A quick primer on how CANDUs fit into Ontario’s windy power grid – 2013 July

July 6, 2013

By: Donald Jones, P.Eng., retired nuclear industry engineer.

To show how the Ontario power grid responds to load changes (variable wind generation is negative load) on the time scale of seconds to hours and how CANDU nuclear units are integrated into the grid.

Sudden fluctuations in load on the grid (maybe from wind gusts) on a seconds to minutes timescale are taken care of by the kinetic energy of all the rotating turbine/generator masses on the grid, they would slow a little if the wind drops and inject energy into the grid to make up for the power loss from the wind. Grid frequency drops a little from 60 Hertz.

If a frequency deadband is exceeded normal turbine governor action kicks in to provide more output from the coal, gas and hydro units on the grid, primary frequency control. The present nuclear units, which operate turbine-following-reactor mode, do not contribute to this primary frequency control – see later.

If frequency offset exceeds a deadband Automatic Generation Control (AGC) will bring it back into spec. This is called secondary frequency control and is normally supplied as an IESO regulation service by select hydro units at Niagara Falls that have the appropriate load governors although coal has been used in the past. Secondary frequency control can also be done manually.

On a longer time scale the dispatching of stored water hydro and Combined Cycle Gas Turbines (CCGTs) at five minute intervals will bring the grid into the range that AGC can handle. Base load is handled by nuclear and by run-of-the-river hydro units.

On a significant load change, maybe from the intermittent characteristic of wind, stored water hydro and/or flexible coal-fired units (soon to disappear from the grid) can be brought up from low power to quickly balance the grid while any CCGTs that have been taken below their dispatchable range are brought back into their dispatchable range. Open Cycle Gas Turbines (OCGTs) are presently not needed on the Ontario grid to cater for wind, even more so now that the Independent Electricity System Operator (IESO) will have the authority to dispatch wind.

When supplying power to the Ontario grid the present CANDU units have two plant operating modes, reactor-following-turbine mode and turbine-following-reactor mode. The units were designed for base load operation with load cycling capability. Load cycling (reducing power overnight and at weekends) was originally intended to be performed with the unit in the turbine-following-reactor mode. Load following (responding to 5 minute dispatches from the IESO) could also have been done in this mode if the units had load following capability as well as in the reactor-following-turbine mode if operation were more stable in this mode – see later. Small power variations, typically +/- 2.5 percent of full power from turbine governor action to stabilize the grid when operating in reactor-following-turbine mode is called primary frequency control and is not load following. Read the rest of this entry »