Technical Measures of Transferring Capability Improvement in Power System

In this paper, a generic load model is presented, suitable for dynamic simulations over a wide range of loading conditions. The proposed model is based on the well-established exponential recovery load model. Contrary to the conventional first-order form of this model, the proposed formulation employs variable-order transfer functions, enabling the accurate simulation of high-order load dynamics. A complete methodology is proposed for the identification of the required load model parameters from real field measurements. For this purpose, the Vector Fitting technique is introduced for the first time as a method for measurement-based load modeling. Several simulation scenarios are performed using the NEPLAN software to validate the accuracy of the proposed model. Numerical studies indicate that the developed load model captures accurately the load dynamic behavior over a wide range of loading conditions.

Summary form only given. Power system load modelling is a mature and generally well researched area which, as many other in electrical power engineering at the present time, is going through a period of renewed interest in both industry and academia. This interest is fuelled by the appearance of new non-conventional types of loads (power electronic-based, or interfaced through power electronics) and requirements to operate modern electric power systems with increased penetration of non-conventional and mostly intermittent types of generation in a safe and secure manner. As a response to this renewed interest, in February 2010 CIGRE established working group C4.605: Modelling and aggregation of loads in flexible power networks. One of the first tasks of the working group was to identify current international industry practice on load modelling for static and dynamic power system studies. For that purpose, a questionnaire was developed and distributed during the summer/autumn of 2010 to more than 160 utilities and system operators in over 50 countries on five continents. This paper summarizes some of the key findings from about 100 responses to the questionnaire received by September 2011 and identifies prevalent types of load models used as well as typical values of their parameters.

The behavior of the load model in power system stability studies has been shown by various investigators to be a very important factor in determining the stability of power system elements. Load models which are presently available in typical stability programs make it difficult if not impossible to model the actual dynamics of power system load. Present load models allow easy static representations, but require detailed individual dynamic models such as induction or synchronous motor models. Thus a simple load model that can represent power system composite load dynamics will be a valuable tool in the stability study Presented herein is a second order state space load model which can represent power system composite load dynamics. In addition, the parameters of this state space load model can be identified from actual system load measurements by a weighted least squares parameter identification process. Several authors recognized the significance of load behavior to changes in voltage as long as 25 years ago. However, most of the attention in stability studies has been directed toward improved generator and control system models. Until recently, little modeling has been considered necessary for power system loads since the effects of load models in stability studies were considered only secondary. At present the load representation in typical stability programs is limited to a static load model where the load is represented as constant MVA, constant current, constant impedance, or some combination of the three.

Comparison of impedance based fault location methods for power distribution systems

The load in power system has the two characters, one is random time-variety and the other one is geographical dispersion. Both have negative effects on the accuracy of load model. In this paper, an improved fuzzy C-means (FCM) algorithm based on density function is proposed to minimize this influence. Firstly, the disadvantage of traditional FCM algorithm is analyzed to show its subjectivity, which is hard to obtain the available global optimal solution. Then, a FCM algorithm based on density function theory is applied to initialize the clustering centers. It can determine the number of load category and optimize the clustering result of power system daily load profile. The simulation result of a real power system load model verifies the accuracy and practicability of the proposed method. It can obtain the objective load category number and effectively solve the non-convergence problem in traditional FCM algorithm.

The importance of having accurate load models in power system stability studies has been well established in the literature as being essential for precise power system transient event investigations. The power industry presently uses composite load models in typical stability programs (e.g. LOADSYN and PSS/E). However, the parameters of the composite load models need to be tuned for each type of disturbance based on an assumed load composition, and are often inadequate for matching the modeled dynamics of the power system disturbance event to actual measured results. A stochastic time series technique in the form of an ARMAX mathematical model is presented in this paper as a novel alternative for dynamic load modeling. The model parameters are estimated using on-line measurement data for a number of disturbance events, collected from five substations in the Victorian electricity network in Australia. The performance of the model is then evaluated for other transient events, and compared against the recorded response for these events. The results achieved show that this heuristic-based model is robust and effective in predicting the dynamic response of a power system load across a range of events spanning various seasons and locations.

Power load is an important element in power system. While, the traditional load model is fixed. So, when the load components change, the load model will be inappropriate. Based on the Multiple Model Theory, this paper proposes the Adaptive Synthesis Dynamic Load Model (ASDLM). In load modeling, the load model set is established, and every model in the model set has a weight. The time-varying classical load model (TVCLM) is proposed as the basic structure of the load models in load model set. The parameters and weights of the power load model can be adjusted when there is a maneuvering in the actual load system. Then, the models in the model set are integrated to ASDLM. The load data fitting results of ASDLM verify the adaptability and practicability of the proposed model structure and modeling ideology.

This paper presents an application of energy function with included effect of nonlinear load models to be incorporated for the calculation of critical fault clearing time, which can be used in the analysis of maintaining power system stability. The conventional transient stability studies have used time domain to find critical fault clearing time for clearing machine under fault condition is not practical because it uses the trial-error method. Energy function methods are used to find the critical fault clearing time, which is superior to the conventional method (time domain approach) in term of computation time. The method is calculated the potential energy boundary surface (PEBS) and the controlling of unstable equilibrium point (CUEP). The system load model is developed which is included non-linearity of power system load. The non-linear load models are made to yield an accurate result more than constant impedance load model.

Modelling of voltage dependent load is always a current topic due to existence of an adequate load model is a prerequisite for calculation of steady states and the analysis of transient regimes of power systems. Given that it is important to know the load model parameters, we have performed the measurements in electric power system of Bosnia and Herzegovina (EPS BIH). The main idea is to identify the values of voltage parameters of dynamic load model. Therefore, this paper describes the dynamic load models and the methodology for exponential dynamic load model parameter identification. It also presents the voltage parameters of dynamic load model for a node in EPS BIH and compares them to similar published studies, concluding that the difference in given parameter values is the consequence of local load characteristics.

Efficient identification of critical load model parameters affecting transient stability

Not only static load model but dynamic load model is necessary for improving the accuracy of time-domain simulation results because only dynamic load model can express the dynamic response of the load. The measurement based auto-derivation method for estimating load model parameters from PQVF monitoring devices has been developed. The PQVF monitoring device which has already been installed into power system captures disturbance data triggered by voltage deviation and frequency deviation. The transfer-function based load model which consists of voltage characteristics index, frequency characteristics coefficient, transient gain, and time constant was used as the dynamic load model. The simple transfer function-based dynamic load model can express 80% of 182 load responses precisely. Therefore, the transfer function-based dynamic model can have applicability to represent the dynamic behavior of the load response concisely and accurately on condition that the quantity of voltage drop during the disturbance is less than 30%.

This paper describes a practical application of the Expectation Maximization algorithm for computing the static load model of large electricity consumers. Such a model is necessary for effective and reliable operational control of electric power systems. The possibility of estimating basic states of electrical load using arrays of measured voltage and power data is shown. The criteria for selecting the most appropriate clusters are formulated and a step by step linear load model parameter estimation method is proposed. A practical method application is illustrated by using measurements of voltage and real power obtained at a large industrial facility. As a result, a load model with two basic states and corresponding linear models is obtained.

In a large-scale transmission network, such as the European power system, the regional characteristics of electrical demand may have an impact on local voltage stability. The dynamic load behaviour depends on the sectoral composition of demand, which is subject to regional and seasonal variation. An aggregate load model is presented, which accounts for the sectoral characteristics of 210 European regions for all hours a year from the perspective of the high, medium, and low voltage levels. The proposed load model is used to better understand the influence of regional characteristics on voltage recovery during a short-circuit event, as well as the static point of voltage collapse. Several countries are considered in terms of their sectoral composition, to represent intermediate regions or such dominated by the industry and households. Moreover, the dynamic load model is benchmarked against two static load models, assuming a constant power (PQ) load or a constant impedance (Z) load. The conducted study demonstrates that regional differences in the composition of demand highly impact local voltage stability.

Guest Editorial: Situational awareness of integrated energy systems

This paper presents a new method to construct dynamic models for large industrial and commercial facilities commonly connected to power transmission systems. These facilities typically draw large amounts of power and have complex dynamic responses to power system disturbances. Traditional load modeling approaches such as those based on load composition or site measurement are not adequate to produce dynamic models for such facilities. In this paper, a facility template based load modeling technique along with template scaling/equivalence algorithms is proposed to solve the facility modeling problem. Oil refinery facilities are used as an example to illustrate the proposed modeling technique. The technique requires minimal user input and can be implemented in a database program.