EPSRC-NSFC Collaborative Research Initiative in Smart Grids 

DC transmission and Distribution Networks with DC/DC Converters for Integration of Large Renewable Sources

Background

UK and China alike have enormous wind power potential which theoretically can well exceed total national energy demand. Much of this energy is located offshore or in remote sites like North Scotland and North West China which have no electrical grid infrastructure. The wind energy generation is by its nature dispersed (large number of small 3-7MW generators), in remote areas and variable/intermittent. These factors have traditionally caused integration challenges, demanded new approaches for the associated transmission/collection grids and substantial use of power electronics.

The offshore environment is the first driver for using HVDC (High Voltage Direct Current) with wind energy and the VSC (Voltage Source Converter) based HVDC is used in radial fashion with BorWin1 and DolWin1 North Sea German wind farms. In addition to reduced losses and costs, HVDC connection provides better control, fault responses, contribution to grid stabilization and ancillary services at the point of common coupling. However the currently employed radial HVDC approach cannot be extended to large offshore wind power parks, like 20GW Dogger Bank. Because of operational, security and cost reasons there is need for a meshed (offshore) DC grid providing interface with dispersed wind farms. A DC grid will further simplify traditional wind integration challenges. A DC grid will appear as a firm power source to connecting national grid because of internal droop stabilizing control at each VSC terminal. Such grid can eliminate both: fast communication of stabilizing signal with wind farms and DC chopper resistors (both are causing operating issues with HVDC connected wind farms). Comparing with multiple radial HVDC, DC grid provides better coordinated control and stability, resilience to cascaded outages, and sharing of power reserve. 

 

Building DC grids and the role of DC/DC converters

DC/DC converters will take one or more of the following functions in future DC grids :

  1. Power flow regulation. If the number of DC lines exceeds the number of AC/DC converters in a meshed DC grid, then we cannot control power flow in each DC line. In order to prevent DC line overloading we need series controlling devices. In Figure 1, DC/DC1 provides power flow control on line 1-5 where busses 1 and 5 have same/similar voltage level. All the remaining DC/DC converters have higher stepping ratio and also provide power flow regulation.
  2. High gain DC/DC stepping. Typical applications include cases where we need to interconnect DC grids of different DC voltage levels. The DC cables have voltage limitation of around 300-400kV and we will need DC/DC stepping to connect with overhead lines which operate at 500-800kV; shown as DC/DC2-4 in Figure 1. Further examples include connection with DC distribution/collection grids like converters DC/DC 8-9 in Figure 1.
  3. Interconnecting different DC technologies. The vast majority of HVDC operate with Line Commutated Converters (LCC), but they are difficult to integrate with DC grids because power direction reversal is achieved with voltage polarity reversal. The preliminary studies have demonstrated that some DC/DC converters facilitate connection of LCC HVDC with DC grids. Converter DC/DC 5 in Figure 1 illustrates this application.
  4. Galvanic isolation and connecting monopolar with bipolar DC systems. The galvanic isolation is of importance for system earthing, fault protection, overvoltage protection and power channelling to different DC poles. A galvanically isolated DC/DC enables connection of monopolar DC segment to bipolar DC system. Most large DC grids will be of bipolar topology and likely with metallic return, like busses 1-5 in Figure 1. However smaller DC systems are frequently monopolar for cost reasons, like busses 8-10. 
  5. DC Fault isolation. It can be assumed that most DC/DC converters will be capable of DC fault isolation, and therefore they can replace one fast DC CB. This property can be strategically used in building DC grid topologies. A DC/DC converter divides a protection zone in two segments, as an example DC/DC 2 in Figure 1, and grid protection strategy becomes linked with location of DC/DC converters and other control functions.
  6. Variable DC voltage. Most DC grids will operate at constant DC voltage however some grid segments can run at a reduced DC voltage. In some cases variable DC voltage is required. Variable speed wind generators must keep flux constant and this implies change in terminal voltage as speed is changing. The other example is the MW size storage plant which in most storage technologies requires variable DC voltage.

The DC/DC converters are expensive and it would be uneconomical to allocate a DC/DC converter for only one of the above functions. The project will address the DC grid design/planning and operation challenge assuming multiple functions on a single DC/DC unit.    

Figure 1. A Hypothetical DC grid showing the role of DC/DC converters. This DC grid is not the project test system, but rather illustrates various positions where DC/DC converters play important role.