The development of large-scale renewable generation and load electrification call for highly efficient and flexible electric power integration, transmission and interconnection. High Voltage DC (HVDC) transmission technology has been recognized as the key technology for this scenario. HVDC transmissions, including both the line commutated converter (LCC) HVDC and voltage source converter (VSC) HVDC have played an important role in the modern electric power system. However, with the inclusion of power electronic devices, HVDC introduces the characteristics of nonlinearity and different timescales into the traditional electromechanical system and thus careful modeling and simulation of HVDC transmission are essential for power system design, commissioning, operation and maintenance. This book focuses on the modeling and simulation of HVDC transmission systems. The development of HVDC technologies is briefly introduced, and then the role of modeling and simulation in the research and development of HVDC systems is discussed. The chapters cover the general practice of HVDC modeling and simulation; electromagnetic modeling of LCC HVDC; VSC HVDC system modeling and stability analysis; electromagnetic modeling of DC grids; electromagnetic simulation of HVDC transmission; electromechanical transient simulation of LCC HVDC; electromechanical simulation of VSC HVDC; dynamic phasor modeling of HVDC; small-signal modeling of HVDC systems; hybrid simulation for HVDC; and real-time modeling and simulation for HVDC systems. The simulation algorithms are explained for each model and case studies and application examples are included. This book is essential reading for engineers and researchers involved with transmission grid construction, as well as advanced students of electrical engineering.
Inspec keywords: power grids; power system stability; power system simulation; hybrid power systems; time-domain analysis; HVDC power transmission
Other keywords: stability analysis; electromechanical transient simulation; real-time modeling; LCC-HVDC electromagnetic modeling; dynamic phasor modeling; small-signal modeling; DC grid electromagnetic modeling; HVDC hybrid simulation; HVDC transmission simulation; VSC system modeling
Subjects: Power system control; Power engineering computing; General and management topics; General electrical engineering topics; d.c. transmission
High-voltage DC (HVDC) transmission technology has been considered as the key technology for bulk electric power transmission and network interconnection [1,2]. HVDC is also referred to as the earliest and most significant application of power electronics in the electric power system. The technology is consistently active in innovation and the development for energy integration, transmission, and grid interconnection. This chapter will first provide a brief description of the config-uration, classification, and development and then the discussions will be oriented to the needs for the modeling and simulation of HVDC.
In this paper modeling and simulation of HVDC transmission have been considered essential for the HVDC project planning, commissioning, and operation. The fundamental difference in the modeling and simulation of the power grid with HVDC is the inclusion of the power electronic systems. According to the selection of circuit variables, the modeling for HVDC can be further classified into electromagnetic modeling (EMT) and electromechanical modeling or transient stability. The properties of the electric power grid with HVDC are different from the traditional electromechanical power system with the small transient time constant, nonlinearity, and time varying. The mathematical modeling for a power system with HVDC means a group of differential and or algebraic, Boolean equations will be established. The simulation procedure usually resorts to the pertinent numerical computation method. The utilization of tensor analysis and Diakoptics facilitates the con-struction of a flexible mathematical model of the complete systems of multi-converter stations and associated AC circuitry. The dynamic phenomena such as oscillation and harmonic instability have been successfully studied with the small-signal analysis. Varieties of models of HVDC have been developed and integrated in the well-recognized digital simulation software such as EMTDC/PSCAD, BPA, MATLAB, and PSS/E.
This chapter describes the electromagnetic transient (EMT) modeling of two-terminal line commutated converter-based high-voltage direct current (LCC-HVDC) transmission systems. The scope of the description is confined to modeling for the computation of EMTs associated with HVDC transmission systems using the widely spread EMT program-type method.
For the modeling of VSC-based HVDC systems with electromagnetic transient (EMT)-simulation tools, the parameters must first be properly designed before any further investigation. This chapter will first discuss all the constraints of the VSC electrical systems, and the corresponding criteria are given to calculate the system parameters. Then the AC dynamics of the VSC system are developed with space-phasor representation in the phasor domain. Using the dq0 transformation, the VSC space-phasor model is transformed from the phasor domain to the dq0 domain, in which the current-mode control is designed to decouple the VSC system in d-axis and q-axis. With this typical dual closed-loop decoupling control, the VSC system can be controlled in d-axis and q-axis independently
Up till now, HVDC has been predominately used for point-to-point transmission with two terminals and, in a few cases, with three terminals in radial configuration [1]. There have been quite a few multi-terminal practices for both line commutated converter (LCC)-HVDC and voltage source converter (VSC)-HVDC transmission [2]. With the fast development of large-scale remote-area renewable generation and wide-area energy transmission as in West Europe, North America, and Mainland China, the scenarios call for the need of multi-point integration, multi-point consumption, and multi-area interconnection. Studies of multi-terminal HVDC or MTDC grid have shown their potential application for the scenarios. MTDC can be defined as HVDC transmission or interconnection with more than two converter terminals. If some of the terminals in the MTDC system have more than one channel to reach each other, or there is one or more meshed structure included, it is referred to as a meshed DC grid. The MTDC can be considered as a radial-type DC grid, a typical example of DC grid. The terminology DC grid will be used in this book for both MTDC and meshed grid. The possible topology and configuration will be illustrated in the following sections.
Electromagnetic simulation is an effective approach to study the dynamic behaviors of high-voltage direct current (HVDC) transmission system. Transient characteristics of HVDC transmission system can be described with a set of differential equations, which are time varying and nonlinear. To include the distributed parameter effects of HVDC transmission line, partial differential equations are also needed. Special programs can be developed for the given type of electromagnetic-transient study. However, it is usually easier to setup the simulation of HVDC transmission system with the help of some general-purpose electromagnetic-transient programs. The general-purpose electro-magnetic-transient simulation programs fall into two categories.
The line commutated converter based high-voltage direct current (LCC HVDC) is widely used in power system to transmit a large amount of power for a long distance, which may have a great effect on the whole power system's stability. The LCC HVDC model has been modeled and simulated in detail in the electromagnetic simulation software [4-8] to ensure the accuracy of LCC HVDC itself. However, because of the great impact on the large scale of power system stability, the LCC HVDC should also be involved in the simulation of the whole system with an electromechanical model. The power system is always in three phases and symmetric in steady state. When the disturbance has happened, the electromagnetic process is usually finished in short term and the focus is changed to the power system stability, then the electromechanical simulation is employed. The electromechanical transient simulation [3] is focused on the reaction between electrics and mechanics, and is usually used for large-scale power system simulation, which is based on the hypothesis that the electric power system is symmetric, and the harmonics are not considered. The time step may be several mil-liseconds and the dynamic process may be extended to several seconds or minutes.
Voltage source converter based high-voltage direct current (VSC HVDC) has been developed rapidly all over the world , and several VSC HVDC projects have been commissioned in China, in which the most complex project is 500 kV and four terminals. VSC HVDC is usually privileged to transmit renewable energy power or supply power to the load center and can be effectively used for power system stability control. The VSC HVDC requires the building of the electromechanical simulation model for a large scale of power system. Because the main purpose is to simulate the interaction between VSC HVDC and the power system, the external characteristics of VSC HVDC should be kept as accurate as possible while the internal characteristics can be simplified. A quasi-steady-state model will also be employed for the electro-mechanical simulation of VSC HVDC, which includes the main converter circuit, DC line circuit and converter control strategy.
The application of dynamic phasors for developing SS models of HVDC systems was presented in this chapter. The chapter started with an introduction to dynamic phasors. Then, the modeling of transmission lines using dynamic phasors was presented. This was followed by modeling of individual components of the HVDC systems. How to combine the individual components together was explained with the aid of block diagrams to illustrate the signal flow between components. The developed SS models have been validated against detailed EMT simulation models.
In this chapter, how the current control loop and the outer loops that feed to it can be analyzed via small-signal modeling was shown. Thereby, an input-admittance matrix is formed, which characterizes the behavior of the VSC-HVDC terminal as seen from the PCC. This matrix is operating-point dependent. By modeling the grid impedance as a matrix as well, the stability of the converter-grid interconnection can be analyzed using the generalized Nyquist criterion. Although this was exem-plified for a purely inductive grid impedance, the grid-impedance matrix can be expanded to include, e.g., models of other converters and synchronous machines that are located in the electrical vicinity of the VSC-HVDC terminal.
Power system transient simulation is an important approach to study the system's dynamic behaviors and guide its daily operation schedule. It can be classified into the following two categories based on the different scales of time step: electromagnetic transient (EMT) simulation and transient stability analysis (TSA). EMT has time step in microseconds, uses instantaneous value of electrical variables, and studies the transient phenomena caused by the interaction between electrical and magnetic fields. EMT is able to present the detailed switch process of power electronic devices in HVDC, achieving very accurate simulation. However, generally it is not suitable for large-scale system simulation because of its large computational burden due to complicated model and small time step. EMT is commonly used in detailed study of HVDC system in a relatively small scale. Currently there are several commercial EMT tools, such as PSCAD/EMTDC, EMTP, MATLABĀ®/SimulinkĀ®, Real Time Digital Simulator (RTDS), HYPERSIM, and RT-LAB. Particularly, some EMT tools can achieve the so-called real-time simulation, such as RTDS, by parallel computation hardware. This capacity makes it possible to connect real control/protect devices via power amplifiers, implementing hardware in loop simulation, which is quite valuable for in-factory tests.
This chapter describes the modeling and simulation methodologies for high-voltage direct current (HVDC) systems on RTSs. In the past 25 years, RTSs have been successfully used in factory and dynamic testing of HVDC controllers by HVDC manufactures and electrical power utilities. It is widely accepted by today's power industry that the RTSs based on ElectroMagnetic Transient (EMT) theory are the mainstream simulation platforms for closed-loop testing of HVDC controllers. These RTSs have sufficient accuracy, while they are more affordable and more flexible compared with previous analogue simulators. As the EMT theory and algorithms are well-documented in many classical publications [1-6], and also in the previous chapters of this book, there is no need to duplicate the EMT metho-dology in this chapter. This chapter focuses on the special techniques used in real-time modeling of HVDC equipment and systems.