26-05-2012, 01:44 PM
Control Scheme of Three-Level NPC Inverter
for Integration of Renewable Energy
Resources into AC Grid
Resources into AC Grid.pdf (Size: 1.1 MB / Downloads: 63)
Abstract
This paper presents a multiobjective control scheme
based on the dynamic model of three-level, neutral-point-clamped
voltage source inverter for integration of distributed generation
(DG) resources based on renewable energy resources to the
distribution grid. The proposed model has been derived from the
abc/αβ and αβ/dq transformation of the AC system variables. The
proposed control technique generates the compensation current
references and by setting appropriate references of DG control
loop, the DG link not only provides active and reactive currents
in fundamental frequency, but also it can supply nonlinear load
harmonic currents with a fast dynamic response.
Introduction
DISTRIBUTED generation (DG) systems based on renewable
energy resources play an important role in electric
power systems. The estimated share of these resources in
electrical networks will increase significantly in the near future
by providing different benefits like cost reduction, reliability
of main grid, and emission reduction [1]. Application of
renewable energy resources such as wind turbines, photovoltaic,
and fuel-cell in a power system may cause major
changes in the design and operation of distribution networks
[2]. Several methods and strategies have been proposed to
integrate renewable energy resources to the distribution grids.
Three-Level Diode-Clamped Inverter Topology
The neutral point converter proposed by Nabae, Takahashi,
and Akagi in 1981 was essentially a three-level diode-clamped
inverter [16]. A three-phase three-level diode-clamped inverter
is shown in Fig. 1. These three inverter phases share a common
DC bus, subdivided by two capacitors into three levels. The
voltage across each capacitor is VDC/2; and the voltage stress
across each switching device is limited to VDC/2 through the
clamping diodes.
Voltages and Currents in the Special
Reference Frames
The proposed space vector PWM control technique of
NPC VSI in this paper is based on analysis of voltage
and current vector components mainly in a special reference
frame, e.g., abc to αβ and dq of the AC system variables.
Synchronous reference frame control uses a reference
frame transformation module, e.g., abc to dq, to transform
the grid current and voltage waveforms into a reference
frame that rotates synchronously with the grid voltage. As
a result, the control variables become DC values; therefore,
filtering and controlling can be achieved more easily [17]–
[19]. The Clarke transformation (αβ transformation) maps
the three-phase instantaneous voltages and currents in the
123 (abc) phases into the instantaneous voltages and currents
on the αβ-axes [18]. The αβ transformations of three-phase
generic voltage and current components are given by (2)
and (3)
Conclusion
In this paper, a flexible control strategy for integration
of renewable energy resources to the distribution grid was
presented. The high performances of the proposed control
strategy in both steady state and transient operation have been
verified through simulation results. The dynamic model of
NPC VSI during integration of renewable energy resource to
the AC grid has been investigated. Simulation results clarified
the ability of the proposed control strategy in compensation
of active and reactive currents in fundamental frequency and
harmonic current components. In addition, simulation results
validated the ability and fast dynamic response of proposed
control technique in tracking reactive power variations. Simulation
results shows, by setting delivered active power from
renewable energy resources as a constant value, and increasing
requested load reactive power in fundamental and harmonic
frequencies, the control loop of active and reactive powers
considerate independent of each other. The proposed control
scheme has the ability to be used in all types of converters
topologies and can be used as a multiobjective strategy for
integration of different types of renewable energy resources
into AC grids.