Evangelos Farantatos

ARMAX-Based Transfer Function Model Identification Using Wide-Area Measurement for Adaptive and Coordinated Damping Control

One of the main drawbacks of the existing oscillation damping controllers that are designed based on offline dynamic models is adaptivity to the power system operating condition. With the increasing availability of wide-area measurements and the rapid development of system identification techniques, it is possible to identify a measurement-based transfer function model online that can be used to tune the oscillation damping controller. Such a model could capture all dominant oscillation modes for adaptive and coordinated oscillation damping control. This paper describes a comprehensive approach to identify a low-order transfer function model of a power system using a multi-input multi-output (MIMO) autoregressive moving average exogenous (ARMAX) model. This methodology consists of five steps: 1) input selection; 2) output selection; 3) identification trigger; 4) model estimation; and 5) model validation. The proposed method is validated by using ambient data and ring-down data in the 16-machine 68-bus Northeast Power Coordinating Council system. The results demonstrate that the measurement-based model using MIMO ARMAX can capture all the dominant oscillation modes. Compared with the MIMO subspace state space model, the MIMO ARMAX model has equivalent accuracy but lower order and improved computational efficiency. The proposed model can be applied for adaptive and coordinated oscillation damping control.

Measurement-Based Real-Time Voltage Stability Monitoring for Load Areas

Summary form only given. This paper proposes a measurement-based voltage stability monitoring method for a load area fed by N tie lines. Compared to a traditional Thevenin equivalent based method, the new method adopts an N+1 buses equivalent system so as to model and monitor individual tie lines. For each tie line, the method solves the power transfer limit against voltage instability analytically as a function of all parameters of that equivalent, which are online identified from real-time synchronized measurements on boundary buses of the load area. Thus, this new method can directly calculate the real-time power transfer limit on each tie line. The method is first compared with a Thevenin equivalent based method using a four-bus test system and then demonstrated by case studies on the NPCC (Northeast Power Coordinating Council) 48-machine, 140-bus power system.

An adaptive three-bus power system equivalent for estimating voltage stability margin from synchronized phasor measurements

This paper utilizes an adaptive three-bus power system equivalent for measurement-based voltage stability analysis. With that equivalent identified online, a measurement-based approach is developed to estimate real-time voltage stability margin for a load-rich area supported by remote generation via multiple tie lines. Compared with traditional Thevenin equivalent based approach, this new approach is able to provide more accurate voltage stability margin for each individual tie line. This approach is validated on a three-bus system and the IEEE 39-bus system.

Short-circuit current contribution of converter interfaced wind turbines and the impact on system protection

Traditional short-circuit modeling techniques and the associated models existing in commercially available packages are not accurate enough and do not accurately represent the behavior of converter interfaced renewable energy resources during short-circuit events. To address those issues, detailed EMT-type, time domain models of Type III and Type IV wind turbines have been developed as part of this work. Those models can be used to achieve an improved understanding of the way in which these devices affect system protection and of improved short-circuit models for system studies. The potential impact of renewable energy resources on relay misoperation and protection coordination is discussed, given that depending on the type of the wind turbines and the associated controls, the short circuit current contribution of a wind park might be the same for different locations of the fault. Finally, presently available modeling techniques of renewables in frequency domain are presented and compared. Their disadvantages are discussed along with an approach for improved modeling.

A study on fluctuations in electromechanical oscillation frequencies of power systems

This paper investigates the fluctuation phenomenon in the electromechanical oscillation frequency of a power system. Analysis on the power system swing equation explains that the oscillation frequency fluctuates around a central frequency due to the nonlinear system nature of a power system. A Phase-Plane Trajectory based method and the Prony method are used to track the real-time frequency fluctuation, based on which a realtime angular stability margin index is proposed for early warning of angular stability issues. That index is tested by case studies on a two-generator system and a 179-bus power system.

Observability for PMU-based monitoring of nonlinear power system dynamics

PMUs placed at different buses enable dynamic monitoring of a power system. The question is whether those limited PMU measurements can provide sufficient information. In this research, observability analysis is used to answer this question. Different observability concepts together with their related state monitoring applications are discussed in this paper. Nonlinear dynamic observability based on Lie derivatives is applied to the PMU-based dynamic monitoring of power system. The smallest singular value of the observability matrix serves as an observability index. A 3-bus system is used to illustrate the observability concepts presented in this paper. The concepts of observability are clarified and the nonlinear observability analysis provides a basis for PMU-based on-line wide area monitoring and control to improve the dynamic performance of power systems.

Methods to establish input-output relationship for system identification-based models

Both model-based and measurement-based methods are presented in this paper to describe the correlation between measurement locations. Transfer impedance (the model-based method) and correlation coefficient (the measurement-based method) are compared and applied to input location selection of power system dynamic modeling for dynamic response estimation. The comparison results show that both methods can describe the correlation between measurement locations effectively.

An Accurate Type III Wind Turbine Generator Short Circuit Model for Protection Applications

The integration of renewables into power systems introduces several technical challenges including the development of appropriate models for system protection studies and accurate evaluation of short circuit contributions. Converter interfaced wind turbines (WTs) produce significantly different current waveform signatures compared to the traditional synchronous or asynchronous generators. This paper proposes a new phasor domain modeling approach for Type III WTs with doubly-fed induction generators (DFIGs) using the concept of control based equivalent circuits. The proposed model accounts for the impact of negative sequence quantities on WT control response to achieve accurate simulation of unbalanced faults.

Adaptive wide-area damping control using measurement-driven model considering random time delay and data packet loss

One of the main drawbacks of the existing wide-area damping controller (WADC) that are usually tuned based on several selected typical operating conditions, is its limited adaptability to continuous variations in operating conditions. An adaptive WADC employing the lead-lag structure using measurement-driven model is proposed in this paper. The state subspace model is identified online using ambient data or ring-down data to represent system oscillatory behaviors. The parameters of the lead-lag time constants can be updated based on the new residue derived from the identified model, while the new control gain can also be determined based on the identified model to achieve maximum damping ratio. Moreover, a delay compensator adopting the lead-lag structure and the quadratic interpolation algorithm are utilized to handle random time delay and data packet loss, respectively. The effectiveness of the proposed adaptive WADC is validated by the case study in the two-area four-machine system.

A Predictive Generator Out-of-Step Protection and Transient Stability Monitoring Scheme Enabled by a Distributed Dynamic State Estimator

A novel predictive generator out-of-step protection and transient stability monitoring scheme is presented. It is based on real-time dynamic monitoring of the system, enabled by a distributed dynamic state estimator (DSE). DSE utilizes local synchronized and nonsynchronized measurements and provides the evolution of the systems dynamic state at rates of 60 times/s. The real-time dynamic model is then utilized to evaluate the systems energy function based on Lyapunovs direct method, estimate the stability barrier, and characterize its stability. The two major components of the scheme are: the calculation of the systems center of oscillations and the derivation in real time of an equivalent, reduced size model used for the calculation of the generator stability barrier and its stability characterization. It is shown that the major advantage of the proposed scheme, compared with legacy generator impedance relay-based out-of-step protection, is that the out-of-step condition is predicted before its occurrence and the generator can be tripped much faster than the traditional scheme. The theoretical background of the scheme and application on an actual system are presented. The proposed scheme characterizes very accurate potential generator instability and predicts generator loss of synchronism before its occurrence, before the generator slips a pole.