW 3 kv dc substation terminals at the end of train line. In fig. 2.8, as model validation, a comparison between line-voltage waveforms (real measurement and simulation result) during a short circuit opened by a circuit breaker is proposed. Figure 1.7 Wcrr pag. 6

7 Figure EMC The evolution in the field of electrical and electronic equipment has brought to light a series of questions related to electrical and electronic interaction between the various apparatuses. During a short circuit measurements suggest condition of big variation of currents and voltages during phenomena. As an example in fig. 3.9 an evaluation - simulation, but in good accordance with measurements of di/dt during a short circuit near substation is presented. Inductive coupling may generate disturbances on near low-voltage circuits, not only on board but on track side too. Figure 3.9 Wcrr pag. 7

8 5. CONCLUSIONS Due to severity of the extreme conditions involving short circuits, these are a critical point when it comes to dealing with railways safety and interoperability, for the aspects related to defining energetic and electrodynamics stresses, and those concerning electromagnetic compatibility. Measurements and theoretic analysis lead to the conclusion that even in a direct current system the short circuit is a phenomenon with elevated dynamic properties, on account of current and voltage fluctuations deriving from both the control devices and the reactive elements present in the system. Experimental and simulation results are now available to describe d.c. short circuit and its effects. Wcrr pag. 8

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