Three-Phase Technology Traction Rolling Stock PDF

4. POWER CONVERTER INTRODUCTION The three phase voltage required for operating the traction motors is generated on the vehicle by means of two traction converters connected between the vehicle‟s main transformer (single phase) and the traction motors. To control the tractiv...

4. POWER CONVERTER INTRODUCTION The three phase voltage required for operating the traction motors is generated on the vehicle by means of two traction converters connected between the vehicle‟s main transformer (single phase) and the traction motors. To control the tractive or braking effort, and hence the speed of the vehicle, both the frequency and the amplitude of the three-phase converter output voltage are continuously changed according to the demands from the driver‟s cab. This allows continuous adjustment of the driving or braking torque of the traction motors, which means that the driving speed changes smoothly. When braking electrically the traction motors act as generators. In the converter the resulting three-phase electrical energy is converted into single-phase energy, which is fed back into the line (regenerative brake). OVERVIEW: STRUCTURE AND COMPONENTS OF THE CONVERTER. (See Fig.4.1) Line converter A11, A12…………… Oil – cooled value sets with 2 pairs of arms each, 2xZV24(12) A111 – A114 A121 – A124………. Gate Units (Forced air – cooling) (227) K01…………………Charging contactor (12.3) K11…………………Converter contactor (12.4) R04…………………Charging resistor (14) U11-U14……………Current transducers (18.2) TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 43 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 44 DC –Link A31………….DC – link capacitor bank (15.5) A33………….Series resonant circuit capacitor bank (part of the converter block) (15.4) H08………….Voltage indicator (15.7) L02………….Series resonant choke (15.3) (outside of the converter block) Q21………….Earthing switch (15.82) R51, R52…….Earthing resistors (90.61/90.62) R71………….Over voltage limitation resistor (15.1) U01, U02……Voltage transducers (measurement of DC-link voltage) (15.6) U05………….Earth fault monitoring voltage transducer (89.4) Motor Converter A21…………Oil-cooled valve set with 2 pairs of arms ZV24(12) A22…………Oil-cooled valve set with 1 arm ZV24 for the motor converter and 1 arm MU23 for the MUB (13) A211- A214 A21-A222……Gate units for the motor converter (228) A223…………Gate units for the MUB-arm (229) U21-U23……..Current transducers (18.5) Additional converter apparatus A01………..Converter bus station with converter control unit, SLG and drive control unit, ALG (415) A04………..Primary voltage transformer module (224) A08………..Gate unit power supply GUSP (219) ABBREVIATIONS The abbreviations used in these operating instructions are explained below: ALG : Drive control unit ASR : Motor converter BUR : Auxiliary converter FLG : Vehicle control unit GU : Gate unit GUSET : Gate unit transmitter / receiver test unit GUSP : Gate unit power supply MUB : Over voltage protection circuitry NSR : Line converter SLG : Converter control unit SR : Converter Ud : DC-link voltage VS : Valve set TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 45 ZK : DC-link ZP : Pair of arms MAIN CONSTITUENT PARTS AND THEIR FUNCTIONS (See main circuit diagram Fig.4.1) The traction converter is largely a modular construction and consists of the following main functional groups:  Line converter  DC – Link  Over voltage limitation circuitry  Motor converter Line converter (NSR) (A11, A12, A111-A124) (See main circuit diagram Fig.4.1) Circuit & Function The line converter consists of two pulse – controlled single – phase full bridge circuits (A11, A12) which are connected to a transformer secondary winding (terminals 1U1, 1V1 resp. 2U1, 2V1). The line converter is a self-commutating 4-quadrant converter. The AC terminals of the two bridge circuits (A11, A12) see AC voltages that consist of square–wave pulses of identical amplitude (see fig.4.2). These pulses are produced by pulse– width –modulating the DC–link voltage. The fundamentals of these alternating voltages are at line frequency and form the counter-e.m.f to the two transformer secondary voltages. The converter input current is in quadrature with EL i.e., voltage across transformer reactor. Since Es is equal to Vector sum of EL & EC (see fig. 4.3), it is possible to ensure that IS is in phase with ES by changing amplitude and phase of EC It could be seen that fundamental component of EC is nothing but modulating wave itself. Its amplitudes and phase angle, referring to the transformer primary voltage, can be changed independently of each other. This allows the adjustment at cos = 1 in either driving or braking mode. The full-bridge circuit GTOs are switched at a frequency much greater than the line frequency. The switching signals for the four pairs of arms are shifted by 90 (quarter of a switching period) in relation to one another. This ensures that the AC current in the transformer primary winding is almost sinusoidal and that the harmonic currents in the line are kept down to a minimum (for the whole converter operating range). The line converter maintains the DC –link voltage at a value, which is dependent on the power, direction of energy flow and line voltage. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 46 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 47 Main constituents parts The two converter bridges A11-A12 each consist of two bipolar switches. Two GTO – thyristors arms and two diodes, connected in anti – parallel, realize each bipolar switch. These two pairs of arms together with their snubber circuit components are housed in a square aluminium tank and form a so-called valve set (A11 and A12). Such a valve set is oil – filled and cooled by forced oil circulation. The four gate units do not belong to the valve set. They are, however, placed in their immediate vicinity. The line converter input currents are measured by means of the current transducers U11, U12 resp. U13, U14. Their output signals are sent to the converter controller in the drive control unit (ALG). DC-link (L02-A33, Q21, R51-R52, U05, U01-U02, Ho8, A31, R71-A22) (See main circuit diagram Fig.4.1) Function The DC-link connects the line converter to the motor converter. Primarily, it serves to compensate both periodic and non-periodic power differences between the motor-side and line-side terminals of the traction converter. Such power differences occur on the one hand as relatively low frequency pulsations caused by the single-phase circuit of the line converter. On the other hand they may occur as irregular transient surges produced by sudden disturbances of the power equilibrium between the motor side and line-side of the converter, e.g. due to pantograph bounce, wheel spin etc. It is not possible to completely avoid these transient power differences but they can be minimized. It is a characteristic of the circuit that the power equilibrium after a disturbance cannot be instantaneously recovered, there is a certain delay. Absorption circuit (A33-LO2) The periodic pulsation in the DC-link occurs because the fundamentals power in a symmetrically loaded three-phase system (traction motor system) is constant, whereas the fundamental power in a single-phase system pulsates at double the line frequency. Referring to the DC-link currents of the converter, this means that the DC-link is fed from the line converter with a pulsating current at double the line frequency, whereas the motor converter draws almost pure DC-current from the DC-link. The A33-L02 series resonant circuit serves to filter out the current at double the line frequency. It must therefore be tuned to this frequency. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 48 DC-link capacitor (ZK) (A31) The DC-link capacitor is used to cater to non-periodic power differences between the motor and line sides of the converter. It also absorbs the harmonic currents produced by both the line converter and the motor converter (frequency higher than double the line frequency). The DC-link capacitance is rated in such a way that the DC-link voltage remains as constant as far as possible under all operating conditions and that there are no inadmissible fluctuations with regard to the operation of the motor converter. Over voltage limitation circuit MUB (R71-A22) If the capacitance of A31 is not sufficient to prevent the DC-link from sudden and inadmissible high transient overvoltages the R71 overvoltage limitation resistor is almost immediately connected across the DC-link by the firing of a GTO in the A22 valve set. The GTO is fired as soon as a certain overvoltage threshold is reached. After the overvoltage has decayed, the GTO is turned OFF again. Overvoltages in the DC-link may occur due to:  Wheel spin  Pantograph bounces  Detuned (defective) series resonant circuit (L02, A33) The MUB-circuit also serves to discharge the converter DC-link if the vehicle is put out of operation (powering-down). Valve of the MUB-resistor is 2.5 m. The temperature of the MUB-resistor is monitored in the control electronics by a thermal model. Furthermore, the DC-link also contains the following measuring, monitoring and protective functions:  DC-voltage measurement, transducer U01, U02 for converter control  Voltage indicator H08 and converter earthing switch Q21  Earth fault monitoring system (U05, R51 + R52) Motor converter (ASR) (A21, A22, A211-A222) Function The motor converter consists of a pulse-controlled three-phase bridge circuit (valve set A21, A22), which is connected to the DC-link. On the AC-side, it is connected to the three motor stator windings (connected in start). All 2 resp. 3 motors are connected in parallel. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 49  Each of the three pairs of arms of the three-phase bridge circuit (two of them in A21, one in A22) generates an AC-voltage from the DC-link voltage. This AC-voltage consists of square pulses of constant amplitude. Both the fundamental frequency and the amplitude of the alternating voltage can be changed continuously and independent of each other. These 3 voltages appear at the converter output terminals 1U2, 1V2, and 1W2. Their fundamentals are shifted against each other b by 1/3 period (120°) and from the phase voltages of the traction motor three-phase system. The torque and the speed of the motors are controlled by continuously changing both the frequency and the amplitude of the fundamentals of the pulse-shaped motor-phase voltages. In motoring mode (driving mode) the fundamental frequency of the motor terminal voltage is higher than the frequency corresponding to the motor speed (positive slip), resulting in a positive motor torque. During braking, the fundamental frequency of the motor terminal voltage will be lowered below the frequency corresponding to the motor speed, resulting in a negative slip and therefore producing a braking torque. The whole control range of the motor voltage is subdivided into three smaller ranges i.e., (indirect self-control), TB_DSR (direct self control), or “tolerance band control” as well as square-wave operation or “field-weakening”-DSR, characterized as follows: ISR (indirect self control) The ISR-range covers the range from standstill (motor voltage = 0) up to approx. 30% of the nominal voltage and type frequency (resp. type speed) of the motors. In this range, the relationship between the motor voltage amplitude and frequency remains roughly constant. This is achieved using the pulse-width-modulation method. The motors are constantly magnetized at nominal induction and can be loaded with the nominal torque over the whole ISR-range. The GTO-switching frequency is constant over the whole ISR-range. Therefore, the motor voltage per half-wave consists of a variable number of pulses having the same amplitude but differing widths. TB-DSR (tolerance band control) The TB-DSR covers the range from approx. 30% up to 98% of the nominal voltage and nominal frequency (resp. nominal speed) of the motors. Over this range the motors are fully magnetized and therefore they can be loaded with the full torque (see Fig.4.4). TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 50  Fig. 4.4 The torque, however, is no longer controlled to an average value but to a set value. The difference between actual torque and set value must always lie within a preset tolerance band whose width is determined by the admissible GTO-switching frequency. Thus, the motor converter is always operated with the largest possible switching frequency, resulting in the smallest possible torque pulsations. “Field weakening”-DSR (square-wave operation) The “field weakening” range is the region between the nominal speed (nominal voltage) and the maximum speed of the traction motors. Over the whole field weakening range the amplitude of the motor voltage is kept at its maximum value. Only the frequency is changed. This has the effect that the motor flux and pullout torque are inversely proportional to the frequency. The line-line motor voltage in square-wave operation is formed by one voltage pulse per half-wave. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 51 TRACTION CONVERTER CONTROL BY THE CONTROL ELECTRONICS (See main circuit diagram Fig. 4.1) The traction converters serve to provide continuous and utmost automatic control of both speed and torque of the three-phase induction motors. The demands from the driver‟s cab, such as driving or braking, and the relevant driving speed are converted into the voltage, current and frequency values required on the traction motor side by means of the control electronics and the converter power electronics. The traction converters of a vehicle are exclusively controlled by the central vehicle control unit (FLG) and the individual converter control units (SLG) as well as the relevant drive control units (ALG). The converter has no hand-operated component except the earthing switch Q21. Basic structure of the control electronics The control electronics is fully based on microprocessors, connected to each other via a data bus system (MICAS vehicle bus). Each vehicle contains a vehicle control unit (FLG). Each converter is controlled by a converter bus station. The A01-converter bus station contains both the converter control unit (SLG) and the drive control unit (ALG), which is controlled by the former. Furthermore, the ALG is also equipped with controllers, one each for the line converter and the motor converter. Motor converter (ASR) control Depending on the demands made in the driver‟s cab and the instantaneous speed of the vehicle, the FLG calculates the required tractive or braking effort. The demanded torque is sent to converter bus station via the vehicle data bus. The SLG compares the demanded torque from the FLG with the effective load torque calculated in the ALG. The demanded torque for the ALG is determined from the difference between these two values. The ALG determines the required firing and turn-off pulses for the GTOs on the basis of the demanded torque. The pulses are sent to the gate-units (A211-A222) via fiber optics. The outputs of the gate-units are at high potential. The actual firing and turn-off pulses for the GTO-thyristors are only generated in the Gate Units (GU). The Gate Unit Power Supply A08 (GUSP) provides the Gate Units with the required energy. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 52 Line converter (NSR) control The line converter is also controlled by the ALG. The ALG controls the line converter in order to maintain a constant DC-link voltage. This DC-link voltage is reduced, if the needed power is below the rated value. Hence, the loading of the line converter has to correspond to the loading of the motor converter. Therefore the flow of active power on both sides of the traction converter is of the same value and direction (driving, braking). MONITORING OF GTO – Thyristers The GTO-thyristors are not only controlled by the drive control unit (SLG, ALG) and Gate Units but are also monitored (GTO feedback signals). This ensure for example that a GTO is only fired if the necessary conditions are fulfilled, and that the converter is immediately shut down (or not powered-up) when a GTO fails. Additional functions performed by the vehicle control unit, converter control unit and drive control unit (FLG, SLG, ALG) Apart from the control functions described above, the vehicle control unit and the converter control units fulfill numerous additional functions. The following functions are important for the converter:  Automatic powering-up and powering-down of both the vehicle and the converter according to the selections from the driver‟s cab.  Monitoring of various variables (limit values). TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 53 AUTOMATIC SYSTEM TESTS The converter is regularly and automatically tested by the control electronics. This takes place when the vehicle is first powered-up and whenever the converter is powered-up again, e.g. after a protective shut-down during driving mode. The functional test consists of two parts, the OFF-LINE TEST and the ON-LINE TEST. The OFF-LINE test is performed before any turn-ON (the converter being de- energies). The ON-LINE test, however, is carried out during the charging of the DC- link as well as during the operation. OFF-LINE TEST The OFF-LINE TEST tests the following functions (amongst others):  The current transducers (U11-U23) by means of simulated actual value signals (test windings).  The comparators for the various protection thresholds.  The power supplies for the gate units and the control electronics. ON LINE TEST The ON-LINE TEST tests the following functions (amongst others):  That the K01 charging contactor closes.  That the DC-link voltage is reached within a predetermined time.  That the K11 main contactor closes.  That the actual value signals of the voltage transducers (U01, U02) are correct (plausibility test).  That the MUB is ready for operation. During operation, different states and values, which allow the release of the GTO firing pulses, are continuously (ON-LINE) monitored. Amongst others they are  The position of the vehicle main circuit breaker, the K01 charging contactor and the K11 main contactor.  The pressure and the temperature in the oil cooling system.  The converter DC-link voltage.  The current and voltage transducers (plausibility test).  The gate unit supply voltages. The ON-LINE test is only initiated when the OFF-LINE test is successfully complete. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 54 CONVERTER TURN-ON/OFF (So called powering-up and powering-down) Powering-up (See main circuit diagram Fig.4.1) When powering-up the vehicle, the converter is powered-up after the OFF-LINE test. For this purpose the K01 charging contactor is closed, so that the converter DC-link is precharged through the R04 charging resistor and the diodes in the A11 valve set. Immediately afterwards the K11 main contactor is closed. Subsequently, the GTO-thyristors‟ control pulses for the line converter are released and approx. 200 ms later those for the ASR (motor converter). The conditions for control pulse release are:  Closed vehicle main circuit breaker.  Converter powered-up (i.e., OFF-LINE and ON-LINE test OK).  Gate unit supply (GUSP) OK.  Driving direction switch (reversing switch) set to “forward” or “reverse”.  DC-link charged. Powering-down The converter is powered down if:  The vehicle is put out of operation.  The converter protection (steps 4 or 5) has reacted.  The line voltage is too low for a considerable time. When powering-down  The demanded torque is reduced to zero.  The firing pulses of both the line and motor converter are inhibited.  Main circuit breaker is opened.  The K11 main contactor is opened, and  The MUB is turned ON (DC-link is discharged). CONVERTER PROTECTION (See main circuit diagram Fig.4.1) Various conditions during converter operation (wheel spin, pantograph bounces, overload) as well as errors in the control electronics may endanger important components of the converter, especially the power GTOs. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 55  In order to prevent this and to affect the converter normal operation as little as possible a five-stage protection concept is used, whose individual stages are described below. Protection stages Protection stage 1: Monitoring of the minimum switching times of the GTO- thyristors and mutual interlocking of the gate units of a GTO-thyristor pair of arms: To ensure the safe function of the converter, when it is alternately switching the plus or minus pole of the converter DC-link to the relevant converter terminals (alternate firing and turning OFF of the GTO-thyristors in a ZP), it is absolutely necessary that minimum turn ON and OFF times are observed including the minimum change-over time of the GTO- thyristors. These times are monitored by the control electronics. A further step to avoid shorting the DC-link is to interlock the firing signals for the two GTO-thyristors of a pair of arms. A firing command is only released if the neighbouring GTO has safely turned off. Triggering: Protection stage 1 responds as soon as the converter reaches its control limits. Effect on the operation: Normal operation is maintained. The diagnostic system will indicate a fault. Protection stage 2: Power and current set value limitation: In order to prevent the converter from thermal overloading, it is important that certain current limits on both the line side and motor side are not exceeded. For this reason, the current and torque set values required by the converter control circuits are limited. Triggering: Protection stage 2 responds either if the overhead line has under voltage or if the motor converter is operated with motor fundamental frequencies that are below 1.1 Hz. Effects on the operation : TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 56  Reduction of the motor torque. No fault is indicated. Protection stage 3: Instantaneous voltage limitation MUB : Normal operation is transiently affected but maintained. No fault is indicated. Protection stage 4: Full reduction of load : (Controlled reduction of the motor torque to zero) To reduce the power flow in the converter as quickly as possible, but without disturbing the power equilibrium between line and motor converter (otherwise there is a danger of overvoltages in the DC-link), the torque of the driving motors is quickly and continuously reduced to zero (ramp function). Subsequently, all the GTO-thyristors of both the line and motor converters are turned off. Triggering: Protection stage 4 responds if:  The plausibility test of the current and voltage transducers indicates a fault (either thermal drifts or defective transducer).  The MUB – resistor overheats.  Either the cooling medium temperature is too high or the inlet pressure is too low.  The converter control circuits attain an abnormal state (e.g. a GTO-thyristor can not be fired). Effects on the operation: The converter is turned off and turned on again, provided it has passed the off-line test. If the test, however, is not successful, the converter will automatically be powered down, which means that it is completely shut down. The DC-link will be discharged. Protection stage 5: Immediate converter shut-down: (Opening the vehicle main circuit breaker and firing the MUB) The converter is fully shut down without delay. For this purpose the MUBs are fired by means of a continuous pulse, all the GTO-thyristors of both line and motor converter are turned off and the vehicle main circuit breaker is opened. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 57  Triggering: Protection stage 5 responds, if:  The maximum admissible DC-link voltage is exceeded despite activation of the MUB (protection stage 3).  The MUB (protection stage 3) responds several times in a row.  The maximum admissible current is exceeded at one of the power terminals of the converter.  The gate units of a pair of arms signal either an inadmissible switching state or a defective GTO-thyristor.  The DC-link voltage inexplicably drops below its minimum admissible value.  Either the gate unit power supply or the supply for the control electronics fails Effect on the operation After vehicle shut-down the converter undergoes an OFF-LINE test. It the test is not successful, it remain shut shown. TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 58 Behavior in case of line under voltage during operation If during operation the line voltage drops below the minimum allowable value. The motor converter and then immediately afterwards, the line converter are turned off (or vice versa depending if the converter is in motoring or braking mode). In the blocked state, the DC-link voltage gradually drops. If the line voltage returns inside the tolerance range before the DC- link voltage has decreased too much, the firing pulses are again released. If however the line voltage remain below the minimum value for more than approximate10 seconds, the converter will be powered down. Once the line voltage has recovered, the normal powering- up process will be initiated. Fig. 4.5 TRACTION ROLLING STOCK : THREE PHASE TECHNOLOGY Page 59