Long distance energy transmission with HVDC technology
A ReFlexion of Carl Barker, Consulting Engineer of GE Vernova
Using Modular Multi-level Converters
Increasing the voltage enables energy transmission over a long distance since it reduces energy losses. High-voltage (HV) transmission can be in Alternating Current (AC) or Direct Current (DC). For transmitting power over approximately 80 kilometres via submarine or underground cables, typically, HVDC technology is preferred (the HVAC versus HVDC break-even point is very project-specific).
The power grid is mostly based on AC technology, and therefore, in order to transmit power with HVDC technology, it is needed to use a piece of equipment, called a converter, that transforms the incoming AC-based power into DC. This is shown in Figure 1 for the example of transmitting power from an offshore wind farm to the onshore grid: at the offshore substation, there is one converter that transforms the energy into DC, and at the onshore substation, there is another converter that transforms it back to AC.
Figure 1 - A two-terminal HVDC link
By interconnecting two (or more) HVDC converters, as shown in Figure 1, active power can be exchanged between these converters by adjusting the relative DC voltage at each terminal. This active power flow is independent of the AC system phase or frequency of each converter-connected AC system. This means that a HVDC connection provides a highly flexible means of transmitting energy between AC connection points. Hence, the technology can be applied not only in an offshore wind farm, but also in many other applications (for example, transmitting power between countries).
Structure of a Modular Multi-level Converter
Today, the Voltage-Sourced Converter (VSC) used in High-Voltage Direct Current (HVDC) transmission systems are typically based on Modular Multi-level Converters (MMC). Each converter consists of six ‘valves’, and each ‘valve’ consists of multiple, series-connected ‘sub-modules’, as shown in Figure 2.
Figure 2 – Structure of a HVDC converter that uses MMC technology: the valve tower, the module and the submodule.
Each submodule, in its basic form, consists of a capacitor and two semiconductor switching devices, typically IGBTs (Insulated Gate Bipolar Transistors). A capacitor is an element that can hold and store some electric charge inside it, which means that it can supply a certain voltage across its terminals. The IGBTs can be switched ON and OFF to insert or remove the submodules, and hence the capacitors, into the circuit.
By switching in or bypassing each individual sub-module, the voltage across the capacitor can be included or removed from the total valve voltage, allowing the valve voltage to vary from zero to the valve’s full rated voltage. By driving the valve voltage of each series pair of valves in opposite directions, the mid-point between the valves can be “pushed” between the HVDC rails, thereby allowing the converter to have a DC voltage between the HVDC rails and an AC voltage at the mid-point of each series valve pair (the AC terminal).
In addition, associated with each valve is an inductor, the ‘valve reactor,’ which limits the instantaneous current change through each valve. The converter is typically connected to the 3-phase AC power system through a power transformer, which allows the converter side AC voltage to be selected to optimise the converter design and provides galvanic isolation. Galvanic isolation means that both sides of the transformer are electrically isolated in such a way that the current on one side of the transformer does not directly flow into the other side of the transformer.
Figure 3 - Electrical schemetic of the elements that compose an MMC converter: a) converter; b) submodule
Controlling the energy through MMC As the energy stored in each sub-module capacitor is limited, they will either charge or discharge when connected in the circuit, dependent on the current flow direction. A key feature of an MMC control is, therefore, the energy balancing. This energy balancing can be thought of as consisting of four parts:
a. Energy balancing between the AC and the DC side of the converter b. Energy balancing between the three phases of the MMC c. Energy balancing between the top and bottom valve within each phase d. Energy balancing between individual submodules within a valve
To sum up, HVDC technology based on Modular Multi-level Converters is a useful tool for transmitting power over long distances. In the FlexH2 project, we are looking at innovations in HVDC technology. Part of the focus is on re-designing a Modular Multi-Level Converter so that it can be directly connected to feed electrolysis plants.
initiative
