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First Benchmack Model for HVDC Controls in ATP program

Guilherme Sarcinelli Luz* Nestor Felippe da Silva

Furnas Centrais Elétricas S.A. Independent Consultant

Brazil Sweden

SUMMARY

ATP program has been steadily developed and improved in the last decades. Today ATPDraw, a graphical framework, is available, which has improved the use of the program significantly, above all for the less experienced users. However many other resources are also available, but just waiting for improvements. Among those resources are the user specified models which allow for a more complex model to be modularized, making available just the essential information to the user. HVDC transmission is one of these complex systems for which a simplified model can be a very useful tool. In 1991 the Working Group 14.02 (Control in HVDC systems) of CIGRÉ Study Committee 14 published the “First Benchmark System for HVDC Controls” in magazine ELECTRA [1]. The purpose of this HVDC benchmark model was to encourage comparisons of performance of different DC control equipment and control strategies of various manufacturers and institutes by means of simulator or digital circuit models. A secondary purpose was to provide reference cases for testing of simulators and digital programs. This paper presents this benchmark system modelled in the ATP program using the ATPDraw platform [2]. Good agreement was obtained with the same model in the PSCAD [3] program for different faults conditions and some comparative results are shown. Adaptation to other AC and/or DC system configurations can be made taking into account the already developed models. Control systems changes can also be implemented through modifications into the files that contain the control programming.

KEYWORDS

Electromagnetic Transients, HVDC transmission, ATPDraw, ATP/EMTP, PSCAD/EMTDC

X SEPOPE

21 a 25 de maio de 2006

May – 21rstto 25th – 2006 FLORIANÓPOLIS (SC) – BRASIL

X SIMPÓSIO DE ESPECIALISTAS EM PLANEJAMENTO DA OPERAÇÃO E EXPANSÃO ELÉTRICA

X SYMPOSIUM OF SPECIALISTS IN ELECTRIC OPERATIONAL AND EXPANSION PLANNING

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1. Introduction There are occasions a user needs to utilize specific models to perform a study for which model the user does not have deep or any knowledge. Even when the user has some knowledge it may happen that it would not be enough to manage its complexity and make changes in the parameters since that this may require an understanding of its details. Due to this, complex systems need versatile models in which new configurations and operations conditions can be easily adapted.

High Voltage Direct Current (HVDC) transmission has been applied everywhere, in Brazil as in other countries and the study of its application requires feasible resources for this analysis. With this goal, CIGRE published in 1995 a benchmark to be utilized as a basis for performance analysis of HVDC systems. However, such analysis demands an appropriate tool with a three-phase modeling and a thyristor bridge representation with which the commutation failure can be simulated. This kind of failure is an ordinary and determinant phenomenon in the performance analysis of an HVDC system.

Some electromagnetic transient programs available on the market allow this three-phase modeling such as ATP/EMTP and PSCAD/EMTDC [3]. The last one brings in its installation the CIGRE HVDC benchmark modeled in one of the example cases. Due to the fact that the ATP program is cost-free and has been receiving the cooperation from many specialists around the world, along with the fact that many experts are giving support to the users and encouraging the interchange between all users, it has become the most spread electromagnetic transient program in the world.

The purpose of this paper is to present this “First Benchmark System for HVDC Controls” developed in ATP program, using the several resources it makes available nowadays, in order to make it accessible to any user. From the publication of this paper in this seminar, this model will be offered to the ATP community to be inserted into the example cases given in the program distribution.

2. PRESENTATION OF THE MODEL:

Figure 1: First Benchmark System for HVDC Controls of CIGRE represented in PSCAD program

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The model in the ATP program was developed based on the case of the PSCAD/EMTDC program found in the HVDC example directory given with the program distribution and shown in Figure 1. System data are described in the mentioned ELECTRA magazine [1]. This example is organized in modules in order to facilitate the access to each different part of the model. Figures 2 and 3 present the Electrical System and Rectifiers Control Modules, respectively. Only the “Inverter Controls” module was not presented, but it follows a structure similar to the “Rectifier Controls” module.

Figure 2: Complement of the Figure 1 – “Electrical System” Module

Figure 3: Complement of the Figure 1 – “Rectifier Controls” Module

Figure 4 presents, in a unique module, the same system of PSCAD case above, but represented in ATP program using the ATPDraw platform. The same structure of modules in different levels could be also utilized, but a more simplified system representation was chosen.

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Figure 4: First Benchmark System for HVDC Controls of CIGRE represented in ATP program

Firstly, some components already existent in the ATPDraw program were utilized, such as ideal sources, transformers and linear branches for A.C. equivalent system and DC line representation. Other components were modeled using the modularization resource called “Data Base Module” in order to create new elements considering both simplicity and flexibility making the task easier for the users. Then the following components were developed:

a) 12-pulse converter bridge – available for the rectifier and for the inverter side, its pattern is quite similar to the PSCAD, with the only difference being a 12-pulse bridge with a unique PLL circuit for both 6-pulse bridge. As parameters it has: 2-digit identifier number (ex: 11), requesting of voltages and/or current for monitoring in the 12 valves (ex: 1, 2 or 3), base voltage of high side of the converter transformer in kV (ex: 345), proportional and integral PLL gains in p.u., deblocking time of the valves in seconds (this may leave always 0), frequency of the connected system in Hz, resistance in ohms and capacitance in µF (or µmho depending on COPT option) of valve damping circuit and deionization valve time in µs (in case of being considered different from zero). The 2-digit identifier number has the purposes to generate different internal nodes and to facilitate the identification of the voltage and current in the valves when they are requested. Some internal functions of the control, such as extinction angle measurement utilized by the inverter control, the firing pulse generation from a defined angle given by the external control and the RMS voltage measurement were implemented using TACS language because it can be utilized into the modularization resource. Since the DC terminals of the bridge are used as TACS control variables, they are nodes that can only be grounded through a resistance or any impedance.

b) A.C. Filters – aiming to give a better look to the model, the A.C. filters were built according to its

type branch and the data are given to each of the converter side. These and other filters configurations can also be modeled using the linear branches.

c) Smoothing reactor for rectifier and inverter with initialization variables – aiming to make

easier the initialization of the HVDC system, voltage source in the rectifier DC side and current sources in the inverter DC side were introduced which are disconnected after 15 ms. In order to avoid overloading the model drawing, such sources were incorporated inside the smoothing reactor component of each side correspondently. They are adjustable, but transparent for the user.

d) Initialization A.C. sources – with the same purpose of making easier the HVDC system

initialization, ideal sources were also introduced in the converter bus of each A.C. side. Amplitude, frequency and permanence time values are defined by the user according to the system operation condition.

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e) Control of the rectifier and inverter – they were developed using MODELS language control and have the same logical structure as this model in PSCAD. They have the following parameters: DC voltage and DC current basis, proportional and integral gains of CCA (there are two pairs of gains in the inverter CCA) and voltage and current measuring time constants. In the inverter the following parameters are also given: current order (there is VDCOL only in the inverter and the current order is sent to the rectifier) and the A.C. system frequency used for extinction angle measurement in the inverter control.

3. RESULTS: Having the complete system modeled, several cases were simulated in both ATP and PSCAD programs in order to compare the results. Three-phase and single-phase faults were performed in the rectifier and inverter considering different voltage drop levels (0%, 25%, 50% and 75%), but only the most severe one, the solid fault (drop to 0% of the voltage), was presented in this paper. In order to make easier the comparison of the results, Figures 5 to 8 present, side by side in the same scale, simulation results in both programs. Graphs of DC voltage and DC current and A.C. voltage where the fault was applied are presented. The left side ones are PSCAD results and the right side ones are ATP results.

The faults in the rectifier side are more sensible to the commutation failure if a 555.5µs of deionization time in the inverter corresponding to 10º is considered. Figure 9 presents the single-phase fault in the rectifier considering this deionization time, where the commutation failure just after the fault clearance is observed in both programs. The commutation failures, however, are not repetitive and appear only once. When different values of deionization time were tried some differences in commutation failures occurred between both programs. This could be explained not only because the complete algorithm is not exactly the same, but also because PSCAD considers interpolation in the first step of the thyristor conduction.

As it can be observed by the comparison between the results hereby presented both programs have quite similar responses for all faults.

4. CONCLUSIONS: The first HVDC Benchmark system presented by the task force of CIGRÉ in 1991 had the purpose to give a model where control strategies and different simulation tools could be analyzed. This paper presents this system modeled in the ATP program, making it available to be utilized by all their users. The comparative results with the PSCAD program were quite satisfactory.

Adaptation to other AC and/or DC system configurations can be made taking into account the already developed models. Control systems changes can also be implemented through modifications into the files that contain the control programming.

This model, developed in the ATPDraw platform, makes this Benchmark system available even for a user less familiarized with HVDC transmission concepts. This way the user can study this kind of technology that is spreading throughout the world, especially in countries with long distance transmission or interconnection between systems with different frequencies. The authors hope that the availability of this model can encourage other users to perform investigations with HVDC transmission system.

BIBLIOGRAPHY

[1] M.Szechtman et al: First benchmark model for HVDC control studies – ELETRA No 135, April, 1991, pp 55-73

[2] Hans Kr. Høidalen - ATPDraw Manual

[3] PSCAD – Circuit Design e Custom Models – Tutorial Manuals – Manitoba Research Center

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Figure 5 – Three-phase Fault in the inverter

D.C. voltage in the rectifier for three-phase fault in the inverter

D.C. current in the rectifier for three-phase fault in the inverter

Firing angle in the rectifier for three-phase fault in the inverter

A.C. voltage in the rectifier for three-phase fault in the inverter

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Figure 6 - Three-phase Fault in the rectifier

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Figure 7 - Single-phase Fault in the inverter

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Figure 8 - Single-phase Fault in the rectifier (without comutation failure)

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Figure 9 - Single-phase Fault in the rectifier (with comutation failure)