method based on current direction there are different number of commutation
steps. The first is a four-step commutation strategy, which was presented in [3].
In general, the switching sequence depends on the voltage level switches involved in
the commutation process. In this strategy both IGBTs of the conducting bi-directional
switch are turned on. When commutation between switch cells occurs, the first stage
is to determine the voltage level at the turned on and the turned off switch cells.
This is needed to identify within the two commutating bi-directional switches the
active devices that will operate as freewheeling devices. In general, the freewheeling
devices are as follows:
1. the devices which allow the current flow from source to load in the lower input
voltage phase;
2. the devices which allow the current flow from load to source in the higher input
voltage phase.
After determination of freewheeling devices, the second action is switch commutation
in the following four steps:
Step 1: the freewheeling device of the incoming switch is turned on;
Step 2: the non-freewheeling device of the outgoing switch is turned off;
Step 3: the non-freewheeling devices of the incoming switch is turned on;
Step 4: the freewheeling device of the outgoing switch is turned off.
The above described commutation process is depicted in Fig. 2.26a. A state diagram
of the commutation process for a four-step commutation sequence between two
bi-directional switches with voltage polarity measurement is shown in Fig. 2.26b [29].
Another input voltage measurement-based commutation strategy was presented
by Ziegler and Hofmann in [152]. It is based on the basic operating principle of
providing a freewheeling path for both output current polarities at any given time,
for devices with either steady- or transient-state combinations. This commutation
method is called METZI and is based on the detection of the six time intervals as