Synchronization and Reactive Current Support
of PMSG based Wind Farm during Severe Grid
Fault
Abstract
In order to produce reactive currents, the wind energy generation system
(WECS) and the STATCOM are normally controlled with the phase locked
loop (PLL)-oriented vector control methods. This paper analyzes the
dynamic synchronization mechanism and stability criteria of the wind farm
and proposes a coordinated current control scheme for the WECS and the
STATCOM during severe grid fault period. The synchronization stability of
both the WECS and the STATCOM is remained by the active power
balancing control of the wind farm. The control objectives of the generatorand grid-side converters for the WECS are swapped to avoid the interaction
between the dc-link voltage control loop and the synchronization loop.
Introduction
It has been reported that the permanent magnet synchronous generator
(PMSG) based WECS has the advantages on the LVRT in contrast to other
types of wind turbines because of its capability and flexibility on the active
and reactive power controls with the full-scale converter [4],[5]. The LVRT
schemes for the PMSG based WECS have been widely discussed in the
previous studies. The main difficulty of the WECS control during fault
period is to maintain the dc-link voltage of the power converter. Different
schemes have been proposed and verified in previous literature, such as the
coordinating active power control of the generator- and grid-side converters
[6],[7],[8], dc chopper based active power dissipation scheme [9][10], etc.
Existing method
Reactive currents should be provided by the WECS during the fault period.
For the voltage sag percentage lower than 50%, twice percentage reactive
currents should be supplied. In serious fault situation with voltage sag
larger than 50%, 100% reactive currents are required. The main difficulty
of the WECS control during fault period is to maintain the dc-link voltage
of the power converter.
Draw backs
• DC link voltage imbalance
Proposed method
This paper analyzes the dynamic mechanism of the LOS for the wind farm
equipped with the PMSG based WECS and the STATCOM, then proposes
a coordinating control scheme for the wind farm to keep synchronizing
during severe fault period. As presented in this paper, the dynamics of the
PLL-based WECS and STATCOM perform similarly with the rotational
dynamics of the conventional synchronous generator (SG).
Advantages
• Synchronization is done at severe grid faults
LITERATURE SURVEY
• H. Geng, C. Liu and G. Yang In this paper, the low-voltage ride-through
(LVRT) capability of the doubly fed induction generator (DFIG)-based
wind energy conversion system in the asymmetrical grid fault situation is
analyzed, and the control scheme for the system is proposed to follow the
requirements defined by the grid codes
• Chinchilla M., Arnaltes S., Burgos J. Wind energy is a prominent area of
application of variable-speed generators operating on the constant grid
frequency. This paper describes the operation and control of one of these
variable-speed wind generators: the direct driven permanent magnet
synchronous generator (PMSG).
• Conroy J. F., Watson R In this paper the frequency response capability of
the full converter variable speed wind turbine generator with permanent
magnet synchronous generator is investigated. A control scheme is
developed that improves the frequency control performance, illustrating the
importance of the initial active power output of the FCWTG
• Ziping W., Wenzhong G., Daye Y., A new practical way of modelling
direct-driven permanent magnet synchronous generator (PMSG) wind
turbines is proposed. The model emphasizes on the wind-rotor-to-PMSGto-converter-to-grid system, which is the main energy flow system of the
direct-driven wind turbine
low-voltage ride through
• In electric power systems, low-voltage ride through (LVRT), or fault ride
through (FRT), sometimes under-voltage ride through (UVRT), is the
capability of electric generators to stay connected in short periods of lower
electric network voltage (cf. voltage dip). It is needed at distribution level
(wind parks, PV systems, distributed cogeneration, etc.) to avoid that a
short circuit on HV or EHV level will lead to a widespread loss of
generation. Similar requirements for critical loads such as computer
systems and industrial processes are often handled through the use of an
uninterruptible power supply (UPS) or capacitor bank to supply make-up
power during these events.
Wind turbine
•
Wind turbines, like aircraft propeller blades, turn in the moving air and power
an electric generator that supplies an electric current. Simply stated, a wind
turbine is the opposite of a fan. Instead of using electricity to make wind, like a
fan, wind turbines use wind to make electricity. The wind turns the blades,
which spin a shaft, which connects to a generator and makes electricity.
•
Modern wind turbines fall into two basic groups; the horizontal-axis variety,
like the traditional farm windmills used for pumping water, and the verticalaxis design, like the eggbeater-style Darrieus model, named after its French
inventor. Most large modern wind turbines are horizontal-axis turbines
Turbine Components
• blade or rotor, which converts the energy in the wind to rotational shaft
energy;
• a drive train, usually including a gearbox and a generator;
• a tower that supports the rotor and drive train; and
• Other equipment, including controls, electrical cables, ground support
equipment, and interconnection equipment
Advantages and Disadvantages of Wind-Generated
Electricity
A Renewable Non-Polluting Resource
Wind energy is a free, renewable resource, so no matter how much is used
today, there will still be the same supply in the future. Wind energy is also a
source of clean, non-polluting,
Cost Issues
Even though the cost of wind power has decreased dramatically in the past
10 years, the technology requires a higher initial investment than fossilfueled generators. Roughly 80% of the cost is the machinery, with the
balance being site preparation and installation
CONT…
Environmental Concerns
Although wind power plants have relatively little impact on the environment
compared to fossil fuel power plants, there is some concern over the noise
produced by the rotor blades, aesthetic (visual) impacts, and birds and bats
having been killed (avian/bat mortality) by flying into the rotors.
Supply and Transport Issues
The major challenge to using wind as a source of power is that it is
intermittent and does not always blow when electricity is needed
Permanent magnet synchronous generator
• A permanent magnet synchronous generator is a generator where the
excitation field is provided by a permanent magnet instead of a coil. The
term synchronous refers here to the fact that the rotor and magnetic field
rotate with the same speed, because the magnetic field is generated through
a shaft mounted permanent magnet mechanism and current is induced into
the stationary armature Synchronous generators are the majority source of
commercial electrical energy. They are commonly used to convert the
mechanical power output of steam turbines, gas turbines, reciprocating
engines and hydro turbines into electrical power for the grid. Some designs
of Wind turbines also use this generator type.
Block diagram
Control diagram of grid side converter
Control diagram of STATCOM
CONCLUSION
The following conclusions can be derived from the theoretical analyses and
simulation verifications:
(1) Variable-speed wind turbines and STATCOM both have in the LOS risk
when the grid voltage dip is severe
(2) The proposed active power balancing control scheme which relies on
the frequency dynamic of the PLL can achieve the synchronization stability
of the WECS. However, reactive current capability of the WECS would be
sacrificed to implement such scheme.
.
CONT.
(3) The coordinated current control between the PMSG based WECS and the
STATCOM can achieve both the synchronization stability and the reactive
current support according to the grid code under severe grid fault. The
analysis results and proposed scheme are also available for the LVRT of
other renewable energy conversion systems.
(4) It should be pointed out that this paper focuses on the symmetrical fault
conditions. In practical applications, unsymmetrical faults occur more often
than symmetrical ones.