Design of Real Time Simulation Platform Based on FPGA for MMC-HVDC System
With a large number of all-controlled switching devices of converters in MMC-HVDC system and the frequency-dependent characteristics of line impedance in the electromagnetic transient model of HVDC transmission line, the real-time simulation platform calls for subtle time step and high calculation accuracy. The calculation parameters are prestored in advance so that they can be obtained by looking up tables in the calculation process, which reduces the computational load of simulation calculation of MMC-HVDC system. According to the characteristics of the calculation formulas, the parallel computing capability are fully exploited in order to improve the parallelism of the whole simulation calculation. A two-terminal MMC-HVDC system was built on the real time simulation platform based on FPGA on 5µs time step to verify the simulation and calculation ability of platform. And the simulation result are consistent with the off-line simulation software PSCAD/EMTDC.
OWADAYS, thefunctioning patterns of traditional energy have radically changed with the emerging of new energies. Compared with conventional HVDC transmission system, flexible HVDC transmission system boasts its more flexible control system, and is suitable for new energy access to grid and supply to passive network . Particularly, flexible HVDC transmission system with modular multilevelconverter (MMC),which has figured prominently in academic communities worldwide in recent years, enjoys the reputation of flexible topology, negligible output voltage harmonics and
being free of additional filters .
It’s generally known that a suite of control and protection system with powerful functionality and high-performance is critical to the safe, reliable and stable operation of flexible HVDC system. Hardware-in-loop (HIL) simulation, which is applied only in real-time simulation system, can assess the action accuracy of protection devices and the effectiveness of control strategies.Hundreds of connected sub-modules (SM) make up of the bridge arm in MMC to ensure higher voltage and power output and better harmonic characteristics[4-5]. High-frequency power electronic switches act individually in different sub-modules, which means the time step of electromagnetic transient real-time simulation should be set in the microsecond range. It’s a great challenge to the real-time simulation of MMC-HVDC system that the high-order nodal admittance matrix of the system has to be updated after the switching state of power electronic switches change during one time step.
Researchers improved the speed of MMC electromagnetic transient off-line simulation by simplifying the bridge arm and decreasing the order of admittance matrix[6-7]. However, off-line simulation can not reflect data sampling accuracy and communication delay. RTDS proposed a novel equivalent circuit of bridge arm which saved large computation resource and realized real-time simulation of high-voltage MMC system [8-9]. OPAL_RTproposed MMC model based on RT-LAB real-time simulator so as to realize real-time simulation of high-voltage MMC system [11-12]. The processors of these simulation platforms are all central processing unit (CPU) , which targets high-speed computation. Cluster CPUs are commonly applied to solve the calculation problem of MMC-HVDC electromagnetic transient real-time simulation. However,the establishment of a MMC-HVDC hard-ware-in-the-loop real time simulation platform on a large scale based on RTDS or RT-LAB costs huge amounts of funding, so it is difficult to popularize the real-time simulation platform in the research institutions and colleges.
The economical and portable field-programmable gate array (FPGA), which completes highly parallel numerical calculation, has fully configurable parallel hardware structures, distributed memory structures, and deep pipeline structure. It is widely used for real-time simulation of power systems[13-15]. In general, a dedicated computation module, according to the mathematical model of specific electrical components, is established to achieve component-level parallelism in real-time simulation system on FPGA. However, these dedicated computation modules are often idle due to the limitation of computing process, which results in the waste of resource in FPGA. A real-time digital simulator based on FPGA (FRTDS) and orders is designed and an order generator is proposed . FRTDS has broken the design concept of converting the simulation object into hardware, which provides a new idea to realize the electromagnetic transient real-time simulation of power system based on FPGA.
The design concept of FRTDS,the calculation characteristics of MMC-HVDC simulation and the arrangement of interior resource in FPGA are all taken into consideration. The calculation parallelism of MMC-HVDC simulation is improved by multiplexing the data output ports of calculators, merging the pipelines of calculators to form functional operation blocks. The computational load of the MMC-HVDC simulation is reduced by prestoring various parameters and designing corresponding addressing circuits. This paper is organized as follows: The hardware design of the FRTDS microprocessor cores and the functionality of the orders flow generator are introduced in Section 2. Section 3 describes the electromagnetic transient simulation model of key elements in the MMC-HVDC system. The redesigning of microprocessor core on basis of formulas of simulation model is shown in Section 4. The simulation process of the MMC-HVDC system is shown in Section 5. Study case is shown in Scetion 6. Conclusions are proposed in Section 7.