Transformer modeling for low- and mid-frequency transients - A review

One of the weakest components of modern transient simulation software is the transformer model. Many opportunities exist to improve the simulation of its complicated behaviors, which include magnetic saturation of the core, frequency-dependency, capacitive coupling, and topologically correctness of core and coil structure. This paper presents a review of transformer models for simulation of low- and mid-frequency transients. Salient points of key references are presented and discussed in order to give an accessible overview of development, implementation and limitations of the most useful models proposed to date. Techniques for representation of the most important behaviors are examined, with the intent of providing the needed foundation for continued development and improvement of transformer models. © 2005 IEEE

Operation of DC Series-Parallel Connected Offshore Wind Farm

© 2018 IEEE. A dc series-parallel wind farm was proposed as an alternative to an ac wind farm. But, the operation of such a wind farm results in significant wind turbine (WT) output voltage variation, which may lead to severe stress being seen by the turbine capacitors. The output voltage variation problem is demonstrated and rigorously analyzed with proposed power flow calculation. In order to mitigate the voltage variation a dc WT with integrated storage along with strategies for control and storage sizing is proposed. The time domain simulation results for 120 MW 2 × 6 dc series-parallel wind farm shows that a large voltage variation, with maximum value reaching close to 2 p.u and low voltage of about 0.2 p.u, is obtained when WT output voltage is not limited. The power input variation of a single turbine affects voltages and currents of all the WTs in the wind farm. A 48 kWh storage integrated with the WT appears to significantly alleviate the WT voltage variation

A DC wind turbine with integrated storage

© 2017 IEEE. The dc series and series-parallel wind farms have been recently proposed as alternatives to ac for offshore applications. A dc wind turbine for use in dc series and series-parallel wind farms has been proposed in this paper. The operation of the dc series and series-parallel wind farm results in continuous variation of the output voltage with possibility of losing maximum power point tracking capability (MPPT). A wind turbine (WT) design with back-to-back connection of voltage source converter (VSC) and dual active bridge (DAB) dc-dc converter is proposed. An integrated storage connected to internal dc bus of the turbine with a DAB is also proposed. Simulation results show that the proposed design ensure maximum energy capture during undervoltage and overvoltage operating conditions. The output voltage variation is limited to a range of 0.7 to 1.5 p.u

Out-of-step detection using Zubov\u27s approximation stability boundaries

This paper presents a new out-of-step detection method for multi-machine power systems based on approximate stability boundaries. The boundaries are constructed based on Zubov\u27s method of stability analysis. The proposed out-of-step detection method first constructs a series of boundaries in power angle - frequency (δ-ω) plane, and tracks the trajectory of each generator in the δ-ω plane after the disturbance to differentiate out-of-step from stable power swings. The electrical parameters needed to construct approximate stability boundaries are obtained directly from Phasor Measurement Units (PMUs) located at the generator\u27s terminal. The effectiveness of the proposed method is tested in a Single Machine Infinite Bus (SMIB) and the IEEE 3-machine 9-bus system. Simulation results show that the proposed algorithm is capable of detecting out-of-step conditions in multi-machine power systems without using network reduction techniques

Transformer model for inrush current calculations: Simulations, measurements and sensitivity analysis

The modeling of inrush currents that occur upon energization of a transformer is a challenge for Electromagnetic Transients Programs due to limitations in available transformer models and the ability to determine and specify initial flux. The estimation of transformer model parameters is also an issue. This paper presents a transformer model for low- and mid-frequency transient studies with a focus on the behavior in saturation and the estimation of residual fluxes. The comparison of the simulation results with analytical calculations and measurements proves the capability of the model to accurately represent energization and de-energization transients of a three-legged-core distribution transformer. A novel property is the ability of auto initialization after disconnection, made possible by the implementation of a hysteretic core model which properly simulates and remembers residual flux from the previous de-energization. Special attention is paid to parameter estimation. Detailed core and winding design data are not used as they are seldom available from the manufacturer. Sensitivity analysis is performed to verify the influence of each parameter on the inrush current envelope curve. It is observed that the parameter that most influences the current amplitude is the slope of the magnetization curve at extreme saturation. © 2010 IEEE

Advanced Leakage Inductance Model for Transformer Transient Simulation

Transformer models in EMTP which provide inter-phase mutual inductance leakage representations have deficiencies with respect to core modeling. Parameter estimation is also a concern when no or incomplete sources of information are available. The hybrid transformer model in EMTP-ATP addresses these issues by allowing the integration of the leakage representation with a duality-derived topologically correct core model, and providing flexible estimation of leakage parameters based on known information. This paper details the development of the leakage model, its integration with the topologically correct core, and the related parameter estimation methods

Comparison of wind farm topologies for offshore applications

Increasing energy demand and environmental factors are driving the need for the green energy sources. The trend, in general, with respect to wind farms is to increase the number and the size of wind farms. The wind farms are also being located offshore with the prospect of more consistent and higher energy capture. The offshore wind farms are likely to move farther off from the shores to reduce visual impacts and increase the size. But, this has implications in terms of design of the collection grid and grid interconnection. Farms of 1 GW size and at distances of about 100 km are envisaged [1]. The design of collection system and turbine interconnection will become very important as the farms move farther offshore. Proper choice of collection system topology is important from the point of view of maximum energy capture while ensuring a high reliability of the design. Different collection system topologies have been proposed by researchers before, with the radial system being most popular. One of the key factors in selection would be the losses in the farm. In order to select the most suitable topology a comparison of different topologies with respect to losses, reliability and costs has to be done. Comparisons of calculations indicate that the DC series and series-parallel wind farm design may be options for future wind farm designs. The DC series and series-parallel design have lower reliability, but can be improved by providing redundancies. The designs have equipment costs almost equal to the AC wind farm costs. The losses in DC series-parallel wind farm are higher by about 12 % when compared to AC wind farms. The DC series design is also very attractive design, but has restrictions with respect to insulation. Also, the required turbine ratings may be significantly and unrealistically high when it comes to designing large wind farms. It can also be concluded that the novel designs require significant amount of work before these can be used in real wind farms. © 2012 IEEE

Transient and harmonic overvoltages related to transformer and transmission line interaction during black start energization

Copyright © 2003 IFAC. Restoration of the power grid following a blackout is a subject of major concern for all power utilities. Such a black start typically begins by starting small gas turbine or hydro generators and using them to backfeed the high-voltage grid, energizing one piece of the system at a time. However, the energization of high-voltage transmission lines and large transformers is a challenge, since this can result in a combination of transient and sustained harmonic resonant overvoltages which may cause damage to equipment and/or lead to surge arrester failure. This paper addresses the transient and harmonic overvoltages occurring during black start procedures, discusses the development of Electromagnetic Transient Program (EMTP) models, and presents simulation results. The results of EMTP simulation using more complex models compare more favorably to the event records than the results using simpler models. Based on comparisons with measurements, modeling requirements and some guidelines necessary for obtaining accurate results are provided

Improved application of surge capacitors for TRV reduction when clearing capacitor bank faults

Current-limiting reactors are placed in series with capacitor banks to limit the rate of rise of current to the values specified in the circuit breaker (CB) standards. But this arrangement has created capacitor bank failures when attempting to clear faults in between the reactor and the capacitor bank. After detailed analyses of failures, solutions have been proposed by researchers: 1) Add a surge capacitor to ground on the capacitor bank side of the breaker and 2) add a surge capacitor across the reactor. These surge capacitors are sized based on the stray capacitances of the bus, the reactor, the circuit breaker, and on the maximum-available fault current at the substation. This paper presents a simplified means of sizing the surge capacitors for method 2), based only on the CB\u27s interrupting current rating and reactor size. This eliminates the need for and uncertainty of stray capacitance values. Also, the design does not need to be revisited when grid enhancements increase the available fault current at a substation. A standard surge protection package, which can also be applied to existing installations, is proposed. This new approach has been verified with studies using Electromagnetic Transients Program/Alternative Transients Program. © 2010 IEEE

Power Engineering Design Projects: Capstone Team Projects Versus Topical Design Courses

Over the years, Michigan Tech has had two different philosophies for teaching design in the power area. Initially, we integrated design into topical courses, teaching the material the students would need to complete the design. Presently, our students complete year-long, industry-sponsored projects. We will give details on the implementation of the two approaches and compare and contrast the outcomes. Both techniques for teaching design have their advantages and disadvantages