FR2486726A1 - HT equaliser for electric vehicle supply line - has thyristor connected to extinction inductor and capacitor forming time constant for trigger pulse controlled by over-voltage detector - Google Patents

Abstract

The equaliser absorbs over voltages across the terminals of a HT filter with a series inductance (L) and a parallel capacitance (C). A thyristor (Th) is connected in parallel with the latter together with a series resistor (r), with a series capacitor (c) and inductor (e) across the thyristor (Th). The thyristor firing control (14) causes successive firing of the thyristor (Th) after a period equal to the time constant of the oscillator formed by the capacitor (c) and the inductor (e). The thyristor (Th) is first fired when a detector (13) detects an overvoltage across the filter capacitance (c) and firing ends when a predetermined balance voltage is detected. The device absorbs over voltages caused when other vehicles are switched off etc.

Description

Surge absorber at the terminals of a high voltage filter
The present invention relates to a surge absorber surge arrester at the terminals of a high voltage filter, in particular a thyristor clipper for equalizing the high voltage coming from an electric vehicle supply line.

 It is known that overvoltages due to external causes such as pantograph detachments, shutdown of the engines of other railway vehicles can occur at the terminals of the capacity of a low-pass filter with inductance in series and capacity in parallel arranged between the high-voltage DC power source and energy conversion circuits such as inverters installed on board railway vehicles. Hitherto, a resistor followed by a thyristor arranged across the capacitance of the filter has been used for clipping overvoltages. The thyristor is triggered only once when the voltage across the filter capacitance exceeds a certain level, which causes a short circuit of the voltage across the resistor. However, this clipping circuit has the disadvantage of stopping the operation of the assembly, the equilibrium voltage at the output of the clipper being zero.

 The clipper according to the present invention overcomes this drawback.

This, in fact, allows very good absorption of overvoltages while ensuring during this absorption the uninterrupted operation of the assembly thus supplied. In addition, the clipper uses only a reduced number of components operating intermittently.

 The subject of the present invention is a clipping device absorbing overvoltages at the terminals of a high voltage filter comprising an inductance in series and a capacitance in parallel, clipping device comprising in parallel on the capacitance of the filter a thyristor followed by a resistor characterized in that an extinguishing inductor and an extinguishing capacity are arranged at the terminals of said thyristor and that an electronic device for controlling the ignition of the trigger of the thyristor causes successive ignitions of said trigger after a period substantially equal to the time constant of the oscillating circuit formed by the extinction capacity and inductance, the first ignition being effected by the detection by means of a sensor of an overvoltage at the terminals of the filter capacity and the stopping of the ignitions being performed when said sensor detects a predetermined equilibrium voltage.

 With reference to FIGS. 1 to 3, attached, an example of implementation of the present invention will be described below, an example given purely by way of illustration and in no way limiting.

 Figure 1 shows an electrical block diagram of the line supply voltage clipping.

FIG. 2 represents simultaneous diagrams of voltage and current as a function of time t in which
- Figure 2A shows the ignition pulses of the thyristor trigger
- Figure 2B represents the intensity Ithy of the current passing
thy by thyristor
- Figure 2C shows the voltage Vthy across the thyristor
- Figure 2D represents the tensi Vr across the resistor r
- Figure 2E represents the intensity of the current Ic passing through the extinction capacity c
FIG. 3 represents the voltage U obtained at the output of the clipping as a function of time t.

 We see in Figure 1 an electric line 10 symbolized by an inductor 11 and whose voltage E can be 1800 volts.

A pantograph 12 takes this voltage for use on board the electric vehicle such as a locomotive, tram or trolley bus.

A low-pass filter consisting of an inductor L in series and a capacitor C in parallel has the role of eliminating the frequencies higher than the cut-off frequency which can come either from the line or from the converter on board the vehicle and risking jamming the low frequency information used for signaling railways. Inductance L can be 8 millihenrys and capacitance C 12 millifarads. The clipper consists of a thyristor Th, anode on the side of the positive pole of the line, in series with a resistor r whose end opposite to the cathode of the thyristor Th is connected to ground.

 The clipper also includes, across the thyristor Th, an extinguishing inductor 1 and an extinguishing capacity c. The output voltage of the clip is U.

 A voltage measurement sensor 13 is arranged across the capacitance C of the filter. The sensor 13 measures a voltage proportional to the voltage across the capacitance C. A control electronics 14 receives the proportional voltage from the sensor 13 and has the role of triggering the trigger of the thyristor Th through a pulse transformer 15 When there are overvoltages on line 10 the sensor 13 measures a voltage which when it exceeds a threshold corresponding, for example, to 2200 volts, causes the triggering pulses of the thyristor Th to trigger at a period T substantially equal to the time constant of the extinction circuit lc.

These ignitions at period T are carried out until the sensor 13 measures an equilibrium voltage UO which can be 2000 volts.

 Figures 2 illustrate the operation of the clipper.

In Figure 2A we see the pulses sent to the trigger of the thyristor Th and spaced from the period T. Times referenced 1, 2, 3, 4, 5 represent
1. the rest time before the presence of the overvoltage
2. the time between the ignition of thyristor Th and the cancellation
of the intensity Ic (curve 2E) crossing the capacitor
c and discharging into inductance 1.

3. The time between the cancellation of 1c (curve 2E) and the start
of the cancellation of Ithy (curve 2B) current intensity
thy
crossing the TH thyristor
4. The time between the start of the cancellation of Ithy (curve 2 B)
and the cancellation of Vthy (curve 2C), terminal voltage
thy
from thyristor Th.

5. The positive Vthy time (curve 2C) between a cancellation
by voltage inversion and cancellation by ignition
thyristor Th
During time 1 the extinguishing capacity c is positively charged on the extinguishing inductance side 1. There are across the thyristor
Th a voltage equal to E, the thyristor Th not being conductive.

 During time 2, following the ignition of the thyristor Th, it becomes conductive and the extinguishing capacity c is traversed by a current Ic (curve 2E) in the capacity c direction towards inductance 1 to close on the capacitance c through the thyristor Th, passing.

The current Ic, by convention, is negative and vanishes at the end of time 2 since the current 10 is an oscillation current created in the loop constituted by the thyristor Th and the oscillating circuit lo.

 However the current Ithy (curve 2 B) crossing the thyristor Th is more important because it is added to the current 10 a current 1r crossing the resistance r and coming from the line 10 through the thyristor Th.

 The voltage Vthy (curve 2C) at the terminals of the thyristor Th is zero because it is conductive.

The voltage Vr (curve 2D), across the resistor r, increases given that the current 1r crosses the resistor r. During time 3 the current 10 (curve 2E) changes direction becomes positive because in the oscillating circuit lc, the inductance l flows through the capacitor c. However Ithy which is equal to I + I remains positive before
thy rc to cancel at the end of time 3 when 1r is equal to Ic (curve 2E).

The thyristor Th is therefore conductive over time 3 and the voltage Vthy (curve 2c) remains zero.

During time 4 the current Ithy (curve 2B) is zero because the thyristor Th is blocked. Current I comes from line 10
c crosses inductance l, capacitance c, resistance r and closes on the ground.

The current Vr (2D curve) passes through a maximum greater than
E. The voltage Vthy is negative.

thy
During time 5 the voltage Vthy is positive, the thyristor Th always being blocked (Ithy, curve 2B, being equal to 0). Voltage V
r (2D curve) across the resistor r decreases and becomes less than E.

The current 10 decreasing towards 0 the voltage Vthy equal to E - Vr tends towards E since V = rI
vs
Time 4 during which Vthy is less than 0 is called
thy thyristor blocking time. This blocking time can be less than 75 microseconees for an operating frequency (determining the period T) of 1.8 kilohertz. In that case
r = 1.7 ohms, c = 88 microfarads 1 = 72 microhenrys.

 As an indication, the weights with a capacity of 88 microfarads with the characteristics required for this assembly is approximately 35 kilograms.

In Figure 2 D we see that V has a lower maximum
r in the following period after the second ignition of thyristor Th.

 In FIG. 3, it can be seen that the voltage U tends towards an equilibrium voltage Uc, for example 2000 volts, following several successive ignitions during the period T.

 The clipper according to the present invention achieves very good absorption of overvoltages. The number of active components is reduced to a single thyristor. The components r, 1, c, Th only work for the duration of the clipping. The logic of the control electronics, known per se, is particularly simple.

 This invention is in the field of railway electrical engineering.

Claims (1)

CLAIM  Surge absorber at the terminals of a high voltage filter comprising an inductance L in series and a capacitance C in parallel, clipper comprising in parallel on the capacitance C of the filter a thyristor Th followed by a resistor r characterized in that a extinction inductance l and an extinction capacitance c are arranged at the terminals of said thyristor Th and that an electronic control (14) of the ignition of the thyristor trigger causes successive ignitions of said trigger at the end of a period substantially equal to the time constant of the oscillating circuit formed by the capacitance c and the extinction inductance l, the first ignition being effected by the detection by means of a sensor (13) of an overvoltage at the terminals of the capacity C of the filter and the ignition stops being effected when said sensor (13) detects a predetermined equilibrium voltage.