Small Signal Disturbances Research Articles

Power Calculation Using RBF Neural Networks to Improve Power Sharing of Hierarchical Control Scheme in Multi-DER Microgrids

All control methods for the decentralized control of distributed energy resources (DERs) in the microgrid need to calculate power to decide whether the power produced will be able to stabilize the system. Unlike the previous research that is limited to the primary and secondary control levels, the presented decentralized droop-based control scheme includes detailed modeling for three hierarchical control levels for either grid-connected or autonomous modes. A new complementary control loop that is added to the hierarchical droop-based control scheme determines and controls the reactive power reference by a novel application of radial basis function neural networks (RBFNNs) for a fast, authentic, and accurate calculation of power to improve power sharing and enhance microgrid stability margins in facing with small and large signal disturbances. This method suppresses the low-pass filter that is normally used to determine high-frequency components of power and replaces it by the power flow nonlinear equation set that is solved by a novel application of RBFNNs, and consequently, power sharing to loads and network is done sufficiently. The simulation studies that have been performed on a microgrid consisting of four DERs and local loads using MATLAB/SIMULINK software demonstrate the effectiveness of the proposed control scheme.

Fuzzy and PID Stabilizers for Power Systems for Small Signal Stability Enhancement

Small signal stability investigation is important for stability analysis of power systems as a small signal disturbance could be an initiating event for a large system outage. Fuzzy logic as a novel method for power system stability has attained wide importance in the recent years. In this paper, stability studies have been carried out for a single machine infinite bus system which has been modeled according to the Heffron Philips model. A comparison between the conventional lead-lag, PID and Fuzzy controllers for damping the system oscillations has also been carried out in this paper. The system is simulated in MATLABTM Simulink environment and the results have been presented.

Development of Robust Excitation Controller for Synchronous Generator<sup></sup>

This paper deals with the design and evaluation of robust excitation controller for a single-machine infinite-bus power system. The design of the regulator guarantees the stability of the closed loop system and ensures the output voltage is maintained within an acceptable threshold. In addition, it damps out local mode oscillations for small signal disturbances. The designed robust controller is also analyzed under change in step input and disturbance, which limits the heavy oscillations on the speed ω and voltage. Glover-McFarlane loop shaping algorithm is applied in designing the robust excitation controller. Two different techniques such as Optimal control and mixed sensitivity approach is used in this paper. The performance of the AVR was analyzed and compared with IEEE type2 Exciter.

Dynamic load modelling of a paper mill for small signal stability studies

This paper considers a power system where about half the demand comprises industrial loads and some include large rotating machines. Therefore, the behaviour of these loads during system disturbances has to be considered for system stability studies. This paper describes a practical approach to the development of a Laplace transfer function (TF) model of a (paper) mill load connected to this power system. A PSCAD/EMTDCTM simulation model of the mill (the detailed model) was built from available information on the types of load within the mill. This detailed model was verified in a piece-wise manner, e.g. against individual motor starts. Instead of using the available generic load models, system identification theory is applied to evaluate the transfer function load model from simulated small signal disturbance responses. The model is developed for both voltage and frequency changes, and considers the actual frequency range of inter-area oscillations within the power system. In this paper the focus of the work is to accurately represent the mill load behaviour for small signal disturbances. It is recognised that the large signal requirements of a load model for transient stability studies are also important but the inevitable non-linearity of such a model would tend to saturate the loads’ behaviour during lesser disturbances. The performance of the developed small signal load model is tested using some practical disturbance profiles from this power system. This modelling approach allows development work to commence before real-life disturbance measurements are available since such data is currently scant. However, modern high bandwidth monitoring has recently come on-line for the mill and the TF load model will be fully verified over time as disturbance data builds up.

A novel stabilization approach for small signal disturbance of power system with time-varying delay

Small signal instability may cause severe accidents for power system if it can not be dealt correctly and timely. How to maintain power system stable under small signal disturbance is a big challenge for power system operators and dispatchers. Time delay existing in signal transmission process makes the problem more complex. Conventional eigenvalue analysis method neglects time delay influence and can not precisely describe power system dynamic behaviors. In this work, a modified small signal stability model considering time varying delay influence was constructed and a new time delay controller was proposed to stabilize power system under disturbance. By Lyapunov-Krasovskii function, the control law in the form of nonlinear matrix inequality (NLMI) was derived. Considering synthesis method limitation for time delay controller at present, both parameter adjustment method by using linear matrix inequality (LMI) solver and iteration searching method by solving nonlinear minimization problem were suggested to design the controller. Simulation tests were carried out on synchronous-machine infinite-bus power system. Satisfactory test results verify the correctness of the proposed model and the feasibility of the stabilization approach.

Damping Inter-Area Oscillations Based on a Model Predictive Control (MPC) HVDC Supplementary Controller

This paper introduces, formulates and evaluates an approach for damping inter-area oscillations of power systems based on supplementary current control of a line-commutated HVDC link. The proposed control is based on the model predictive control (MPC) strategy. The salient feature of the MPC as compared with other optimal control strategies, e.g., the linear quadratic Gaussian (LQG) control, is that it adjusts the control signal to achieve the objectives while explicitly respecting the plant constraints. This paper also compares the performance of the MPC with that of the LQG control. The two approaches are tested on the Western System Coordinating Council (WSCC) 9-bus system and the IEEE 14-bus system. Small-signal disturbance and large-signal disturbance stability studies are performed to demonstrate and compare the performances of the LQG and MPC methodologies. The study results show the effective and superior performance of the MPC for damping poorly damped oscillatory modes of the test systems.

Interleaved Buck Converter Applied to High-Power HID Lamps Supply: Design, Modeling and Control

As the output current levels of power converters increase, interleaved topologies become widely used. A well-known topology is the interleaved buck converter (IBC), which presents as main characteristic an inherent low ripple output current source behavior in addition to high efficiency and small volume. At the same time, it is necessary to drive high-intensity discharge (HID) lamps with nearly constant current/power and low-frequency square-wave shape due to HID lamp voltage source characteristics and their acoustic resonance issue. This way, an IBC can be advantageous for supplying such lamps, particularly in high power levels where efficiency and volume become more critical parameters. The goal of this paper is a proposal of an electronic ballast based on a two-cell IBC for high-power HID lamps, with current control loop, voltage sensing, and constant power. A dc model based on averaged state-space technique is obtained for the ballast, as well as an ac model based on small-signal disturbances. A complete concept with a digitally implemented closed loop has been developed and a prototype has been built, successfully achieving performance targets. This paper describes the most relevant aspects of the complete design and presents results both experimental and from simulation in MATLAB/Simulink software.

A New Optimum Design for a Single Input Fuzzy Controller Applied to DC to AC Converters

In this paper, the design of an optimum single input Fuzzy controller for application in dc to ac converters is presented. Contrary to conventional Fuzzy controllers, the proposed controller has a smaller number of rules and tuning parameters but is capable of performing identically to a conventional controller. These benefits lead to a simpler controller design. The controller is designed as a PI controller for small-signal disturbances. However, for optimum large-signal performance, heuristic tuning is used. The tuning is less complicated and hence optimum large-signal performance is achievable. The system is simulated and a hardware prototype was developed for comparison purposes.

NR-PSS (Nonlinear Robust Power System Stabilizer) for large synchronous generators and its large disturbance experiments on real time digital simulator

On the foundation of nonlinear robust control and exact generator model, this paper presents a design principle of NR-PSS (Nonlinear Robust Power System Stabilizer) for multi-machine power system, based on which an industrial NR-PSS equipment is developed. For popularizing it, the proposed parameter setting method of NR-PSS is completely the same as the widely used parameter adjustment rule of PSS. By virtue of real time digital simulator (RTDS), large disturbance experiments are carried out to compare the performances between generator excitation system equipped with NR-PSS and PSS in order to verify the correctness of design theory. The results show that compared with classical PSS, the proposed NR-PSS can dramatically improve the generator damping and attenuate the oscillation much faster, enhance the generator damping and raise both the small signal and large disturbance transient stability transmission power limit remarkably. The NR-PSS equipment with independent intellectual property right has been successfully put into operation on a 300 MW generator in Baishan Hydro Plant of Northeast China Grid more than 10 months.

Dynamic range optimization of weakly nonlinear, fully balanced, gm-c filters with power dissipation constraints

This paper presents an analytical algorithm for optimizing the dynamic range of continuous-time active filters under a fixed power dissipation constraint. Assuming weak third-order nonlinearities, we model distortion as a small signal disturbance of the linear solution and use linear techniques in the distortion analysis. Due to the generality of the formulation many important distortion phenomena can be described, e.g., the compression of the desired in-band signal due to large out-of-band interferers, intermodulation distortion, etc. Simple, closed-form expressions are derived for the noise and the distortion of the filter as functions of power dissipation. These expressions are used to optimize the noise, distortion and dynamic range, of the filter. The validity of the proposed algorithm is verified with simulation.

Enhancing small signal power system stability by coordinating unified power flow controller with power system stabilizer

Advancement of power electronic technologies makes ac systems to be more adaptive and flexible as new forms of power controller emerges in recent years. Though not yet well developed, underpinned research suggests that unified power flow controller (UPFC) is a promising power system controller. This paper proposes a method to coordinate UPFC with power system stabilizer (PSS) so as to damp down oscillations caused by small signal disturbance. It starts by deriving a mathematical model of UPFC, with which its linearized differential equations are integrated to design an overall control strategy for enhancing small signal stability of the so connected power systems. The approach is to identify the eigenvalue of the largest real part and then minimize it as a nonlinear optimization problem. In order to ensure the robustness of the control system, different operating conditions are simultaneously considered in the parameter optimization process. Results of a case study on the New England test system show that the model is correct and the method is promising for coordinating the model parameters to enhance the small signal stability of the complex power system.

Enhancing small signal power system stability by coordinating unified power flow controller with power system stabilizer

Advancement of power electronic technologies makes ac systems to be more adaptive and flexible as new forms of power controller emerges in recent years. Though not yet well developed, underpinned research suggests that unified power flow controller (UPFC) is a promising power system controller. This paper proposes a method to coordinate UPFC with power system stabilizer (PSS) so as to damp down oscillations caused by small signal disturbance. It starts by deriving a mathematical model of UPFC, with which its linearized differential equations are integrated to design an overall control strategy for enhancing small signal stability of the so connected power systems. The approach is to identify the eigenvalue of the largest real part and then minimize it as a nonlinear optimization problem. In order to ensure the robustness of the control system, different operating conditions are simultaneously considered in the parameter optimization process. Results of a case study on the New England test system show that the model is correct and the method is promising for coordinating the model parameters to enhance the small signal stability of the complex power system.

Improving power system dynamics by series-connected FACTS devices

This paper examines improvement of power system dynamics by use of unified power flow controllers, thyristor controlled phase shifting transformers and thyristor controlled series capacitors. Models suitable for incorporation in dynamic simulation programs for studying angle stability are analysed. A control strategy for the damping of electromechanical power oscillations using an energy function method is derived. The achieved control laws are shown to be effective both for the damping of large signal and small signal disturbances and are robust with respect to loading condition, fault location and network structure. Furthermore, the control inputs are easily attainable from locally measurable variables. The effectiveness of the controls are demonstrated for model power systems.

Design of a Fuzzy PI Controller for Power System Applications

A new adaptive fuzzy excitation controller for synchronous generators is proposed using fuzzy set theory. In the proposed linguistic fuzzy controller, generator speed deviation Δω and acceleration Δω are taken as controller inputs. A simple rule base consisting of a minimum number of rules is used for designing the fuzzy controller. A predictive routine is used for generating an adaptive compensating control when the system operating conditions or parameters change. To demonstrate the effectiveness of the fuzzy excitation controller, digital simulations of a power system subject to large and small signal disturbances over a wide range of operating conditions are performed. It is found that the adaptive fuzzy excitation controller produces significant damping in comparison to the conventional controller using a fixed gain structure.

An adaptive stabilizer for thyristor controlled static VAR compensators for power systems

A static VAR systems play an important role as a stability aid for small signal and transient disturbances in an interconnected power system. Further the introduction of an adaptive stabilizer loop to its controller can significantly improve power system performance under a variety of abnormal situations such as 3-phase short-circuits, voltage collapse, load shedding, reactance switching, etc. This paper presents the analytical development of a new controller scheme for a static VAR system and presents detailed simulation results to highlight its effectiveness in comparison with a discrete PI stabilizer under a variety of operating conditions for power transmission systems. The adaptive stabilizer is suitable for microprocessor digital implementation as the numerical steps involved are minimal.

Steady-State Stability Analysis of HVDC Systems with Digital Control

This paper derives and linearizes a mathematical model of an HVDC link with digital control. The discrete state equations for the multi-variable control with various pure time-delays are established by using a state-averaging method. The stability of operation has been evaluated analytically for small-signal disturbances. The effects of various parameters of control, dc link and the receiving ac system on stability of the overall system are discussed.

Improvement of the Dynamic Stability Performance of Multimachine Power Systems by the Use of Optimized Static Var Compensators

This paper presents the dynamic performance analysis of a multimachine power system operating with thyristor controlled static VAR compensators at the machine terminals. The synchronous machines of the multimachine power system are modelled using d-q axes quantities and the thyristor controlled static, VAR compensator as a inertialess variable voltage source with a fixed reactance. A biasing capacitor is connected in parallel with the controlled reactor to extend the operating range to both inductive and capacitive range, of the reactive power. The magnitude of' the voltage source however, depends on the firing angle of the thyristors forming the reactor compensator arms. In order to make the thyristor controlled static compensator effective over a wide range of system operating conditions, a variable structure stabilizing control loop with several gains and time constants is used along with auxillary feedback signals proportional to speed, terminal voltage and reactive power deviations.An eigenvalue search technique is used for the conbined state space model of the multimachine power system and static VAR compensator to optimize the structure of the stabilizing control loop for the best system damping. A two machine power system is considered for digital simulation to highlight the static compensator perfermance for improving the performance of the system for both small and large signal disturbances. From the simulation it is found that the system dynamic performance can be improved considerably during small disturbance conditions and faults and load rejection at the machine terminals using a static compensator. However when the rating of the compensator is limited to a smaller value( for economic considerations) it operates primarily in the bane-bang control node and therefore in place of a continuous controller, a switching controller With fixed inductor and capacitor component is feasible.

Evaluating alternative controllers for an electric power system

The effectiveness of optimal, suboptimal and modal controllers is examined for their suitability of implementation to electric power systems. The techniques for synthesizing the controllers are discussed at length and their performances for small signal disturbance are illustrated for the test system under study. The merits and disadvantages of the different controllers are examined so that definite recommendations may be made on the basis of system specifications and performance requirements.