CONTROL OF ACTIVE POWER AND FREQUENCY Copyright

CONTROL OF ACTIVE POWER AND FREQUENCY Copyright © P. Kundur This material should not be used without the author's consent 1539 pk

Active Power and Frequency Control § The frequency of a system is dependent on active power balance § As frequency is a common factor throughout the system, a change in active power demand at one point is reflected throughout the system § Because there are many generators supplying power into the system, some means must be provided to allocate change in demand to the generators F speed governor on each generating unit provides primary speed control function F supplementary control originating at a central control center allocates generation § In an interconnected system, with two or more independently controlled areas, the generation within each area has to be controlled so as to maintain scheduled power interchange § The control of generation and frequency is commonly known as load frequency control (LFC) or automatic generation control (AGC) C- 1 1539 pk

Primary Speed Controls § Isochronous speed governor F an integral controller resulting in constant speed F not suitable for multimachine systems; slight differences in speed settings would cause them to fight against each other F can be used only when a generator is supplying an isolated load or when only one generator in a system is required to respond to load changes § Governor with Speed Droop F speed regulation or droop is provided to assure proper load sharing F a proportional controller with a gain of 1/R F If precent regulation of the units are nearly equal, change in output of each unit will be nearly proportional to its rating F the speed-load characteristic can be adjusted by changing governor settings; this is achieved in practice by operating speed-changer motor C- 2 1539 pk

ωr = rotor speed Pm = mechanical power Y = valve/gate position Figure 11. 6 Schematic of an isochronous governor Figure 11. 7 Response of generating unit with isochronous governor C- 3 1539 pk

Figure 11. 8 Governor with steady-state feedback (a) Block diagram with steady-state feedback (b) Reduced block diagram Figure 11. 9 Block diagram of a speed governor with droop C- 4 1539 pk

Percent Speed Regulation or Droop where ωNL = steady-state speed at no load ωFL = steady-state speed at full load ω0 = nominal or rated speed For example, a 5% droop or regulation means that a 5% frequency deviation causes 100% change in valve position or power output. Figure 11. 10 Ideal steady-state characteristics of a governor with speed droop C- 5 1539 pk

Load Sharing by Parallel Units Figure 11. 11 Load sharing by parallel units with drooping governor characteristics Figure 11. 12 Response of a generating unit with a governor having speed-droop characteristics C- 6 1539 pk

Control of Generating Unit Power Output § Relationship between speed and load can be adjusted by changing "load reference set point" F accomplished by operating speed-changer motor § Effect of load reference control is depicted in Figure 11. 14 F three characteristics representing three load reference settings shown, each with 5% droop F at 60 Hz, characteristic A results in zero output; characteristic B results in 50% output; characteristic C results in 100% output § Power output at a given speed can be adjusted to any desired value by controlling load reference § When two or more units are operating in parallel: F adjustment of droop establishes proportion of load picked up when system has sudden changes F adjustment of load reference determines unit output at a given frequency C- 7 1539 pk

(a) Schematic diagram of governor and turbine (b) Reduced block diagram of governor Figure 11. 13 Governor with load reference control Figure 11. 14 Effect of speed-changer setting on governor characteristic C- 8 1539 pk

Composite System Regulating Characteristics § System load changes with freq. With a load damping constant of D, frequency sensitive load change: PD = D. f § When load is increased, the frequency drops due to governor droop; Due to frequency sensitive load, the net reduction in frequency is not as high. § As illustrated in Figure 11. 17, the composite regulating characteristic includes prime mover characteristics and load damping. An increase of system load by PL (at nominal frequency) results in F a generation increase of PG due to governor action, and F a load reduction of PD due to load characteristic C- 9 1539 pk

where The composite frequency response characteristic β is normally expressed in MW/Hz. It is also sometimes referred to as the stiffness of the system. The composite regulating characteristic of the system is equal to 1/β Figure 11. 17 Composite governor and load characteristic C- 10 1539 pk

Supplementary Control of Isolated Systems § With primary speed control, the only way a change in generation can occur is for a frequency deviation to exist. § Restoration of frequency to rated value requires manipulation of the speed/load reference (speed changer motor). § This is achieved through supplementary control as shown in Figure 11. 22 F the integral action of the control ensures zero frequency deviation and thus matches generation and load F the speed/load references can be selected so that generation distribution among units minimizes operating costs § Supplementary control acts more slowly than primary control. This time-scale separation important for satisfactory performance. C- 11 1539 pk

Figure 11. 22 Addition of integral control on generating units selected for AGC C- 12 1539 pk

Supplementary Control of Interconnected Systems § The objectives of automatic generation control are to maintain: F system frequency within desired limits F area interchange power at scheduled levels F correct time (integrated frequency) § This is accomplished by using a control signal for each area referred to as area control error (ACE), made up of: F tie line flow deviation, plus F frequency deviation weighted by a bias factor Figure 11. 27 illustrated calculation of ACE § Bias factor, B, set nearly equal to regulation characteristic (I/R + D) of the area; gives good dynamic performance § A secondary function of AGC is to allocate generation economically C- 13 1539 pk

Figure 11. 27 AGC control logic for each area C- 14 1539 pk

Figure 11. 28 Functional diagram of a typical AGC system C- 15 1539 pk

Underfrequency Load Shedding § Severe system disturbances can result in cascading outages and isolation of areas, causing formation of islands § If an islanded area is undergenerated, it will experience a frequency decline F unless sufficient spinning generation reserve is available, the frequency decline will be determined by load characteristics (Fig. 11. 30) § Frequency decline could lead to tripping of steam turbine generating units by protective relays F this will aggravate the situation further § There are two main problems associated with underfrequency operation related to thermal units: F vibratory stress on long low-pressure turbine blades; operation below 58. 5 Hz severely restricted (Fig. 9. 40) F performance of plant auxiliaries driven by induction motors; below 57 Hz plant capability may be severely reduced or units may be tripped off C- 16 1539 pk

Fig. 11. 30 Frequency decay due to generation deficiency ( L) Fig. 9. 40 Steam turbine partial or full-load operating limitations during abnormal frequency, representing composite worst-case limitations of five manufacturers ©ANSI/IEEE-1987 C- 17 1539 pk

Underfrequency Load Shedding (cont'd) § To prevent extended operation of separated areas at low frequency, load shedding schemes are employed. A typical scheme: F 10% load shed when frequency drops to 59. 2 Hz F 15% additional load shed when frequency drops to 58. 8 Hz F 20% additional load shed when frequency reaches 58. 0 Hz § A scheme based on frequency alone is generally acceptable for generation deficiency up to 25% § For greater generation deficiencies, a scheme taking into account both frequency drop and rate-of-change of frequency provides increased selectivity F Ontario Hydro uses such a frequency trend relay Fig. 11. 31 Tripping logic for frequency trend relay C- 18 1539 pk