40220140505005

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 – 6545(Print),
ISSN 0976 – 6553(Online) Volume 5, Issue 5, May (2014), pp. 36-43 © IAEME
36
A REVIEW: LOW VOLTAGE RIDE THROUGH (LVRT) IN WIND FARM
Pravin S. Phutane
PG student, Department of Electrical & Electronics Engineering, RKDF College of Engineering,
Bhopal, M.P. India
A.K. Jhala
Associate Professor, Department of Electrical & Electronics Engineering,
RKDF College of Engineering, Bhopal, M.P. India
ABSTRACT
In concept of green energy, wind energy is one of the promising energy sources of electrical
power. Renewable wind power is fastest growing generation globally. Wind generation system has
the potential for grid support. However, when the wind power is connected to an electric grid affects
the power quality and poses problems like stability, grid voltage disturbances. It become very
important to address these challenges and problems arise due to wind generation as Induction
generators are very sensitive to the grid voltage disturbances and need retrofit solution to enhance
their low voltage ride through (LVRT) capability.
This paper analyses the extent to which the low voltage ride through (LVRT) capability of
wind farms using induction generators can be enhance by the use of FACTS controllers such as
STATCOMs or MERs (Magnetic Energy Recovery Switch),compared to the thyristors controlled
Static Var Compensators(SVSs). The transient stability margin is proposed as indicator of LVRT
capability. To know the effect of STATCOMs or MERs on transient stability margins a torque slip
characteristic approach is considered. The method for estimating the required rating of different
compensation devices to ensure stability after fault is suggested based on same approach.
Index Terms: Wind Farm, Grid Code, STATCOM, Low Voltage Ride Through (LVRT), MERS.
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING &
TECHNOLOGY (IJEET)
ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)
Volume 5, Issue 5, May (2014), pp. 36-43
© IAEME: www.iaeme.com/ijeet.asp
Journal Impact Factor (2014): 6.8310 (Calculated by GISI)
www.jifactor.com
IJEET
© I A E M E

37
I. INTRODUCTION
Research of grid connected wind turbines has gained great interest in the recent years. Due to
this, new guidelines and regulations has been set up for connection of large wind farms including
guidelines for fault tide through or low voltage ride through (LVRT) capability [1] As the wind
power plants increase in size and command a larger share of supply portfolio, they are required to
stay operational and not disconnect from the grid supporting the grid with reactive power during and
after voltage sags [2]. Such requirements are known as Fault Ride-Through (FRT) or Low Voltage
Ride-Through (LVRT) capability. Low voltage ride-through is a condition required to the wind
generators when the voltage in the grid is temporarily reduced due to a fault or large load change in
the grid. The required low voltage ride-through (LVRT) behavior is defined in grid codes.
In this paper, a survey on recent LVRT solutions for different generator topology is
discussed. LVRT capability must be provide to wind turbines as penetration of wind power
continues. Crowbars are commonly used to protect the power converters during voltage dips and
their main drawback is that the DFIG absorbs reactive power from the grid during grid faults. we
know that there are emergency grid code requirement, as per this requirement if any fault which may
be external or any voltage failure condition occurs in that case wind generator should continue its
generation and should remain connected to grid and continue own reactive power supply.
II. TYPES OF WIND GENERATORS
Following are the types of generator used for wind generation
a) Squirrel Cage Induction Generators (SCIG)
b) Doubly Fed Induction Generator (DFIG)
c) Permanent Magnet Synchronous Generators (PMSG)
Figure 1: Schematic diagram of the electric system of a fixed speed wind turbine
Figure 2: WECS with squirrel cage induction generator (SCIG)

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Figure 3: WECS with Doubly-Fed Induction Generator (DFIG)
Figure 4: WECS with Permanent Magnet Synchronous Generator (PMSG)
Low voltage ride through (LVRT) of squirrel cage induction generators in wind farms is
challenging due to the large required reactive power after low voltage period [3].Using power
electronic converters with reduced capacity in Doubly-Fed Induction Generator (DFIG) based wind
turbines makes them vulnerable to over-current during grid disturbances[4]. Solutions on LVRT
such as blade pitch angle control, control of fully rated converters, and capacitor sizing. Other
solutions on LVRT include active crowbar rotor circuit and the DC bus energy storage circuit are
illustrated for permanent magnet synchronous generators (PMSG) in [1].
III. WIND TURBINE TECHNOLOGY
The kinetic energy of moving air molecules are converted into rotational energy by the rotor
of wind turbine. This rotational energy in turn is converted into electrical energy by wind electric
generator. The amount of power, which the wind transfers to the rotor, depends on the density of air,
the rotor area, and the wind speed. The power contained by wind is given by,
P =0.5*(air mass flow rate)*(wind velocity)2
= 0.5* (ρ*A*V) * (V)2
=0.5ρAV3
where,
P = power contained in the wind (W)
ρ = air density (kg/m3)
A = rotor area (m2)
V=wind velocity before rotor interference (m/s)

39
The power coefficient (Cp) describes the efficiency of a turbine that converts the energy in
the wind to rotational power. Hence power output of the turbine is given by
Pο = 0.5ρAV3Cp ----------------1
The tip speed ratio of the wind turbine is given as
λ =ωR/V ------------ 2
Where R = radius of the swept area in meters
ω= angular speed in radians per second [6]
IV. COMPARISON OF LVRT TECHNOLOGIES
In the past, wind generators were regarded as devices for distributed generation and thus were
made to trip following even minor disturbances. However, as the penetration of wind power
penetration increases, their impact on the grid can no longer be ignored. Today, wind turbines are
required to operate like conventional generators in many utilities; consequently, many new grid
codes for wind generators have been established [8], governing their LVRT capability, reactive
power capability, and real power control. Of these requirements, LVRT capability represents the
greatest challenge to wind turbine manufacturers. LVRT requires that wind generators must connect
to the grid and stable during most grid faults, so that they contribute to the grid stability [7]. To
enhancement of wind farms the LVRT capability, several approaches have been proposed, which can
be divided into three categories: installation of additional hardware, improvement of the control of
the power converter, and determination of the optimal point of common coupling (PCC). A
commonly used solution for satisfying the LVRT requirement by the additional hardware installation
method involves the use of an active crowbar circuit [10], which provides a low-impedance path for
the rotor current during the fault. This method is relatively simple and cost-effective. However, upon
activation of the crowbar, control over the machine is lost, and the DFIG behaves as a squirrel cage
induction machine; therefore, the issue of reactive power consumption during grid faults must be
addressed. Another method that involves the installation of hardware is to insert an energy storage
system (ESS) [11], such as super capacitors or batteries , at the dc bus inside the rotor-side
converter, where the energy produced during the fault is momentarily stored and is later exported to
the grid following fault clearing. The ESS can also be applied under steady-state conditions to
attenuate power fluctuation caused by wind variations. However, larger ESS systems require more
space; accordingly, a dc chopper can be added inside the dc-link [12]. This dc chopper dissipates the
excess power through its resistor and maintains the dc-bus voltage in a safe range during the critical
period.
V. WIND FARMS AND GRID CODES
For any public electricity network, technical requirement must comply and agreed by its
connected consumers and generators. It is well known that to provide many control functions electric
networks are depends on generators, this led to increase complexity in technical requirements for
generators. These technical requirements are termed ‘Grid Codes’ [13]. Above stated complex
technical requirement which governs relationship between system operator and generators, should be
defined clearly. This process makes more complicated with the introduction of renewable generation.
Generators used in renewable generation have different characteristics than conventional
synchronous generators. Hence in some countries like Germany a specific grid code has been

40
developed for wind farms, and in others countries the aim has been to define these grid codes which
is nothing but the technical requirement in which renewable generators technical requirements is
independent of conventional generators.
The technical requirements within grid codes and related documents vary between electricity
systems. The typical requirements for generators can be classified as follows for simplicity:
• Tolerance - the range of conditions on the electricity system for which wind farms must
continue to operate;
• Control of reactive power - often this includes requirements to contribute to voltage control on
the network;
• Control of active power - often this includes requirements to contribute to frequency control on
the network;
• Protective devices; and
• Power quality.
Basic Requirement of the Grid Code on LVRT
Wind turbine low voltage ride through (LVRT) means that, when the voltage at point-of-
common coupling of wind farm drops, wind turbine should maintain its grid-connected state and
even provide a certain amount of reactive power to the grid to support the grid recovery until its
normal state is reached. Thus low-voltage time/regional is crossed
Figure 5: LVRT Requirements of Wind Farm

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Figure 5 is a Chinese standard [9] on LVRT for wind farms, given as follows:
a) For different types of power system faults, if the voltage at point-of-common coupling of wind
farm stands above the contour line in Figure 6, wind turbine must ensure not escaping from the grid
and maintain continuous operation, otherwise, should allow wind turbine to cut out.
b) For the wind farm not cutting out the grid during power system fault, after the fault is cleared, its
active power should restore at least 10% of rated power per second until to the value of pre-failure.
c) For the wind farm group whose total installed capacity is beyond one million kW, when three-
phase short-circuit fault of power system causes its voltage dropping, each wind farm should have
dynamic reactive supporting ability during LVRT process.
VI. METHODOLOGY USED
Basically different techniques are used to know the transient behavior of cage induction
generators if the grid voltage is going to be changed. Following are the techniques:
A. SVC Static Var Compensator
B. STATCOM
C. MERS type series FACTS controller
A. SVC Static Var Compensator
Traditionally, the use of synchronous condensers, SCs, and/or static var compensators, SVCs,
have been the most commonly employed techniques for the stability improvement of ac systems. The
disadvantages of the synchronous condensers systems are known as slow response and high losses.
Apart from being costly solutions, the use of SVCs has the disadvantage that it becomes less
effective when the ac voltage level is reduced, i.e. during faults and large disturbances. The SVC
consists of a thyristor controlled reactor, and thyristor or mechanically switched capacitors. For the
purpose of this investigation, the SVC can be considered as shunt impedance determined by the
parallel connection of the capacitor and the effective inductance of the thyristor controlled reactor
[14].
B. STATCOM
The STATCOM is a power electronics device based on the voltage source converter
principle. The technology typically in use is, depending on voltage level and total rating, a two- or
three-level voltage source converter, controlled by digital techniques and connected to the power
system in shunt through a filter and possibly a coupling transformer [14].
C. MERS type series FACTS controller
Reactive power compensation with shunt compensation can improve the LVRT capability,
but requires a large rating. The alternative use of series compensation is studied. Series compensation
is based on increasing the reactive power transfer from the grid, and can by this reduce the required
rating of the compensator compared to shunt compensation. A series compensator called magnetic
energy recovery switch (MERS) is found suitable due to large control range and good over-current
capability.

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VII. ANALYSIS
The DFIG is an induction machine which requires reactive power compensation during grid
side disturbances. STATCOM is a feasible option to provide the necessary reactive power
compensation when connected to a weak grid [15].
STATCOM can support the fixed speed wind power plant in order to fulfilment the required
voltage-dip ride-through capability. An 85% Low Voltage Ride Through (LVRT) for 150 ms on the
grid side is studied based on E.ON grid code [16].MERS can improve the LVRT capability by
injecting a series voltage during and/or after the low voltage period [3].
VIII. CONCLUSION
Finally, evaluation of series compensation suggests cost reductions compared to shunt
compensation; however, the influence on power system stability of increased reactive power transfer
from the grid after low voltage period should be investigated in future works.
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