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In recent years, the large penetration of wind power system is considered as an effective means for power production due to this continuously growing nature of wind power system, power utilities does not consider the power quality problems that are issued by the system. They are going for other power system stability analysis. This will produce an integration of large wind power system with the electrical network that are series compensated for ensuring bulk power transfer. But this series compensated transmission system will produce an unwanted effect of subsynchronous resonance. SMES (Super Conducting Magnetic Energy Storage) with STATCOM (Static Compensator) controller provide an efficient damping for Subsynchronous Resonance that will enhance power system stability in addition to real and reactive power compensation. And it will reduce MVA rating required by the STATCOM when it is operated alone. Here output of PV cell is used instead of a constant dc source. This work mainly concentrate for the coordinated operation of STATCOM control with SMES scheme for the series compensated wind power system for damping power system oscillation is simulated using MATLAB/SIMULINK in power system block set.
Index terms: Superconducting Magnetic Energy Storage (SMES), Doubly-Fed Induction Generator (DFIG), Subsynchronous Resonance (SSR), Wind Turbine generator (WTG).
Flexible Ac Transmission Systems (FACTS) can provide an effective solution to alleviate SSR  and thyristor-based FACTS controllers have been employed in the field for this purpose . Wind turbines are subject to mechanical modes of vibrations related to turbine blades, shaft, gear train and tower . In the case of wind turbine generators operating radially on the end of a series compensated transmission line, there is the potential for induction-machine self-excitation SSR . The issue of power quality is of great importance to the wind turbine . The Section II introduces the subsynchronous resonance. The wind speed is transmitted as fluctuations in the control technology have been proposed for improving the concept of SSR and its causes. Section III deals with SSR in DFIG based wind generation. The Section IV, V, and VI discuss SMES and STATCOM control.VII denotes simulation results.
II. SUBSYNCHRONOUS RESONANCE (SSR)
Subsynchronous resonance occurs in a power system network, when the mechanical system generator exchanges its energy with the electrical network. So there will be a resonant condition occur between mechanical and electrical systems. That will results an unwanted effect of subsynchronous resonance. Series compensation in the line results in the excitation of subsynchronous currents at an electrical frequency 'fe' is given by
fe = f0* Xc / Xl
Where Xc is the reactance of the series capacitor, Xl is the reactance of the line including that of the generator and transformer and 'f0' is the nominal frequency of the power system. Currents at the resonant frequency 'fr' is given by
fr = f0-fe
These rotor currents are responsible for the production of subsynchronous armature voltage components which may enhance subsynchronous armature currents to produce SSR. There are two ways for the production of SSR.
1) Self excitation involving both an induction generator effect and torsional interaction.
2) Transient torque (also called transient SSR).
Induction Generator Effect
Generator Self-excitation in the electrical system alone is the cause for induction generator effect. This can be explained in the case of a wind farm comprising self-excited induction generators (SEIGs). Negative resistance is offered by the induction generator for armature currents will cause this effect. As the rotating MMF produced by the subsynchronous frequency armature currents moves at speed Ns which are slower than the speed of the rotor Nr. The resistance of the rotor (at the subsynchronous frequency viewed from the armature terminals) is negative, as the slip 'S' of the induction generator is negative. Negative resistance is offered by the induction generator for armature currents will cause the effect of Torsional interactions.
When the magnitude of this resistance exceeds the sum of the armature and network resistances at a resonant frequency there will be self-excitation and the subsynchronous electrical current will tend to increase rapidly.
This form of self excitation involves both electrical and rotational dynamics. This may occur when the electrical resonant frequency 'fs' is very close to the complement of a torsional resonant frequency 'fr' of the turbine-generator (TG) shaft system. The torques at rotor frequencies may get amplified and potentially lead to shaft damage.
Transient SSR generally refers to transient torques on each segments of the T-G shaft resulting from subsynchronous oscillating currents in the network caused by faults or switching operation. This is mainly due to system disturbance, will alter the network topology also.
III.ANALYSIS OF SSR IN A DFIG
Now-a-days, the majority of wind turbines are equipped with Doubly Fed Induction Generators (DFIGs). In the DFIG concept, there will be separate winding employed for both stator and rotor. The wound rotor induction generator is grid-connected at the stator terminals as well as at the rotor mains via a partially rated variable frequency AC/DC/AC converter (VFC). This only needs to handle a fraction (25%-30%) of the total power to achieve full control of the generator. The VFC consists of a Rotor side Converter (RSC) and a Grid-Side Converter (GSC) connected back-to-back by a dc-link capacitor, In order to meet power factor requirement (e.g. âˆ’0.95 to 0.95) at the point of connection. In order to get a complete control of entire transmission line, voltage across the dc link capacitor must be double the value of grid voltage. Most wind farms are equipped with switched shunt capacitors for conventional reactive power compensation. Moreover, because many wind farms are connected to weak power system networks which are characterized by low short circuit ratios and under-voltage conditions. Dynamic power electronic devices such as Static Var Compensator (SVC) and a Static Synchronous Compensator (STATCOM) have been increasingly used in wind farms to provide rapid, smooth reactive power compensation and finally increase the voltage profile when it is connected to the grid. During a grid fault the voltage sags and swells at the connection point of the wind farm can cause a high current in the rotor circuit and the converter. Since the power rating of the variable frequency converter is only about 25%-30% of the total induction generator power rating. This over-current can lead to the damage of the converter. Therefore, one of the important issues related to the wind farms equipped with DFIGs is the grid fault or low voltage ride through capability and system stability. One technique of blocking the RSC is short circuiting the rotor circuit by a crow-bar circuit to protect the converter from over current. WTGs continue their operation to produce some active power and the GSCs can be set to control the reactive power and voltage.
ÂÂÂÂÂÂÂ Fig. 1.Wind turbine model
IV. Superconducting Magnetic Energy Storage (SMES)
A SMES device is a dc current controlled device that stores energy in the magnetic field. The dc current flowing through a superconducting coil in a large magnet creates the magnetic field. The inductively stored energy (E in Joule) and the rated power (P in Watt) are commonly given in specifications for SMES devices and they can be expressed as follows:
where L is the inductance of the coil, I is the dc current flowing through the coil and V is the voltage across the coil.
Fig. 2. Components of a typical SMES system
A SMES system consists of a superconducting coil, cryogenic system and the power conversion or conditioning system (PCS) used for control and protection functions.
IEEE defines SMES as "A Superconducting Magnetic Energy Storage device containing electronic converters that rapidly injects and/or absorbs real and/or reactive power or dynamically controls power flow in an ac system". Energy storage device like SMES have a capability to compensate real power also.
V. STATCOM/SMES Topology
A Static Compensator (STATCOM) is a shunt-connected device, which injects reactive current into the AC system. This leading or lagging current can be controlled independently by the AC system voltage supplied through power electronics based variable voltage source. The STATCOM does not employ a capacitor or reactor banks to produce reactive power as the Static VAR Compensators (SVC) do. In the STATCOM the capacitor is used to provide a constant DC voltage in order to allow the operation of the voltage-source converter. A STATCOM controller with SMES is similar to an ideal synchronous machine, which generates a balanced set of (three) sinusoidal voltage at fundamental frequency with controllable amplitude and phase angle. Fig. 3 indicates the proposed model of the SMES controller for the application in a distributed power network. This model consists of the PCS and the SMES coil with its damping and protection system.
Fig. 3. Detailed model of the proposed SMES system including the PCS and the SC.
A. Power Conditioning System
The PCS provides a power electronic interface between the WTG and the Short circuit Coil which have an ability to achieve two goals: one is to convert electric power from dc to ac and the other is to charge/discharge the coil efficiently. The major component of the proposed PCS is the well-known three-phase voltage source inverter (VSI) or converter (VSC) shunt-connected to the distribution network by means of a step-up Î”-Y coupling transformer as depicted in Fig. 3. The major component of a VSI is the Isolated Gate Bipolar Transistors (IGBTs). Due to its lower switching losses and reduced size when compared to other devices its operation have high priority. Output voltage control of the VSI can be efficiently achieved through sinusoidal pulse width modulation (SPWM) techniques. A standard three-level six-pulse inverter structure is used here. This three-level six-pulse VSI topology generates more smooth sinusoidal output voltage waveform than a conventional structure without increasing the switching frequency. It effectively doubles the power rating of the VSI for a given semiconductor device. This is used to analyses the output waveform structure so that not only the harmonic performance of the inverter is improved but also the efficiency and reliability of the system.
The inclusion of the SMES coil into the dc bus of the VSI account the use of an improved interface to adapt the wide range of variation in voltage and current levels between both devices. A new two-quadrant three-level IGBT DC/DC converter or chopper is proposed to be employed in this work as shown in Fig. 3. This converter have an ability to decrease the ratings of the overall PCS (specifically the VSI and transformer) by regulating the current flowing from the SMES coil to the inverter of the VSI and vice versa. Major advantages of the three-level dc/dc chopper topologies is the reduction of voltage stress of each IGBT by half, permitting to increase the chopper power ratings while maintaining high dynamic performance and decreasing the harmonics distortion produced with traditional two-level ones include . Furthermore, it includes the availability of redundant switching states which allows the generation of the same output voltage vector through various states. This feature is very important to reduce the switching losses and VSI dc current ripple but used to maintain the charge balance of the dc-bus capacitors thus avoiding the contribution of additional distortion into ac power system.
Fig. 4. Three-level chopper output voltage vectors and their corresponding IGBT's switching states
Fig. 4. shows all possible combination of the chopper output voltage vectors Vab (defining the SMES side of the circuit as the output side) and their corresponding IGBT switching states. As derived the chopper can be thought of a switching device that combines various states for getting either a positive, negative or null voltage to the SC coil. As a result the charge balance of the dc-bus capacitors can be controlled by using extra switching states. The output voltage vectors can be selected based on the required SMES coil voltage. In the third mode of operation i.e. the discharge mode, the chopper works as a step-up (BOOST) converter. Since power is feedback from the SC to the electric grid this mode can also be called regenerative mode and makes use of a combination of negative and null vectors. Consequently only one semiconductor device is switched per switching cycle in the same way as the charge mode.
B. SMES Coil
An SMES system consists of several sub-systems which must be carefully designed in order to obtain a high-performance compensation device under the phenomenon of superconductivity. The base of the SMES unit is a large Superconducting Coil (SC), whose basic structure is composed of the cold components itself (the SC with its support connection components and the cryostat) and the cryogenic refrigerating system in Fig. 3. The equivalent circuit of the SMES coil makes use of a lumped parameters network represented by a four-segment model comprising self inductances (Li ), mutual couplings between segments (i and j, Mij ), ac loss resistances (Rp ), skin effect related resistances (Rpi ), turn-ground (shunt-CShi), and turn- turn capacitances (series-CSi) are shown in Fig. 3.
VI. PROPOSED CONTROL SCHEME OF THE SMES SYSTEM
The proposed hierarchical control schemes of the SMES unit consist of external, middle and internal level controllers. Its design based on the concepts of instantaneous power theory on the synchronous rotating d-q reference frame. This structure has the goal of rapidly and simultaneously controlling the active and reactive power flow provided by the SMES. For achieving this aim the controller must ensure the instantaneous energy balance among all the SMES components. In this way the stored energy is regulated through the PCS in a controlled manner for regulating the charging and discharging of the SC coil. So power system oscillation gets damped out.
The external level controller is outlined in Fig. 5. is primarily responsible for determining the active and reactive power exchange between the SMES and Wind Turbine generating system. This control strategy is designed for performing two major high-prioriy control objectives: one is Voltage Control Mode (VCM), which have a capabilities to compensate reactive power (case of a traditional static synchronous compensator with no ESS) and the Frequency Control Mode (FCM) which used for both active and reactive power exchange aiming to controlling (regulating and supporting) the power system frequency (case of an SMES operating as a stabilization device). The standard control loop of the external level is the VCM and consists for controlling the voltage at the point of common coupling (PCC) of the SMES to the distribution feeder of the WTG through the stabilization of the quadrature part of reactive component iq. This control mode has proved a very good performance in conventional static compensators (with no energy storage).
The control mode is subordinated to the FCM when the device is working as a power system stabilizer. A standard Proportional-Integral (PI) compensator is including in the system to enhance the dynamic performance of the VCM system. This mode compares the reference voltage 'Vr' set by the SMES operator with the actual measured value at the Point of common coupling (vm) in order to eliminate the steady-state offset in the voltage waveform via a PI compensator/controller. A voltage regulation droop Rd (typically5%) is included in order to allow the terminal voltage of the VSI to vary in proportion with the compensating reactive current. Droop gain is also used to divide the loads equally when a parallel connection of device is employed.
Fig. 5. General structure of the external-level control of the SMES system.
The FCM is the high priority control mode that aims to regulate the WTG frequency through the modulation of both the active and reactive components of the output current 'iq' (as a conventional var compensator) and the active component 'id' (case of an SMES). The voltage reference signal Vr is adjusted with a stabilizing voltage signal proportional to Î”f (defined as frâˆ’f) which directly represents the power oscillation of the power system. This added signal responds the output quadrature current of the VSI 'iq' to vary around the operating point defined by Vr, purpose of this variation being to improve the damping of the power system oscillations. In this way the voltage at the PCC is forced to decrease when frequency deviation Î”f is positive aim to reducing the transmitted power through the distribution system and thus providing an effective fast-acting voltage reduction reserve that opposes the deceleration of generators in the WTG. The phase locked loop (PLL) is used to produce The frequency deviation signal Î”f . In all cases the frequency signal is derived from the positive sequence components of the ac voltage vector measured at the PCC of the SMES through a PLL. The design of the PLL is based on concepts of instantaneous power theory in the d-q reference frame.
The middle level control makes the expected output
i.e. positive sequence components of id and iq to dynamically track the reference values set by the external level controller. The middle level controller design, is shown in Fig. 5.1. In this model shows a cross-coupling of both the components of the SMES voltage source inverter output current through the synchronous angular speed of the grid voltage at the fundamental frequency. Therefore, in order to achieve a fully decoupled active and reactive power control it is simply required to decouple the control of id and iq through two conventional PI controllers.
Fig. 5.1.General structure of the middle-level control of the SMES system
Fig. 5.2 shows a basic scheme of the internal-level control of the SMES unit. The internal level provides a dynamic control of the input signals for the DC/AC and DC/DC converters. This level is responsible for generating the triggering/firing control signals for the three-level VSI, according to the Sinusoidal Pulse Width Modulation Techniques (SPWM).
Fig. 5.2.General structure of the internal-level control of the SMES system
In SPWM technique, output voltage waveform which is obtained from the middle level controller is compared with the triangular waveform generated by the carrier generator. These states are decoded by the states-to pulses decoder for getting the corresponding firing pulse for each IGBT in the VSI.
VII. SIMULATION STUDY AND RESULTS
Fig. 6.Simulation block of SMES with STATCOM controller
Fig. 6.1.Simulation result of VSI
Fig. 7.Simulation block of SMES with STATCOM controller with PV cell input
Fig. 7.1 Simulation Result of PV cell
Fig. 8. Real power variation in a Transmission Line
Fig. 8.1. Reactive power variation in a Transmission Line
Fig. 9.Simulink block of wind power system with SMES and STATCOM
Fig. 9.1 Grid Voltage waveform
Fig. 9.2.Current waveform
Fig. 9.3.Wind speed waveform
Fig. 9.4 Real power variation
Fig. 9.5.Reactive power variation
This paper has presented an effective SMES controller for the stabilization and control of the power flow in a three phase transmission line and series compensated wind power systems. SMES controllers have an ability to control both active and reactive power in the line in addition to damp power system oscillation in a power system so it will provide power system stability. It also provide a cost effective mechanism instead of a STATCOM alone operation. Here PV cell input Is also used instead of a constant dc supply. Simulation results will show the variation of active and reactive power variations.
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