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A NEW APPROACH FOR MODELING COMPLEX POWER SYSTEM COMPONENTS IN DIFFERENT SIMULATION TOOLS Per-Erik Bjorklund ABB Power Systems Ludvika, Sweden

Jiuping Pan ABB Corporate Research Raleigh, USA

Chengyan Yue ABB Corporate Research Beijing, China

Kailash Srivastava ABB Corporate Research Vasteras, Sweden

Abstract – This paper presents a new modeling concept underlining the development of complex models for advanced power system components using HVDC Light as an example. Instead of writing the model for HVDC Light in different simulation tools as user defined component, a “common component” is developed which represents the detailed control functionality of HVDC Light. This common component is then linked to different simulation tools through appropriate user model interfaces. The common component is quite general and can be interfaced with any simulation tool that permits linking of an external application. This tool independent modeling approach is particularly useful for upgrade and maintenance of models with utmost quality especially when the product is under constant development. The performance of the HVDC Light models implemented in this novel way was evaluated for PSS/E and Power Factory and found to be satisfactory for power system dynamic stability analysis.

by the equipment manufacturers. This situation has resulted in limitations in power system simulation studies for exploring more efficient transmission grid expansion alternatives. The following are the two problems for modeling complex power system components in different dynamic simulation tools:

Keywords: HVDC transmission, dynamic response, modeling, simulation, PSS/E, Power Factory

The work presented in this paper is based on the concept proposed in [6] concerning complex models for advanced power system components in electromechanical transient programs. Instead of writing the device model in different tools as user defined component, a “common component” is developed which represents the detailed functionality of the device. This common component is then linked to different simulation tools through appropriate user model interfaces. The feasibility of such a tool independent modeling approach has been investigated in this work with HVDC Light transmission system as an example. The common component is general and can be interfaced with any simulation tool that permits linking of an external application. The performance of the HVDC Light model implemented in this novel way was evaluated for PSS/E and Power Factory and found to be satisfactory for power system dynamic stability analysis. The common component based HVDC Light model has also been implemented in Netomac.

1 INTRODUCTION The recent development in advanced transmission technologies, such as HVDC Light® and FACTS opens up new possibilities for improving the reliability and utilization of power grids. Detailed models for various dynamic simulation tools such as PSS/E, Power Factory, PSLF, Simpow, Netomac, etc. [1-5], are needed to enable electric utilities and regional transmission organizations evaluate the operational benefits of incorporating HVDC Light and FACTS devices as feasible planning alternatives using the simulation tools of their choice. A model for power system simulation purposes can cover different aspects from steady-state analysis to dynamic response simulation. Also, a wide range of simulation tools are used by different utilities. The focus of this paper is on the dynamic representation of complex power system components in different simulation tools. Dynamic response simulations are used on a very wide range of applications, from initial planning studies by electric utilities to detailed project studies by manufacturers. The credibility of simulation studies is affected by the modeling accuracy of virtually every major power system components. However, due to the complexities of the device characteristics and the issue of intellectual property protection, accurate models for advanced power system components such as HVDC Light and FACTS are mainly developed and maintained

1) Models available in the tool-dependent model libraries are either designed for specific projects or over simplified, and thus may not adequately represent the characteristics of updated power system technologies. 2) Development of complex power system components in different tools as user defined component is a challenge because of significant implementation and maintenance effort and difficulties to ensure consistent performance.

2

TOOL INDEPENDENT MODELING FOR HVDC LIGHT

2.1 HVDC Light Technology HVDC Light is a technology to transmit power underground and under water, also over long distances [79]. It offers numerous environmental benefits, including “invisible” power lines, neutral electromagnetic fields, oil-free cables and compact converter stations. With extruded DC cables, power ratings from a few tens of megawatts up to more than thousand of megawatts are

available. The converter station design is based on voltage source converters (VSCs) employing Insulated-Gate Bipolar Transistors (IGBTs) that operate with high frequency pulse width modulation (PWM). HVDC Light has the capability to rapidly control both active and reactive power independently of each other, to keep the voltage and frequency stable. Reference [10] gives detailed description of HVDC Light technology. HVDC Light was introduced in 1997. A number of underground transmissions up to 350 MW are in commercial operation and more are being built. One recent project is the Estlink Transmission System which operates at ±150 kV DC and is rated at 350 MW of active power in either direction. The link interconnects the national grids of Estonia and Finland, enabling the exchange of electric power between the Baltic states and the Nordel electric system for the first time. 2.2 HVDC Light Control System Each HVDC Light converter is provided with an identical control, independent of rectifier or inverter operation. The principal control scheme of one converter station is shown in Figure 1. PCC is the Point of Common Connection, i.e. the point of converter connection to the ac system, and the reference point for ac voltage, active and reactive power orders.

power and voltage references are normally picked from the load flow solution as the initial condition; the user can modify these inputs to represent power order changes or other step changes. The auxiliary inputs ΔPref, ΔQref and ΔUacref can be used for modulation to achieve desired frequency control, damping control and voltage stability enhancement. 2.3 Tool Independent Modeling Approach The technical concepts of tool independent model development strategy for complex power system products involve the following two main steps: 1) Common Component - This tool independent code represents the full control functionality of the power system products described by the block diagram or the system of differentialalgebraic equations (DAE). It performs computations related to initialization, time derivatives and numerical integration. 2) Tool Dependent Interface - This is in general a user defined model which directly communicates with the simulation engine and the common component. Figure 2 illustrates the technical approach of tool independent model development strategy for HVDC Light. Main Circuit and Control System HVDC Light Simulation Models Tool Dependent Implementation

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Figure 2 Tool Independent Modeling for HVDC Light

Figure 1 Principal Control Scheme of HVDC Light The control functions of HVDC Light include ac and dc voltage control, active and reactive power control, and inner current control. The control system also recognizes current output limitation and internal converter voltage limitations. Figure 1 also shows the reference inputs Pref, Qref, Uacref and Udcref as well as the auxiliary inputs ΔPref, ΔQref and ΔUacref. For a twoterminal HVDC Light system, one of the converters should control dc voltage (Udcref) and the other active power (Pref). Each of the converters can be independently set as ac voltage control mode (Uacref) or reactive power control mode (Qref). In dynamic simulation, the

The scope of common component is limited to the control system of HVDC Light which communicates with the ac system and the dc system through appropriate tool dependent user model interfaces (UMI). For different tools, the ac system, the dc system and the converters may be modeled in different ways depending on the standard components available in the tool dependent model libraries. In the design of common component, robust internal integration method is one of the key requirements. This is because some of the simulation tools use numerical solution methods with a fixed time step. To minimize the total simulation time, users often want to use a time step in the range 5-10 ms for system stability studies. For HVDC models in general, including HVDC Light, such a large time step is a challenge in combination with the time constants in HVDC control. Thus, in order

to ensure result accuracy, time steps in the range of 0.55 ms are recommended. In some other simulation tools, variable time step algorithm is supported for numerical integration, permitting the time step to increase automatically once the transients have died out. This feature hastens the simulation process while maintaining the same accuracy. The common component can be implemented in different languages such as FORTRAN, C/C++, and Matlab/Simulink In some simulation tools, such as PSS/E and Power Factory, the developed common component can be linked into the simulation process as external functions via appropriate user model interface mechanisms as shown in Figure 3. PSS/E

interface which communicates with the common component (not shown in Figure 4). User has the flexibility to select either the standard PSS/E integration method or the internal integration method. In principle, if standard PSS/E integration method enabled, the performance of common component based HVDC Light model can be expected to match that of the existing, tool dependent PSS/E model as will be shown in the next Section.

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Figure 4 Load Flow and Dynamic Models for HVDC Light in PSS/E

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Figure 3 UMI of PSS/E and Power Factory 3

MODEL IMPLEMENTATION

3.1 Model Implementation in PSS/E The modeling approach of HVDC Light in PSS/E has been discussed in Reference [6, 11]. As shown in Figure 4, in load flow analysis, the HVDC Light transmission is modeled by two generic generators, each representing a converter with user specified active and reactive power levels and voltage setting points. In this version of implementation, DC system is not explicitly represented in the PSS/E load flow model. The total dc system losses are a priori assumed/estimated and considered by the difference in active power levels of the two generators. The active power of sending end generator is negative valued to represent rectifier operation. The main circuit of HVDC Light station includes a converter transformer and a shunt filter. The dynamic behavior of the HVDC Light system is modeled by two user models: CHVDCL and DC_HL2. • The converters are represented via the PSS/E generic generator model, and the user model CHVDCL is used to calculate the current injection by each generator. • The dynamic behavior of the dc system is represented by a simple 1/sT block, and the user model DC_HL2 is used to calculate the dc voltages at both ends. With the tool independent modeling approach, the user model CHVDCL is designed as the user model

3.2 Model Implementation in Power Factory In Power Factory, the main circuit of HVDC Light station can be explicitly represented by standard components available in its library such as DC buses, DC cables and PWM converters as shown in Figure 8. Since the standard PWM converter is lossless, the converter losses are a priori assumed/estimated and modeled by equivalent resistance loads at DC buses. Figure 5 shows the user-defined composite frame for the converter control system, which consists of measurements, controller and PWM converter [12]. Controller receives the network input signals from measurement devices and sends out the modulation index to the PWM converter.

Figure 5 User Defined Composite Frame for HVDC Light Control System in Power Factory

With the tool independent modeling approach, the controller, implemented as a DSL model and associated C++ interface, functions as the tool dependent interface and communicates with the common component (not shown in Figure 5). In principle, with appropriate setup of simulation parameters, the performance of common component based HVDC Light model in Power Factory can be expected to be close to that of the PSS/E model as will be shown in the next Section. It should be mentioned that the models for HVDC Light are continuously improved and developed as the technology itself moves forward and more flexible usermodel interfaces are available in different simulation tools. The focus of this paper is the principle of tool independent modeling approach based on the concept of common component. 4

MODEL VERIFICATION

4.1 Benchmark System The benchmark system used to verify the implemented simulation models in different tools is shown in Figure 6. This simple test system consists of two separate ac systems connected by the HVDC Light link. The rated power of HVDC Light is 373MVA and the rated dc voltage is +/-150kV. The strength of ac system is weak on both sides represented by a short circuit capacity 3 times that of the converter rating at the point of common connection.

Figure 6 Benchmark System for Model Verification The single line diagram of the benchmark system implemented in PSS/E and Power Factory is shown in Figure 7 and Figure 8 respectively.

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Figure 8 Benchmark System Single Line Diagram in Power Factory 4.2 Model Verification A number of test cases have been used to verify the performance of implemented HVDC Light models in PSS/E and Power Factory. The defined test cases include step changes in active power reference, reactive power reference, ac voltage reference, dc voltage reference, three-phase ground fault, etc. For each case, three simulations were performed: • PSS/E-BM – PSS/E with the existing, tool dependent HVDC Light model as described in [6] • PSS/E-CC – PSS/E with the common component based HVDC Light model • Power Factory – Power Factory with the common component based HVDC Light model In this verification, the performance of common component based HVDC Light models is benchmarked with the results of the existing, tool dependent HVDC Light model in PSS/E. As concluded in Reference [1], the existing, tool dependent HVDC Light model in PSS/E has demonstrated good agreement with PSCAD/EMTDC simulation results for different time steps from 0.5ms up to 10ms. The PSCAD/EMTDC model is a replica of the complete HVDC Light control system. 4.3 Step Change Examples Figure 9 through Figure 11 give example cases involving step changes in active power reference, ac voltage reference and dc voltage reference respectively. 450

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Figure 7 Benchmark System Single Line Diagram in PSS/E

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4.4 Three-Phase Ground Fault Example Figure 12 through Figure 14 give an example case involving three-phase ground fault at the PCC bus of rectifier. The fault was applied at 0.1s and the remaining ac voltage is about 10%. The fault was cleared at 0.2s.

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The following conclusions have been obtained: The performance of HVDC Light model in PSS/E implemented based on the common component matches that of the existing, tool dependent PSS/E model perfectly as expected (red lines not visible in plots). • The performance of HVDC Light model in Power Factory implemented based on the common component is quite close to that of the existing, tool dependent PSS/E model, and thus satisfactory for stability simulation studies.

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5 CONCLUSIONS Accurate models of advanced power system technologies such as HVDC Light and FACTS are needed in utility and regional transmission organization planning studies to explore more efficient transmission grid expansion alternatives. A new modeling concept is introduced in this paper concerning the development of complex models for advanced power system components. The advantages of the proposed model development strategy can be summarized as follows. • Only one set of common component needs to be developed so that the functionality and quality of the common component can be guaranteed for simulation studies in different tools. • Implementation in any tool is limited to the tool specific part of the user model interface. With a

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proven common component, the debugging will be limited to the tool specific code. With a common component, the model of products being continuously developed such as HVDC Light can be updated in different tools at the same time.

As an example, the HVDC Light model has been successfully implemented in PSS/E and Power Factory based on the common component concept. The developed simulation models have been verified and found to be satisfactory for power system dynamic stability simulation studies. The common component based HVDC Light model has also been implemented in Netomac. REFERENCES [1] [2]

PSS/ETM (Power System Simulator for Engineering), http://www.pti-us.com/pti/software/psse. Power Factory/DIgSILENT, http://www.digsilent.de/Software.

[3]

PSLF (Positive Sequence Load Flow Software), http://www.gepower.com/prod_serv/products/utility_software/en /ge_pslf. [4] Simpow®, http://www.stri.se. [5] Netomac, http://www.simtec.cc. [6] Per-Erik Bjorklund, Kailash Srivastava, William Quaitance, “HVDC Light Modeling for Dynamic Performance Analysis”, IEEE T&D Conference, Atlanta, USA, 2006. [7] Asplund G, Erilsson K, Svensson K. “DC transmission based on voltage source converter”. CIGER SC14 Colloquium in South Africa, 1997. [8] Stefan G Johansson, Gunnar Asplund, Erik Jansson, Roberto Rudervall, “Power System Stability Benefits With Vsc DCTransmission Systems”, CIGRE conference 2004, Paris. [9] U. Axelsson, A. Holm, C. Liljegren, K. Eriksson, L. Weimers, “Gotland HVDC Light Transmission – Worlds First Commercial Small Scale DC Transmission”, CIRED Conference, Nice, France, 1999. [10] It’s time to connect – Technical description of HVDC Light® technology, ABB Power Technologies AB, 2006. (http://www.abb.com/hvdc) [11] ABB User Guide for the PSS/E Implementation of the HVDC Light Model Version 1.1, October 2006. [12] ABB User Guide for the DIgSILENT PF Implementation of the HVDC Light Model Version 1.1, August 2007.