Underexcitation Limiter Research Articles

Izbor parametara i analiza dinamičkih karakteristika limitera minimalne pobude primenom računarskog modela

This paper presents a method for selecting parameters and analyzing the dynamic characteristics of a under-excitation excitation limiter using a computer model. The parameters are determined using the pole cancellation method for the worst-case operating regime from the perspective of the under-excitation limiter to ensure stability in all operating regimes. The analysis of the dynamic characteristics presented in the paper first obtained the time responses and frequency characteristics of the excitation control system when the limiter is active. Based on these, the values of dynamic performance indicies that indicate the quality of regulation are determined. The paper also provides a brief description of the implementation of the limiter and the computer models used in the analysis.

A New Admittance Approach Applicable to the Coordination Between the Loss-of-Excitation Protection and the Underexcitation Limiter

The performance of salient-pole synchronous generators protection is of great importance for the stability of power systems, as they are present in large hydropower plants, subject to various abnormal operating conditions such as three-phase or two-phase faults, electromechanical disturbances and loss-of-excitation. In this context, this work proposes a testing philosophy of salient-pole synchronous generators, in order to define more accurately the setting of the protections and controls of this asset. Thus, in the developed methodology, the admittance plan (G-B) proves applicable to analyze the coordination between the loss-of-excitation protection and the underexcitation limiter, as an alternative to the power (P-Q) and impedance (R-X) planes, normally employed in literature. In modeling the problem, real data for both the hydrogenerators and the electrical network are used. The simulations were performed in a hardware-in-the loop scheme integrated by a Real Time Digital Simulator and a physical relay. Thus, analyzes are made for situations of stable power swing and partial and total loss-of-excitation. The results are useful to prove, through the monitoring of relevant variables of the hydrogenerators, the feasibility and accuracy of the G-B plane in the coordination between the protection and the control of the synchronous machine.

Self-Excitation of a Synchronous Generator During Power System Restoration

This paper analyzes synchronous generator self-excitation during power system restoration, caused by a large capacitive load and the impact of generator voltage control on the voltage rise. The paper analyzes measurements from a recent self-excitation incident in the Finnish 400-kV transmission system, models the system using an electromagnetic transient model and a linear d -axis generator model, validates the models using the measurements and analyzes the impact of generator voltage control during self-excitation. Self-excitation is rare during a normal system operation but may occur in a weak system during system restoration after a blackout. Since high voltages caused by the self-excitation may delay the restoration or even damage the equipment, self-excitation should be taken into account in restoration planning. This paper shows that the voltage controller and especially the under-excitation limiter settings of the generator have significant impact on the voltage during self-excitation. This paper also shows that details in the excitation system response influence the simulations significantly and reference measurements from the specific generator are required for detailed modeling. However, all measured incidents with digital static excitation systems are important references for simulations. In addition, special attention should be given to protection relays during the restoration.

Development of an Adaptive Underexcitation Limiter in Excitation System

A generator is designed to operate in the underexcitation region within its capability according to the system conditions. However, a sustained underexcitation condition owing to the partial and complete loss-of-field in the generator can cause thermal damage and system instability. Thus, an automatic voltage regulator and an underexcitation limiter should control the underexcitation condition of the generator, and the loss-of-field relay should be utilized to guarantee reliable operation. Therefore, in this paper, we discuss the coordination of generator underexcitation protection among various factors. In addition, in order to enhance the conventional underexcitation limiter, we proposed an adaptive underexcitation limiter. Through this method, a possibility of miscoordination can be avoided and the operation is modified adaptively according to the loss-of-field condition. The performance of the proposed method was verified via a 345-kV Korean nuclear power plant and transmission system using the Electromagnetic Transients Program.

A Theoretical and Practical Approach for Underexcitation Protection and Control Studies of Hydrogenerators in a Real-Time Environment

The installation and settings of protection devices and excitation control systems require experience since a failure can cause damage to the synchronous generator and negative impact on the power grid. A test bed implemented in a real-time digital simulator (RTDS) is presented in this paper as an alternative for the training and learning process that involves the knowledge of an effective coordination between such protection and control systems of synchronous generators. A complete generation system is implemented in a RTDS, including an excitation system model with its limiters. These limiters are coordinated with the protection functions implemented in a physical relay. The contributions of this work are the demonstration of the importance of this type of study (i.e., real-time simulations results through hardware-in-the-loop tests), the consolidation of the impedance (R-X) and admittance (G-B) planes as the more appropriate ones to analyze the coordination between the ANSI 40 function and the underexcitation limiter, and the presentation of a study focused on hydrogenerators (including novel settings for the ANSI 40 protection).

Effective Micro Grid Stability Under Excitation Limiters in Islanded and Connected Modes

In this paper the authors tried to design a under excitation limiter and a power system stabilizer which can operate without any kind of interaction. The under excitation limiter (UEL) is intended to prevent reduction of generator excitation to a level where the steady state stability limit or the stator core end-region heating limit is exceeded. The power system stabilizer (PSS) uses auxiliary stabilizing signals to control the excitation system so as to improve power system dynamic performance.

Coordinated Design of Under-Excitation Limiters and Power System Stabilizers

In this paper the authors tried to design a under excitation limiter and a power system stabilizer which can operate without any kind of interaction. The under excitation limiter (UEL) is intended to prevent reduction of generator excitation to a level where the steady state stability limit or the stator core end-region heating limit is exceeded. The power system stabilizer (PSS) uses auxiliary stabilizing signals to control the excitation system so as to improve power system dynamic performance.

Design of Volts per Hertz limiter with consideration of the under-excitation limiter control actions

The dynamical relationship between the Volts per Hertz limiter and the under-excitation limiter control loops in a synchronous generator is examined using the frequency response technique. From the analysis, it is concluded that sustained oscillations due to the interaction between the two limiters can occur should the loading level of the generator exceeds a certain threshold value. For generator operation below this value, a systematic design procedure has been proposed for the limiters which will ensure network stability.

Coordinated design of under-excitation limiters and power system stabilizers

Summary form only given as follows. The inherent dynamical relationship between the under-excitation limiter (UEL) and the power system stabilizer (PSS) control loops in synchronous generators is studied using the frequency response technique. While the stabilizers are effective in introducing electromechanical damping torque components into the generators, it is shown that the limiters should be designed to effect a much slower response characteristics as their main function is to prevent excessive stator end-core heating. The analysis also shows that a reduction in the values of the slope of the boundary curves, which prescribe the operating region of the limiters, is accompanied by a decrease in the damping level of the closed-loop excitation control systems. A method which decouples the design procedure of the UEL from that of the PSS is then proposed.

Under excitation limiter and its role in preventing excessive synchronous generator stator end-core heating

The process of synchronous generator stator end-region (SER) heating in the under-excited condition is investigated using a thermal model based on flux densities consideration. The parameter of the model are identified using the least-square method and from which the SER steady-state and transient thermal responses are determined. A new SER heating limit is derived and used for the design of the under-excitation limiter. The proposed method makes more effective use of the generator capability to absorb reactive power without causing instability, SER overheating, and serious system overvoltage problems.

Under-excitation limiter design by variable structure control

A new technique to the design of under-excitation limiters (UEL) for synchronous generators using variable structure control is presented. Based on a linearized model of the a radial power system, the design problem has been formulated as one where the UEL gains are allowed to vary. Some useful expressions relating the gains with system parameters have been derived. The approach has been generalized to include operating conditions and parameter variations likely to occur in power systems. The VSC UEL is shown to be robust and provide much improved damping compared to that based on the conventionally designed UEL.

Underexcitation limiter models for power system stability studies

Underexcitation limiter models suitable for use in large scale power system stability studies are presented. These models are compatible with current IEEE recommended excitation system models. Using these models, it is expected that most underexcitation limiters in use on large system-connected synchronous machines can be represented.

Steady state stability of a remote gas-turbine generating station: field tests and validation of computer simulation results

This paper examines the steady state stability aspects of a remote power station. Results of staged-tests, performed on two gas turbine units, are utilized to validate and improve on the generator and excitation system models used in simulation studies. The validated models allow stability limits for these units to be determined much more accurately. A new underexcitation limiter scheme is then proposed. It permits the reactive power absorption capability of the generators to be fully utilized. The device would also prevent the gas turbines from operating beyond their stability limit.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Extended use of Generator Reactive Capability By a Dual Underexcitation Limiter

A dual underexcitation characteristic limiter scheme for voltage regulators is introduced which extends the usable reactive capability of generators for the control of transmission voltage levels. A field installation at Commonwealth Edison's Powerton Station is described which demonstrates additional underexcited capability of approximately 300 Mvars from two 992 MVA 90% power factor generators.