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EMTP-RV Newsletter November 2011
Edito
The Development Coordination Group (DCG), group of companies leading the development of EMTP-RV decided unanimously at the beginning of 2011 to mandate POWERSYS to commercialize EMTP-RV. The role of POWERSYS is triple: to sell the software worldwide, to organize user group activities, to take charge of the maintenance and integration of the software under the guidance of the DCG. But its role is also to create a new dynamic: to be active, to be close to the users, to facilitate exchanges in the EMTP-RV community, to be ambitious. 2011 has been a year of hectic activity for EMTP-RV with a lot of initiative for the benefits of the users. Now that we are coming close to the end of the year, it is time to have a close look at what was done. An international user group meeting took place in Paris where around 50 users met and shared their experience about the applications of the software. EMTP-RV was also present at the conference IPST in Delft (Netherlands). This conference is a major reference for the simulation of electromagnetic transients and is a great opportunity to listen to the needs of the users in terms of simulation. Two events were organized by POWERSYS in partnership with Opal-RT at the IEEE PES (Detroit, USA) to present case studies from users and to exchange around simulation topics. Three courses were set up with the support of EMTP-RV developers and experts of the software applications. The two first ones took place in France and in Australia; the next one of this year will take place in December 12-16 in New-Orleans, Louisiana (USA), but a course is already planned for May 2012 at University of Wisconsin-Madison (USA). Regarding the maintenance and development of the software, POWERSYS works with the key developers of EMTP-RV. A new version of EMTP-RV will be released soon, including significant improvements of various models as well as an increased set of examples illustrating the applications of the software. After this brief portray I would like to thank, on behalf of the DCG, POWERSYS, but also all the actors: developers, presenters, course organizers, etc. who have helped with their enthusiasm and energy to make of this year 2011 a success for EMTP-RV. Alain Xemard Chairman of the DCG
INSI DE :
TE CHNICA L
P UB L ICA TIONS :
E HV AC ca b les a n d
HV DC l ink s i ns ta l l a t io ns
o n t h e F ren ch g r i d :
E M TP -RV too ls
a n d s tu d i es a t R te
A New Hy s te res is
M o de l in EM TP -RV
EVENTS AT A GLANCE
COMING EVENTS
POWERSYS is the worldwide commercializer or EMTP-RV www.emtp.com Offices France: +33 (0) 442 610 229 (worldwide sales except USA) Offices USA: +1 727 412 8202
[email protected] | www.powersys-solutions.com | [email protected]
EMTP-RV Newsletter |November 2011 |page 2/28
Table of contents
EHV AC cables and HVDC links installations on the French grid: EMTP RV tools and studies at Rte ........................................................................................................................................... 3
1 Introduction ...................................................................................................................................... 3
2 Risks of failure during transformer energization .............................................................................. 3
3 EMTP-RV studies ............................................................................................................................ 4
3.1 Network Modeling .................................................................................................................... 4
3.2 Frequency scan studies ........................................................................................................... 5
3.2.1 Study set-up ..................................................................................................................... 5
3.2.2 Studies analysis ............................................................................................................... 5
3.3 TRANSFORMER ENERGIZATION, TIME-DOMAIN SIMULATIONS ..................................... 6
4 Specific tools developed in EMTP-RV ............................................................................................. 6
4.1 EMTP Works scripts and Parametric toolbox .......................................................................... 6
4.2 DLL Based Equivalent Harmonic Impedance .......................................................................... 7
A New Hysteresis Model in EMTP-RV .................................................................................................. 11
1 Existing hysteresis models in EMT programs ............................................................................... 11
1.1 EMTP Type-96 .......................................................................................................................... 11
1.2 EMTP Type-92 ....................................................................................................................... 12
1.3 PSCAD/EMTDC Jiles-Atherton Model................................................................................... 12
2 New hysteretic reactor model ........................................................................................................ 12
2.1 A modified Preisach Model ........................................................................................................ 12
2.2 Physically correct hysteresis model .......................................................................................... 13
2.3 Memory-dependent and Wipe-out properties ............................................................................ 14
3 Implementation of the model in the EMTP-RV .............................................................................. 14
4 Validation of the new hysteresis model based on the experimental results .................................. 19
4.1 Determination of the core remnant flux ..................................................................................... 19
4.2 Ferroresonance test of a 33kV voltage transformer .................................................................. 22
5 Conclusions ................................................................................................................................... 26
6 References: ................................................................................................................................... 27
EMTP-RV Events .................................................................................................................................. 28
1 Events at a glance: ........................................................................................................................ 28
2 Coming events: .............................................................................................................................. 28
EMTP-RV Newsletter |November 2011 |page 3/28
EHV AC cables and HVDC links installations on the French grid: EMTP RV tools and studies at Rte
Authors:
Simon Deschanvres, RTE- National Center For Grid Expertise - Substation Department
Yannick Vernay, RTE- National Center For Grid Expertise - Substation Department
Sébastien Dennetière, RTE-National Center For Grid Expertise - Substation Department
1 Introduction
The French TSO RTE (Réseau de Transport d'Electricité) operates, designs and maintains the largest network in Europe, at the center of the European electricity market. To carry out needed reinforcements in the French grid in a context of dwindling public acceptance for new overhead infrastructure, many installations of long EHV cables, capacitor banks and HVDC links are planned in the next few years. These new components may give rise to unusual electromechanical and electromagnetic phenomena with potential adverse effects on the existing network. Therefore, specific studies have to be conducted, from the earlier planning to final commissioning stages. The installation of several 225 kV cables up to 80 km long and 400 kV cables up to 20 km long are presently studied by Rte. In a near future, connections of offshore wind farms are going to come. In this article, the problems for operating the network and protecting equipment caused by the integration of long EHV AC cables are presented. EMTP-RV functionalities developed to improve these studies are also presented.
2 Risks of failure during transformer energization
The grid is mainly composed of inductive equipments such as transformers, shunt reactors, and overhead lines. However, underground cables are highly capacitive elements. From a very simple point of view, we can consider that a LC oscillating circuit is created and can lead to harmonic resonance. This issue can be problematic for energization of the transformer especially for transformer of production units. After switching a transformer, saturation is induced in its magnetic core. In other words, a transformer generates harmonic currents during its energization. If the grid impedance is high enough at a harmonic frequency, these currents generated by the transformer could result in transient overvoltages that can be dangerous. During the saturation of a transformer, low order harmonic currents are more important than high level ones. So the higher the harmonic frequency of resonance, the lower this resonance is problematic.
EMTP-RV Newsletter |November 2011 |page 4/28
3 EMTP-RV studies
3.1 Network Modeling
With the increase of cable insertion in the transmission grid, EMTP studies take a growing part alongside system planning studies. These studies have to be performed in the most efficient way to meet the deadlines requirements for the projects. Modeling of the network is a particular sensitive point in terms of time consuming for EMTP studies. The questions which arise are the size of the network to model, the initial load flow, generation and load conditions, model of the network components … As the literature is not always very clear on these critical points, network modeling is not an easy task. Assumption and comparison has to be made by taking into the sensitivity of the parameters. The low frequency phenomena (from 50 to 2000 Hz) need to be studied with an extensive network since it is very hazardous to determine frequency equivalents for the network impedance at the frontier nodes of the model. For the frequency scan studies on the 400kV network, RTE will use a model of the entire 400 kV French grid and of the 225 kV grid at minimum up to 1 substations away from the cable ends.
Figure 1: EMTP-RV modeling of the complete 400kV French network
The EMTP-RV model of the complete 400kV French network is used to save time in building EMTP network model for futures installation project (long EHV AC cable, HVDC links, SVC, offshore wind farms connection... etc) and to be able to run load-flow, frequency scan and time-domain simulation from the same EMTPWorks environment. Lines are modeled with CP-line elements. The propagation mode data in zero sequence and positive sequence are easily available from RTE line data base. However, in the close area of the cable insertion substations, the long lines (over 50 km) are modeled with frequency dependant line model FD-line. This line modeling is not feasible for the complete network in terms of time as it requires accurate electrical and geometrical data of the line. The frequency dependence of the cable has to be modeled as well. FDQ-cable or Wide-band cable models are used.
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C: \ M esDocum ent s\ EM TP\ Tools\ Reseau_RTE\ Donnees\ LI T_RXH_20100409( 1) . xls
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change Subst at ion Nam es
C: \ M esDocum ent s\ EM TP\ Tools\ Reseau_RTE\ Donnees\ LI T_RXH_20100409( 1) . xls
EMTP-RV Newsletter |November 2011 |page 5/28
RTE has developed a XML based interface in order to configure automatically the SCADA data from the models used in the system studies into EMTPWorks. Bus-bar connections, production and consumption are up to date with project data. This point is very important in such time constrained projects and will avoid also the risk of human error when building large networks.
3.2 Frequency scan studies
3.2.1 Study set-up
The direct sequence impedance of the grid is investigated in these studies. Various grid schemes are studied (contingencies, load levels) in order to identify critical situations. For instance, the impedance is highly dependent on grid topology. If a transmission line close to the connection point is not connected to the grid, the impedance becomes very different. The resonance frequency becomes lower and the resonance amplitude is higher. This impedance can more easily lead to harmonic overvoltages.Harmonic overvoltages are limited in a meshed network.
3.2.2 Studies analysis
An objective criterion is used to analysis the frequency scan results. From the levels of harmonic currents generated during the energization of a transformer and the maximum bearable overvoltages, we can deduce the maximum eligible direct sequence impedance for each harmonic frequency. Harmonic currents are found through a time-domain study of typical transformer switching. Time-domain simulations are very time consuming, so frequency scan studies allow us to save time by selecting critical grid schemes. A criterion example used for 400 kV connections is presented on Figure 2. A 600 MVA autotransformer is used to set this criterion.
Figure 2: Maximum positive sequence harmonic impedances for transformer energization
0
1000
2000
3000
4000
5000
6000
50 100 150 200 250 300 350 400 450 500
Zd (Ω)
Frequency (Hz)
EMTP-RV Newsletter |November 2011 |page 6/28
3.3 TRANSFORMER ENERGIZATION, TIME-DOMAIN SIMULATIONS
Next step is to study the critical network configurations in time-domain simulations. The saturation of the transformer is due to a dc component of the flux in the transformer magnetic core, which depends on the switching time and the residual fluxes initially present in the transformer core. A statistical study is performed on switching times and residual fluxes. The statistical simulation option and scripts in EMTPWorks are used to run multiple simulations and define the worst case.
4 Specific tools developed in EMTP-RV
4.1 EMTPWorks scripts and Parametric toolbox
Multiple run simulations can be performed in EMTP-RV by script usage. The script language is JavaScript with EMTPWorks extensions. Any model and simulation parameters can be controlled through this language. Post-processing can also be performed with MPLOT controlled through JavaScript. A set of scripts dedicated to multiple run simulations have been developed at Rte. Cable installations and HVDC studies require an extensive usage of these scripts. Although scripts offer a very flexible solution for multiple run simulations, validation and maintenance of these functionalities can be difficult. Moreover programming skills are required to develop these scripts. That is why Rte collaborates with Hydro-Québec on a parametric toolbox. The toolbox is under development and should be ended in the beginning of 2012. This toolbox is composed of an independent GUI:
To specify the multiple run simulation parameters (changes can be applied on any device : value, connection, state),
To specify post-processing analysis,
To visualize final results. A set a functions have been developed to control EMTPWorks through an ActiveX interface. The toolbox takes advantage of multi-core architectures by applying multi-threading.
Figure 3: Parametric toolbox – Multiple run simulation parameters
EMTP-RV Newsletter |November 2011 |page 7/28
Figure 4: Parameteric toolbox – Post-processing GUI
4.2 DLL Based Equivalent Harmonic Impedance
Large scale network modeling is mandatory to properly assess harmonic impedances. As presented in section 3.1 an interface between EMTPWorks and Rte's SCADA tool has been developed to model large network. In case of an interconnection with a foreign grid this interface is not used to model the foreign grid because data are not available in the same platform. This situation occurred in the France-Spain HVDC interconnection studies. Installation of this 2000 MW VSC-HVDC link (www.inelfe.eu) imposed harmonic impedance calculations at both converter stations.
Figure 5: The France-Spain HVDC link
EMTP-RV Newsletter |November 2011 |page 8/28
As presented in Figure 5 this link is included in a meshed AC network. As a consequence the Spanish grid must be taken into account in the harmonic impedance calculation at Rte BAIXAS substation. A large part of the Spanish grid should have been modeled for this harmonic study. This work would take too much time and would require a transfer of large amount of network data. To solve this problem the DLL interface in EMTP-RV has been used to represent the Spanish grid by an harmonic equivalent circuit.
Figure 6: Spanish grid represented by an harmonic equivalent circuit
The Spanish TSO REE provided Rte with harmonic impedances values at VIC substation when the line between substations VIC and BAIXAS is disconnected. The DLL interface has been used to develop a component called DLL Based Equivalent Impedance to:
Read the Spanish harmonic data provided in an Excel file (R and X values from
harmonic rank 1 to 50),
For each harmonic rank :
o Connect an R-L impedance at VIC substation to ground,
o Update R and X values in relation with the Spanish harmonic impedance data.
This component can only be used for frequency scan studies. This solution enables Rte to accurately represent the Spanish grid without any detailed description of the foreign grid. Harmonic impedance data are read in the excel file as presented below. This section is included in the section corresponding to the request "DLL_INITIALIZE_NEW".
BAIXA
LF
Phase:0
P=-505MWQ=79MVAR
LF8
Pa
ge
BAIXAY762
Pa
ge
BAIXAY763
Pa
ge
BAIXAY761
Pa
ge
BAIXAY732
Pa
ge
BAIXAY733
Pa
ge
BAIXAY731
Z(f) ?z
ZIN1
Z(f )
SPANISH_EQ
CaseNb = 12
VIC
HL
71
BA
IXA
EMTP-RV Newsletter |November 2011 |page 9/28
The source code used to update R and X values at each frequency is presented below. It is included in the section corresponding to the core code request "DLL_PUT_IN_YN_SS". A similar source code has been developed for the zero-sequence harmonic impedance data.
EMTP-RV Newsletter |November 2011 |page 10/28
EMTP-RV Newsletter |November 2011 |page 11/28
A New Hysteresis Model in EMTP-RV
Authors: Afshin Rezaei-Zare, University of Toronto, Canada Reza Iravani, University of Toronto, Canada
The hysteresis phenomenon is one of the significant properties of the iron cores. The nonlinearity and an important part of the core loss correspond to this phenomenon. As such, the accurate analysis of some electromagnetic transients in which the iron core plays a significant role, cannot be feasible without a precise representation of the hysteresis.
1 Existing hysteresis models in EMT programs A few hysteresis models have been implemented and used in the existing ElectroMagnetic Transient (EMT) programs. The properties of these models and the corresponding drawbacks are discussed as follows:
1.1 EMTP Type-96
This model is the first hysteresis model which was implemented in the EMTP. The model is first presented in [1] and then modified in [2]. The model is constructed based on a look-up table including a few flux-linkage vs. magnetizing current points of the upward hysteresis major loop trajectory. Between each pair of the points, the trajectory is defined as a linear characteristic and therefore, Type-96 is a piece-wise linear hysteresis model. The model is simple and needs only the hysteresis major loop data. In the Type-96 model, the hysteresis minor loops are formed based on scaling the major loop trajectory. However, the experimental results show that the formation of the minor loops is independent of the major loop. Therefore, the model can cause erroneous simulation results in some transient analyses. Furthermore, the implementation of the model in previous versions of the EMTP, e.g. DCG EMTP-V3 [3], not only does not provide a true nonlinear solution with the rest modeled power system but also introduces a noisy behavior when the operating point switches between the linear branches of the magnetization trajectory [4]. In addition, in a piece-wise linear model, changing the operating branches is equivalent to switching in/out of an inductance and such a fictitious switching can cause numerical oscillations and erroneous simulation results.
EMTP-RV Newsletter |November 2011 |page 12/28
1.2 EMTP Type-92
The current hysteresis model in the EMTP-RV is of this type. The model represents a true nonlinear solution with the network equations. The implementations of the model in the EMTP programs are different in terms of the utilized nonlinear solution techniques. In the EMTP-V3 [3], the model is solved based on the compensation method. Such a solution method imposes significant topological and numerical limitations such as the inability of including the hysteresis model and another nonlinear element in the same sub-network [4]. The numerical solution technique has been significantly improved in the EMTP-RV version of the program. However, the inherent drawbacks of the model still exist which make the model simulation results unreliable in some transient analyses. The Type-92 model is based on the contribution of two parts, i.e., an unsaturated and a saturated characteristic, with the defined parameters “unsaturated flux” and “saturated flux” [3]. The unsaturated part is a function of unsaturated flux and represents the hysteresis behavior. The saturated part is a relationship between the unsaturated and saturated fluxes which forms the core saturation. Both parts of the model are represented by hyperbolic equations, which in turn impose additional limitations to the model since the accuracy of the model in representing various shapes of the hysteresis loops is limited. Furthermore, the model is constructed based on the positive flux part of the major loop upward trajectory, and the negative part is assumed to be a symmetry of the positive part which is not correct for many hysteresis major loops [5].
1.3 PSCAD/EMTDC Jiles-Atherton Model
In the PSCAD/EMTDC program [6], there is no hysteresis reactor available to users to construct an iron core of a power equipment model. However, in the program there is a current transformer (CT) model based on the Jiles-Atherton hysteresis representation [7]. The model is treated as a control/metering block of a protection/control system rather than a power equipment. The model has an input control signal which is the CT primary current, and calculates the CT secondary current as an output control signal which can be applied to the protective/control system.
2 New hysteretic reactor model To cope with the above mentioned drawbacks of the existing hysteresis models, a new hysteresis model is proposed based on the Preisach theory of hysteresis [8]. Various aspects of the new hysteresis model and the validation of the model based on the experimental results are presented and discussed in [5] and [9]-[12]. The properties of the new hysteresis model can be summarized as follows:
2.1 A modified Preisach Model
The new model is a modified version of the classical Preisach model [13]. In the classical model, a two-variable function is preliminarily determined which represents the distribution of magnetic dipoles in the ferromagnetic material. Once such a function is determined, the magnetization trajectories are deduced based on a double-integral over the dipole distribution plane [13] which applies a high computational burden on the program solver, and significantly reduces the suitability of the model for the implementation in an electromagnetic transient program.
EMTP-RV Newsletter |November 2011 |page 13/28
In the new hysteresis model, instead of defining the distribution function, the attempt is made to directly deduce the double-integral result as a function of the excitation and thereby significantly reduce the computational burden. The new model is based on the following two-term function:
)(*, xMinorxMajorxF (1)
n
i
iiii xcxBxMajor1
2 )(sech)tanh()( , to form major loop, (2)
))_(tanh()( shiftxxpDCxMinor , to form minor loops, (3)
Where the magnetization trajectories are constructed based on function F(x) with ascending and descending excitation parameters α and β, respectively. Function Major(x) forms the major loop and Minor(x) represents the minor loops. The parameters of Major(x) are deduced based on curve fitting to the major loop upward trajectory and to achieve a good fit, as many terms as required can be used. The parameters of Minor(x) are then calculated based on curve fitting to a minor loop data, e.g. the hysteresis loop at the rated excitation. Accordingly, as a new feature, in the new hysteresis model the formation of the minor loops is independent of the major loop shape. This is consistent with the observed hysteresis loops of the magnetic materials [13]. Figure 1 depicts an example of representing two sets of hysteresis loops with the same major loop shapes but different minor loop formations. Based on the same parameters of (2) but different parameters of (3) and starting from a de-magnetized condition, the minor loop shapes and the final magnetic remnant fluxes are different for the same applied exciting voltage.
(a) (b)
Figure 1: The same major loops but different formations of minor loops
2.2 Physically correct hysteresis model
Unlike Type-96 and Type-92 hysteresis models which are pure mathematical models, the proposed model is based on a physically correct and extensively validated theory of ferromagnetic hysteresis, i.e., Preisach theory. Over several decades, the Preisach model has been significantly improved and some aspects have been added to the early classical model. As such, the Preisach model is the most accurate hysteresis model to date [13].
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2.3 Memory-dependent and Wipe-out properties
As observed in the magnetic materials, the formation of hysteresis loops depends on the history of the magnetization in the material. In other words, based on two different past excitation conditions, starting from the same operating point, the new magnetization trajectories are different. The new model stores the past excitation extrema in the memory and used them to generate the magnetization trajectories. Another well-known physical property of ferromagnetic materials which is also incorporated in the new hysteresis model is Wipe-out property. It means that if the start and the end operating points of an excitation scenario are the same, the excitation condition does not impact the subsequent magnetization trajectories and can be erased from the material history.
3 Implementation of the model in the EMTP-RV Based on the DLL user-defined modeling feature of the EMTP-RV, the new hysteresis model has been implemented in the program. The model participates in the nonlinear iterations of the network equations and results in true nonlinear solutions of the system under study. Figure 2 shows a simple circuit within the EMTP-RV including the new developed hysteresis reactor. In the illustrated implementation, four parameters are available to be observed including the reactor voltage, inductance, flux and current. Figure 3 depicts the dialog box of the hysteretic reactor. The dialog box is designed such that by moving mouse over each parameter, a help statement appears (as shown for the flux tolerance by the yellow box). In addition, some parameters are evaluated for the validity of the user entered values. If a parameter value is out of the associated acceptable range, the entered value is replaced by a pre-specified default value.
Figure 2: A simply circuit within the EMTP-RV including the new developed hysteresis reactor
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EMTP-RV Newsletter |November 2011 |page 15/28
Figure 3: Dialog box of the hysteretic reactor
In the first part of the dialog box, i.e. initialization, the user can select starting simulation from steady-state, with initial condition, or with no initial condition (remnant flux=0). For the steady-state solution, if a negative inductance is entered, the program calculates the inductance for the steady-state solution. A minimum frequency is also defined below which the source is treated as a DC source (in figure 3 it is 1Hz). Since the Preisach model is initialized from a full saturation level, a tolerance is required for finding a symmetric hysteresis loop under steady-state conditions, i.e., the positive current peak should be equal to the negative peak magnitude. The last row of the part “initialization” includes:
i) The total number of the excitation extrema, ii) Number of the past extrema that the model iterates in a controlled manner to reach
the specified steady-state conditions, iii) The convergence tolerance for the flux.
In the second part “Hysteresis Parameters Type”, the user specifies that the parameters provided in the next part, i.e. “Hysteresis parameters”, are deduced based on:
i) The actual data of the core characteristic, ii) Mapped data which means for example the B_H curve is available and the
characteristic should be scaled based on the core dimensions and so on, iii) No-load test data are available.
Another advantage of the option “based on the mapped data” is that once a set of hysteresis parameters is found, only by changing the flux and current scaling factors, other transformer cores with the same material can be modeled.
EMTP-RV Newsletter |November 2011 |page 16/28
Furthermore, using this option, various parts of the transformer core, such as the core yokes and legs of a three-phase transformer in a detailed transformer model, can be represented without further curve fitting process. In part “Hysteresis Parameters”, the parameters of the hysteresis major and minor loops are entered. In the implemented model, the number of the tanh-sech terms representing the major loop (2) is limited to three terms, but it can be extended to as many terms as required. Starting from steady-state conditions, figure 4 illustrates the capability of the new model for the representation of different shapes of the hysteresis major loop; with wide area around the knee region (gooseneck shape), (top two figs), gooseneck shape with the intersection of the downward and upward trajectories at a high-current level (bottom left) and anti-symmetric hysteresis loop which is similar to the hysteresis shape of the Type-92 model (bottom right). Figure 5 illustrates the hysteresis loops when the core maintains different values of the remnant flux. Figure 6 shows the capability of the implemented model to be initialized based on multiple voltage sources with different frequencies. Accordingly, the model can directly start the time-domain analysis from the steady-state conditions without unwanted transients.
Figure 4: Various formations of the hysteresis major loop
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Figure 5: Hysteresis loops based on different remnant flux values
Figure 6: Hysteresis loops starting from a steady-state condition, in the presence of multiple
sources with different frequencies
EMTP-RV Newsletter |November 2011 |page 18/28
Figure 7: Ferroresonance demo of the EMTP-RV
Figure 8: Simulation results of the system of figure 7 based on the new reactor model, demonstrating the ferroresonance of the 230kV power transformer
Figure 7 illustrates the ferroresonance demo system which is available in the EMTP-RV program. The 230kV transformer core is represented based on the new reactor model. The corresponding simulation results are presented in figure 8 demonstrating the ferroresonance oscillations of the 230kV power transformer under study.
EMTP-RV Newsletter |November 2011 |page 19/28
4 Validation of the new hysteresis model based on the
experimental results The behavior of the developed hysteretic reactor model has been verified against the experimental results under various test scenarios which are discussed as follows:
4.1 Determination of the core remnant flux
To show the flexibility and the capability of the model to follow the actual trajectory of the flux, a laboratory test was performed on a transformer core [10]. Figure 9 shows the transformer which is energized through a circuit breaker. First the core was biased on one direction to represent half cycle saturation. The current was then interrupted by opening the circuit breaker. This case was chosen to investigate the core behavior under the “transformer inrush” or “CT saturation due to fault current with dc offset” conditions. Figure 10 illustrates the current interruption conditions. Following the negative part of the current, the current is interrupted and the voltage reaches zero.
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Figure 9: Transformer de-energization test for the evaluation of the core remnant flux
Figure 10: Interruption of the transformer current
EMTP-RV Newsletter |November 2011 |page 20/28
Figure 11: Hysteresis trajectory and the core remnant flux associated with the current
interruption of figure 10.
The measured hysteresis loop associated with the current interruption condition of figure 10 is shown in figure 11. In addition to the new reactor, the EMTP Type-96 was simulated based on the test condition for comparison purpose. The Type-96 was chosen because it can precisely represent the major loop. However, as shown in figure 11, it can not follow the minor loop trajectory and therefore cannot precisely predict the core remnant flux. Although, the error of the Type-96 model is 9%, the error can significantly increase under multiple current interruption scenarios such as the following case study. Figure 12 shows the operation of an auto-recloser on a permanent 12kA fault. The recloser dead-times are 0.6sec and 1.0sec, figure 12. As the second attempt, the recloser recloses on the fault at t=1.8sec.
Figure 12: Auto-reclosure operations on a 12kA Fault Current
EMTP-RV Newsletter |November 2011 |page 21/28
Figure 13: Remnant Flux subsequent to the second current interruption
In this study, the fault current measuring CT is modeled based on different formations of the hysteresis minor loops but the same major loop. Figure 13 depicts the CT core remnant flux, prior to the last reclosing on the fault (t=1.8sec in figure 12). Figure 13 shows that the remnant fluxes deduced from the models are noticeably different. Figure 14 illustrates the CT behavior subsequent to the second reclosure on the fault. Based on different remnant fluxes of figure 13 and different minor loop shapes, the CT represents significantly different output signals. In the first model which corresponds to the model with the highest remnant flux, figure 13(a), the CT is saturated in the first cycle. However, the third model, figure 13(c), does not show a significant saturation, figure 14(c). Based on figure 14(c), the CT is slightly saturated in the 4th cycle but it is rapidly recovered since the dc component has already decayed. This case study highlights the fact that a hysteresis model which is only based on the major loop data can cause erroneous results under some transients. The formation of minor loops determines the core remnant flux and as shown in figure 14, it results in completely different impacts on protection systems. A similar scenario can also be considered for energization/de-energization of power transformers, where the different remnant fluxes result in significantly different magnitudes of the transformer inrush current.
EMTP-RV Newsletter |November 2011 |page 22/28
Figure 14: CT output current subsequent to the second reclosure on the fault
4.2 Ferroresonance test of a 33kV voltage transformer
Another validation of the new reactor model is based on a ferroresonance test on a 33kV Voltage Transformer (VT) [5]. In the test, the source voltage was gradually increased up to a level at which the ferroresonance occurred. Figure 15 illustrates the VT voltage and power loss when the ferroresonance initiates. The VT voltage jumps to a transient peak of 2.36pu and settles down on 1.94pu. The VT power loss also increases from 29W under normal operating condition to 103W under ferroresonance condition. The transient power loss is also important because it plays a significant role to determine which steady-state operating point is reached, i.e., normal or ferroresonance operating point. In addition, the transient loss highly influenced the final ferroresonance operating mode, i.e., fundamental, sub-harmonic, or chaotic mode. To investigate the accuracy of various hysteresis models to represent the VT test conditions, the new reactor model, Type-92, and Type-96 were adopted in the VT model. Figure 16 depicts the type-92 model fitted to the measured hysteresis upward trajectory data. Figure 16 clearly shows a poor capability of Type-92 to accurately represent the measured trajectory. However, figure 17 shows a good agreement between the new reactor and the measured hysteresis trajectories.
EMTP-RV Newsletter |November 2011 |page 23/28
Figure 15: Test results illustrating the VT voltage and power loss during the transient period
from normal operation to steady-state ferroresonance condition
Figure 16: Fitted Type-92 model to the measured upward trajectory data demonstrating an inaccurate representation of the VT magnetization characteristic
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EMTP-RV Newsletter |November 2011 |page 24/28
Figure 17: Fitted hysteresis loops of the new reactor to the measured loops
Figure 18: Bifurcation points deduced from the models
Figure 18 shows the predicted bifurcation points obtained from the models, which are significantly different. The bifurcation point is the source voltage at which ferroresonance initiates. The Type-96 presents the largest error among the models. The EMTP-RV model (Type-92) voltage before ferroresonance is close to the measurement and the ferroresonance occurs at higher voltage than the Type-96. However, the most accurate ferroresonance occurrence voltage is predicted by the new reactor model. The voltage under steady-state ferroresonance condition is also precisely represented by the new reactor. Figure 19 depicts the modeled power losses compared with the measured loss. Since the power loss plays an important role in some transients, the core model should be able to accurately represent the core loss, under the transient regimes. The EMTP Type-96 is fairly accurate for the representation of the core loss under ferroresonance conditions. However, before ferroresonance and during the transient, it introduces a noticeable error. The EMTP-RV (Type-92) represents the largest error among the models. Under none of the operation conditions the Type-92 model is accurate. This is due to the inaccurate hysteresis loops of this model, figure 16. However, the new reactor model is accurate for the entire depicted transient period.
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Figure 19: Comparison of power losses of the models with the measurement accurate for the entire depicted transient period.
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5 Conclusions
The features of the new hysteretic reactor model can be summarized as:
• The model is based on an accurate, physically correct, and widely-verified model of
hysteresis, i.e., the Preisach model.
• Independent formation of hysteresis major and minor loops
• True nonlinear solution within the EMTP-RV
• It can accurately represent the physical properties of the magnetic core materials
For precise EMTP studies, the following subjects are the potential applications of the new
reactor model:
De-energizing/re-energizing of transformers
Ferroresonance phenomena in power and instrument transformers
Determination of the core remnant flux
Precise modeling of VTs, CTs, and CVTs for protection studies
Accurate modeling of electrical machines
Effective design of control systems for power-electronic based drives by taking into
account the machine nonlinearity and a realistic estimation of the power loss
It is also worth to be noted that:
• Evidently, a more detailed model needs more parameters. Although, a model with
simple implementation and with less required parameters is generally preferable, the
accuracy of such models is limited.
• For a sophisticated hysteresis model, the dependency to the minor loop data is
necessary and an advantage, although minor loop data are not always available. Due
to different behavior of minor loops (extensively verified by experiments), neglecting
the minor loop parameters can result in completely different and unexpected results.
• For the new reactor, if the minor loop data are not available, a set of pre-specified
default values can be considered.
EMTP-RV Newsletter |November 2011 |page 27/28
6 References: [1] S.N. Talukdar, and J.R. Bailey, “Hysteresis Models for System Studies”, IEEE Trans. Power Apparatus and Systems, vol. PAS-95, no. 4, pp. 1429 - 1434, July-August 1976.
[2] J.G. Frame, N. Mohan, and T. Lui, “Hysteresis Modeling in an Electro-Magnetic Transients Program”, IEEE Trans. Power Apparatus and Systems, vol. PAS-101, no. 9, pp. 3403 - 3412, Sept. 1982. [3] Electromagnetic Transients Program (EMTP), EMTP-V3, Development Coordination Group of EMTP.
[4] S. Dennetière, J. Mahseredjian, M. Martinez, M. Rioual, and A. Xémard, “On the implementation of a hysteretic reactor model in EMTP”, International Power Systems Transients Conference, New Orleans, USA, 2003.
[5] A. Rezaei-Zare, R. Iravani, M. Sanaye-Pasand, H. Mohseni, and Sh. Farhangi, “An accurate hysteresis model for ferroresonance analysis of a transformer,” IEEE Trans. Power Del., vol. 23, no. 3, pp. 1448–1456, Jul. 2008.
[6] PSCAD/EMTDC, Version 4.3 Manitoba HVDC Research Center Inc. Winnipeg, MB, Canada, 2010.
[7] D. C. Jiles and D. L. Atherton, “Theory of ferromagnetic hysteresis,” J. Magn. Magn. Mat., vol. 61, pp. 48–60, 1986.
[8] F. Preisach, “Uber die magnetische nachwerikung,” Zeitschrift Phys., vol. B 94, pp. 227–302, 1935.
[9] A. Rezaei-Zare, M. Sanaye-Pasand, H. Mohseni, S. Farhangi, and R. Iravani, “Analysis of Ferroresonance Modes in Power Transformers Using Preisach-Type Hysteretic Magnetizing Inductance”, IEEE Trans. on Power Delivery, Vol. 22, No. 2, pp. 919-929, April 2007.
[10] A. Rezaei-Zare, R. Iravani, M. Sanaye-Pasand, H. Mohseni, S. Farhangi, “An Accurate Current Transformer Model Based on Preisach Theory for the Analysis of Electromagnetic Transients”, IEEE Trans. on Power Delivery, Vol. 23, No. 1, pp. 233 – 242, Jan. 2008.
[11] A. Rezaei-Zare, R. Iravani, M. Sanaye-Pasand, “Impacts of Hysteresis Loop Formation on Stability Domain of Ferroresonance Modes”, IEEE Trans. on Power Delivery, Vol. 24, No. 1, pp. 177-186, Jan. 2009.
[12] A. Rezaei-Zare, and R. Iravani, “On the Transformer Core Dynamic Behavior During Electromagnetic Transients”, IEEE Trans. on Power Delivery, Vol. 25, No. 3, pp. 1606-1619, July 2010.
[13] I. D. Mayergoyz, Mathematical models of hysteresis, New York: Springer-Verlag, 1991.
EMTP-RV Newsletter |November 2011 |page 28/28
EMTP-RV Events
1 Events at a glance: EMTP-RV Course in Lille - October 24-27, 2011 Using EMTP-RV for simulation and analysis of Power Systems. Over 25 participants took part to this comprehensive course. Organized in collaboration with Ecole Centrale de Lille, this course was presented by Sébastien DENNETIERE from RTE, France, Xavier GUILLAUD Professor from Ecole Centrale de Lille, France, Xavier LEGRAND from Electricité de France, France, Jean MAHSEREDJIAN, Professor from Ecole Polytechnique de Montréal Québec, Canada.
2 Coming events: EMTP-RV COURSE in New Orleans, USA Simulation and Analysis of Power System Transients using EMTP-RV December 12-16, 2011 - New Orleans, Louisiana – USA Analysis of Transients in Power Systems May 21-24, 2012 - Madison, Wisconsin – USA This course is organized by the University of Wisconsin-Madison. EMTP-RV Academic User Group May 17, 2012 - Sarajevo, Bosnia and Herzegovina
Find more information at http://www.emtp.com For any information about this Newsletter, please contact [email protected]
