Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
The main feature of the voltage source inverter supply is that the inverter supplying DC voltage is nearly constant, relatively high capacitance capacitor energy storage is built in, to filter the transient load change. To achieve the field oriented control, the switching elements of the inverter constrain voltage to the motor terminals with pulse width modulation control. The higher the switching frequency of the pulse width modulation, the faster and more punctual the achievable field oriented current vector control.
The circuit diagram of the two level voltage source inverter can be seen in Fig.5.6. This is the most commonly used, well-known circuit for supplying three-phase induction motors.
Generally the T1…T6 switching elements are IGBT voltage controlled transistors, as in the previous figure, but at high power vehicles the GTO gate-turn-off thyristors are commonly used in voltage source inverters. Thee phase control is applied for the voltage source inverter, each motor phase terminal is connected to the positive or negative bar. If three-phase control is used the number of the switching states, achievable by the pulse width modulation, is k=8, the number of the different voltage vectors (ū=(2/3)(ua+āub+ā2uc)) can be switched to the motors is seven as can be seen in Fig.5.6.c. The ū(7)=0 state is identical with the ū(8)=0 state, at ū(7) all the three phase terminals are connected to the positive bar while at ū(8) all the three phase terminals are connected to the negative bar.
Fig.5.6.b presents a simplified diagram, can be frequently found in circuit diagrams of rail vehicles, where each “box” contains one branch of the two level voltage source inverter. Each box has three terminals (+, - and ~), and contains one branch (in a dashed box in Fig.5.6.a).
The circuit diagram of the three level voltage source inverter with GTOs can be seen in Fig.5.7. It is commonly used at high power vehicles.
At three level inverter, the number of the switching states is k=27, but the number of the different voltage vectors can be switched to the motors is only 19, including the 0 vector, as can be seen on Fig.5.7.c. The magnitude of the maximal voltage vector is (2/3)ue. The larger number of the switchable voltage vectors involve that smoother voltage control can be achieved by the three level inverter even if the allowed switching frequency for the high power semiconductor elements is more limited.
The simplified diagrams are also applied for three level inverter circuits that can be found in Fig.5.7.b, where each “box” contains one branch of the three level voltage source inverter. Each box has four terminals (+, -, 0 and ~), and contains one branch (in a dashed box in Fig.5.7.a).
There can be several practical solutions of field oriented controlled, induction motor driven vehicle drive. Fig.5.8. represents one possible solution with space-vector pulse width modulation controlled voltage source inverter and general machine model. The figure presents a simplified block diagram of a vehicle drive, suitable for speed control.
The speed control shall always be complemented with torque limitation to achieve favorable acceleration and deceleration features for passengers. The field oriented control is divided to two main channels: the flux control (lower channel, related to α component), and torque control (upper channel, related to β component). The reference signal of the rotor flux magnitude (ψrα) is determined depending on the speed range, according to the field-weakening strategies, that were mentioned in chapter 5.1.1. There are vehicles, where the speed control can be switched to direct torque control, i.e. the m a reference signal can be set directly. The trolley and electric car are such vehicles, where the direct torque setting imitates the function of the accelerator pedal.
In the followings some specific vehicle controls are presented.
Fig.5.9. presents the main circuit diagram of the induction motor drive. The motor is supplied by a two level voltage source inverter. For the control and calculating the rotor flux an encoder – mounted on the motor shaft - is required.
The voltage source inverter is connected to the overhead line with a pole through a charging-circuit and a network protecting circuit. The charging-circuit limits the switching-on transient current of C smoothing capacitor until it reaches the normal charge state. The network protecting circuit is a diode bridge rectifier circuit, its two diodes are by-passed with two IGBT elements. The diode bridge protects the main circuit against reversed polarity, which can be occurred at intersections for a short time. However the diode bridge does not allow the possibility of the regenerative braking. The two IGBT elements allow the regenerative brake at normal overhead line polarity with reverse current.
The voltage source inverter fed trolley-bus drive has field oriented control, suitable for the motor mode and braking mode control. The resistive brake only operates if the network is not suitable to consume the regenerative energy.
The trolley operates with torque control, the torque reference signal is defined by the accelerator pedal position.
Fig.5.10 presents the schematic circuit diagram of two motors that belong to one bogie, and a picture of a motor bogie. According to the picture two motors in a motor bogie drives two wheels one behind the other, because of the low-floor design. Two motors are connected parallel to one inverter. The structure of the main circuit is similar to the circuit of the trolley, can be seen in Fig.5.9, only the network protecting circuit is missing. The reversed polarity of the supplying voltage cannot happen in trams.
Nowadays, most of the new locomotives are energy-efficient and network-friendly, that manifests itself in three ways:
capable for regenerative electric braking,
connected to the network with network-friendly line-side converter,
they have energy-efficient motor torque control.
The Siemens 1047 (Taurus) is a good example for an energy-efficient, network-friendly, dual-voltage locomotive connected to AC voltage. Fig.5.11 presents the main circuit diagram of the electric locomotive drive. The figure shows the drive system of one bogie. The 6400kW, dual-voltage locomotive has secondary number of turn switch, can be switched to 15kV 16 2/3Hz or 25kV 50Hz supplying system and it is connected to the network with a 4qS converter. The role of the 4qS network-friendly converter is detailed in Chapter 3.3.5, the circuit diagram with IGBT switching elements can be found in Fig.3.4. On the other hand the 4qS converters and the motor-side inverters of the locomotive, presented in Fig.11, are implemented by GTO turn-off thyristors. Shifting the PWM control of the three parallel 4qS converters can be applied for the reduction of the network current harmonics. The network current phase angle can be set, its optimal value is accessible (cosφ=±1).
There are two ways for tuning the filter smoothing the DC-link voltage, to 33Hz or 100Hz, depending on the frequency of the overhead line voltage (16 2/3Hz or 50Hz). The single-phase supply is the origin of the double frequency pulsating input power that shall be filtered.
The E186D/A/PL type Bombardier locomotive is an example for a quad-voltage locomotive. Fig.5.12 presents the circuit applied at AC voltage overhead line. The circuit of the locomotive is the same at 15kV, 16 2/3Hz and 25kV, 50Hz overhead line voltage.
The figure presents the drive of one bogie. For simplifying the figure, the built-in switches for the two AC voltages (15/25kV) and the charging circuits cannot be seen. On the other hand the brake resistor circuit and the supplying system of the auxiliaries are in the figure. The permissible range of the DC-link voltage is 2,1…2,8kV, the nominal (and maximum) voltage of the motors is 2183V. Fig.5.13 presents the circuit applied at 3kV DC voltage overhead line.
The two parallel connected converters - providing the 4qS function previously - now operate as a DC/DC step-down converter, since the permissible range of motor-side inverters DC-link voltage is lower than 3000V. The brake circuit is on the input line-side. This and the auxiliaries converter shall be designed for 3000V.
Fig.5.12 presents the circuit of the same locomotive applied at 1500V DC voltage overhead line. The two parallel connected converters - providing the 4qS function previously - now operate as a DC/DC step-up converter. The brake circuit is connected to the DC-link. The secondary coil of the input transformer operates as a smoothing choke.