HVDC-MMC Interconnection (1000-MW, +/- 320 kV)
This example shows a model of a High Voltage Direct Current (HVDC) interconnection using Voltage-Sourced Converters (VSC) based on the Modular Multi- level Converter (MMC) technology. The simulation is optimized by the use of an aggregate MMC model.
DC links are becoming the preferred means for power exchange between countries and transmission of power from renewables (hydroelectric plants, off-shore wind power and solar farms) to power grids. Several of these systems are presently in operation, for example the INELFE project (France-Spain interconnection), the Dolwin1 project (offshore wind HVDC link), or planned for future projects such as the Northern Pass (Canada-USA 1090-MW DC link).
The MMC converter is implemented using an aggregate model to simulate the power modules of one arm. With this aggregate model, control system dynamics, converter harmonics and circulating currents phenomena are all well-represented. However, having only one virtual capacitor to represent all capacitors of one arm, the model assumes that capacitor voltages of all power modules are well-balanced, and therefore capacitor voltage-balancing scheme cannot be simulated. The aggregate model runs much faster than a detailed model that would use two switching devices and one capacitor for each individual power module of one arm. This aggregate model is also well-suited for real-time simulation.
The electrical grid is modeled using a 400-kV, 50-Hz equivalent and two breakers to energize Converter 1.
Converter 1 is implemented using 6 half-bridge MMC blocks, each representing 36 power modules. This custom SPS block uses a switching-function model where only one equivalent module is used to represent all power modules. The control signal is a two-element vector [ Nin, Nbl ], where Nin represents the number of inserted modules and Nbl represents the number of modules in the blocked-state. The output Vc (capacitor voltage) has only one element and gives the average value of the capacitor modules.
Inside the DC Circuit subsystem, you will find a simplified model of the cable, as well as the second converter modeled using DC sources and ideal switches. You will also find a switch to apply a fault on the cable.
The Controllers subsystem contains the various control systems required to operate the interconnection. It includes the Active & Reactive Power Regulators, the DC Voltage Regulator, a dq current regulator with feed-forward, PLL & measurement subsystem, PWM Generators to control Converter1 half-bridge MMC’s. You will also find the Sequencer Area where the various modes of operation of Converter1 are programmed.
Scopes & Measurements contains all the scopes used to observe several signals during the simulation. Power and fundamental voltage calculations are also performed in the subsystem
Simulating our SPS model for 10 seconds allows observation of the operation of the interconnection during start-up (capacitor charging), voltage regulation, and power regulation. All parameters required to run the model can be found in the following file: HVDC_MMC_param.m. This file is automatically executed in the MATLAB workspace when the example is open. Run the model and observe the following events:
At 0.1s, Breaker 1 is closed and Converter 1 is energized through a resistor to reduce the charging current. Capacitors are being charged and at 1s, the start-up resistor is short- circuited by closing Breaker 2.
At 1.5s, Converter 1 is deblocked and the Voltage Regulator is enabled.
At 2s, the Voltage Regulator setpoint is ramped to the nominal DC operating voltage of the interconnection: 640 kV (+/-320 kV).
At 4s, PQ regulators are enabled and Converter 2 switches are closed.
At 4.2s, the Active Power Regulator setpoint is ramped to 1pu (1000 MW).
At 7.5s, the Reactive Power Regulator setpoint is ramped to 0.25pu (250 Mvar).
If you set the value of parameter Tfault to 7 (actual default value=9999) in the workspace and re-start the simulation, a DC fault will be applied at the middle of the cable at 7s. The half-bridge MMC will be blocked and the interconnection will be shut down after two cycles (Brk1 will be opened).
VSC-HVDC Transmission with Cascaded Two-Level Converters, Bjorn Jacobson, Patrik Karlsson, Gunnar Asplund, Lennart Harnefors, Tomas Jonsson ABB,Sweden, CIGRE 2010 B4-110
Setup and Performance of the Real-Time Simulator used for Hardware-in-Loop-Tests of a VSC-Based HVDC scheme for Offshore Applications., O. Venjakob, S. Kubera, R. Hibberts-Caswell, P.A. Forsyth, T.L. Maguire Siemens, Germany & RTDS Technologies, Canada. Paper submitted to the International Conference on Power Systems Transients (IPST2013) in Vancouver, Canada July 18-20, 2013