15-06-2013, 12:49 PM
Power Quality Improvement of Grid Connected Wind Energy System by Statcom for Balanced and Unbalanced Linear and Nonlinear Loads
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Abstract
A Power quality problem is an occurrence manifested as a nonstandard voltage, current or frequency that results in a failure or a mis-operation of end user equipments. Utility distribution networks, sensitive industrial loads and critical commercial operations suffer from various types of outages and service interruptions which can cost significant financial losses. With the restructuring of power systems and with shifting trend towards distributed and dispersed generation, the issue of power quality is going to take newer dimensions. Injection of the wind power into an electric grid affects the power quality. The performance of the wind turbine and thereby power quality are determined on the basis of measurements and the norms followed according to the guideline specified in International Electro-technical Commission standard, IEC-61400. The influence of the wind turbine in the grid system concerning the power quality measurements are-the active power, reactive power, variation of voltage, flicker, harmonics, and electrical behavior of switching operation and these are measured according to national/international guidelines. The paper study demonstrates the power quality problem due to installation of wind turbine with the grid. In this proposed scheme STATIC COMPENSATOR (STATCOM) is connected at a point of common coupling with a battery energy storage system (BESS) to mitigate the power quality issues.
INTRODUCTION
One of the most common power quality problems today is voltage dips. A voltage dip is a short time (10 ms to 1 minute) event during which a reduction in r.m.s voltage magnitude occurs. It is often set only by two parameters, depth/magnitude and duration. The voltage dip magnitude is ranged from 10% to 90% of nominal voltage (which corresponds to 90% to 10% remaining voltage) and with a duration from half a cycle to 1 min. In a three-phase system a voltage dip is by nature a three-phase phenomenon, which affects both the phase-to-ground and phase-to-phase voltages. A voltage dip is caused by a fault in the utility system, a fault within the customer‘s facility or a large increase of the load current, like starting a motor or transformer energizing. Typical faults are single-phase or multiple-phase short circuits, which leads to high currents. The high current results in a voltage drop over the network impedance. At the fault location the voltage in the faulted phases drops close to zero, whereas in the non-faulted phases it remains more or less unchanged. [1]
STATIC COMPENSATOR (STATCOM)
Principle of STATCOM
A STATCOM is in principle a voltage source converter (VSC) connected via an inductance to a grid. The concept
has been known for many years and is described in detail Figure 1 shows an example of a STATCOM connected to a grid;
Figure 2 shows the simplified single line diagram.The inductance can represent a reactor or a transformer. Reactive power
can be altered by modifying the voltage amplitude of the VSC.
The phasor diagram in Figure 3 helps to understand the principle of the STATCOM. For this purpose, a
transformer with a turns-ratio of 1:1 or a reactor is assumed. In addition, constant grid voltage is assumed. Therefore, the
grid voltage vector UGrid remains at a constant value. If the value of the compensator voltage vector UComp is higher than the
grid voltage vector, the vector of the voltage drop across the inductance XT is in the same direction as the compensator
voltage vector. Therefore the compensator current IGrid flows in positive direction as per the definition in Figure 2. In this
situation, the STATCOM acts like a capacitor. . If the value of the compensator voltage vector UComp is lower than the grid
voltage vector, the vector of the voltage drop across the inductance XT is in the opposite direction compared to the
compensator voltage vector. Therefore the compensator current IGrid flows in negative direction as per the definition in
Figure 2. In this situation, the STATCOM acts like an inductor.
BESS-STATCOM
The battery energy storage system (BESS) is used as an energy storage element for the purpose of voltage regulation. The BESS will naturally maintain dc capacitor voltage constant and is best suited in STATCOM since it rapidly injects or absorbed reactive power to stabilize the grid system. It also controls the distribution and transmission system in a very fast rate. When power fluctuation occurs in the system, the BESS can be used to level the power fluctuation by charging and discharging operation. The battery is connected in parallel to the dc capacitor of STATCOM. The STATCOM is a three-phase voltage source inverter having the capacitance on its DC link and connected at the point of common coupling. The STATCOM injects a compensating current of variable magnitude and frequency component at the bus of common coupling
Controller for STATCOM
The control scheme approach is based on injecting the currents into the grid using ―bang-bang controller.‖ The controller uses a hysteresis current controlled technique. Using such technique, the controller keeps the control system variable between boundaries of hysteresis area and gives correct switching signals for STATCOM operation. The control system scheme for generating the switching signals to the STATCOM is shown in Fig2.The control algorithm needs the measurements of several variables such as three-phase source current, DC voltage, inverter current with the help of sensor. The current control block, receives an input of reference current and actual current are subtracted so as to activate the operation of STATCOM in current control mode.
Modeling of Control Circuit
The control scheme approach is based on injecting the currents into the grid using ―bang-bang controller.‖ The
controller uses a hysteresis current controlled technique. Using such technique, the controller keeps the control system
variable between boundaries of hysteresis area and gives correct switching signals for STATCOM operation. The control
system scheme for generating the switching signals to the STATCOM is shown in Fig2.The control algorithm needs the
measurements of several variables such as three-phase source current, DC voltage, inverter current with the help of sensor.
The current control block, receives an input of reference current and actual current are subtracted so as to activate the
operation of STATCOM in current control mode Once the reference supply currents are generated, a carrier less hysteresis
PWM controller is employed over the sensed supply currents and instantaneous reference currents to generate gating pulses
to the IGBTs of STATCOM. The controller controls the STATCOM currents to maintain supply currents in a band around
the desired reference current values. The hysteresis controller generates appropriate switching pulses for six IGBTs of the
VSI working as STATCOM
CONCLUSION
STATCOM system is an efficient mean for mitigation of PQ disturbances introduced to the grid. STATCOM
compensator is a flexible device which can operate in current control mode for compensating voltage variation, unbalance
and reactive power and in voltage control mode as a voltage stabilizer. The latter feature enables its application for
compensation of dips coming from the supplying network. The simulation results show that the performance of STATCOM
system has been found to be satisfactory for improving the power quality at the consumer premises. STATCOM control
algorithm is flexible and it has been observed to be capable of correcting power factor to unity, eliminate harmonics in
supply currents and provide load balancing. It is also able to regulate voltage at PCC. The control algorithm of STATCOM
has an inherent property to provide a self-supporting DC bus of STATCOM. It has been found that the STATCOM system
reduces THD in the supply currents for non-linear loads. Rectifier-based non-linear loads generated harmonics are
eliminated by STATCOM. When single-phase rectifier loads are connected, STATCOM currents balance these unbalanced
load currents.