Evaluation of a New Definition for a Multi-Infeed Short Circuit Ratio

KTH Electrical Engineering Evaluation of a new definition for a Multi-Infeed Short Circuit Ratio Master of Science Thesis by Mercedes Sánchez Illanas XR-EE-ES 2007:005 KTH Electrical Engineering Electrical Power Systems Acknowledgements This thesis work is part of my Master of Science degree and is carried out at the School of Electrical Engineering, Division of Electrical Power Systems, Royal Institute of Technology (KTH) in Stockholm in cooperation with ABB HVDC, Sweden. Firstly, I would like to thank Professor Lennart Söder, the Head of the Division and my examiner Dr Mehrdad Ghandhari for letting me perform my work in this Division. I wish to express my gratitude to my supervisor Paulo Fischer De Toledo of ABB HVDC for the inspiration, support, advice, and sharing his expertise and deep knowledge related to the design and operation of HVDC. This thesis work would not have been carried out without his contribution. I am thankful to Dr Valerijs Knazkins, my supervisor at KTH for the given support and encouragement. Finally, I would like to thank all my colleagues in the Division of Electrical Power Systems for their support and advice during these six months that we have shared. Stockholm, March 2007 Mercedes Sánchez Illanas i Abstract A detailed methodology and consistent results related to the evaluation and validation of the Multiple Infeed Short Circuit Ratio as an index of the system strength in a particular point for Double Infeed HVDC systems are presented in this thesis. The evaluation will be carried out by comparing the critical MESCR with respect to the critical Short Circuit Ratio for a Single Infeed HVDC system. These critical values represent the weakest AC-network that connected to the inverter is able to keep the system stability after a disturbance. These stability limits are obtained by studying the risk of voltage instability. The results presented in this work conclude that the validation is positive and the stability limit can be set in 1.3. ii Table of contents 1 Introduction 3 2 The HVDC transmission concept 5 2.1. General overview 5 2.1.1. Rectifier operation 6 2.1.2. Inverter operation 7 2.2. An HVDC system model: The Cigre Benchmark model 8 2.2.1. The AC networks 9 2.2.2. The HVDC transmission link 9 2.2.3. The transformers 10 2.2.4. The control system 10 2.2.5. The Benchmark model extended to a double infeed HVDC system 12 2.3. Main technical aspects of concern 13 2.3.1. Tendency to voltage instability and voltage collapse 14 3 The Multi-Infeed Short Circuit Ratio 17 3.1. Definitions 17 3.2. Discussion 20 4 Methodology for calculating stability limit in Single and Double Infeed HVDC models 22 4.1. The Simulation Software: PSCAD 23 4.2. First Task: Obtaining stability limit for the Single Infeed HVDC model 24 4.2.1. Simulation case 27 4.2.2. Procedure 1 27 4.2.3. Procedure 2 28 1 4.3. Second Task: Obtaining stability limit for the Double Infeed HVDC model 29 4.3.1. Symmetrical configuration 29 4.3.2. Asymmetrical configuration 30 4.3.3. Procedure 1 31 4.3.4. Procedure 2 32 5 Results and Discussion 33 5.1. Single Infeed HVDC system 33 5.2. Double Infeed HVDC system 36 5.2.1. Symmetrical configuration 36 5.2.2. Asymmetrical configuration 41 5.3. Final discussion 43 6 Conclusions and Forward Researches 46 References 48 Table of Figures 50 2 Chapter 1 Introduction The HVDC power transfer has become the most feasible way to transmit a large amount of power over long distances. The enormous energy demand is increasing the number of interconnections between power systems, leading to complex and risky configurations regarding the appearance of adverse power systems phenomena. There are some technical aspects related to multiple infeed HVDC links that can severely destabilize the system, especially in critical situations, when the AC-network is weak in comparison with the DC-power supplied by the HVDC station. In fact, this relation provides information concerning the cooperation between both parts into the system performance referred to the prevention or resistance to anomalies such as temporary overvoltages, commutation failures, voltage instability or resonance, among others. The Short Circuit Ratio (SCR), or Effective Short Circuit Ratio (ESCR), represents the strength of the system as the ratio between the short circuit capacity of the ac-network and the nominal power of the HVDC link. This index is valid for single infeed HVDC systems, but can be extended to multiple infeed HVDC by the so called Multiple Infeed Short Circuit Ratio (MSCR) or Multiple Infeed Effective Short Circuit Ratio (MESCR). Such indices were introduced in [3]. The scope of this thesis consists of evaluating the definition of Multiple Infeed Short Circuit Ratio in a Double Infeed HVDC system, and validating it as a way to define the 3 real strength of a system in a particular point, and thus, estimate the performance like for the single-infeed HVDC case. The validation will be accomplished in the way that the definition involves all the interactions between HVDC stations that could affect the system stability. The evaluation will be made by determining the critical MSCR and critical SCR related to the risk of voltage instability in the system, and comparing them. Critical MSCR or critical SCR corresponds to the weakest AC system that connected to the inverter of a HVDC station still has stable operating conditions. This evaluation is made by applying small increases in current order in the HVDC stations. Two different procedures were put in practise, since the way to apply these increments to get the most realistic conclusions entails another goal in this thesis. Results for both procedures and different increment sizes are presented and discussed in this work, obtaining important conclusions which can serve as a starting point for further extended research in this field. 4 Chapter 2 The HVDC transmission concept The HVDC concept appeared in order to find solutions to some of the weaknesses of the HVAC power transmission. The HVDC power transfer is optimal for long distances, because the bulk of transmitted power is almost unlimited for practical purposes. In addition, an HVDC station is the best solution for linking two power systems working at different frequencies or not synchronized. On the other hand, this kind of installation is essentially more expensive [1]. 2.1 General overview A conventional HVDC station consists of two 12- pulse converters, the rectifier that is the positive pole and the inverter, which constitutes the negative pole, linked each other by a DC line. Both 12-pulse stations are formed by two 6-pulse, line-frequency bridge converters connected by Y-Y and a ∆ –Y transformers. The station is united at each terminal to the AC-network and a set of filters and shunt capacitors banks needed to reduce the current harmonics from the converters and supply the reactive power required by them [2]. 5 Figure 2.1 An HVDC transmission system [2] 2.1.1 Rectifier operation Assuming that the transformer’s reactance and the voltage drops through the thyristores are negligible, the average rectifier DC-voltage, in a 12-pulse station, follows the equation 6 2 6ωL V= Vcos α − S I (2.1) dπ LL π d where: VLL is the RMS line-to-line ac-voltage at the commutation bus. α is the firing angle LS is the ac-side inductance Id is the dc-current The AC-filters located at the high-voltage side of the HVDC transformers absorbed most of the harmonic currents. Consequently, it is assumed that the fundamental harmonic is just the responsible for both active and reactive power supply to the rectifier, resulting the equations below, which are simplified supposing LS = 0. 6 Pd= 2.7 V LLd I cos α (2.2) Q= 2.7 VLL I d sin α (2.3) From equations (2.2) and (2.3), we conclude that an HVDC converter acts as a load connected to the grid, and controlled in both magnitude and power factor. 2.1.2 Inverter operation All equations above can be extended to the inverter, knowing that the firing angle in this case is larger than 90°. Changing the polarity of the voltage with respect to that in the rectifier, the inverter average DC-voltage can be written in terms of the extinction or commutation margin γ as follows: 6 2 6ωL V= Vcos γ − S I (2.4) dπ LL π d and the relation between γ and DC-current, as I2ω L d S =cosγ − cos () γ + u (2.5) 2VLL where u is the overlap angle that represents the time in which more than two thyristors are conducting at the same time in a 6-pulse bridge because of LS. Equation (2.5) shows the functional relationship between the current and the overlapping between the conducting thyristors. The relation between inverter firing angle, overlap angle and commutation margin is 180 °=αi + γ + u (2.6) 7 which explains why the commutation margin is of paramount important for the reliable performance of HVDC stations. It must be large enough to permit thyristors the recovery from conduction to withstand forward blocking voltage. Otherwise, they could prematurely conduct, resulting in a failure in commutation of current between thyristors fail and causing large overcurrents [8]. The active power from equation (2.2) can be rewritten in terms of γ, taking into account that, in this case, that power goes from the DC side to the AC side. The reactive power does not change the polarity with respect to the rectifier because this is as well absorbed by the converter.

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