LUND 2015
21, rue d’Artois, F-75008 PARIS http : //www.cigre.org
262 – Session 2.6 MODELING and DYNAMIC PERFORMANCE ASPECTS OF INTEGRATING HVDC INTO THE AC POWER SYSTEM
Standard Model Structures for Power Flow and Stability Analysis of HVDC Systems in Transmission Planning Studies – A WECC Task Force Effort Topic: SC C4 - System Technical Performance; Modeling P. POURBEIK1 EPRI USA
Y. KAZACHKOV Siemens PTI USA
J. SENTHIL Siemens PTI USA
D. DICKMANDER ABB Inc. USA
W. PRICE Consultant USA
J. WEBER PowerWorld USA
D. DAVIES WECC USA
J. SANCHEZ-GASCA GE USA
SUMMARY The Western Electricity Coordinating Council (WECC) is the Regional Entity (RE) responsible for coordinating and promoting bulk electric system reliability in the Western Interconnection, in North America. For more than a decade, the WECC Modeling and Validation Working Group has been very active in developing various generic and standard model structures for use in planning study simulations. Some examples are the development of the three standard models for static Var systems for power flow and time domain stability simulations [1], [2] and the recent development of the second generation generic models for wind turbine generators [3], [4]. These models have to date been adopted by three major commercial software platforms and are being used throughout North America, and possibility elsewhere in the world. Similar to the previous efforts, WECC initiated the WECC HVDC Task Force. The TF is working to develop standard models to facilitate the analysis of conventional linecommutated converter (LCC) and voltage-source converter (VSC) based High-Voltage Direct-Current (HVDC) systems for power flow and stability studies for planning, when investigating alternative means of transmission. The intended scope of the models is to allow for power flow and transient stability analysis, typically performed in positive sequence programs. The dynamic models should be suitable for phenomena occurring in a timeframe ranging from a fraction of a second to many tens of seconds, with dynamics in the range of 0.1 to 10 Hz, and simulated such that the integration time step for the full interconnected system model does not need to be smaller than ¼ cycle – the frequency range and suggested integration time step is given to allow for appropriately considering the closed-loop control dynamics, of the HVDC system, for simulation within a transient stability program framework. HVDC systems are quite complex and for detailed studies associated with an actual HVDC installation vendor specific models will in many cases be required with close collaboration with the equipment vendor. Such vendor specific models are typically proprietary and not shared publicly. The simple public models to be developed by this group are not intended to be equipment specific, and furthermore are not meant to be used for detailed design or 1
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interaction studies. These models are intended for scoping studies related to planning for HVDC based transmission, particularly for future planned transmission expansion. When the WECC HVDC TF started this work, the commercial software platforms used in WECC already had well defined and well tested capabilities for modeling of LCC in power flow and dynamics. Also, all the tools had the capability of modeling VSC HVDC in dynamics. In fact, vendor-specific user-written models for both LCC and VSC HVDC have been developed, well tested and successfully used in many studies in these platforms. At least one of the tools also had a power flow model for VSC. However, the goal of the TF was to come up with a uniform set of simple and public models that could be adopted in all commercial software tools. Thus, the TF is a forum within which the collective experiences of all the stakeholders became a starting point for developing the uniform model specifications. To date the TF has developed a power flow model specification for VSC-HVDC, which has been implemented and tested in three commercial software platforms used within WECC. In addition, a user-written version of a dynamic model structure for LCC-HVDC has been developed and tested. This model will soon also be implemented in the commercial tools for testing and release. The focus of the TF at present is on point-to-point transmission, with the potential of extending the models to multi-terminal representation in the future. The paper presents these models and some basic simulation results.
KEYWORDS HVDC, Modeling, Voltage Source Converters
INTRODUCTION The goal of this effort can be summarized as follows: To develop basic building block models and documentation to facilitate the analysis of LCC and VSC HVDC systems in power system power flow and stability studies for planning studies, when investigating alternative means of transmission. The intended scope of the models is to allow for power flow and transient stability analysis, typically performed in positive sequence programs. Power flow models should be suitable for both contingency and post-transient analyses. Dynamic models should be suitable for phenomena occurring in a timeframe ranging from a fraction of a second to many tens of seconds, with dynamics in the range of 0.1 to 10 Hz, and simulated such that the integration time step for the full interconnect system model does not need to be smaller than ¼ cycle . The frequency range and suggested integration time step is given to allow for appropriately considering the closed-loop control dynamics, of the HVDC system, for simulation within a transient stability program framework. It is well known that HVDC systems are quite complicated systems and for detailed studies associated with an actual HVDC installation vendor specific models will in many cases be required, with close collaboration with the equipment vendors. Such vendor specific models are typically proprietary and not shared publicly. The simple public models being developed by this group are not intended to be equipment specific or used for detailed design or interaction studies. They are intended for scoping studies related to the investigation of the potential use of HVDC particularly for future planned transmission expansion. The power flow models developed, however, may be applicable even for use with detailed vendor specific dynamic models.
POWER FLOW MODELS FOR HVDC Power flow models for LCC HVDC have existed in commercial power system simulation tools for years, and are relatively well established. Some of the commercial tools, hitherto, did not have formal VSC HVDC power flow models and in many cases the VSC HVDC was modelled as a PV bus with reactive power limits (i.e. a synchronous generator model) on both ends. What this meant is that the user would need to essentially keep track of the two ends 2
and manually solve the simple power flow exchange between the two ends to ensure that the solution was reasonable. This is clearly cumbersome at best and error prone at worst. Thus, a power flow structure was proposed, discussed and finalized within the group and is shown in brief below in Figure 1.
Figure 1: VSC HVDC power flow structure. The VSC HVDC power flow model shown in Figure 1 has three parts: (i) the dc converter, (ii) the dc bus and (iii) the dc line/cable. The model captures the important aspects of VSC-HVDC. That is, 1. The ability to independently regulate ac voltage at the converter station; within the current and MVA ratings of the converter. 2. The ability to regulate voltage or maintain power factor. 3. Dc power and voltage setpoints on the dc side (one converter must always regulate voltage) 4. The dc converter (and line) losses. The details of the specification have been documented, but are too detailed to provide in this summary paper. The model has been implemented in three commercial tool platforms that are used in the WECC. In one of the tools, since the existing LCC HVDC power flow model allows for multiterminal HVDC modeling, the VSC power flow model was also extended to multi-terminal representation. A simple test case was built to test the model in the three platforms. The test case is shown in Figure 2.
Figure 2: Simple test system for VSC HVDC power flow simulations.
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This was modeled in the three programs and three different scenarios were simulated, with the results as shown in Table 1. All three platforms gave the same results, which also agree with hand calculations. One minor difference was found in one of the tools. In the beta release of two of the commercial tools, the parameter dcset has been implemented such that it specifies the dc MW setpoint of the converter. That is, as shown below in Table 1, when the inverter dcset is set to 200 MW, the dc power at the inverter end is kept at 200 MW, and thus the ac power injected into the grid by the inverter = 200 – converter losses (1 MW) = 199 MW (first row of the table). However, in the third tool when dcset = 200 MW, this is interpreted by the program as being the setpoint for the ac power being injected by the inverter, and so the ac power injected into the grid by the inverter is 200 MW, and the DC power = 200 + converter losses (1 MW) = 201 MW. This was discussed in the group and the agreement was that in future program releases all three would allow the user to toggle between a dc or ac setpoint for this parameter for consistency and flexibility. Table 1: VSC test case powerflow results.
Aloss (kW) 0 250 250
Bloss (kW/A) 0 0.5 2
Minloss Rectifier Side Inverter Side (kW) Pac (MW) Qac (MVAr) Pac (MW) Qac (MVAr) 1000 ‐202 50 199 ‐25.7 1000 ‐202 50 199 ‐25.7 1000 ‐203.3 50 197.7 ‐25.2
DYNAMIC MODELS FOR HVDC When the TF started the work of developing dynamic models, some of the software tools had several general purpose dynamic models both for LCC and VSC HVDC. However the aim of the TF was to develop new simple, standard and public model structures for LCC and VSC HVDC that could be implemented uniformly in all the software platforms. The initial specifications for the LCC HVDC dynamic model has been developed. The models were also developed as user-written models and initially tested to see if they provided reasonable response [5]. This was then discussed and reviewed with the task force and generally agreed to. Thus, work is presently progressing to start implementation of the dynamic models as beta versions in the commercial tools used in WECC. Once the beta implementations are completed the models will be benchmarked across the platforms and tested for appropriateness before the models are presented to WECC for final approval. Two alternative simple dynamic LCC HVDC models were developed. The first, called chvdc1, is shown below in Figures 3 to 5. This model adopts the high-level control philosophy used in the past by some vendors (e.g. BBC) where the various controls (current, voltage, extinction angle) are effected by separate PI loops and then the final firing angle command selected by a high/low value gate at the rectifier/inverter, respectively. The second simple planning model, called chvdc2, is shown below in Figures 6 and 7. This model adopts the high-level control philosophy used by vendors such as ABB, where a single PI loop controls the firing angle and the other control objectives are achieved by dynamically controlling the main PI loop controller limits. Note that for this case the Voltage Dependent Current Order Limit (VDCOL) model remains the same as that shown in Figure 5. The actual VDCOL function is a look-up table; likewise for the non-linear gains in chvdc1. This short paper does not allow for a detailed discussion of the various features of the models, which will be documented in more detail by the TF once the models are finalized and released. It is also quite possible that further changes may be made once the beta versions are implemented and tested in the commercial software platforms. The interested reader may refer to other references such as [6] or [7] for a more detailed explanation of the control philosophies, particularly of that employed in chvdc2. A simple benchmark test case system, based on the CIGRE benchmark case [8], was established for testing these models. The results of an example simulation are shown in Figure 8. The simulation results in Figure 8 are simple simulations to establish that the models are working – they do not represent optimized parameters or tuning of the controllers, nor do they in any way represent actual
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equipment behavior for any HVDC installation, they are illustrative examples for testing the model implementation.
CONCLUSION AND SUMMARY The WECC HVDC TF is working on the development of simple powerflow and dynamic models for HVDC systems for the purpose of being used in steady-state and time-domain positive sequence simulations for planning studies. Hitherto model specifications have been developed for the VSC power flow model and for two simple LCC HVDC dynamic models. These are being implemented and tested presently at the time of issuing this paper. In due course the VSC dynamic model is also expected to be completed and then all the models, once implemented, tested and approved by all stakeholders, will be submitted for WECC approval and thus hopefully implemented as standard library models in the commercial software platforms used in North America, and hopefully also adopted by other software tools as well. Idc_margin_r
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Figure 3: Rectifier controls for chvdc1 Idc_margin_i
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Figure 4: Inverter Controls for chvdc1
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If dV/dt ≥ 0 T = Tu else T = Td
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Figure 5: VDCOL control logic Idc_margin_r
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Figure 7: Inverter controls for chvdc2 model.
chvdc1 model
chvdc2 model
Figure 8: Simulation of a rectifier side fault on the ac system on the CIGRE benchmark case.
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BIBLIOGRAPHY [1] Generic Static Var System Models for the Western Electricity Coordinating Council https://www.wecc.biz/Reliability/GenericStaticVarSystemModelsforWECC.pdf [2] P. Pourbeik, D. J. Sullivan, A. Boström, J. Sanchez-Gasca, Y. Kazachkov, J. Kowalski, A. Salazar, A. Meyer, R. Lau, D. Davies and E. Allen, “Generic Model Structures for Simulating Static Var Systems in Power System Studies—A WECC Task Force Effort”, IEEE Transactions on PWRS, August 2012. [3] Specification of the Second Generation Generic Models for Wind Turbine Generators https://www.wecc.biz/Reliability/WECC%20Second%20Generation%20Wind%20Turbine%20Model s%20012314.pdf [4] A. Ellis, P. Pourbeik, J.J. Sanchez-Gasca, J. Senthil and J. Weber, “Generic Wind Turbine Generator Models for WECC – A Second Status Report” Accepted for publication at the IEEE PES General Meeting 2015, paper 15PESGM0126, Denver, CO, USA [5] P. Pourbeik, “Final Proposed Model Specification for LCC HVDC”, memo sent to WECC HVDC TF and EPRI P40.016, January 16th, 2015. [6] P. Kundur, Power System Stability and Control, McGraw-Hill, 1994. [7] C. T. Wu, P. R. Shockley and L. Engstrom, “Intermountain Power Project 1600 MW HVDC transmission system”, IEEE Trans. PWRD, pp: 1249 – 1256, 1988. [8] M. Szechtman, T. Wess, C. V. Thio, H. Ring, L. Pilotto, P. Kuffel and K. Mayer, “First Benchmark Model for HVDC Control Studies”, CIGRE Report of WG 14.02, Electra Magazine, 1991.
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