ZIP Load Research Articles

Homotopy‐enhanced short‐circuit calculation for general distribution networks with non‐linear loads

Many existing short-circuit calculation (SCC) methods may encounter the issue of divergence in solving distribution systems with distributed generators (DGs) and ZIP (constant power, constant current, and constant impedance) loads. To address this problem, this study presents a robust homotopy-enhanced numerical SCC method for general distribution networks with non-linear loads. Numerical studies reveal that the difference of SCC results between ZIP loads and constant impedance loads can be as large as 37.9% for the IEEE 8500-node test system, highlighting the importance of load modelling in calculating short-circuit currents. To demonstrate its robustness and effectiveness, the proposed method is evaluated on the IEEE 13-bus network and the IEEE 8500-node test system. The evaluation results are very promising. One distinguished feature of the proposed method is that it can assist several existing SSC methods to solve the short-circuit equations of distribution networks with DGs and ZIP loads.

Distribution System Nodal Prices Determination for Realistic ZIP and Seasonal Loads: An Optimal Power Flow Approach

The impact of deregulation in power system has changed the electricity pricing scheme where the prices are governed by the rules of market operation. The prices must take into account the effect of load and must be based on the change in the operating state of the system. In this paper an optimal power flow approach is used to determine the marginal clearing price along with the nodal prices by minimizing the total system operation cost. The method uses marginal loss coefficients (MLCs) to get these prices, the results are compared with realistic ZIP, residential, industrial and commercial (RIC) load patterns which are also considered with seasonal variations such as winter, summer and spring seasons. The entire study is conducted on IEEE 33 bus radial distribution system (RDS) using GAMS 23.4 and the seasonal load is taken from IEEE Reliability Test System (RTS) data.

PV-WT Based Distribution Generator Location Minimizing Transmission Loss in Pool/Bilateral Electricity Market Model

In this paper, analysis has been carried out for transmission loss minimization with the integrated hybrid PV and Wind turbine in the power system network. For obtaining the power output from the PV and Wind turbine, a probabilistic model for the solar irradiation and wind has been considering using Beta and Weibull distribution function. A Mixed Integer Nonlinear Programming (MINLP) approach has been utilized for determining optimal location and number of distributed generators considering minimization of transmission loss. The analysis has been carried out with constant PQ load and Zip load model. The impact of different load models has been studied. The analysis has been carried out for IEEE24 bus test system in pool/bilateral electricity market model.

Multistage time‐variant electric vehicle load modelling for capturing accurate electric vehicle behaviour and electric vehicle impact on electricity distribution grids

Accurate electric vehicle (EV) load modelling is a daunting task because the proportion and characteristics of these extra electrical loads in the grids are continuously changing. In many recent EV studies, EV load is assumed to be a constant power load to analyse the effect of EV charging on electric network. However, such an assumption would not be effective in real-time application and can give misleading results. This study proposes a multistage time-variant EV load modelling technique based on the characteristics of EV load demand due to EV battery charging. To capture the transitions between different stages of battery charging, the EV loads are modelled with a multistage scheme and implemented ZIP model. A comparative study of energy losses and voltage profile is carried out on the IEEE 69-bus test system for the use of a traditional constant load model, conventional ZIP load model and the proposed multistage time-variant EV load model. Results show that the load models can significantly affect the energy losses and terminal voltages, and provide misrepresentation if EV load is not accurately modelled. Therefore, for proper analysis of EV loads during unidirectional charging, it is essential to have accurate models for predicting true EV behaviour.

An approach for an efficient hybrid AC/DC solar powered Homegrid system based on the load characteristics of home appliances

In this paper, a new and efficient hybrid AC/DC solar powered Homegrid model is proposed based on the load characteristics of a typical set of home appliances working on a 220V, 50Hz system. The load characteristics of home appliances are represented by a static ZIP (constant impedance (Z), constant current (I) and constant power (P)) coefficient model, a useful load model in power flow studies. A Homegrid system is formed by analysing the distortion produced by modern household equipment, and a K-factor with a significant role in the overheating of residential/distribution transformers is determined. Further, the ZIP load model is validated by a simulation and the results are compared with experimental data. To improve the efficiency and quality of the traditional Homegrid system, a solar powered hybrid AC/DC supply network model is proposed. This system eliminates redundant power conversion stages and mitigates the loss due to the harmonics by a shift of harmonic intensive loads to the Direct Current (DC) network side. Its effectiveness has been investigated with a simulation in MATLAB as well as with hardware implementation. This proposed energy efficient Homegrid model could be quite helpful in fulfilling the idea of net zero or surplus energy buildings.

FACTS Devices Cost Recovery During Congestion Management in Deregulated Electricity Markets

In future electricity markets, flexible alternating current transmission system (FACTS) devices will play key role for providing ancillary services. Since huge cost is involved for the FACTS devices placement in the power system, the cost invested has to be recovered in their life time for the replacement of these devices. The FACTS devices in future electricity markets can act as an ancillary services provider and have to be remunerated. The main contributions of the paper are: (1) investment recovery of FACTS devices during congestion management such as static VAR compensator and unified power flow controller along with thyristor controlled series compensator using non-linear bid curves, (2) the impact of ZIP load model on the FACTS cost recovery of the devices, (3) the comparison of results obtained without ZIP load model for both pool and hybrid market model, (4) secure bilateral transactions incorporation in hybrid market model. An optimal power flow based approach has been developed for maximizing social welfare including FACTS devices cost. The optimal placement of the FACTS devices have been obtained based on maximum social welfare. The results have been obtained for both pool and hybrid electricity market for IEEE 24-bus RTS.

Quasi real-time ZIP load modeling for Conservation Voltage Reduction of smart distribution networks using disaggregated AMI data

This paper aims to investigate quasi real-time ZIP load models for new Smart Grid-based Volt-VAR Optimization (VVO) techniques. As recent VVO solutions are able to perform in quasi real-time using Advanced Metering Infrastructure (AMI) data, more accurate load modeling could give distribution network operators and/or planners more precise Conservation Voltage Regulation (CVR) and energy saving values at each operating time stage. Furthermore, more accurate load modeling of each quasi real-time stage could improve VVO efficiency. As type, amount and operating time of each residential appliance varies throughout a day, this paper aims to discover ZIP load model of each quasi real-time stage separately through disaggregated data (i.e. decomposing residential load consumption into home appliance consumptions). This paper shows that the energy conservation achieved by CVR operation through presented quasi real-time ZIP load modeling could lead AMI-based VVO solutions to higher level of accuracy and data resolution compared with conventional techniques. Therefore, this paper primarily introduces a new quasi real-time AMI-based VVO engine. Then, it investigates ZIP load model of each quasi real-time stage through statistical data to conserve energy consumption. To check the authenticity and the applicability of presented model in a whole system, 33-node distribution feeder is employed.

Behavioral Characterization of Electric Vehicle Charging Loads in a Distribution Power Grid Through Modeling of Battery Chargers

The plug-in electric vehicle (PEV) is a new atypical load in power systems. In future, PEV load will play a significant role in the distribution grids. This integrated load into the power grid may overload the system components, increase power losses, and affect the voltage profile in the distribution systems. Currently, the constant power load model is most commonly used for the modeling of the electric vehicle (EV) load that considers the EV loads as constant power elements without considering the voltage dependence of the EV charging system. EV load demand cannot be considered as a constant power due to the fact that modeling as a constant power load will not provide accurate information about the behavior of the charging system during the charging process. In this paper, an accurate model representing the realistic behavior of EV load is developed which is based on the ZIP load model with the ZIP parameters established through the realistic EV load data. The proposed model can be used to analyze the true behavior of the EV charger integrated to an electricity grid and determine the impacts of EV charging load on the grid. A realistic charging system was used to test and capture the EV load behavior and extract the coefficients of the EV ZIP load model, which have been verified using computer simulations and laboratory experiments. Additionally, a comparative study between the proposed ZIP load model and the constant power load model was carried out, and the results were verified with the practical EV load data. The results confirm that EV represented using constant power load will not provide the true reflection of the EV load behavior and the EV impacts on the power grid.

An Improved L Index for the Static Voltage Stability Assessment of Power System with ZIP Loads

This paper proposes an improved L index which is adapted to analyze the voltage stability of power system with the ZIP loads. First it introduces derivation of the traditional L index. Then, it analyzes the differences between ZIP loads and PQ loads and then proposes an improved version of L index based on the derivation. Furthermore, the proposed improved index is used to analyze the voltage stability under different operation conditions of the power system with ZIP loads. The simulations show the effectiveness of the proposed improved L index.

Capacitor Allocation in Radial Distribution System with Time Varying ZIP Load Model and Energy Savings

Abstract In this paper, novel method based on loss sensitivity approach is utilized to determine optimal locations and size of the capacitors in radial distribution system to reduce power losses and improve voltage profile. Reduction in power loss enhances energy efficiency in distribution system also releases feeder capacity. Distribution system has combination of different type of loads (Industrial, commercial and Residential). Load models have significant impact on selecting capacitor size for better deployment of reactive power support. The main contribution of the paper is: (i) Capacitor allocation for radial distribution network using sensitivity approach, (ii) capacitor allocation and sizes calculation for radial distribution system with load growth and winter load variations, (iii) comparison of the results obtained with single and multi-capacitors placement with load growth and load variations, (iv) The loss savings and overall cost savings per annum with capacitors placement. In this paper we considered the impact of time varying load flow with realistic load model. The load growth factor is considered in the study which is essential for the planning and expansion of the existing systems. The impact of the realistic load model as ZIP load model has been considered for study of the systems. The results have been obtained for the distribution network of UK Distribution Corporation consisting of 38 buses.

Performance Analysis of Unbalance Radial Feeder with Time Varying Composite Load

The focus of this paper is to present performance indices for unbalance radial feeder having different characteristic and composition of time varying static ZIP load models. These provide a framework for benchmarking of distribution automation projects. 15 minutes characteristics time interval for load flow and load modeling are considered to meet smart grid implementation criterion. A forward-backward sweep method is employed for load flow solution. Developed performance indices were illustrated on modified IEEE 37 node test feeder. Performance indices are useful for analysis, operational, planning and integration of stochastic renewable sources.

Load Models for Flat-Panel TVs

A realistic representation of power system loads has proven to be essential for ensuring reliable and economic operation of a power system. Although the exact load model is nearly impossible to obtain, it is still possible to find a suitable load representation that can improve the accuracy of steady-state and dynamic simulations. Load model development has been a continuous effort among industry since the 1980s. However, with the ever increasing development of technology, electric appliances and equipment behavior have changed. In particular, home electronics, such as televisions, are a good example of these changes not only for their cutting edge technology but also for their penetration into the system. Electronic loads tend to behave mostly near constant power with respect to voltage and frequency variations, which may have significant impact on the system stability. Devoting efforts to include the load models of new appliances become a critical issue in the field of computer simulation. This paper devotes its efforts to develop ZIP load models for liquid-crystal-display and light-emitting-diode televisions to be added into the family of the load models to improve the accuracy of system simulation. A computer simulation of a power system illustrates the impact of load models on the stability of the power systems.

Approximate Representation of ZIP Loads in a Semidefinite Relaxation of the OPF Problem

Recent research has applied semidefinite programming (SDP) to the optimal power flow (OPF) problem. Extending SDP formulations to include the ZIP load model, which consists of constant impedance, constant current, and constant power components, is necessary for many practical problems. Existing SDP formulations consider constant power components and can be easily extended to incorporate constant impedance components. With linear dependence on voltage magnitude, constant current components are not trivially formulated in a manner amenable to SDP. This letter details an approximate representation of ZIP loads in an SDP relaxation of the OPF problem.

Analysis of Mesh Distribution Systems Considering Load Models and Load Growth Impact with Loops on System Performance

The distribution system is the final link between bulk power system and consumer end. A distinctive load flow solution method is used for analysis of the load flow of radial and weakly meshed network based on Kirchhoff’s Current Law (KCL) and KVL. This method has excellent convergence characteristics for both radial as well as weakly meshed structure and is based on bus injection to branch current and branch-current to bus-voltage matrix. The main contribution of the paper is: (i) an analysis has been carried out for a weekly mesh network considering number of loops addition and its impact on the losses, kW and kVAr requirements from a system, and voltage profile, (ii) different load models, realistic ZIP load model and load growth impact on losses, voltage profile, kVA and kVAr requirements, (iii) impact of addition of loops on losses, voltage profile, kVA and kVAr requirements from substation, and (iv) comparison of system performance with radial distribution system. Voltage stability is a major concern in planning and operation of power systems. This paper also includes identifying the closeness critical bus which is the most sensitive to the voltage collapse in radial distribution networks. Node having minimum value of voltage stability index is the most sensitive node. Voltage stability index values are computed for meshed network with number of loops added in the system. The results have been obtained for IEEE 33 and 69 bus test system. The results have also been obtained for radial distribution system for comparison.

Mesh distribution system analysis in presence of distributed generation with time varying load model

With the implementation of competitive electricity supply system all over the world, there is the need of optimal utilization of the existing generation sources at the same time to curtail the generation from the conventional power plants and add the renewable sources generation looking into the environmental concern and limitation of the fossil fuels. This has lead to the motivation for the studies on the integration of distributed resources to the grid. In this paper optimal locations and sizes for DG is determined for weakly meshed distribution networks based on the sensitivity method. Novel method based on loss sensitivity is used in this paper to determine optimal size and location of DGs. The modified Novel method is proposed for DG allocation. The main contribution of the paper is: (i) distributed generation allocation for mesh network using sensitivity approach, (ii) modified Novel method for DG allocation and sizes calculation for meshed distribution system with load variations, (iii) comparison of the results obtained with single and two DG placement with load variations, (iv) the loss savings and overall cost savings per annum with single and two DGs placement with load variations. In this paper we considered the impact of time varying load flow with realistic load model. The realistic ZIP load model has been considered for study. The results have been obtained for a distribution network of UK Distribution Corporation consisting of 38 buses. The results have also been obtained for radial distribution system for comparison.

Modeling and simulation of static loads for wind power applications

This study aims to model and simulate static nonlinear loads with wind power generation to evaluate the impact of load models on wind power systems. Nonlinear loads are modeled as exponential load model, ZIP load model and combination of exponential/ZIP with an induction motor. The wind power generator is represented with a reduced-order doubly fed induction generator (DFIG) model. Developed models have been implemented in a grid-integrated wind power plant and simulated in MATLAB/SIMULINK. The effects of nonlinear loads into wind power plant are investigated in terms of bus voltages, angular speed, electrical torque, and d---q stator axes currents. Additional analyses are conducted to compare the behaviors of full- and reduced-order DFIG models under a selected loading condition. The results of this study indicate that the response of a system with DFIG is dependent of the load modeling and reduced-order DFIG model shows more stable trend than full-order DFIG model.

Static Load Model Determination Using Functional Approximation Based on Gaussian Pulses and its Comparison with ZIP Load Model

This paper describes a static load model based on functional approximation. The functional approximation has been achieved by selecting Gaussian pulses as basis function. Based on this a single layer neural network has been constructed, which can easily trained using δ-rule. A case study has also been reported. The load model as obtained using the proposed neural network has been compared with ZIP load model as obtained by least square estimator.

Capacitor Allocation in Unbalanced Distribution System under Unbalances and Loading Conditions

It is well known fact that distribution systems can operate under unbalanced loading conditions, means that the distribution system planner have to account for any unbalances in the system for reliable operation of the system. In this paper optimal sizes and locations of capacitors is determined using Index Vector method in unbalanced radial distribution system (ubds). In this paper two types of load unbalances and different loading conditions are considered to investigate its impact on optimal capacitor sizes and locations. The main objective of this paper is:•Finding optimal size and locations of capacitors in ubds.•Impact of different loading conditions (low, medium and high loading) on optimal placement of capacitors in ubds is evaluated.•Impact of type-A and type-B unbalances on voltage profile, voltage unbalance, total power losses and cost of energy losses in ubds is determined.•Impact of type-A and type-B unbalances on optimal capacitor allocation problem is addressed.•Impact of realistic load model as ZIP load model has been considered in optimal capacitor placement problem.•Computing required amount of capacitive reactive power to minimise total power losses and improve voltage profile under different load unbalances, different loading conditions and realistic ZIP load model.

Congestion management with generic load model in hybrid electricity markets with FACTS devices

The load variations during the entire day specially during the peak hours have substantial impact on the loading pattern of the transmission system. The voltage profile become poor during such situation of peak loading of the network and can lead to congestion during such events. This paper attempts congestion management considering the impact of constant impedance, current, and power (ZIP) load model along with the load variation pattern in a day-a-head hybrid electricity market. The main contribution of the proposed work is: (i) Optimal rescheduling based congestion management for a hybrid market model with three bid offer from generators, (ii) study of the impact of ZIP load model and load variations on rescheduling and congestion cost, (iii) impact of third generation FACTS devices on congestion management, (iv) comparison of results obtained for hybrid market model without and with ZIP load model. The generators offer three block bid structure to the ISO in a day-a-head market for congestion management. The base case economic load dispatch has been obtained for generators and is taken as base case generation output data during the congestion management to obtain new generation schedule. The three block bid structure submitted to the ISO has been modeled as a linear bid curve which is a function of increment/decrement (inc./dec.) of generation within the upper and lower bounds offered for congestion management. The results have been obtained for IEEE 24 bus test systems.

ATC with ZIP load model – A comprehensive evaluation with third generation FACTS in restructured electricity markets

In this paper, an optimal power flow based approach has been applied for multi-transactions in deregulated environment for ATC determination with third generation FACTS devices. The main contribution of the paper is (i) OPF based approach for evaluation of ATC with multi-transactions, (ii) ATC enhancement with third generation FACTS devices viz. Static Compensator (STATCOM), Static Synchronous Series Compensator (SSSC), and Unified Power Flow Controller (UPFC) for intact and line contingency cases (iii) Impact of ZIP load on ATC determination and comparison of ATC obtained with ZIP load model and constant P,Q load model, (iv) Comparison of ATC obtained with DC/AC-PTDFs based method along with FACTS devices for comparison. The results have been determined for intact and line contingency cases taking simultaneous as well as single transaction cases for IEEE 24 bus RTS.