Single H-bridge Cell Commutation Process

A summary of the operation and forced current commutation process of a phase H- bridge cell is given in this section. It is assumed that the machine phase commutating inductance is not negligible, i.e. for desirable operational performance of the machine, there is a need for forced commutation of the phase current. It is also assumed that dc current in all the other H-bridges connected in series with the H-bridge undergoing commutation will stay constant during the commutation process. At this stage, atten- tion is given to the forced commutation operation only, the energy recovery circuit operation is ignored for now. For this multilevel multiphase machine converter topol- ogy, all machine H-bridge cells operational characteristics are exactly identical. As such, a single machine phase and its associated H-bridge cell will be considered.

Although a generating mode topology is used here for illustration purposes only, similar operational characteristics will apply for the motoring mode. Figure 4.29 below shows the power electronic circuit topology of a single machine phase winding and its H-bridge converter cell. The machine phase voltage and machine commutating

4.6 Forced Commutation and Energy Recovery

& T4 although assumed to be of a Thyristor type with forced commutation capability, any semiconductor power electronic device with reverse voltage blocking and forced commutation capability can be used. These devices are configured to form the machine

phase power electronics H-bridge cell. In this generating topology, an extra diodeD1

has been included to reduce conduction losses during the zero vector states.

The diode rectifier comprising ofd1,d2,d3,d4and its output capacitorC-clamp

forms the voltage clamping circuit for the H bridge cell which is essential for absorbing

the energy ((0.5L_ci2) trapped in the machine phase commutating inductance during

the commutation process. The energy transferred to the clamp circuit can then be either recovered by the energy recovery circuit or wastefully dissipated resistively as heat. The commutation process described in the subsequent sections will reference the circuit component labelling highlighted in figure 4.29.

Machine Phase Lc Vph

Machine Current From other cells in series T1 T2 T3 T4 D1 d1 d2 d3 d4 C clamp Energy Recovery Circuit

Fig. 4.29 Machine H-bridge cell with Voltage Clamp and Energy recovery Auxiliary circuits

When the current commutation occurs, the machine phase current polarity reverses. The rate at which the phase current commutates depends on the voltage available to aid commutation, the magnitude of the phase current and the effective commutating inductance of the respective machine phase undergoing commutation. The machine phase winding current reversal commutation process is illustrated in a number of H-bridge device states below.

Commutation Sequences Reference to figure 4.30 will be made in the explanation

4.6 Forced Commutation and Energy Recovery

process, the H-bridge devices T2 & T3 are ON and carrying the machine phase winding current and at the end of the commutation process devices T1 & T4 will be ON and carrying the machine phase winding current in the opposite direction.

State 1: Commutation starts with T2 & T3 ON and carrying phase current. Subject

to the operating firing angle, the current may start naturally commutating into diode D1 at this stage if the voltage is sufficient to forward bias the diode.

State 2: Starts when device T1 is fired resulting in three devices being ON (T1, T2

& T3). When T1 is fired, current does not immediately transfer from T3 to T1 instantly. In fact the current will stay in T3 until the phase voltage reaches a sufficient level for diode D1 to become forward biased and the current will then starts to naturally commutate from T3 to D1. An interval when either both the top or bottom arms of the H-bridge cells are ON at the same time will be referred to as the overlap period. In this case both T1 & T3 devices are ON.

State 3: At this stage, the phase current still has not reversed polarity and device T3

is now force commutated. At this point, separate current paths are formed: most of the constant load current from other seriesed H bridge cells not undergoing commutation diverts into the diode D1 and a small fraction (if any) diverts into T1 (dictated by magnitude of on state voltage drop of 2 series devices T1 & T2 versus one diode D1). The clamping circuit diodes d2 & d3 become forward biased and all the phase winding current of this cell undergoing commutation is rapidly diverted into the clamp circuit and charges up the clamp capacitor, C- clamp. In this state, the energy trapped in the machine inductance is transferred to the cell clamp capacitor and increases the capacitor voltage up. At this stage, the rate of change of the phase winding current is strongly influenced by the voltage difference between the peak phase voltage and the clamp capacitor voltage magnitude. The phase winding current continues to be diverted into the clamp circuit and will eventually get to zero and stays at zero, assuming no other devices are switched. This state creates a zero vector state where the load current bypasses the respective machine phase winding, i.e. machine phase is essentially open circuited and no phase current is present. The duration of this

4.6 Forced Commutation and Energy Recovery

zero vector state can be beneficially exploited to aid regulation of the machine average output power.

State 4: At this stage, device T4 is now fired. Again the period when both T2 & T4

are ON is another overlap time state. When T4 is turned on, current will now start to naturally commutate from diode D1 to the H-bridge phase, subject to magnitude of phase voltage.

State 5: T2 is turned OFF at this stage. Note, since most of the current from other

seriesed H-bridges will be through D1, the devices T4, T2 will have very low or no current at this stage, so T2’s forced commutation duty is very low, i.e. force commutates very low current if any.

State 6: The current in D1 has effectively naturally commutated to zero and devices

T1 & T4 now carry the phase current. This completes the H-bridge cell current commutation process. 1 T4 D1 E R C T2 T3 T1 T4 D1 E R C T2 T3 T1 T4 D1 E R C T2 T3 T1 T4 D1 E R C T2 T3 T1 T4 D1 E R C T2 T3 T1 T4 D1 E R C T2 T3 T1

T2 & T3 ON 2 T1, T2 & T3 ON 3 T1 & T2 ON

4 T1, T2 & T4 ON 5 T1 & T4 ON 6 T1 & T4 ON

Fig. 4.30 Phase Cell H-Bridge Cell Commutation Sequence States

Device Commutation Duty It can be noted that in the example illustrated in figure

4.30 , the commutation started with T2 & T3 ON, of these two devices the device that turns off first (T3 in this case) always force commutates a significant amount of current

4.6 Forced Commutation and Energy Recovery

State 1 State 2 State 3 State 4 State 5 & 6 State (active vector) (overlap) (zero vector) (overlap) (active vector)

Sequence 1 T2,T3 T1,T2,T3 T1,T2 T1,T2,T4 T1,T4

Forced Commutation T3 T2

Sequence 2 T2,T3 T2,T3,T4 T3,T4 T1,T3,T4 T1,T4

Forced Commutation T2 T3

Table 4.1 Alternative Forced Commutation Sequences and Active & Zero Vector States

in comparison to the other device (T2 in this case). This commutation characteristic has a direct impact on the duty and rating of the device gating system if Thyristor type devices with gate assisted current commutation are used. For such cases, in order to ensure that all devices and their associated gating systems are equally exercised during the operation of the machine, the commutation phase sequence can be made to alternate between sequences 1 & 2 highlighted in Table 4.1 below.

A similar alternating commutation sequence can also be deduced when the current polarity is opposite, i.e. start commutation with devices T1 and T4 ON. Thus, for a given phase current over two fundamental cycles all four arms of the H-bridge cell will have undergone the same forced commutation duty.

Analysis of the operational characteristics shows that apart from the machine com- mutating impedance, phase voltage and load current, there are three other parameters that also affect the behaviour and performance of the machine, namely; the operating

phase control angle(α), the commutation overlap period and the commutation zero

vector state periods. These parameters add extra degrees of freedom that can be exploited to give desired optimum machine and converter operating performance.

It can be readily appreciated that instead of employing the above commutation sequences and strategy, where all four arms have the same forced commutation duty, alternative commutation strategies where either only the top or bottom arms of the H-bridge cell are force commutated is also feasible. For applications requiring single quadrant mode of operation, the other arms can be naturally commutated and employ simple diodes or Thyristors with no controlled current turn off capability as highlighted in earlier sections. This can be beneficial in applications where simplicity and reliability of the H-bridge cell is of paramount importance. In this case current control is achieved by the two arms of the H bridge cell with controlled gate turn on/off functionality.

4.6 Forced Commutation and Energy Recovery

However, as alluded to earlier, this simplicity comes at the expense of the possibility of inability of the converter cell to interrupt fault current under certain converter cell failure modes, such as short circuit device failure on the arms with current turn off capability.