The control over frequency in an interconnected power system is usuallydivided into a hierarchy of three subsequent control levels: primary control,secondary control and tertiary control 4, 5. Primary control is taken care inlarge by generator turbine governors and partly by frequency dependent loads.The system natural frequency response 6 is the combined effect of the above mentioned. The area frequencyresponse characteristic coefficient ?scorresponding to this response is defined as (1)where Rs is the turbine governor droop, in Hz per pu power, and Ds is the dampingcharacteristic of load in pu power per Hz.
Following a step load change, therewill be a deviation in frequency which cannot be eliminated completely by the action ofprimary control. Steady-state frequency deviation is therefore expressed as (2)where is active power mismatchin pu power, and is system frequency deviationin pu frequency. Secondary control is done by Automatic GenerationControl (AGC), which actuates the reference positions of valve in governor unitsand restores the system frequency to nominal value. Tertiary control refers tothe dispatch of units subject to economical aspects, where the operating pointsof selected (mostly the generators with low running cost) on-line units arechanged periodically (e.g.
, every 15 minutes in India) to realize thefeasibility of transactions in real-time markets. Secondary control plays animportant role in a balancing area’s active power and frequency control in realtime.The measure of mismatch between generationand demand within a balancing area is termed as Area Control Error (ACE), in MW. AGC follows necessaryaction according to ACE.
The standardAGC control strategy in an interconnected system is originally developed byNathan Cohn known as tie-line bias control (TBC) 7, 8, which is based onprinciple that all operating pool members must contribute their share tofrequency in addition to taking care of their own net interchange. In TBC, thecontrol error for each area is a linear combination of frequency and tie-lineerror. So, ACE is given by : (3)where Ta is the tie-line’s actual power exchange , Ts is the tie-line’s scheduled power exchange, f isthe actual system frequency, f0is the nominal system frequency, and B is the frequency bias of a particular balancingarea in MW/0.1Hz, which is a negative value. The value of B is chosen to be 10B=?Aso as to match the balancing area’s frequency response coefficient ?A,where ?A is Balancing AreaA’s frequency response coefficient. Alsoas shown in 9 value of B should notbe less than 1% of the balancing area’s estimated yearly peak demand per 0.1Hzchange . According to 7 TBC has three functions:absorbing the changes in load locally, sharing frequency control along withothers, and coordinating with natural frequency response of each plant toremote load changes.
We illustrate these functions for a two-area system, i.e.,areas A and B, in what follows. ACEin each area are expressed