TRACTION BRAKING RESISTORS
Braking a Railway vehicle is mainly done by electric resistors. Mechanical brakes wear out too quickly and require expensive maintenance. During braking, the kinetic energy of the train is transformed into electric energy and if possible, fed back to the overhead line but this can only be done if another vehicle is connected to the same overhead line and can use the energy. If this is not possible, then the energy must be dissipated as heat in onboard resistors.
EXAMPLE OF IN-FRAME BRAKING RESISTORS
There are multiple railway vehicles application, each with its own demands, legislation and directives. Although in the real meaning of the word trolley and (hybrid) electrical busses are not railway application, the same type of resistors can be used here too. Each type of transport, whether it is tram, metro, light rail or (high speed) train, have unique demands on the type of resistor. Ambient conditions such as air temperature, humidity, mechanical forces, national and international directives, play an important role in the selection of the materials to use and the construction of the resistor.
CHARGING AND (DE)CHARGING
Starting up electronic systems would in general cause high im-pulse loads that could damage electronic components. To prevent this, charging resistors are used to safely but fast charge the system. For safety, de-charging electrical circuits is done with the same principle but normally over a longer time. In stationary sys-tems (substations), resistors are used to check the overhead-lines before powering them. This to prevent damage to the High Speed Circuit Breaker in case of any faults.
A DC capacitor bank must be charged in a controlled manner to prevent damage to the rectifier bridge. High inrush currents could destroy components such as diodes or thyristors. Placing a resistor in the charging circuit limits this current and protects the semiconductors, avoids line-voltage sag in the case of a weak grid and prevents the capacitor voltage from over-shooting. The dangerous high voltage due to the energy stored in a capacitor bank needs to be discharged in a controlled way. Safety regulations require that dangerously high voltages must be below a safe voltage level within a certain time (normally 1 or 2 minutes). A discharge resistor switched across the capacitor bank after the system is turned off, is a reliable and cost-effective way of ensuring this.
The difference between charging and de-charging is mainly the duration time the process takes. Normally, charging a capacitor banks is done within one second up to a few seconds. De-charging is allowed to take a longer time, however, normally it is lim-ited to 2 minutes. The longer de-charging time requires high ohm resistors. If the resistance values are too high to make the resistor with steel grid plates, ceramic wirewound resistors are used. The resistor parts, whether they are steel grid types or ceramic wirewound types, are fitted into a box. Creepage and clearance distances are observed and at the same time minimum distances from the active (hot) elements not to heat up any neighboring components.
DAMPING AND FILTER RESISTORS
Harmonic Filter Resistors are used to attenuate harmonic frequencies that would otherwise perturb the power grid. In any application where power semiconductor switches are used, such as in propulsion drive inverters and onboard auxiliary pow-er supply inverters, harmonic voltages and currents are generated causing additional losses and interference with other elec-trical circuits. Harmonic Filter Resistors are used in combination with capacitors and inductors. The filter passes the funda-mental frequency through and dissipates the harmonic currents into heat. The power dissipation depends on the sum of all attenuated harmonics.
The difference between damping resistor and filter resistors is that damping resistors limit any current and voltage peaks where filter resistors take out higher harmonics that disturb the voltage shape. Damping resistors prevent any damage on other (electronic) components where filter resistors improve the quality of the power grid.
CROWBAR AND LINE-TEST RESISTORS
Crowbar resistors are used to suppress transient or short-duration voltage peaks and can also be used to control longer lasting over-voltages. Two different types of crow bar resistors can be defined; a soft crowbar that handles short duration voltage peaks and a hard crowbar that will be switched on to eliminate the longer lasting over-voltages. If the hard crowbar is switched on it will short circuit the whole inverter and by that protecting it against high voltages. Other, necessary installed protection devices like fuses will safely interrupt the power to the inverter.