The is used to improve the stability of

The
main objective of this paper is to establish a predictive power control
strategy which is used to improve the stability of power systems. This has been
done in the presence of wind farms based on Doubly Fed Induction Generator
DFIG using Static Synchronous Series Compensator SSSC and Super Capacitor
Energy Storage System SCESS. In our proposed system, SCESS is used to control
the active power in the Grid Side Converter GSC and SSSC is used to reduce
the low frequency oscillations. SCESS is composed of a capacitor bank and a
dual switch DC/DC converter which is connected to DFIG through the DC link. In
this method, it is used to control the active and reactive power of the Rotor
Side Converter RSC as well as damping controller design for SCESS and SSSC.
For improving the power system stability, an SSSC is used, which is an modern power quality FACTS device that employs a voltage
source converter connected in series to a transmission line through a
transformer. Thus the system has been simulated and tested on MATLAB/SIMULINK.

1. Introduction

1.1
Motivation

Energy
is a key to human development, and renewable energy can ensure energy needs are
met while protecting local environments and populations. Even other sources of
energy are finite and will someday be depleted. Now a day’s renewable energy is
the major source for the power generation. Among renewable resources, wind
energy and solar energy is emerging technology. In that wind energy is the
common energy to produce electrical signals all around the world. Due to
oscillating behaviour of wind power, there will be some oscillations in
electrical grids while transmitting the electrical signals. In a power systems,
these fluctuations from stable system and considering the power quality, it is
an serious issue in power system 1, 2.  For this we are using an Super Capacitor
Energy Storage System (SCESS) and Flexible AC Transmission System(FACTS)
devices, which is used as a balancer to diminish oscillations and enhance
damping in power system 3,4. To eradicate this fluctuations

and
to improve damping, we can done more number of studies in the field of power
system. In those studies there will be some pros and cons in it. The main
objective of this paper is to increase the stability of the power system with
the help of predictive control strategy. Because of the cost effectiveness and
direct power control, Doubly Fed Induction Generator DFIG is taken as a
common type of variable speed wind turbine 5. Here, rotor of DFIG is coupled
to the grid along with the back-to-back bi-directional converter and stator is
coupled to grid. More number of studies have been done for the DFIG based
control systems to improve stability such as Sensitivity Analysis and Hybrid Wavelet-PSO-ANFIS
6, eigen value sensitivity analysis 7, state feedback 8. We have analayzed
the PI controllers, neural networks and fuzzy logic types but it also having
some complexity and cons in it 9,10. Now a days, Super Capacitor Energy
Storage System plays an vital role in reducing the fluctuations of DFIG power
system 11,12. For compensating the reactive power, Fly-Wheel Energy Storage
System (FESS) 13, 14 and Super conducting Magnetic Energy Storage(SMES)
15-17. For reducing the low frequency oscillations in power system, we can
use the FACTS devices. SSSC is an advanced type of FACTS devices in which it is
connected in series along transmission lines 18,19. There are various methods
have been developed for designing an controller of SSSC 20-25. Advanced
control method known as model predictive control (MPC) is mostly used in
industries and in research 26.   

2. Power System Design

2.1
Doubly Fed Induction Generator Type

In
DFIG based wind turbine system, the grid side converter and rotor side
converter are connected as back-to-back converter through DC link. To maintain
the balance between grid side converter and rotor side converter, DC link is
used.

 

2.2
Design of Rotor Side Converter

Rotor
Side Converter is used to extract the maximum power from wind, to control the
active and reactive power of DFIG and it gives an reactive power needed by the
induction generator. In this controller, active power and voltage are regulated
by Pqr and Pdr respectively. Using the reactive power
control, the voltage control is made by measuring Xsw and by
measuring the turbine speed Trw, the reactive power control is made.
These control signals are compared along with the reference signals by PI
controller for all the parameters, the reference signals (Kqr_ref, Kdr_ref)
are measured. For producing the error signal, current reference signal is
compared to the reference signals which are in d-q axis. These reference
signals are amplified by current signals to produce Pqr and Pdr
signal and it is send to Pulse Width Modulation. Signals from PWM is send to
inverter to act as switching mode 7.

 

 

2.3
Design of Grid Side Converter

Grid
Side Converter is used to keep the constant DC voltage link and for controlling
the reactive power of the system. In this paper, reference signal of DC link (Pdc_ref)
is compared with the Pdc by PI controller. The current reference
signals (Kqg_ref, Kdq_ref) is compared along with measured
value. After combining the current signals, Pdg, Pqg are
produced and these signals are send to Pulse Width Modulation. The pulses from
PWM is send to inverter to act as switching mode 7.

 

2.3
Design of SCESS converter

Super
Capacitor Energy Storage System is used to control the DC link voltage. In this
paper, damping controller is designed, which is used to control the reactive
power of the system. SCESS converter consists of capacitor bank and dual switch
DC/DC converter. This is connected to DFIG through DC link. The switches S1
and S2 is used as boost and buck modes in the system. The converter
will works as boost mode, when S1 opens. The converter will works as
buck mode, when S2 opens.

 

2.4
Design of Static Synchronous Series Compensator

SSSC
is a type of FACTS devices which is connected in series with transmission line.
Depending upon voltage source inverter, SSSC is used as a reactive power
compensator. SSSC can operate in capacitive and inductive modes. SSSC consists
of voltage source inverter, series transformer, capacitor and control block.

 

3. Controller Section

For
determining the future response of the system, a new powerful technique known
as model predictive control was used in the system 27, 28. For each problem,
optimal point can be determined by using this method. For state estimation of
the process, we can predict the input and output of the system. By using this
control strategy, for each sampling interval, a optimization problem is solved in
it. The changes can be made in the system as an error signal. In this method, predictive
horizon is measured in the system for future response.