Matrix Converter for More Electric Aircraft

1.1. Green technology

The aim of More Electric Aircraft (MEA) is to support green technology by replacing other powers usage of aircraft with electrical power usage. The conventional aircraft requires mainly four powers such as electrical power, pneumatic power, hydraulic power, and mechanical power. The concept of MEA is to replace other powers with electrical power using green technologies. This chapter is focused on green technology for aerospace applications such as aircraft surface actuation control systems. The reason is that regenerative power from the MC drive causes stability problems at aircraft power supply. Overcoming this limitation of MC drive is vital. For example, the host drum drive motor (HDDM) regenerates power when the tanker aircraft (TA) refueling hose trails and winds at air. Figure 2 shows circuit of the tanker aircraft with regeneration control circuit (RCC), which is used to dissipate regenerative power of MC drive using proposed methods.

The host drum drive system (HDDS), which is controlled by Refuelling Control Unit (RCU) and Aeronautical Radio Incorporated Commands (ARINC), controls refueling hose and has three units such as motor control unit (MCU), dump resistor pack (DRP), and two motors. The schematic circuit of HDDS of TA is depicted in Figure 3.

Regeneration occurs only whenever refueling hose winds and trails and this action is commanded by MCU with RCU. It means that the MCU supervises direction of motors based on input from RCU commands. Hence, HDDS must dissipate the regenerated power; otherwise, it can cause below mentioned problems:

1. The input supply to HDDS will be increased during regeneration.

2. There is possibility of deactivation of HDDS system because of instability in the input supply of HDDS.

The HDDS is using DC link converter, which is not favorable to transient operations of HDDS drive, and this system is bulky and more weight, which are not desirable characteristics for aircraft. The MC drive is proposed to address above problems. However, inherent regeneration capability of matrix converter limits itself being used for above application because bidirectional switches directly fed back to regenerated power to aircraft input power supply without requiring any additional power electronic components.

According to aerospace power quality specifications, this regeneration onto aircraft input power supply must be limited. For this reason, avoidance of regeneration is vital for aircraft surface actuation systems of aircraft. Hence, to avoid regeneration in the matrix converter drive, three novel methods are proposed:

1. Bidirectional switch (BDS) method

2. Input power clamp (IPC) method

3. Standard clamp circuit (SCC) method

To detect regeneration in MC drive, two novel techniques are proposed:

1. Power comparison (PC) technique

2. Input voltage reference (IVR) technique

Each and every method has its own regeneration control circuit (RCC). For example, RCC for BDS method consists of three bidirectional switches (BDS) in series with three resistors, and this setup is connected across small input filter of MC drive. The RCC for IPC method requires one conventional uncontrolled six-pulse rectifier and a unidirectional switch (UDS) in series with a resistor, and this setup is connected in between input power supply and small input filter of MC. SCC method does not require additional components, which means that no separate RCC is required to do the same action.

The simulation results prove that the matrix converter is a suitable alternative to conventional HDDS converter topologies. The BDS method is experimentally adopted to verify the proposed concept by laboratory prototype matrix converter, which is built at Smiths Aerospace laboratory (later called GE Aviation laboratory) in University of Nottingham. This MEA project strongly supports the green environment by adopting abovementioned green technologies to obtain reduced aircraft emissions.

2.1. Basic rules

The matrix converter consists of nine bidirectional switches with 29 (512) possible switching states. However, only 27 switching states can be used because two basic rules have to be followed [13].

1. No short circuit of two inputs

2. Never open circuit the outputs

Because of the above rules and also inductive loads nature of MC drive, each output line must always be connected to an input line. Under these basic rules, space vector modulation (SVM) for MC drive has 27 switching states.

2.2. Space vector modulation

The space vector modulation (SVM) is defined as type of pulse width modulation (PWM) to generate gate drive signal to trigger the bidirectional switches (BDS) in MC [14]. SVM is also preferable to control and analyze machines with vector control (VC) or field oriented control of machines and allows visualization of the spatial and time relationships between the resultant current and flux vectors (or space phasors) in various reference frames.

2.3. Vector control

Decoupling flux and torque is feature of VC to overcome sluggish torque response of Induction Motor (IM) to work like a separately excited DC machine. To achieve an independent control of the flux and torque, the direct axis (d axis) is aligned to a rotor flux vector (Ψr) and the concept of the indirect field-oriented vector control (IFOVC) is depicted in Figure 5 [15, 16, 17]. The rotating reference frame is rotating at synchronous angular velocity (ω_e). The sensed three-phase output currents of MC drive are converted into stationary reference frame (i_sα, i_sβ) and then viewed as two “dc” quantities (i_sd, i_sq). The direct axis or real axis component is responsible for the field producing current (i_sd) and is ideally maintained constant up to the motor synchronous speed. If d-axis is aligned with rotor flux vector (Ψr), the system is said to be field oriented. The q-axis component is responsible for torque producing current (i_sq). These two vectors are orthogonal to each other so that the field current and torque current can be controlled independently [16, 17].

2.4. Closed-loop IFOVC

Both faster current control loop and speed control loop outputs [12] provide the reference voltages (V_sd, V_sq) and hence (V_a, V_b, V_c) to SVM to get the stable operation of MC drive as shown in Figure 6.

3.1. Power comparison (PC) technique

To calculate the absolute value of output power of MC drive to achieve power comparison (PC) technique, the torque producing current (i*_sq) and measured rotor speed (ω_re) are sensed. Figure 7 shows the gate drive signal, which is generated for RCC of input power clamp (IPC) method. Here power dissipation through a resistor in the regeneration control circuit (RCC) is directly proportional to the duty cycle of unidirectional switch (UDS), as in Eq. (1),

where D = duty cycle of the unidirectional switch and P_dis = power dissipation through the resistor.

The duty cycle calculation requires the maximum electrical braking power (P_mb) to be calculated, as shown in Eq. (2). The duty cycle of the switches is then less than or equal to unity under all operating conditions.

where T_me = electromagnetic torque and ω_mre = speed of MC drive.

The gate drive signals for RCC switches are generated by using field programmable gate array (FPGA) with digital signal processor (DSP). Here FPGA that receives input parameters (ω_re, T_e, i*_sq) from sensors is fed into DSP, which does all mathematical calculations to generate gate drive signal as shown in Figure 7, and again fed back to FPGA that is sending gate drive signal to the gate drive of UDS. The duty of UDS/BDS is linearly varying with respect to output negative power. The MC drive is not capable to output whole of regenerated power because of it losses such as friction, windage, iron, switching and conduction losses. Because of above reason, the braking resistor dissipates less than the actually regenerated power. As written in Eqs. (3) and (4), the design of braking resistor (R_b) relies on maximum regenerative power during regeneration.

where the braking current (I_b) and input power (P_in,max) are directly proportional to the input voltage. The braking resistor design also depends on the braking time, thermal capacity of the resistor, and heat sink. And the current rating of UDS/BDS in RCC must be higher than the braking current.

3.2. Input voltage reference (IVR) technique

The voltage across small input filter capacitor is measured and compared to the MC supply voltage to generate gate drive signal for RCC of IPC method as shown in Figure 8.

The IVR technique can be used to detect the regeneration in the matrix converter for electrical braking methods. The duty cycle variation is directly proportional to the increase in the line to line voltage across the input filter capacitor of the matrix converter under regeneration with respect to the output power (P_o), as shown in Eq. (5).

Here, VAB is line voltage across small input filter capacitor.

The RCC is turned on only if any voltage difference is detected between MC supply voltage and voltage across the small input filter capacitor for each sampling period. In addition, if the small input filter capacitor voltage is equal to the MC supply voltage, then the duty cycle is set to zero. Gate drive signal is generated using FPGA and DSP control platform similar to PC technique. The power dissipation through RCC happens only if the MC drive is operating under regenerative mode.

4.1. Bidirectional switch (BDS) method

The power circuit for the BDS method [18], regeneration control circuit (RCC) or electrical braking circuit, is shown in Figure 9. The regeneration control circuit (RCC) is introduced across the input filter capacitors (C_AB, C_BC, and C_AC). The regeneration control circuit (RCC) is responsible for power dissipation when regeneration takes place in the MC motor drive.

The RCC consists of three bidirectional switches (BDS_AB, BDS_BC, and BDS_AC) in series with three resistors (R_AB, R_BC, and R_AC) connected across the input lines, in parallel with the input filter capacitor. The schematic of the regeneration control circuit (RCC) or electrical braking circuit (EPC) is depicted in Figure 9.

4.2. Input power clamp (IPC) method

The input power clamp (IPC) method [19] is used for braking the electrical energy in a matrix converter motor drive. The IPC method requires only one braking resistor and a UDS, as shown in Figure 10, when compared to the BDS method, which requires three switches in series with three resistors. Electrical braking circuit or regeneration control circuit (RCC) for the input power clamp (IPC) method is located across the input filter capacitors (CAB, CBC, and CAC). The RCC of IPC is controlled using either power comparison (PC) technique or the input voltage reference (IVR) technique.

The main power electronic components for RCC of IPC are conventional uncontrolled six-pulse rectifier and a UDS in series with a braking resistor (R), as shown in Figure 10. This braking resistor does not have inductive property to help to achieve better electrical braking when regeneration happens in MC drive. It is believed that IPC method is the best method when compared to BDS method because it requires only fewer power semiconductor switching components but not suitable for aerospace applications because it has electrolytic capacitor in the RCC.

4.3. Standard clamp circuit (SCC) method

Similar to the BDS method and the IPC method [19], the proposed standard clamp circuit (SCC) method is using two techniques to detect the regeneration in the matrix converter drive. These are (1) power comparison (PC) technique and (2) input voltage reference technique. However, here the PC technique is only considered and the simulation results of the SCC method with PC technique are discussed. The block diagram for standard clamp circuit is shown in Figure 11. The electrical braking circuit for SCC is shown in Figure 14.

Power dissipation through resistor is directly proportional to duty cycle of UDS, which is already given in Eq. (1). To prove the performance of the SCC method for electrical braking in the MC drive, a 2.2-kW vector-controlled induction motor fed by MC drive is considered.

4.4. Comparison of BDS, IPC, and SCC methods

When compared to earlier methods, called the BDS method and IPC method, no auxiliary hardware is required for SCC method for electrical braking in the matrix converter drive as shown in Table 1. The BDS method [18] has three drawbacks:

1. It requires three BDS in series with three resistors.

2. Also, it requires complex control platform, which controls six PWM for electrical braking.

3. This auxiliary circuit increases size, weight, and cost.

Similarly, the IPC method has three main drawbacks:

1. It requires conventional uncontrolled diode rectifier and UDS with a resistor.

2. Separate control platform for a PWM for electrical braking is required.

3. This auxiliary circuit increases the size, weight, and cost.

The SCC method requires only one UDS switch in series with a resistor. Because of using SCC in the MC drive, achieving electrical braking using this method is easy and no complicated control platform is required. The SCC is considered as a safety device to protect the matrix converter under abnormal conditions such as overvoltage in the input side or output side.

5.1. Regeneration

The regeneration can be demonstrated at V_in = 240 V, q = 0.75, and f_s = 10 kHz by applying step transient to reverse the speed at 1.2 s as shown in Figure 12. A step transient lasts upto 2.4s, no load speed reversal (from +188.5 rad/s to -188.5 rad/s), as shown in the Figure 13 which also shows developed torque which is direclty proportional to torque producing current (i_sq) of IM during VC. The torque producing current (i_q) of the induction motor reaches the maximum limit of 35 A during acceleration as shown in Figure 13. Here regenerative power depends upon large inertial load (j = 0.089 kg m²) of the induction motor, which is created coupling IM with the same rating of DC motor (4 kW). The output current waveforms of the matrix converter drive is depicted in Figure 14(a), which also indicates during speed reversal, the four-quadrant operation, inherent property, of MC from motoring mode to regenerating mode is smoothly achieved. The control of dq-currents (i_d, i_q) with no coupling effects is demonstrated. Figure 14(b) shows input phase currents (i_A, i_B, i_C) of the matrix converter during the four-quadrant operation. The input regenerative powers (P_A, P_B, P_C) to be dissipated using the regeneration control circuit (RCC) are shown in Figure 15(a). During regeneration, the phase opposition (180° phase displacement) between the input phase voltages (V_A, V_B, V_C) and input phase currents (i_A, i_B, i_C) can be seen in Figure 15(b).

5.2. BDS method with PC technique

The generation of the required identical six pulses using PC technique for the regeneration control circuit (RCC) of BDS method is shown in Figure 16(a). Here duty cycle is linearly varying with respect to the output power of the MC drive as shown in Figure 16(b). In order to verify the regenerative energy dissipation, the input phase powers are calculated using input phase voltages and the input phase currents. The resulting input phase currents and calculated input phase power are shown in Figure 17(a) and (b), respectively. When compared to Figure 15(b), Figure 18 proves that regenerative power is dissipated using novel RCC of BDS method, hence input phase voltages (V_A, V_B, V_C) and input phase currents (i_A, i_B, i_C) are in phase. However, there is some input power left, as shown in Figure 17(b), because of constant losses (such as friction, windage losses, and inertial losses) in the IM and switching noises.

5.3. IPC method and SCC method with IVR technique

The generation of a pulse to trigger the RCC, duty cycle variation, and the output power (P_o) variation during regeneration for the IPC method with the IVR technique is shown in Figure 19. Figure 20 shows input phase powers (P_A, P_B, P_C) of MC drive after regeneration control. From simulation results, the IVR technique for both methods (IPC and SCC) is producing acceptable results to avoid regeneration with a MC drive similar to PC technique.