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

1539pk C- 1 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  speed governor on each generating unit provides primary speed control function  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)

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

1539pk C- 3 ω r = rotor speedY = valve/gate position P m = mechanical power Figure 11.7 Response of generating unit with isochronous governor Figure 11.6 Schematic of an isochronous governor

1539pk C- 4 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

1539pk C- 5 Percent Speed Regulation or Droop where ω NL = steady-state speed at no load ω FL = steady-state speed at full load ω 0 = 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 Ideal steady-state characteristics of a governor with speed droop

1539pk C- 6 Load Sharing by Parallel Units Figure 11.12Response of a generating unit with a governor having speed-droop characteristics Figure 11.11Load sharing by parallel units with drooping governor characteristics

1539pk C- 7 Control of Generating Unit Power Output  Relationship between speed and load can be adjusted by changing "load reference set point"  accomplished by operating speed-changer motor  Effect of load reference control is depicted in Figure  three characteristics representing three load reference settings shown, each with 5% droop  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:  adjustment of droop establishes proportion of load picked up when system has sudden changes  adjustment of load reference determines unit output at a given frequency

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

1539pk C- 9 Composite System Regulating Characteristics  System load changes with freq. With a load damping constant of D, frequency sensitive load change:  P D = 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  P L (at nominal frequency) results in  a generation increase of  P G due to governor action, and  a load reduction of  P D due to load characteristic

1539pk C- 10 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 where Figure Composite governor and load characteristic

1539pk C- 11 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  the integral action of the control ensures zero frequency deviation and thus matches generation and load  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.

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

1539pk C- 13 Supplementary Control of Interconnected Systems  The objectives of automatic generation control are to maintain:  system frequency within desired limits  area interchange power at scheduled levels  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:  tie line flow deviation, plus  frequency deviation weighted by a bias factor Figure 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

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

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

1539pk C- 16 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  unless sufficient spinning generation reserve is available, the frequency decline will be determined by load characteristics (Fig )  Frequency decline could lead to tripping of steam turbine generating units by protective relays  this will aggravate the situation further  There are two main problems associated with underfrequency operation related to thermal units:  vibratory stress on long low-pressure turbine blades; operation below 58.5 Hz severely restricted (Fig. 9.40)  performance of plant auxiliaries driven by induction motors; below 57 Hz plant capability may be severely reduced or units may be tripped off

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

1539pk C- 18 Underfrequency Load Shedding (cont'd)  To prevent extended operation of separated areas at low frequency, load shedding schemes are employed. A typical scheme:  10% load shed when frequency drops to 59.2 Hz  15% additional load shed when frequency drops to 58.8 Hz  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  Ontario Hydro uses such a frequency trend relay Fig Tripping logic for frequency trend relay