How it works

Response to Disturbances

  • During normal conditions, each HVDC tie is operated in a conventional manner with power flow held at scheduled levels on each tie until a disturbance is sensed on either side of the HVDC link.
  • The disturbance is indicated by changes in frequency beyond a small threshold. Power transfer is automatically changed to support a falling frequency or limit a rising frequency.
  • Power flows can be increased up to the ratings of the HVDC ties or to the thermal or voltage limits imposed by adjacent ac systems on either side of the HVDC ties.
  • Without the protection assured by grid segmentation, disturbances can propagate without bound, causing tie power flows to increase to levels exceeding thermal limits, and eventually violating voltage and angular stability limits and thereby triggering cascading.
  • With HVDC ties, power transfers are limited to safe levels that will often be beyond the stability limits of ac ties.
  • Unlike unsegmented grids where the possibility of any one ac tie reaching a thermal or voltage limit requires constraining the loading of other parallel ties to levels substantially below their thermal capabilities, and which are thus underutilized, each HVDC tie can operate up to its local thermal or voltage limit.
  • The ability to control power flows at the perimeter of an area (ac sector) allows adjustment of power flows to counter disturbances and internal problems that could otherwise result in intra-area cascading.
  • Centralized control and coordination of power flows over the HVDC ties will be limited to normal dispatching of inter-area power flows and remedial adjustments to limit or reduce power flow in weakened areas within a sector. (The latter capability is generally available for today’s grids only from phase angle regulators in ac ties.) This degree of flow controllability is much faster and more manageable with HVDC ties.
  • Inter-area support (that is traditionally provided by synchronizing power with its problematic lack of controllability) is provided by what is effectively a governor control on each HVDC tie that responds to frequency differences between asynchronous areas. When a disturbance in one area triggers increased power flow from adjacent ac systems via these controls, the frequency change in the adjacent systems will cause increased support from more remote ac systems.
  • A major disturbance that causes frequency to drop in an area may load HVDC ties to their local power transfer limits. The first result is cascading avoidance. The second result is the activation of spinning reserve in the affected system as frequency is allowed to drop. Between increasing tie flows and greater intra-area generator's production from spinning reserve, the frequency drop may be arrested. If it is not, under-frequency load shedding that normally responds after a grid breakup, will come into play and arrest the frequency decay with a relatively lesser loss of customers.

 

 
 


Three sector segmented interconnectiion
 


Loss of generation in sector A, sector A spinning reserve activates
 


Sector B spinning reserve activates to support sector A
 


Sector C spinning reserve activates to support sSector A