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A HYBRID PWM BASED DC-LINK VOLTAGE BALANCING
ALGORITHM FOR A 3-LEVEL NEUTRAL- POINT-CLAMPED
(NPC) DC/AC TRACTION INVERTER DRIVE
ABSTRACT
This paper presents a hybrid pulse-width modulation (H-PWM) technique, for a 3-level
neutral point clamped (NPC) electric vehicle (EV) traction inverter drive. The proposed strategy
depicts the advantages of both classic space-vector pulse width modulations (SV-PWM) as well
as carrier-based PWM. The duty-cycles for the traction inverter switches are calculated using
carrier-based PWM, to reduce the computational time and control complexity of the system. The
redundancies of the switching states are then used to balance the 2 DC-link capacitor voltages,
similar to the SV-PWM based strategy. The proposed scheme is capable of maintaining the
difference between the 2 DC-link capacitor voltages for a wider range of machine speedtorque
variations. Furthermore, a single carrier is used for PWM, instead of multiple carriers, which
further reduces computational complexity.
A detailed comparative study is carried out, to prove the performance difference between
the low switching loss-based SV-PWM techniques, which is previously proposed by authors and
the present proposed hybrid-PWM strategy. In addition, total harmonic distortion of voltage and
current, duty-cycles of the switches, total inverter power loss as well as DC-link voltage
balancing capabilities are also compared. The balancing ability of the proposed strategy by
changing the neutral point potential control is also demonstrated. Detailed simulation and
experimental studies are carried out to prove the performance of the proposed control strategy
with a 6.0 kW, surface-mounted permanent magnet synchronous machine (SPMSM). Simulation
and experimental test results prove the desired performance of the proposed scheme.