By: Donald Jones, P.Eng., retired nuclear industry engineer – 2014 August
Ontario’s Independent Electricity System Operator (IESO) has a pilot project that uses motor/generator flywheels, batteries, and aggregate loads as short term energy storage to, it says, provide regulation services (reference 1). These short term energy storage systems should not be confused with longer term storage systems like pumped water storage, compressed air, thermal etc. Regulation is secondary frequency control (reference 2) and can be automatic (AGC – automatic generation control) or manual and brings grid frequency back into its narrow control band after an under frequency or over frequency event has been arrested and the grid stabilized by fast acting primary frequency control/response. This regulation service is normally supplied by selected hydroelectric units at Niagara Falls and even by Ontario’s coal fired units before they were shutdown. Combined cycle gas turbine (CCGT) units can also be used to provide regulation service.
The addition of large amounts of unreliable wind generated electricity to the Ontario grid will/has caused a deterioration in frequency control (reference 3). Wind does not provide any passive inertial response/energy storage capability or active primary frequency control. The wind generation displaces conventional generation on the grid, initially the CCGTs and then some hydroelectric, with the consequent loss of the passive inertial response (energy storage capability) of the rotating masses of the conventional units to help limit frequency perturbations from such things as wind gusts on the wind generators. As well this results in the loss of the active primary frequency control capability of the conventional units. Primary frequency control is automatic and is provided by the speed governors of individual generating units to very rapidly arrest any drift in frequency due to mismatches in supply and demand on the grid. Primary frequency control is essential for grid stability. Reduced amounts of primary frequency response on the system can result in under-frequency load shedding and cascading outages. Since the nuclear units are presently operated in their turbine-following-reactor mode of operation they cannot provide primary frequency control and only provide passive inertial response (reference 2). This means that during periods of surplus baseload generation (SBG) that usually occur when demand is low and wind conditions are favourable all the large CCGT units and some hydro units are shutdown so frequency response on the Ontario grid will solely depend on the primary frequency response of the operating hydroelectric units with enabled speed governors and the energy storage (inertial response) capability of the nuclear and hydroelectric units. This results in a jittery grid caused by wind gusts and larger swings in frequency after an upset resulting in the need for more megawatts of secondary frequency control/regulation to return grid frequency back into its narrow control band. During periods of SBG without wind generation the grid would still not have the inertial response of most of the gas-fired generators but would be more stable because it would not be subject to the rapid frequency upsets from wind. The shutdown of the CCGTs and some hydro because of SBG, with or without wind generation, also means that the reactive power support and voltage control provided by these units is no longer available with, apparently, little if any affect on grid voltage? If voltage support were a concern and the CCGTs and hydro units did not have a synchronous condenser mode of operation, which they likely don’t, then other equipment that provide local voltage support would have to be used. Read the rest of this entry »