Power-current controller based sliding mode control for DFIG-wind

energy conversion system

 ABSTRACT
This study presents an overall sliding mode control scheme for stator power-current control, grid
power-current control and dc-link voltage regulation to operate a doubly-fed induction generator
(DFIG) based wind energy conversion system. At the generator rotor side, the stator power
control is achieved through controlling the rotor currents. The rotor current state model is carried
out from the stator and the rotor equations of the generator under the condition of stator voltage
alignment. At the grid side, a cascade control loop is applied for the dc-link voltage regulation
and the power transfer using grid and dc-link modelling. The structure of the sliding mode
control law, combination of compensating, sliding and integral terms, enhances the tracking
performance and the robustness to uncertainties. The proposed control strategy is validated using
an experimental DFIG wind turbine system and the results are provided to demonstrate the
capabilities of the proposed control system in tracking and control under different operating
conditions and robustness to uncertainties.
 CHAPTER 1

INTRODUCTION

Doubly-fed induction generators (DFIGs) based wind turbine systems are the most
popular machines in high power generation due to their operation at variable speed, independent
regulation active and reactive power capability, small power converters and reduced power
losses [1–3]. Despite the DFIG advantages, challenges remain in the strategy of controlling the
overall system for smooth and efficient operation. Conventional control of grid connected DFIG
wind turbine systems is based on stator voltage oriented, or stator-flux-oriented vector control
(VC). In this control scheme, instantaneous stator active and reactive powers can be
independently controlled through decoupling the rotor current and regulating its components
using proportional–integral (PI) controllers. The tracking performance highly depends on the PI
parameters tuning, the exact machine parameters, such as stator and rotor resistances and
inductances, and disturbances in the system. Hence, performance may deteriorate due to the
deviation of the actual machine parameters from the nominal values used in the control system
especially when operating the PI controllers under fixed gains [4–8]. In [7], an interesting
overview on the applications of DFIG in wind energy systems was provided, where issues of
control systems for stand-alone operation, connection to balanced or unbalanced grids, sensorless
control, and frequency support from DFIGs and low-voltage ride-through were discussed. In [8],
the stator active and reactive powers were regulated by controlling the machine inverter with
three different controllers (PI, polynomial reference signal tracking (RST) based on pole
placement theory and linear quadratic Gaussian approach) with simulation validation.

Sliding mode control (SMC) strategy is an effective control strategy for non-linear
systems subject to uncertainties, parametric mismatch and external unknown disturbances. It is
based on a switching term, to maintain the state on the manifold surface, and a compensation
term can be added to deal with system modelling [9– 12]. The SMC strategy has been applied on
different wind energy conversion system (WECS) configurations based on permanent magnet
synchronous generator [13–15] and DFIG [16–22]. In [16], a sliding surface, based on the
tracking error and its integration, has been used in the SMC approach to control only the active–
reactive power at the DFIG stator without discussion about the dc-link voltage regulation and the
power transfer though the grid inverter, and their effect on the overall control system. In [17], a
similar SMC scheme was used for the rotor side control based on the rotor speed tracking and the
grid side control for the dc-link voltage regulation.

In both studies, the proposed control scheme has been validated through only computer
simulation. In [18], SMC algorithms for the rotor- and grid-side power converters of a DFIG
based wind turbine under non-ideal grid voltage conditions were proposed and verified by
simulation. In [19], the DFIG stator side active–reactive powers were controlled through the
electromagnetic torque and the d-component of the stator current, using a second-order SMC
scheme. In this work, the grid side control and its effect on the proposed control system were not
included to ensure efficient power transfer through the back-to back converter. In [20], similar
work to [19] was proposed but with variable control gains. The control design was complicated
by the gains computation and the control scheme was verified only by simulation. The same
second-order SMC was applied in [21] for the power control at the grid side of a DFIG-driven
wind turbine. In [22], a discrete second-order SMC scheme was used as an observer to construct
the reference value for the extractable power based on the operation condition. In [23], simple
SMC method based on the rotor current error as the control surface was proposed for stator
power regulation, whereas PI regulators were used for the dc-link voltage regulation and power
control at the grid side. In the aforementioned works, an overall control system for operating the
DFIG system through the back-to-back converters was not detailed to demonstrate a smooth
power transfer from the DFIG rotor and stator to the grid. This issue will be investigated, where
a complete SMC scheme is proposed to control the power transferred through the stator and the
rotor to the grid, while maintaining a constant dc link voltage at the back-to-back converters.

Research is still ongoing on studying different methods to apply and implement SMC for
DFIG applications as specified in recent works [24–29]. In [24], a non-linear perturbation
observer, based on a sliding mode state, is integrated into the first-order SMC law for robustness
and applied to a DFIG wind energy system for controlling the rotor speed and the stator reactive
power. Although the proposed control scheme showed significant response improvement
compared to conventional VC, feedback linearization control and sliding-mode control, it was
only verified by simulation. The perturbation observer is based on Luenberger observer and a
sliding mode state, whereas, in the proposed SMC, the robustness is achieved through a simple
integral term in the second-order SMC. In [25], the chattering phenomenon, due to the
conventional SMC, has been reduced using an exponential reaching law, as an adaptive sliding
gain, and applied to control the torque and the stator reactive power reactive of a DFIG wind
turbine. In the proposed SMC, the chattering is less compared to conventional VC and SMC with
no requirement to adapt the gains. In [26], a high-gain SMC is applied for stator power control of
a DFIG with unknown flux and rotor currents. In [27], second-order SMC is applied to DFIG
under grid voltage non-idealities with good performance. In [28], an adaptive SMC is used for
DFIG to track the grid voltage without the current control loop. In [29], a second-order SMC
scheme is applied to DFIG to regulate the rotor speed and the stator reactive power and verified
by simulation. The objective of this paper is to develop a SMC scheme, which includes a
switching term, an integral term and a compensating term, to efficiently operate a DFIG-based
WECS and deal uncertainties, parametric mismatch and external disturbance. The control
scheme will be used to regulate the active–reactive powers at the generator stator and rotor and
maintain a constant dc-link voltage at the back-to-back converter.
 CHAPTER 2
LITERATURE SURVEY
1) Trends in wind turbine generator systems:
This paper reviews the trends in wind turbine generator systems. After discussing some
important requirements and basic relations, it describes the currently used systems: the constant
speed system with squirrel-cage induction generator and the three variable speed systems with
doubly fed induction generator (DFIG), with gearbox and fully rated converter, and direct drive
(DD). Then, possible future generator systems are reviewed. Hydraulic transmissions are
significantly lighter than gearboxes and enable continuously variable transmission, but their
efficiency is lower. A brushless DFIG is a medium speed generator without brushes and with
improved low-voltage ride through characteristics compared with the DFIG. Magnetic pseudo
DDs are smaller and lighter than DD generators, but need a sufficiently low and stable magnet
price to be successful. In addition, superconducting generators can be smaller and lighter than
normal DD generators, but both cost and reliability need experimental demonstration. In power
electronics, there is a trend toward reliable modular multilevel topologies.
THE objective of this paper is to review the trends in wind turbine generator systems and
to describe a number of possible future generator systems. Although there are also smaller wind
turbines, this paper focuses on large wind turbines. Fig. 1 shows how the wind turbine size has
grown over the past decades [1]. Also the wind turbine market has grown significantly over the
past decades [1]. This paper starts with discussing some important requirements and basic
relations for wind turbine generator systems. Next, it describes the four most commonly used
generator systems in wind turbines. Subsequently, it reviews some important possible future
wind turbine generator systems. It closes with concluding remarks.
 The key objective of the developments in wind turbines is to minimize the cost of energy
delivered to the power system. The contribution of the generator system to this objective is to
convert the mechanical input energy from the blades into electrical energy, again enabling
minimization of the cost of energy. This has a number of important implications. 1) Capital
expenditures (such as manufacturing, transportation, and installation) are important, but not
decisive, because operational expenditures (such as repair and maintenance) also have to be
considered. 2) What is the best generator system varies over time because the material cost varies
over time, as we have seen for permanent magnets (PMs). Uncertainty about these price
developments influences decisions. 3) What is the best generator system depends on the location
where the turbine is installed, because the total energy produced depends on the wind speed. 4)
The efficiency of the system is important, but not decisive, because a system with a lower
efficiency that delivers energy at a low cost of energy is better. Besides fulfilling this key
objective, wind turbine generator systems have to meet a number of other requirements. 1) Grid
Connection: To enable large-scale application of wind energy without compromising power
system stability, power system operators have grid codes to describe the requirements for the
quality and the form in which the power is delivered to the system [2]. Wind turbines are
required to grid-fault ride-through [or low-voltage ride-through (LVRT)] capability: they have to
stay connected and contribute to the grid in case of a disturbance such as a voltage dip. On the
long term, wind farms should—similar to conventional power plants—supply active and reactive
power for frequency and voltage control in the power system.
2) Reliability and Availability: Especially offshore, operational expenditures may form a
significant part (in the order of 30%) of the cost of energy. Therefore, requirements
related to reliability, availability, and maintainability are getting more attention and more
research in this field is necessary [3]–[9]. Proper protection against the aggressive humid
and salty offshore environment is extremely important. 3) Variable Speed: To enable an
optimal match between the generator system and the aerodynamic of the rotor, the
generator system is required to have a variable speed. The power that can be captured
from the wind with a wind turbine is given by [1] P = 1 2 ρairCp(λ, θ )πr 2 b v3 w (1)
where ρair is the air mass density, vw is the wind speed, rb is the rotor radius (or the
blade length), and Cp is the power coefficient, which depends on the specific design of
the blade, the blade pitch angle θ, and the tip speed ratio λ (blade tip speed divided by
 wind speed). The power coefficient is maximum for a constant tip speed ratio, and
therefore at a rotational speeds proportional to the wind speed.
Brushless DFIG
In it has been proposed to use the brushless doubly fed induction generator (BDFIG), also known
as the brushless doubly fed machine, as a generator for use in wind turbines. The BDFIG has two
stator windings, one of which is connected to the grid (the so-called power winding) and the
other (the so-called control winding) is supplied via a converter, in the same manner as a DFIG.
The machine has two principal fields, associated with the two stator windings, of different pole
numbers which cross couple via the rotor. The rotor has a short-circuited winding consisting of
so-called nested loops as shown in Fig. 7. The machine operates in a synchronous mode with a
fixed ratio between shaft speed and the two stator frequencies, again like the DFIG. The machine
was proposed for wind turbine use around 1990 by a group at Oregon State University [68] and
has been developed since then. The machine is not easy to analyze despite its simple construction
and only recently more straightforward design procedures have emerged. Following the
description of relatively small experimental machines [69], several larger machines have recently
been built, including a 70-kW machine from Brazil [70], a Chinese machine rated at 200 kW and
what is believed to be the largest machine to date namely a 250-kW machine built in the UK
[67]. These larger machines demonstrate that the BDFIG can be built in larger sizes but a
machine with a MW rating remains to be demonstrated. There are restrictions on the allowable
pole numbers of the two principal fields, with the highest available natural speed (corresponding
to the synchronous speed of a DFIG) with a 2-pole/6-pole combination being 750 rpm on a 50-
Hz system. Therefore, the BDFIG is seen as a natural part of a medium speed drive with a
natural speed in the order of 300 rpm. Research is in progress to develop this approach [71]. The
BDFIG shares with the DFIG the benefits of low cost construction in that no PMs materials are
used and only a fractionally rated converter need be employed. Simultaneously, the absence of
brush-gear obviates one of the main failure modes of the DFIG. Use of the BDFIG therefore
gives a low cost but reliable option [66]. The BDFIG also has a significantly improved LVRT
performance compared with an equivalent DFIG, further reducing system cost and complexity
[67]. Furthermore, it is a medium speed generator, which increases the efficiency and the
reliability because the high-speed gear stage of the gearbox is avoided. Compared with a DFIG
of the same speed, a BDFIG has the advantages that it is brushless and that the LVRT
capabilities are better and the disadvantage that it probably is slightly larger because of the
additional winding.
There is no convergence toward a single best wind turbine generator system, but instead
the variety of wind turbine generator systems is increasing. The three currently used variable
speed systems (with gearbox and DFIG, with gearbox and full converter and DD) are expected to
remain for the coming years. Hydraulic transmissions enable continuously variable transmission
and are significantly lighter than gearboxes, but their efficiency is lower. A brushless DFIG is a
medium speed generator without brushes and with improved LVRT characteristics compared
with the DFIG. Magnetic PDDs are smaller and lighter than DD generators, but need a
sufficiently low and stable magnet price to be successful. Also superconducting generators can
be smaller and lighter than normal DD generators, but both cost and reliability need experimental
demonstration. In power electronics, there is a trend toward reliable modular multilevel
topologies.
2) Overview of control systems for the operation of DFIGs in wind energy applications:
Doubly fed induction generators (DFIGs), often organized in wind parks, are the most
important generators used for variable-speed wind energy generation. This paper reviews the
control systems for the operation of DFIGs and brushless DFIGs in wind energy applications.
Control systems for stand-alone operation, connection to balanced or unbalanced grids,
sensorless control, and frequency support from DFIGs and low-voltage ride-through issues are
discussed.
T HE DOUBLY fed induction machine (DFIM), also known as the wound-rotor or slip-
ring induction machine, is an induction machine with both stator and rotor windings [1], [2]. The
DFIM is nowadays widely used as a generator, particularly in variable-speed wind energy
applications with a static converter connected between the stator and rotor. Currently, this
topology occupies close to 50% of the wind energy market [3]. Table I shows some of the
commercially available wind energy conversion systems (WECSs), with power in the range of
1.5–3 MW, which are based on doubly fed induction generators (DFIGs). In total, in Table I,
there are 93 models of WECSs based on DFIGs for that power range. In Table I, “NM” stands
for number of models. DFIGs are also used in higher power ranges (> 3 MW). The German
company Repower manufactures two models of WECSs based on DFIGs, the model 6M with a
total output power of 6150 kW and the model 5M with a total output power of 5 MW [4]. For
WECSs based on DFIGs, gearboxes are required because a multipole low-speed DFIG is not
technically feasible [5]. The design of a DFIG-based WECS with a one-stage gearbox was
proposed in [6], but no commercial WECS has been implemented with this concept. However,
even with the problems associated with a three stage (3S) gearbox, the DFIG still has some
advantages when compared with other generators used in wind energy applications [3]. For
instance, in [7] and [8], three generators suitable for wind energy applications are studied: a
direct-drive synchronous generator (SG) (which is one of the solution offered by Enercon [9]), a
direct-drive permanent magnet generator (PMG) [10]–[14] (marketed by several companies, e.g.,
Vestas [13], Clipper [14], and Dewind), and a 3S-geared DFIG (see Table I). The results in terms
of weight, cost, size, and losses obtained in [7] and [8] are presented in Table II. Notice that the
3S-Geared DFIG is considered the base for the comparison.
This paper has summarized the most recent research in the field of control systems for
DFIGs in wind energy applications. After reviewing the papers related to conventional control
methods for DFIGs connected to balanced systems, it is concluded that vector control, typically
orientated along the stator flux, is still the most adopted method for regulating the rotor currents
of DFIGs. With this control methodology, decoupling of the reactive power and electrical torque
is simple to achieve. However, as discussed in Section II, most of the control schemes presented
in Section II-D–F can provide good overall performance. Regarding sensorless control of
variable-speed DFIGs, the most popular methods are based in MRAS schemes, with the RCMO
providing good performance in both stand-alone and grid-connected operation of DFIGs. The
TBMO is also an interesting method for sensorless vector control, particularly because the direct
and quadrature rotor currents can be directly obtained from the α–β components of the signals
without resorting to transformations to a synchronous rotating axis. Concerning sensorless
methods, more research can be required in some areas, particularly because the performance of
the rotor position observers proposed in the literature have not been evaluated for LVRT
operation. In this paper, the control systems for the operation of DFIGs connected to unbalanced
grid or loads, have also been assessed. Several control targets for unbalanced operation have
been proposed in the literature, e.g., to eliminate the oscillations in the total active power output
from the DFIG, to reduce the oscillations in the total reactive power supplied to the network, or
to supply a grid current with no negative-sequence components. To fulfill these control targets,
the RSC and/or the GSC can be used. The current trend is to use both power converters
simultaneously because more degrees of freedoms are available in this case. Control systems for
ancillary services and grid-frequency support have also been discussed in this paper. In the past,
DFIGs where mostly controlled for MPPT operation. Nowadays, it is expected that WECSs
based on DFIGs can provide droop control and inertia emulation. This has been reviewed in this
paper. Finally, in this paper, LVRT control systems for DFIGs have been discussed. The
operation of the elements typically used for LVRT compliance, such as crowbars, choppers,
static switches, and other elements, has been analyzed and extensively discussed in this paper.
 CHAPTER 3
DOUBLY FED INDUCTION GENERATOR

3.1 PRINCIPLE OF OPERATION

The block diagram of the doubly fed generator, operating in the super synchronous mode
is shown in Figure 6.1 (Leonhard 1996 and Liexu et al 2006). The stator is directly connected to
the grid. The rotor is also connected to the grid but by means of two back-to-back pulse width
modulation converters. The rotor side converter is current controlled to inject the desired currents
into the rotor (Fernando Valenciage 2007).

When the machine is operating in the generating mode, the mechanical power Pm gets
converted into electrical power in the stator (Pstator) and in the rotor (Protor). The rotor power is
processed by the PWM converters and the grid side converter can be controlled to feed this
power as both real and reactive powers (Pr and Qr) (Rajib Datta et al, 2002).

Figure .1 Structure of the doubly fed induction generator

Thus, the induction generator system is capable of generating a limited amount of

reactive power, unlike the pitch control or rotor resistance controlled wind energy systems. 115
The system can usually be made to operate at a unity power factor with a ± 10 % control range
on the power factor for the entire system. Figure .1 Structure of the doubly fed induction
generator

3.2 DFIG STEADY STATE THEORY

An analytical method for the determination of the steady-state control laws of the doubly
fed induction generators (DFIG) used in wind turbines is presented. The analytical model is used
to derive the converter control laws of the generator in terms of rotor voltage and control angle
(real and reactive power) overall operation speed range.

The DFIG design it needs suitable compromises between the wind turbine performance
and the respective characteristics of the DFIG, the gearbox, the static converters and the
associated control strategy. The optimal solution in terms of performance and cost must be
derived from global design approach (Mustafa Kayikci et al 2008).

Such an analytical formulation is also very efficient in terms of execution time and
robustness of the global optimization process. The application of the proposed methodology is
illustrated by the study of the optimal reactive power allocation between the converters, which is
an important design challenge of DFIG system.

Most induction generators in the world are cage-type machines. Special classes of
induction generators with a three-phase wound rotor, called doubly fed induction generators
(DFIG), have become very popular for use as wind generators as shown in Figure .2. These
machines usually have a three phase inverter connected to the rotor windings, which allows
direct control of the rotor currents. Control of the rotor currents allows for variable speed and
reactive power control.
 Figure .2 Doubly fed induction generator

A cage-type induction generator draws a fixed amount of reactive power, which will
cause the power factor to be lagging over all operating conditions. In addition, a cage-type
induction machine has a very small speed range, typically only a few percent variations from the
synchronous speed. However, direct control of the rotor currents, as allowed by the DFIG, allow
for reactive power control and variable speed operation.

A DFIG can operate at lagging, unity, or leading power factor and can vary its speed by a
much larger margin (usually around 20 to 25 percent above or below the synchronous speed).
These characteristics make the DFIG ideal for use as a wind generator. Reactive power control
allows a DFIG to help provide voltage support for the grid, and variable speed operation allows
the DFIG to operate at a higher efficiency over a wide range of wind speeds.

The main component of the DFIG system and the conversion chain is a wind turbine, a
gearbox, a DFIG and a four-quadrant power converter. The DFIG is usually designed with a low
pair pole number (two or three) to obtain acceptable performance in terms of reactive power
consumption. A gearbox is then necessary to adapt the low rotating speed of the wind turbine (in
a range of ~ 10-20 rpm for high-power wind turbines) to the medium-rotating speed of the DFIG.
The power converter is connected between the grid and the DFIG rotor winding terminals by
using slip rings.
 The grid side converter (GSC) is usually controlled to operate at unity power factor and
to regulate the DC link voltage. The rotor side converter (RSC) controls the electrical frequency
in the rotor windings and the real and reactive power flows. The rotor variable frequency supply
allows the variable rotating speed operation of the wind turbine. Its rotating speed is imposed by
the real power flow controlled by the RSC that is used to provide a suitable torque control loop.
The reactive power managed by the RSC controls the power factor of the whole system, seen by
the grid (GPF) (Aguglia et al 2007).
This analysis highlights two of the DFIG’s main advantages. First, a small amount of
reactive power from the rotor becomes a large amount of reactive power in the stator. Second,
the rotor power rating is required to be only a fraction of the entire generator rating.
A DFIG is a special type of induction generator with a wound rotor. By proper control of
the rotor converter, a DFIG’s can achieve reactive power control and a wider speed range than
for a cage-type induction generator. Variable speed operation allows the DFIG to capture a
greater amount of power in the wind for a given wind speed.
There are three main advantages of a DFIG. First, variable speed operation. Second, a
small amount of rotor reactive power becomes a large amount of stator reactive power. Third, the
rotor converter only needs to be rated for a fraction of the total generator rating.
2.3 Wind energy

2.3.1 Wind Energy and Wind Power

Wind is a form of solar energy. Winds are caused by the uneven heating of the atmosphere by
the sun, the irregularities of the earth's surface, and rotation of the earth. Wind flow patterns are
modified by the earth's terrain, bodies of water, and vegetative cover. This wind flow, or motion
energy, when "harvested" by modern wind turbines, can be used to generate electricity.

2.4 How Wind Power Is Generated

The terms "wind energy" or "wind power" describe the process by which the wind is used to
generate mechanical power or electricity. Wind turbines convert the kinetic energy in the wind
into mechanical power. This mechanical power can be used for specific tasks (such as grinding
grain or pumping water) or a generator can convert this mechanical power into electricity to
power homes, businesses, schools, and the like.

2.5 Wind Turbines

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.

2.6 Wind Turbine Types

Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the traditional
farm windmills used for pumping water, and the vertical-axis design, like the eggbeater-style
Darrieus model, named after its French inventor. Most large modern wind turbines are
horizontal-axis turbines.

2.7 Turbine Components

Horizontal turbine components include:

 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.
2.8 Turbine Configurations

Wind turbines are often grouped together into a single wind power plant, also known as a
wind farm, and generate bulk electrical power. Electricity from these turbines is fed into a utility
grid and distributed to customers, just as with conventional power plants.

2.9 Wind Turbine Size and Power Ratings

Wind turbines are available in a variety of sizes, and therefore power ratings. The largest
machine has blades that span more than the length of a football field, stands 20 building stories
high, and produces enough electricity to power 1,400 homes. A small home-sized wind machine
has rotors between 8 and 25 feet in diameter and stands upwards of 30 feet and can supply the
power needs of an all-electric home or small business. Utility-scale turbines range in size from
50 to 750 kilowatts. Single small turbines, below 50 kilowatts, are used for homes,
telecommunications dishes, or water pumping.

2.10 Wind Energy Resources in the United States

Wind energy is very abundant in many parts of the United States. Wind resources are
characterized by wind-power density classes, ranging from class 1 (the lowest) to class 7 (the
highest). Good wind resources (e.g., class 3 and above, which have an average annual wind
speed of at least 13 miles per hour) are found in many locations (see United States Wind Energy
Resource Map). Wind speed is a critical feature of wind resources, because the energy in wind is
proportional to the cube of the wind speed. In other words, a stronger wind means a lot more
power.
2.11 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,
electricity. Unlike conventional power plants, wind plants emit no air pollutants or greenhouse
gases. According to the U.S. Department of Energy, in 1990, California's wind power plants
offset the emission of more than 2.5 billion pounds of carbon dioxide, and 15 million pounds of
other pollutants that would have otherwise been produced. It would take a forest of 90 million to
175 million trees to provide the same air quality.
 CHAPTER 4

POWER QUALITY
Electric power quality, or simply power quality, involves voltage, frequency, and
waveform. Good power quality can be defined as a steady supply voltage that stays within the
prescribed range, steady a.c. frequency close to the rated value, and smooth voltage curve
waveform (resembles a sine wave). In general, it is useful to consider power quality as
the compatibility between what comes out of an electric outlet and the load that is plugged into
it.[1] The term is used to describe electric power that drives an electrical load and the load's ability
to function properly. Without the proper power, an electrical device (or load) may malfunction,
fail prematurely or not operate at all. There are many ways in which electric power can be of
poor quality and many more causes of such poor quality power.

The electric power industry comprises electricity generation (AC power), electric power
transmission and ultimately electric power distribution to an electricity meter located at the
premises of the end user of the electric power. The electricity then moves through the wiring
system of the end user until it reaches the load. The complexity of the system to move electric
energy from the point of production to the point of consumption combined with variations in
weather, generation, demand and other factors provide many opportunities for the quality of
supply to be compromised. While "power quality" is a convenient term for many, it is the quality
of the voltage rather than power or electric current that is actually described by the term. Power
is simply the flow of energy and the current demanded by a load is largely uncontrollable.

Power Quality Deviations

Voltage

 Variations in the peak or RMS voltage are both important to different types of equipment.

 When the RMS voltage exceeds the nominal voltage by 10 to 80% for 0.5 cycle to 1
minute, the event is called a "swell".
 A "dip" (in British English) or a "sag" (in American English the two terms are equivalent)
is the opposite situation: the RMS voltage is below the nominal voltage by 10 to 90% for 0.5
cycle to 1 minute.
 Random or repetitive variations in the RMS voltage between 90 and 110% of nominal
can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible
changes of light level. Definition of the characteristics of voltage fluctuations that produce
objectionable light flicker has been the subject of ongoing research.
 Abrupt, very brief increases in voltage, called "spikes", "impulses", or "surges", generally
caused by large inductive loads being turned off, or more severely by lightning."Under
voltage" occurs when the nominal voltage drops below 90% for more than 1 minute. The
term "brownout" is an apt description for voltage drops somewhere between full power
(bright lights) and a blackout (no power – no light). It comes from the noticeable to
significant dimming of regular incandescent lights, during system faults or overloading etc.,
when insufficient power is available to achieve full brightness in (usually) domestic lighting.
This term is in common usage has no formal definition but is commonly used to describe a
reduction in system voltage by the utility or system operator to decrease demand or to
increase system operating margins.
 "Overvoltage" occurs when the nominal voltage rises above 110% for more than 1
minute.

Frequency

 Variations in the frequency.
 Nonzero low-frequency impedance (when a load draws more power, the voltage drops).
 Nonzero high-frequency impedance (when a load demands a large amount of current,
then stops demanding it suddenly, there will be a dip or spike in the voltage due to the
inductances in the power supply line).
 Variations in the wave shape – usually described as harmonics at lower frequencies
(usually less than 3 kHz) and described as Common Mode Distortion or Interharmonics at
higher frequencies.

Waveform

 The oscillation of voltage and current ideally follows the form of a sine or cosine
function, however it can alter due to imperfections in the generators or loads.
 Typically, generators cause voltage distortions and loads cause current distortions. These
distortions occur as oscillations more rapid than the nominal 60 Hz, and are referred to as
harmonics.
 The relative contribution of harmonics to the distortion of the ideal waveform is called
total harmonic distortion (THD).
 Low harmonic content in a waveform is ideal because harmonics can cause vibrations,
buzzing, equipment distortions, and losses and overheating in transformers.

Each of these power quality problems has a different cause. Some problems are a result of the
shared infrastructure. For example, a fault on the network may cause a dip that will affect some
customers; the higher the level of the fault, the greater the number affected. A problem on one
customer’s site may cause a transient that affects all other customers on the same subsystem.
Problems, such as harmonics, arise within the customer’s own installation and may propagate
onto the network and affect other customers. Harmonic problems can be dealt with by a
combination of good design practice and well proven reduction equipment.

Linear and non linear loads:

HARMONICS:

Defined as deviations from the fundamental frequency sine wave, expressed as additional sine
waves of frequencies that are a multiple of the generated frequency. They are expressed as third,
fifth, seventh etc harmonics, denoting their frequency as a multiple of the primary wave
frequency.

HARMONIC DISTORTION:

Non linear distortion of a waveform characterized by the appearance in the output of harmonics
other than the fundamental component when the input wave is sinusoidal.

HARMONIC CONTENT:

A measure of the presence of harmonics in the waveform expressed as a percentage of the

fundamental frequency. The total harmonic content is expressed as the square root of the sum of
each of the harmonics amplitudes, expressed as percentage of the fundamental. Non linear
distortion of a waveform characterized by the appearance in the output of harmonics other than
the fundamental component when the input wave is sinusoidal.

LINEAR LOAD:

AC electrical loads where the voltage and current waveforms are sinusoidal. The current at any
time is proportional to voltage. Linear Loads are: POWER FACTOR IMPROVEMENT
CAPACITORS, INDESCENT LAMPS, HEATERS ETC

NON-LINEAR LOAD:

Applies to those ac loads where the current is not propotional to the voltage. Foremost among
loads meeting their definition are gas discharge lighting having saturated ballast coils and
thyritor (SCR) controlled loads. The nature of non-linear loads is to generate harmonics in the
current waveform. This distortion of the current waveform leads to distortion of the voltage
waveform. Under these conditions, the voltage waveform is no longer proportional to the current.
Non Linear Loads are : COMPUTER, LASER PRINTERS, SMPS, REACTIFIER, PLC,
ELECTRONIC BALLAST, REFRIGERATOR, TV ETC.

EFFECTS OF HARMONICS ON NEUTRAL EARTHING

In a 4-wire three phase system, the fundamental currents at any instant will always add
up to zero in the neutral. However, the third harmonic of each phase is always in phase with
those of the other two phases. As a result, rather than canceling each other (as is the case with the
fundamental), they are additive and may well lead to serious neutral loading problems. As an
example: a three phase system has 100 amperes load and each phase contains 30% third
harmonic. The harmonic current flowing through the neutral will be three times. 30% of 100, or
90 amperes at the three times of the fundamental frequency (150 Hz for 50 Hz systems) Ref.
current wave diagram. Therefore, it is recommended to use neutral of minimum double the
capacity since total load on the system in consideration of Linear loads and Non Linear loads. It
is also advised to use single phase UPS/Loads wherever possible with individual neutral in place
of using three phase UPS/Loads. Due to the high frequency and skin effect the current flows on
the other surface of the conductor and also to minimize neutral impedance, it is preferred to use
number of conductors in parallel in place of single neutral conductor.
Earth fault:

The system shall be earthed through conducting material suitable to carry the fault current. The
minimum cross section of the earth conductor shall be calculated based on maximum current,
which can flow at the time of short circuit/earth fault.

Micro grid:

Microgrid is a localized grouping of electricity sources and loads that normally operate
connected to and synchronous with the traditional centralized electrical grid (macrogrid), but can
disconnect and function autonomously as physical and/or economic conditions dictate. By this
way, it paves a way to effectively integrate various sources of distributed generation (DG),
especially Renewable Energy Sources (RES). It also provides a good solution for supplying
power in case of an emergency by having the ability to change between islanded mode and grid-
connected mode. On the other hand, control and protection are big challenges in this type of
network configuration, which is generally treated as a hierarchical control.

Types of micro grids

A typical scheme of microgrid with renewable energy resources in grid-connected mode

Campus Environment/Institutional Microgrids

The focus of campus microgrids is aggregating existing on-site generation with multiple loads
that located in tight geography in which owner easily manage them.

Remote “Off-grid” Micro grids

These microgrids never connect to the Macrogrid and instead operate in an island mode at all
times because of economical issue or geography position. Typically, an "off-grid" microgrid is
built in areas that are far distant from any transmission and distribution infrastructure and,
therefore, have no connection to the utility grid.

Military Base Microgrids

These microgrids are being actively deployed with focus on both physical and cyber security for
military facilities in order to assure reliable power without relying on the Macrogrid.

Commercial and Industrial (C&I) Microgrids

These types of microgrids are maturing quickly in North America and Asia Pacific; however, the
lack of well –known standards for these types of microgrids limits them globally. Main reasons
for the installation of an industrial microgrid are power supply security and its reliability. There
are many manufacturing processes in which an interruption of the power supply may cause high
revenue losses and long start-up time.

Basic components in microgrids

The Solar Settlement, a sustainable housing community project

Local generation
It presents various types of generation source that feed electricity to user. These sources are
divided into two major groups – conventional energy sources (ex. Diesel generators) and
renewable generation sources (e.g. wind turbines, solar).

Consumption

It simply refers to elements that consume electricity which range from single devices to lighting,
heating system of buildings, commercial centers, etc. In the case of controllable loads, the
electricity consumption can be modified in demand of the network.

Energy Storage

In microgrid, energy storage is able to perform multiple functions, such as ensuring power
quality, including frequency and voltage regulation, smoothing the output of renewable energy
sources, providing backup power for the system and playing crucial role in cost optimization. It
includes all of electrical, pressure, gravitational, flywheel, and heat storage technologies.

Point of common coupling (PCC)

It is the point in the electric circuit where a microgrid is connected to a main grid. Microgrids
that do not have a PCC are called isolated microgrids which are usually presented in the case of
remote sites (e.g., remote communities or remote industrial sites) where an interconnection with
the main grid is not feasible due to either technical and/or economic constraints.

Advantages and challenges of microgrids

Advantages
 Microgrid paves a way to integrate Wind, solar, and hydroelectricity, etc. to the main grid.

First of all, a microgrid is capable of operating in grid-connected and stand-alone modes,

and handling the transitions between these two modes. So that it provides good solution to
supply power in case of an emergency and power shortage during power interruption in the main
grid. 

In the grid-connected mode, ancillary services can be provided by trading activity of

microgrid and the main grid. In the islanded mode of operation instead,
the real and reactive power generated within the microgrid, including the help of energy storage
system should be in balance with the demand of local loads.

In islanding mode, there are intentional (scheduled) or unintentional in which intentional

islanding can occur in situations such as scheduled maintenance, or when degraded power
quality of the host grid can endanger microgrid operation or because of economical reason. On
the other hand, unintentional islanding can occur due to faults and other unscheduled events that
are unknown to the microgrid.Both of those situation can be dealt actively by using microgrid.

All of above mentioned points and by means of modifying energy flow through microgrid
components, microgrid allows and facilitates integration of renewable energy generation such as
photovoltaic, wind and fuel cell generations without requiring re-design of the distribution
system.

Challenges:
 Microgrids, and integration of DER units in general, introduce a number of operational
challenges that need to be addressed in the design of control and protection systems in order to
ensure that the present levels of reliability are not significantly affected and the potential benefits
of Distributed Generation (DG) units are fully harnessed. Some of these challenges arise from
invalid assumptions typically applied to conventional distribution systems, while others are the
result of stability issues formerly observed only at a transmission system level.The most relevant
challenges in microgrid protection and control include:

• Bidirectional power flows: The presence of DG units in the network at low voltage levels can
cause reverse power flows that may lead to complications in protection coordination, undesirable
power flow patterns, fault current distribution, and voltage control.

• Stability issues: Interation of control system of DG units may create local oscillations, requiring
a thorough small-disturbance stability analysis. Moreover, transition activities between the grid-
connected and stand-alone modes of operation in a microgrid can create transient stability.
Recent studies have shown that direct-current (DC) microgrid interface can result in significantly
simpler control structure, more energy efficient distribution and higher current carrying capacity
for the same line ratings.

• Modeling: Many characteristic in traditional scheme such as prevalence of three-phase

balanced conditions, primarily inductive transmission lines, and constant-power loads are not
necessarily hold valid for microgrids, and consequently models need to be revised.

• Low inertia: The microgrid shows low-inertia characteristic that are different to bulk power
systems where high number of synchronous generators ensures a relatively large inertia.
Especially if there is a significant share of power electronic-interfaced DG units, this
phenomenant is more clear. The low inertia in the system can lead to severe frequency deviations
in stand-alone operation if a proper control mechanism is not implemented.

• Uncertainty: The operation of microgrid contains very much of uncertainty in which the
economical and reliable operation of microgrids rely on that. Load profile and weather forecast
are two of them that make this coordination becomes more challenging in isolated microgrids,
where the critical demand-supply balance and typically higher component failure rates require
solving a strongly coupled problem over an extended horizon. This uncertainty is higher than
those in bulk power systems, due to the reduced number of loads and highly correlated variations
of available energy resources (limited averaging effect).

Micro grid control:

Regarding to architecture of microgrid control or any control problem there are two
different approaches can be identified: centralized and decentralized. A fully centralized control
relies on a big amount of information trasmittence between involving units and then the decision
is made at a single point. Hence, it will present big problem in implementation since
interconnected power systems usually cover extended geographic and involves enormous
number of units. The fully centralized control is currently considered as infeasible solution. On
another hand, in a fully decentralized control each unit is controlled by its local controller
without knowing the situation of others.  The fully decentralized control is also irrelevant in this
context due to strong coupling between the operations of various units in the system. A
compromise between those two extreme control schemes can be achieved by means of a
hierarchical control scheme consisting of three control levels: primary, secondary, and tertiary

Primary control:

The primary control is designed to satisfy the following requirements:

• To stabilize the voltage and frequency.

• To offer plug and play capability for DERs and properly share the active and reactive power
among them, preferably, without any communication links.

• To mitigate circulating currents that can cause over-current phenomenon in the power

electronic devices

The primary control provides the setpoints for lower controllers which are the voltage and
current control loops of DERs. These inner control loops are commonly referred to as zero-level
control.

Secondary control:
 Secondary control has typically seconds to minutes sampling time (i.e. slower than the
previous one) which justifies the decoupled dynamics of the primary and the secondary control
loops and facilitates their individual designs. Setpoint of primary control is given by secondary
control in which as a centralized controller, it restores the microgrid voltage and frequency and
compensate for the deviations caused by the primary control. The secondary control can also be
designed to satisfy the power quality requirements, e.g., voltage balancing at critical buses.

Tertiary control:

Tertiary control is the last (and the slowest) control level which consider economical concerns in
the optimal operation of the microgrid (sampling time is from minutes to hours), and manages
the power flow between microgrid and main grid.
 CHAPTER 5
PROPOSED METHOD
The objective of this paper is to develop a SMC scheme, which includes a switching term, an
integral term and a compensating term, to efficiently operate a DFIG-based WECS and deal
uncertainties, parametric mismatch and external disturbance. The control scheme will be used to
regulate the active–reactive powers at the generator stator and rotor and maintain a constant dc-
link voltage at the back-to-back converter.

Proposed block diagram

System modeling:
The wind energy conversion system, depicted in Fig. 1, consists of wind turbine coupled to the
DFIG, back-to-back converter, a filter and a grid. 2.1 DFIG modeling The DFIG dynamics,
represented in a synchronously rotating (d–q) reference, is expressed at the stator and the rotor
sides such as
where vsd and vsq are the d–q components of the stator voltage, isd and isq are the d–q
components of the stator current, φsd and φsq are the d–q components of the stator flux, Rs is the
stator resistance, vrd and vrq are the d–q components of the rotor voltage, ird and irq are the d–q
components of the rotor current, φrd and φrq are the d–q components of the rotor flux, Rr is the
rotor resistance, ωs is the synchronous angular speed, ωr is the rotational speed and (ωs−ωr) is
the slip angular speed.
The stator and rotor fluxes are given, in terms of the stator and rotor currents, by

Where Ls, Lr and Lm are the stator inductance, rotor inductance and mutual inductance,
respectively. In the stator flux orientation, the q-axis of the stator voltage is aligned with the
reference frame such as

Then, the stator flux can be approximated to

 In order to define the rotor current dynamics (ird, irq), the rotor fluxes (φrd, φrq), defined
in (2b), are substituted into the rotor dynamics (1b). Furthermore, the stator currents (isd, isq) are
carried out from (1a) and substituted into the rotor dynamics (1b) with the conditions (3) and (4)
for stator voltage (vsd, vsq) and stator flux (φsd, φsq), respectively. Finally, these substitutions
lead to the rotor current dynamics

Where σ = 1 − (Lm 2 /(LsLr)) is the leakage factor, s = (ωs − ωr/ωs) is the slip, a =
((RrLs 2 + RsLm 2 )/(σLs 2Lr)) and b = (Lm/σLsLr). The stator power can be expressed, as a
function of the stator voltage and the current, by

Where Ps and Qs are the stator active and reactive powers, respectively. In the power
calculation (6), the stator voltage (vsd, vsq) are defined in (3), and the stator current (isd, isq) are
carried out from (2a) with the stator flux (φsd, φsq) approximated by (4). Finally, the stator side
active and reactive powers can be approximated to become,

From (7), it can be seen that the active and reactive powers can be controlled through the
(d–q) components of the rotor current such as
 Therefore, developing a control system for tracking the rotor currents will allow the
control of the DFIG stator power using the expression (8).
2.2 Grid and dc-link modeling:
The dynamics of the currents at the grid converter, shown in Fig. 1, is represented using
the condition (3) by

Where vd, vq are the grid converter voltage (d–q) components, id, iq are the grid
converter current (d–q) components, L and R are the inductance and resistance of the filter,
respectively, and ω is the grid angular frequency. The dc-link voltage dynamics is carried out as
 CHAPTER 5
SIMULATION RESULTS:
The procedure adopted while starting and running the setup is as follows: (i) charge the
capacitor by the grid side, (ii) turn ON the rotor side controller, (iii) turn ON the grid side
controller and (iv) run the DFIG.
In this control scheme, the rotor currents (ird, irq) were directly controlled by the
proposed SMC control system, while the stator powers (Ps, Qs) were indirectly controlled by the
rotor currents through the reference model 16.
5.1 Operation under constant rotor speed, constant dc-link voltage and variable
rotor currents first:
The proposed SMC system, to operate the DFIG-based WECS, was tested under a
constant rotor speed (1800 rpm), a constant reference for the dc-link voltage and variable profiles
for the d–q components of the rotor current. It can be observed for the currents responses, shown
in Fig. 4a, that the tracking was successfully achieved with good performance in terms of
transient regime, overshoot, and settling and rise time. Furthermore, the dc-link voltage was
successfully regulated to be constant as shown in Fig. 4b.
 (a) d–q currents

(b) Voltage

Fig. 4 Rotor current and dc-link voltage responses

5.2 Operation under constant rotor speed, variable dc-link voltage and constant rotor
currents:
In order to test the SMC dc-link voltage controller under variable voltage profile, the rotor speed
and the rotor currents were kept constant, and the voltage reference was varied down and up at
different step levels. Fig. 5a illustrates the regulation of the rotor currents to follow the constant
references despite the variation of the dc-link voltage. The powers in the system are shown in
Fig. 5b and concur with currents responses. The voltage tracking, shown in Fig. 5b, occurs with
good performance in terms of transient regime, overshoot, settling and rise time. Although in a
real application, the dc-link voltage is required to be maintained constant, this experiment shows
the capability of the proposed SMC system to operate under different voltage levels. The
waveforms of stator and rotor currents of the proposed control approach are given in Fig. 6, and
the frequency spectrum with total harmonic distortion (THD) calculations are given in Fig. 7.
The computed THDs are 3.3 and 3.39%, for stator current and rotor current, respectively. These
quantities concur with the standard limits.

Current Id
 Current Iq

Dc link voltage
 Rotor speed
5.3 Operation under variable rotor speed, constant dc-link voltage and constant rotor
currents:
For the emulation of the wind turbine operation under variable wind speed, the rotor
speed was varied with a down-up profile, as shown in Fig. 8a. The objective was to maintain all
quantities (dc link voltage and d–q rotor current) constant despite the variations of the rotor
speed. From Figs. 8a and b, it can be seen that the objective was successfully achieved with a
good performance at the rotor speed transitions. Fig. 9 represents the d–q rotor current results,
taken from [4], using the conventional VC scheme based on PI regulators, where it can be
observed that the q-current tracking is affected by the rotor speed transition through the
appearance of picks at each transition, which was eliminated by the proposed SMC system. This
result demonstrates the advantage of the proposed control system compared to conventional VC
method.
 Dc link voltage

Fig. 5 Current, power and voltage responses

Stator current, Rotor current

5.4 Robustness to parametric mismatch:

In order to verify the robustness of the proposed SMC system to parametric mismatch,
the values of the parameters were changed, in the control implementation, as follows: an increase
in the rotor resistance (Rr) by 50%, and a decrease in the rotor inductance (Lr) by 20%. The
objective was to maintain all quantities (dc-link voltage and d–q rotor current) constant despite
the variations of the rotor speed and the parameters used in the control implementation. Fig. 10
shows the results for dc-link voltage and d–q current tracking, where it can be observed that the
regulation was successfully achieved despite the parametric inaccuracies in the control law,
which demonstrates its robustness.

Selected signal: 600 cycles. FFT window (in red): 1 cycles

5

-5
0 2 4 6 8 10
Time (s)

Fundamental (60Hz) = 4.992 , THD= 3.32%

0.4
Mag (% of Fundamental)

0.3

0.2

0.1

0
0 200 400 600 800 1000
Frequency (Hz)
 Selected signal: 600 cycles. FFT window (in red): 1 cycles
20

-20
0 2 4 6 8 10
Time (s)

Fundamental (60Hz) = 17.49 , THD= 2.88%

0.4
Mag (% of Fundamental)

0.3

0.2

0.1

0
0 200 400 600 800 1000
Frequency (Hz)

Fig. 7 THD calculation for current waveforms

(a) Rotor current(b) Stator current,
 CONCLUSION
A SMC scheme for power control and dc-link voltage regulation, of a DFIG-based wind energy
conversion system, has been proposed in this study. The power control has been performed
through the control of the currents at the rotor side and the dc-link voltage regulation has been
conducted at the grid side. The rotor current state representation has been carried out from the
stator and the rotor equations of the system under the condition of stator voltage alignment.
Stability analysis has been conducted for all closed loops generated from this control structure.
The proposed control strategy has been simulation validated and the results have shown good
performance under different operating conditions and robustness to uncertainties.
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