ABSTRACT
This paper focused on the development of capacitor voltage
balancing methods in a flying capacitor multilevel inverter
(FCMLI) fed induction motor drive. For improving the
performance of flying capacitor multilevel inverter, a switching
pattern selection scheme is implemented. The proposed method
has been designed a seven-level flying capacitor multilevel
inverter by using sinusoidal pulse width modulation technique.
The selected pattern has been exposed to give superior
performance in load voltage, total harmonics distortion and
capacitor voltage fluctuation. The performance of proposed
strategies is confirmed through simulation investigations.
Keywords
AC Drive, Flying Capacitor Multilevel Inverter (FCMLI), Total
Harmonic Distortion, Sinusoidal pulse width modulation
(SPWM).
1. INTRODUCTION
In recent year, multilevel power inverters are popular due to their
high-power, high voltage capacity, low switching losses and low
cost. The various different topologies of inverters are neutral point
clamped inverters, flying capacitor inverters and cascaded multi
level with separated dc source inverter [1]-[2]. Midway the Flying
Capacitor Multi-level Inverter (FCMI) does not require isolated
dc sides and additional clamping diodes. However, these
properties may be quite limited by the voltage unbalancing of
flying capacitors which the most serious problem.
Hence the FCMLI has to ensure the voltage balancing of flying
capacitors under all the operating conditions. The FCMLI offers a
great advantage with respect to the availability of voltage
redundancies. They are defined as different combinations of
capacitors allowing the charging or discharging of the individual
flying capacitors in order to produce the same phase leg voltage.
This advantage provides the special opportunity for controlling
the individual voltage on flying capacitors [3]-[4]. Many studies
have publicized that under certain conditions, a simple open loop
control guarantees natural balancing of the flying capacitor.
A filter circuit of the RLC type tuned at the switching frequency,
connected in parallel with the load may be used to achieve the
natural balancing under all conditions. However, the extra filter
increases the cost of the overall system. Another popular method
of capacitor voltage balancing is to vary duty cycles of the
switches to charge or discharge the corresponding capacitors.
There are many ways such as carrier rotation strategy [5]-[7],
modulating signal modification strategy [8], etc. In the control
scheme discussed in [9]-[10], balancing is achieved by
preferential charging or discharging of the capacitors.
This paper highlights the sinusoidal pulse width modulation
technique. This scheme does not require any modification in the
carrier or modulating signal.
2. FLYING CAPACITOR MULTILEVEL
INVERTER (FCMLI)
The FCMLI requires a large number of capacitors to clamp the
device voltage to one capacitor voltage level, provided all the
capacitors are equal values. The size of the voltage increases
between two consecutive legs of the clamping capacitors. Hence
the size of voltage steps in the output waveform.