This paper presents the experimental determination of the ZIP coefficients model to represent (static) modern loads under varying voltage conditions. ZIP are the coefficients of a load model comprised of constant impedance $Z$ , constant current $I$ , and constant power $P$ loads. A ZIP coefficient load model is used to represent power consumed by a load as a function of voltage. A series of surveys was performed on typical residential, commercial, and industrial customers in New York City. Household appliances and industrial equipment found in the different locations were tested in the laboratory by varying the voltage from 1.1-p.u. voltage to 0 and back to 1.1 pu in steps of 3 V to obtain the individual $P$ – $V$ , $Q$ – $V$ , and $I $ – $V$ characteristics. Customer load tables were built using seasonal factors and duty cycles to form weighted contributions for each device in every customer class. The loads found in several residential classes were assembled and tested in the lab. It was found that modern appliances behave quite differently than older appliances even from only 10 years back. Models of the different customer classes were validated against actual recordings of load variations under voltage reduction.

Experimental Determination of the ZIP Coefficients for Modern Residential, Commercial, and Industrial Loads

CIGRE WG C4.605 : “Modelling and aggregation of loads in flexible power networks”

The objective of this paper is to present a new “event-oriented” method of online load modeling for the Illinois Institute of Technology (IIT) microgrid based on synchrophasor data produced by phasor measurement units. Several load models and their parameter estimation methods are proposed. The online load modeling process is based on an adjustable sliding window applied to two different types of load step changes. The load modeling tests and related analysis of the synchrophasor data from the IIT microgrid are demonstrated.

An Event-Oriented Method for Online Load Modeling Based on Synchrophasor Data

In the measurement-based approach, given a load model structure, its parameter values are derived using actual measurements. The issue of multiple local optimal solutions may arise in parameter estimation when the parameter estimation task is formulated as a nonlinear least squares problem. In this paper, the multiple local optimal solutions issue and a plateau phenomenon in developing load models are addressed in detail and linked to the local identifiability problem. Examples of multiple local optimal solutions in estimating parameter values of several static, dynamic and composite load models are demonstrated. In order to address the local identifiability at an obtained local optimal solution, output sensitivity and correlation between parameters are analyzed. Finally, several suggestions are proposed to resolve these issues.

Multiple Solutions and Plateau Phenomenon in Measurement-Based Load Model Development: Issues and Suggestions

This paper presents parameter values and ranges of static load models used in a power system analysis for the representation of both individual electrical devices and aggregate system loads. The paper discusses and correlates load models and their parameters following statistical processing of the responses of around 100 transmission system operators and utilities around the world, who participated in a survey initiated by CIGRE Working Group C4.605, with the corresponding information on load models and their parameter values from the existing literature. According to the survey, the most frequently used static load models are voltage dependent, and voltage and frequency dependent exponential load models. The identified typical parameter values and ranges of these static load models, both for low voltage devices and for aggregate loads at higher voltage levels, are results of the analysis of a large number of data. Based on these results, the paper provides recommendations for their further use in power system studies and also introduces a novel method for obtaining the mean values and ranges of parameters of the aggregate system load models.

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Recommended Parameter Values and Ranges of Most Frequently Used Static Load Models

Load representation has a significant impact on power system analysis and control results. Currently, static load models are still popular in power industry for transient stability analysis. Dynamic load models are recommended in both industry and academia for possible improvement in analysis accuracy. The accuracy of using static (nonlinear) load models with suitably identified parameters for transient stability analysis is examined. Numerical studies conducted using on-line measurement data indicate that static load models, as compared with induction motor load models, are acceptable for modelling real power behaviours during disturbances and hence are adequate for transient stability analysis. Using multiple sets of on-line measurements, representative model parameters of five static nonlinear load models are derived and their performance in modelling dynamic behaviours of both real and reactive powers is compared. A method is presented to determine a representative set of parameters of static (nonlinear) load models for each loading condition.

Y.-t. Chen

Papers

Representative static load models for transient stability analysis: development and examination