Elizabeth P. Cheriyan

Brief paper: Adaptive RTRL based neurocontroller for damping subsynchronous oscillations using TCSC

Modern interconnected electrical power systems are complex and require perfect planning, design and operation. Hence the recent trends towards restructuring and deregulation of electric power supply has put great emphasis on the system operation and control. Flexible AC transmission system (FACTS) devices such as thyristor controlled series capacitor (TCSC) are capable of controlling power flow, improving transient stability and mitigating subsynchronous resonance (SSR). In this paper an adaptive neurocontroller is designed for controlling the firing angle of TCSC to damp subsynchronous oscillations. This control scheme is suitable for non-linear system control, where the exact linearised mathematical model of the system is not required. The proposed controller design is based on real time recurrent learning (RTRL) algorithm in which the neural network (NN) is trained in real time. This control scheme requires two sets of neural networks. The first set is a recurrent neural network (RNN) which is a fully connected dynamic neural network with all the system outputs fed back to the input through a delay. This neural network acts as a neuroidentifier to provide a dynamic model of the system to evaluate and update the weights connected to the neurons. The second set of neural network is the neurocontroller which is used to generate the required control signals to the thyristors in TCSC. This is a single layer neural network. Performance of the system with proposed neurocontroller is compared with two linearised controllers, a conventional controller and with a discrete linear quadratic Gaussian (DLQG) compensator which is an optimal controller. The linear controllers are designed based on a linearised model of the IEEE first benchmark system for SSR studies in which a modular high bandwidth (six-samples per cycle) linear time-invariant discrete model of TCSC is interfaced with the rest of the system. In the proposed controller, since the response time is highly dependent on the number of states of the system, it is often desirable to approximate the system by its reduced model. By using standard Hankels norm approximation technique, the system order is reduced from 27 to 11th order by retaining the dominant dynamic characteristics of the system. To validate the proposed controller, computer simulation using MATLAB is performed and the simulation studies show that this controller can provide simultaneous damping of swing mode as well as torsional mode oscillations, which is difficult with a conventional controller. Moreover the fast response of the system can be used for real-time applications. The performance of the controller is tested for different operating conditions.

Synthesis of power system model for SSR analysis

In this paper, the systematic synthesis of network model from a power system database for subsynchronous resonance (SSR) analysis in large power systems is discussed. The model is convenient to incorporate in a computer program using MATLAB sparse-matrix function. The formulation does not require preprocessing of data for removal of network state redundancy. This formulation is also suitable for selective generalized eigen analysis using widely available tools. A case study is carried out on a large practical system to illustrate the formulation. It is seen that selective eigen analysis of the network oscillatory modes gives information to identify the parts of the network to be modeled in sufficient detail for SSR studies. Advanced techniques can be applied on the identified sub-network to obtain a reduced study zone, retaining the essential dynamics of the power system for SSR analysis. The reduced study zone finds potential application for real-time simulation and also for testing of SSR damping controllers.

Design of a discrete Linear Quadratic Gaussian (DLQG) compensator for SSR damping using a sample invariant discrete model of TCSC

The paper presents the design of a discrete Linear Quadratic Gaussian (DLQG) compensator for SSR damping which includes a full order Kalman filter to estimate all the states of the system and a full state feedback regulator. The controller is tested on IEEE First Benchmark model for SSR studies. A Discrete Linear-Time Invariant (LTI) model of a Thyristor-Controlled Series Capacitor (TCSC) in the synchronously rotating DQ reference frame is used in this system in which, the sample invariance is achieved by a simple transformation of the zero sequence variables in the discrete domain. This model of Discrete TCSC is then integrated with discretised LTI model of the rest of the system under study. Thyristor firing is based on current synchronised phase-locked loop and the sampling frequency is six times the synchronous frequency. The eigen value analysis is done for this integrated system for different angles of conduction of the thyristor. The modeled system is having 27 state variables including the PLL states. The TCSC model and DLQG compensator design is validated using time domain simulation of the system.

Damping of interarea oscillations using SSSC and STATCOM with supplementary controllers

The paper investigates the effect of Static Synchronous Series Compensator (SSSC) and Static Synchronous Compensator (STATCOM) on damping interarea oscillations. Supplementary controllers for each of the flexible ac transmission system (FACTS) devices are developed. An objective function is formulated for tuning the controller parameters of the FACTS devices. The optimization is done based on the promising technique- Particle Swarm Optimisation (PSO). The performances of the proposed controllers are evaluated in a multimachine power system.

Wind energy system for a laboratory scale micro-grid

In the modern power system environment, distributed generation technologies including wind turbines, photovoltaic cells and fuel cells are gaining wide interest due to the benefits such as high reliability and good quality power supply. In the deregulated environment, micro-grids are getting more importance and the use of wind turbines plays a major role in the concept of green energy. Wind energy is one of the fastest emerging renewable alternatives for the electrical supply around the world. A single phase laboratory scale micro-grid system is essential to provide a test facility for demonstrations of advanced distributed generation system integration strategies. A wind turbine emulator is required to study the behaviour of wind energy system in the laboratory.

Modelling and simulation of direct driven wind electric generator for grid integration

The paper presents a model of direct driven wind electric generator (WEG) integrated to grid. Higher efficiency, less maintenance and better performance of the direct driven Permanent magnet synchronous generator (PMSG) make it most attractive among wind electric conversion systems. Pitch angle controlled variable speed wind turbine, PMSG and a back to back converter for optimum operation under integrated environment are modelled. Maximum power point tracking has been made available by drive train speed adjustment. The entire system is developed in per unit using classical modelling approach. The present model is analysed for varying wind conditions and found to be operating in the desired manner. The model is implemented in MATLAB-Simulink.

Identification and analysis of subsynchronous oscillations in DFIG based wind power plants

The increasing penetration of wind power into grid systems made it necessary for the integration of wind farms with series compensated transmission networks to ensure reliable and efficient power flow. In this paper the phenomenon of Sub Synchronous Resonance (SSR) observed in series capacitor compensated Doubly-Fed Induction Generator (DFIG) based wind energy systems has been investigated. The study system based on IEEE First Bench Mark (FBM) model for SSR studies with integrated DFIG wind turbine to grid through series compensated transmission network is simulated in MATLAB and the eigen value analysis is carried out. The various modes of oscillations such as subsynchronous resonance (SSR), supersynchronous resonance (SupSR), electromechanical, shaft and system modes are identified and analysed.

Bilateral converter to interface small battery energy storage system with micro-grid

A power electronic circuit to interface small battery energy storage (BES) system with grid is proposed in this paper. BES is essential in micro grid to have power balance and stable operation when there is an intermittent power generation and demand. A single phase bilateral converter proposed here is operated as an inverter when there is no supply from grid and operated as a charging circuit to charge the BES connected at one side of the converter when there is a grid supply. Two main switching schemes are suggested and a comparison is made between them while the converter is operated in inverter mode.

RTRL Algorithm Based Adaptive Controller for Non-linear Multivariable Systems

The paper presents a new design of adaptive and dynamic neural network-based controller architecture with feedback connection for non-linear multivariable systems. The network is trained online at each sampling interval using the desired output trajectory and the training method used is the Real Time Recurrent Learning Algorithm (RTRL). The recurrent network is a fully connected one, with feedback from output layer to the input layer through a delay element. Since the synaptic weights to the neurons are adjusted on-line, this controller has potential applications in real time control also. Moreover, it can be used for both continuous and discrete systems. The simulation results obtained by applying the algorithm to a non-linear multivariable system demonstrate the effectiveness of the proposed method.

FPGA based hardware implementation of solar PV inverter to act as STATCOM

Solar voltaic panels are getting common as a micro grid energy source day by day. Ample requirement of power quality improving devices are there in the grid. The scenario gives rise to a question that how we can avail more power quality improving devices, like static reactive power compensators by effective utilization of available devices. All the grid connected solar plants requires a three phase inverter and a capacitor which are the essential components of a voltage source inverter. All the voltage source inverters can act as a static reactive power compensator. The grid connected inverter in solar plant can also function as a reactive power compensator whenever rated active power transfer is not happening. The normal control scheme of solar plant inverter should be modified, and it should be implemented using FPGA board.