This paper discusses the modeling of linear and nonlinear load used in wind power plant.Harmonis analysis of the wind farm system is essential to study the behavior of equipments connected in the non sinusoidal system environment for designing and optimal location of STATCOM.Simulation models are developed for linear and nonlinear load. Analysis of voltage and current harmonics is performed for these loads individually. Wind turbine connected to synchronous generator is modelled using PSCAD simulation software to analyse the said issues where STATCOM is introduced as an active voltage and reactive power supporter to increase the power system stability. STATCOM can significantly mitigate the harmonis issues.STATCOM unit is developed to inject reactive power for mitigation of harmonis issues and to get stable grid operation.
Keywords |
| Synchronous generator; PSCAD; Wind Turbine Generator (WTG); Static Synchronous Compensator
(STATCOM); harmonics analysis. |
INTRODUCTION |
| It is the objective of the electric utility to supply its customers with a sinusoidal voltage of fairly constant
magnitude and frequency. The generators that produce the electric generate a very close approximation to a sinusoidal
signal. However, there are loads and devices on the system which have nonlinear characteristics and result in
harmonics distortion of both the voltage and current signals. As more nonlinear loads are introduced within a facility,
these waveforms get more distorted.In recent years wind power generation has experienced a very fast development in
the whole world. As the wind power penetration into the grid is increasing quickly, the influence of wind turbine on the
power quality is becoming an important issue. Induction generator is more attractive than synchronous generator for
wind turbines due to their robust construction, low cost, low maintenance, long life and low power to weight ratio.
Wind power penetration is the impact on power system stability. To facilitate the investigation of the impact of a wind
farm on the dynamics of the power system to which it is connected, an adequate model is required. In order to avoid the
necessity of developing a detailed model of a wind farm with tens or hundreds of wind turbines and their
interconnections, aggregated wind farm models are needed [1]. Two of the main requirements are reactive power
control in normal operation conditions [2] and fault ride-through capability during fault conditions [3].
The main purpose of normal operation requirements is to maintain the voltage between admissible limits both for
security and power quality purposes. Since reactive power cannot be transmitted over long distances, it has to be
provided locally. Therefore, in grid connection specifications, wind farms are generally required to contribute to
reactive power (and sometimes voltage) control. Concerning fault condition requirements, they are aimed at avoiding
as much as possible the loss of generation capacity in case of a fault in the transmission grid. |
| There are a number of possible interconnection structures for wind farms and thus it is not possible to cover every
type of network configuration, load, and interconnection point of the wind farm. Frequently wind parks are connected
to weak systems, as they are typically located far from major load centres and central generation. This reflects itself in
the short circuit ratio (SCR) of the interconnection. For weak systems the SCR will usually be less than 6 having over
speed of generator [4].Reactive power is the most important aspect in today’s condition. Reactive power consumption
in a Wind farm is mainly due to the use of induction generators for energy conversion. The basic principle of induction
generator is that they consume reactive power in order to generate real power. The magnetizing currents drawn by step
up transformers also contribute to reactive power consumption to some extent. This reactive power consumption leads
to increased T & D losses, poor voltage profile over loading of T & D equipment and blocked capacity and over loading and reduction in life of T & D equipment [5].With the rapid increase in penetration of wind power in power
grids, tripping of many turbines in a large wind farm during grid faults may begin to influence the overall power system
stability [6]. |
| Indian grid system is very weak also having poor infrastructure. Considering the increasing share of wind
generation interfaced to grid it is necessary to study an overall prospective on various types of existing wind generator
systems and possible generator configuration, critical power quality issues, problems related with grid connections
[7].Use of more intelligent controller for STATCOM and its interface to large power systems addressing various issues
such as security, stability, and voltage profile improvement and power quality [8]. |
| In this paper a wind turbine fed synchronous generator is modelled using PSCAD and harmonics for linear load &
nonlinear load, are analysed. The STATCOM used as a device for mitigate these problems and simulation results prove
that STATCOM is an effective means to mitigate these problems during continuous operation of grid connected wind
turbine. |
SIMULATION OF WIND FARM SYSTEM INCLUDING SYNCHRONOUS GENERATOR |
| A wind farm typically consists of a large number of individual wind turbine generators (WTGs) connected by an
internal electrical network. To study the impact of wind farms on the dynamics of the power system, an important issue
is to develop appropriate wind farm models to represent the dynamics of many individual WTGs. The major issues
considered for the work for grid connected wind turbine generators (WTGs), equipped with synchronous generators are
harmonics. |
 |
| The model is developed and compared by simulation studies in the PSCAD/EMTDC environment under different
wind velocity and fluctuation conditions.
In this case study, large scale wind farm with synchronous generator having capacity 75 MW is connected to 33/132
kV substation to 220 kV electric grid system is modelled by PSCAD as shown in figure 1. |
HARMONICS ANALYSIS |
| Harmonics are sinusoidal voltages or currents that are integer multiples of the fundamental frequency. It is the
objective of the electric utility to supply its customers with a sinusoidal voltage of fairly constant magnitude and
frequency. The generators that produce the electric power generate a very close approximation to a sinusoidal signal.
However, there are loads and devices on the system which have nonlinear characteristics and result in harmonic distortion of both the voltage and current signals. As more nonlinear loads are introduced within a facility, these
waveforms get more distorted. The objective of this paper is to consider the effect of linear and nonlinear loads on the
utility voltage and current harmonics. Some of the commonly used loads in the wind farm systems are modelled in
PSCAD/ EMTDC, by considering the voltage and current waveforms. Harmonic analysis of complete wind farm
system is performed. |
 |
 |
| Table I indicates the simulated results of THDs of Individual Currents and Voltages of Linear Load and table II
indicates the simulated results of THDs of Individual Currents and Voltages of Non Linear Load. |
MITIGATION OF HARMONICS ISSUES BY USING STATCOM |
 |
| The Static Synchronous Compensator (STATCOM) is a shunt connected reactive compensation equipment which is
capable of generating and/or absorbing reactive power whose output can be varied so as to maintain control of specific
parameters of the electric power system. It consists of VSC connected in shunt to a bus through a coupling transform.
The static compensators are devices with the ability to both generate and absorb reactive and active power, but the most
common applications are in reactive power exchange between the AC system and the compensator. The compensator
control is achieved by small variations in the switching angle of the semiconductor devices, so that the fundamental
component of the voltage produced by the inverter is forced to lag or lead the AC system voltage by a few degrees.
This causes active power to flow into or out of the inverter, modifying the value of the DC capacitor voltage, and
consequently the magnitude of the inverter terminal voltage and the resultant reactive power. If the developed voltage
is higher than system voltage the STATCOM will supply reactive power like a rotating synchronous compensator and
improve the voltage and conversely if lower it will remove reactive power. Figure 2 indicates large scale wind farm
with Synchronous generator connected to the grid with STATCOM is modelled. |
| Mitigation of Harmonics |
| According to the guidelines IEC 61400-21 or IEEE STD 519-1992 harmonic measurements are not required for
fixed speed wind turbines where the induction generator is directly connected to the grid. Harmonic measurements are
required only for variable speed turbines with electronic power converter. The limits of VTHD for 132 kV systems are
3.0 % & individual harmonic of any particular frequency is 2.0%. Also ITHD having voltage level greater than 69 kV is
5.0%. |
 |
 |
 |
 |
| Table III and IV indicates the mitigation of THDS of individual voltages and currents of linear load by using
STATCOM.Table V and VI indicates the mitigation of THDS of individual voltages and currents of nonlinear load by
using STATCOM |
CONCLUSION |
| This paper has investigated the application of STATCOM to wind farm equipped with Synchronous Generators for
analysis of voltage and current harmonics of linear and nonlinear loads individually.A simulation on model of large
scale wind farm is designed in PSCAD software to study the harmonics mitigation using STATCOM. Wind turbine
connected to synchronous generator is modelled using PSCAD simulation software to analyse the said issues where
STATCOM is introduced as an active voltage and reactive power supporter to mitigate the harmonics issues. |
ACKNOWLEDGMENTS |
| Author thanks ...To Prof. Dr. B.E. Kushare, Prof. & Head, Electrical Engineering Department, K.K.Wagh Institute of
Engineering Education and Research, Nasik, Principal Prof. Dr. K. N. Nandurkar and management for supporting to
conduct such research work. |
References |
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