Development of a dynamic power system load model

Load Model for Medium Voltage Cascaded H-Bridge Multi-Level Inverter Drive Systems

Abstract: Medium voltage drives (MVDs) are commonly used in high-power applications and show significant impact on the overall system dynamics due to their large size and high power demand. Although detailed switching models for MVDs can be built using MATLAB/Simulink, such models cannot be used in large-scale simulation software for power system dynamic studies. To solve this problem, the dynamic load model for the medium voltage cascaded H-bridge multi-level inverter drive and induction motor systems, which is suitable for power system dynamic studies, is proposed in this paper. Analytical formula of the model is presented. The model includes the aggregated effect of an MVD, an induction motor, and their control system, and thus, it can accurately represent the dynamic responses of the motor drive system under disturbances. Both voltage and frequency dependence are considered in the model. The accuracy of the model is verified by a case study. A sensitivity study is conducted to evaluate the impact of the model parameter variation on dynamic response characteristics. The developed load model can be readily inserted in the large-scale power system simulation software for power system dynamic studies.

Dynamic Power System Load -Estimation of Parameters from Operational Data

Abstract: The significance of load modeling for voltage stability studies has been emphasized by several disturbances, which have taken place in the past years. They have shown that the loads in combination with other dynamics are among the main contributors of prolonged low voltage conditions, voltage instability and collapse in the power system. As a result of these disturbances new investigations have come up to better understand the nature of the load. However, power system loads keep being very difficult to model; the load generally aggregates a large number of individual components of different nature, different load dynamics are excited depending on the time frame of actuation and the type of disturbance affecting the system, and the load is highly dependent on external factors such as weather conditions. This thesis investigates the load-voltage characteristic during two different time scales, long-term over several minutes, and short-term covering ms to several seconds, for different sized disturbances, and its impact on the calculation of transfer limits and security margins in voltage stability studies. The accurate determination of transfer limits will be an increasingly important task to maintain the operational security and economic dispatch of the power system. The location of the stability limits and the determination of transfer limits depend on the load-voltage characteristic since load relief due to the load-voltage dependency results in larger transfer limits. Moreover, the importance of using dynamic load models instead of static ones in stability studies is highlighted in this thesis. Due to the large amount of electrical heating loads in Sweden and its effect on voltage stability, a dynamic load model with exponential recovery, previously proposed by Hill and Karlsson, [Karlsson and Hill, 1994], has been the starting point for the investigations. Field measurements from continuous normal operation at the 20 kV-level from a substation in Sweden have provided a large amount of data covering all seasons during the time period July 2001-June 2002, and have resulted in extensive, unique and interesting recordings of active and reactive load characteristic and its dependency with small voltage variations. The data have revealed the variation of the load parameters and their dependency with weather and season of the year. The work has also contributed to a better approach for the normalization of traditional reactive load models. Furthermore the load-voltage characteristic during large disturbances has been investigated based on field measurements of phase-to-phase faults in a non-effectively earthed 50 kV system in Sweden. Three-phase currents and voltages have been used to estimate the active and reactive power. The recordings exhibited voltage dips up to 30% in the positive sequence voltage. The severity of the disturbances accentuates the nonlinear behavior of the load; the active and reactive power rapidly increase after fault clearing to levels even above the pre-disturbance value due to the re-acceleration of motors. The full recovery of the voltage is delayed due to the re-connection of tripped load. Moreover, it is shown that traditional load models do not accurately reflect the load behavior during these disturbances, for voltage dips around 12 % or larger due to the nonlinearities. An alternative load model, which represents the nonlinearities, has been tested. The superior behavior is demonstrated with the field measurements. Finally, some guidelines for industry to better account for the load in future stability studies have been included as a corollary of this thesis. (Less)

Dynamic Load Models for Industrial Facilities

Abstract: 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.

Modeling variable frequency drive and motor systems in power systems dynamic studies

Abstract: Variable frequency drives (VFD) are widely used in industrial facilities, however, dynamic models for VFD-motor systems suitable for power system dynamic studies are not available. In this paper, a generic VFD-motor system modeling technique is proposed for the case that VFDs are able to ride through the fault. To illustrate the proposed technique, a dynamic model for a low voltage 6-pulse voltage source inverter (VSI) based drive and induction motor system with voltage per Hz control is created. To verify the accuracy of the developed dynamic model, a case study is conducted using a sample VFD-motor system, both derived dynamic model and detailed switching model for the same system are simulated using Matlab/Simulink, and their dynamic responses are compared. It is found that there are good agreements between the two models, and thus the accuracy of the developed dynamic model is verified. For the case that VFDs will trip out of lines, a VFD trip characteristic curve is proposed, and a simple screening procedure is recommended for evaluating whether the VFD-motor system shall remain in service or trip out of lines in power systems dynamic studies.

Derivation of load model parameters using improved Genetic Algorithm

Abstract: Load components have strong effects on the power system's behavior and should be modeled accurately in system studies. With more and more disturbance measurement equipments have been installed in transmission systems, it opens up an opportunity to use the measurement data to derive load model parameters. This method is termed as the measurement- based approach. The theoretical foundation of the measurement- based approach is system identification. In this paper, we propose to apply an improved Genetic Algorithm (GA) to derive the parameters of load models using the measured disturbance data. The improved genetic algorithm is based on following aspects: (i) the strategy of keeping the best individual (ii) the adaptive rates of mutation and crossover (iii) the strategy of immigration (iv) the optimal search direction. The improved GA method is compared with the Levenberg-Marquardt method using a 23-bus test system.