18-04-2013, 02:52 PM
Constant Power Control of DFIG Wind Turbines With Supercapacitor Energy Storage
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Abstract
With the increasing penetration of wind power into
electric power grids, energy storage devices will be required to
dynamically match the intermittency of wind energy. This paper
proposes a novel two-layer constant power control scheme for a
wind farm equipped with doubly fed induction generator (DFIG)
wind turbines. Each DFIG wind turbine is equipped with a supercapacitor
energy storage system (ESS) and is controlled by the
low-layer wind turbine generator (WTG) controllers and coordinated
by a high-layer wind farm supervisory controller (WFSC).
TheWFSC generates the active power references for the low-layer
WTG controllers according to the active power demand from or
generation commitment to the grid operator; the low-layer WTG
controllers then regulate each DFIG wind turbine to generate the
desired amount of active power, where the deviations between the
available wind energy input and desired active power output are
compensated by the ESS. Simulation studies are carried out in
PSCAD/EMTDC on a wind farm equipped with 15 DFIG wind
turbines to verify the effectiveness of the proposed control scheme.
INTRODUCTION
WIND TURBINE generators (WTGs) are usually controlled
to generate maximum electrical power from
wind under normal wind conditions. However, because of the
variations of the wind speed, the generated electrical power
of a WTG is usually fluctuated. Currently, wind energy only
provides about 1%–2% of the U.S.’s electricity supply. At such
a penetration level, it is not necessary to require WTGs to
participate in automatic generation control, unit commitment,
or frequency regulation.
DFIG WIND TURBINE WITH ENERGY STORAGE
Fig. 1 shows the basic configuration of a DFIG wind turbine
equipped with a supercapacitor-based ESS. The low-speed
wind turbine drives a high-speed DFIG through a gearbox. The
DFIG is a wound-rotor induction machine. It is connected to
the power grid at both stator and rotor terminals. The stator is
directly connected to the grid, while the rotor is fed through
a variable-frequency converter, which consists of a rotor-side
converter (RSC) and a grid-side converter (GSC) connected
back to back through a dc link and usually has a rating of a
fraction (25%–30%) of the DFIG nominal power. As a consequence,
the WTG can operate with the rotational speed in
a range of ±25%–30% around the synchronous speed, and its
active and reactive powers can be controlled independently.
SIMULATION RESULTS
Simulation studies are carried out for a wind farm with
15 DFIG wind turbines (see Fig. 9) to verify the effectiveness
of the proposed control scheme under various operating conditions.
Each DFIG wind turbine (see Fig. 1) has a 3.6-MW
power capacity [14], [15]. The total power capacity of the
wind farm is 54 MW. Each DFIG wind turbine is connected to
the internal network of the wind farm through a 4.16/34.5-kV
voltage step-up transformer. The high-voltage terminals of all
transformers in the wind farm are connected by 34.5-kV power
cables to form the internal network of the wind farm. The
entire wind farm is connected to the utility power grid through
a 34.5/138-kV voltage step-up transformer at PCC to supply
active and reactive powers of P and Q, respectively. In this
paper, the power grid is represented by an infinite source. The
ESS of each WTG is designed to continuously supply/store
20% of the DFIG rated power for approximately 60 s. Then,
the total capacitance of the supercapacitor bank can be obtained
from (1). The parameters of the WTG, the ESS, and the power
network are listed in the Appendix. Some typical results are
shown and discussed in this section.
CONCLUSION
This paper has proposed a novel two-layer CPC scheme for
a wind farm equipped with DFIG wind turbines. Each wind
turbine is equipped with a supercapacitor-based ESS, which is
connected to the dc link of the DFIG through a two-quadrant
dc/dc converter. The ESS serves as either a source or a sink
of active power to control the generated active power of the
DFIG wind turbine. Each individual DFIG wind turbine and its
ESS are controlled by low-layer WTG controllers, which are
coordinated by a high-layer WFSC to generate constant active
power as required by or committed to the grid operator.
Simulation studies have been carried out for a wind farm
equipped with 15 DFIG wind turbines to verify the effectiveness
of the proposed CPC scheme. Results have shown that
the proposed CPC scheme enabled the wind farm to effectively
participate in unit commitment and active power and
frequency regulations of the grid. The proposed system and
control scheme provides a solution to help achieve high levels
of penetration of wind power into electric power grids.