Droop or Regulation
Droop can be defined as the percentage change in speed for a change in load. Whenever there is a mismatch in power, speed changes. As mentioned earlier, the governing system senses this speed change and adjusts valve opening which in turn changes power output.
This action stops once the power mismatch is made zero. But
the speed error remains. What should be the change in power output for a change in speed is decided by the ‘regulation’. If 4% change in speed causes 100% change
in power output, then the regulation is said to be 4 % (in per unit 0.04).
The regulation can be expressed in the form of power – frequency characteristic as shown in Figure below. At 100 % load the generation is also 100 %, frequency (or
speed) is also 100%. When load reduces frequency increases, as generation remains the same. When load reduces by 50 %, frequency increases by 2 %, in the characteristic shown. When load reduces by 100 %, frequency increases by 4 %.
In other words 4 % rise in frequency should reduce power generation by 100 %. This
4 % is called ‘droop’ of 4 %. The characteristic is of‘drooping’ type. Droop or regulation is an important parameter in the frequency regulation. In thermal power
plants droop value is generally 4 % to 5 %. The sensitivity of the governor for a given change in load varies inversely with the percentage droop. The droop of the hydro turbines will be around 2 to 3% where as that of the steam turbines will be 4 to 5%
Operation of the Governor
The governor operation of a turbo-generator can be explained using the following diagram below. Consider that a unit is delivering a power ‘P’ at a frequency of ‘N’ Hz denoted by the operating point ‘A’. Let us assume that due to some reasons, the frequency dips to ‘NB’. Now because of the droop characteristics of the governor, the operating point of the generator will move to ‘B’ and the generation will increase from ‘P’ to ‘PB’.
This response of the governor is called the primary
response as discussed earlier. The operator subsequently will try to restore the generation to the original value ‘P’, the new operating point of the set will be ‘C’ and the frequency will drop further to ‘NC’. This response of the operator is called the secondary response. A secondary response of 1% of the capacity per minute of the set is prescribed by the Load Despatch Center. If no corrective action has been initiated by the Grid Managers by way of tripping out excess demand, then the frequency will settle at ‘NC’.
If the generator is at a higher load denoted by the point ‘F’ and its load limiter is set at ‘PL’, the rise in generation along the droop line will be limited to only
‘PL’ and the operating point will only be ‘L’, whatever be the dip in frequency. As the generation does not compensate the additional load, the frequency will further
drop to NH.
The load limiter action is a classic example of restricting the FGMO.
On the other hand, if any of the feeders tripped, then the frequency will rise to ‘ND’ and the primary response governing action will reduce the load to ‘PD’.
The new operating point of the set now will be D. If the frequency is within the operating range, the operator will bring the unit to the point E and the frequency will further rise to ‘NE’.
The effect of further increasing the generation under high frequency conditions will only raise the frequency and several procedures like Availability Based Tariff, Guidelines for Unscheduled Interchanges etc., have been laid down by the regulatory commissions in this regard.
Frequency control requires provision of primary regulation and supplementary regulation as basic requirement. Primary regulation is provided through speed governors which respond to frequency changes by varying turbine outputs. Keeping governors free to operate in the entire frequency range enables smooth control of frequency fluctuations as well as security against grid disturbances.
In India, due to wide range of frequency fluctuations, speed governors were prevented from responding by the utilities with dead band configuring from47.5 Hz to 51.50 Hz with emergency unloading available only when frequency goes above 51.50 Hz. Efforts have been made to enable speed governors responding in the entire frequency range which has come to be known as free governor mode of operation (FGMO).
The introduction of Availability Based Tariff (ABT) though stabilized frequency in a narrower band, the rapid fluctuations continued to occur with frequency excursions of 0.5 Hz over a period of 10 minutes and frequency shooting up to 51 Hz and above when sudden bulk load shedding or maximization of generation takes place before evening peak hours. Dipping of frequency takes place during onset of peak loads or unit tripping.
Such frequency fluctuations during normal operation in the grid leads to complex counter actions by the control center operators at regional and state level. Further, the fluctuating frequency even in an interval of 15 minutes does not give out clear signals to operators to plan generation changes, load shedding or to draw/inject Unscheduled Interchange (UI) power responding to signals generated by the commercial mechanism (ABT).
Under ABT mechanism, frequency is allowed to float between 49 Hz and 50.5 Hz and drawing / injection of UI power is permitted in this frequency range. However,fluctuating frequency masks the frequency based ABT signals.
In most of the grid disturbances over the last few years, Southern regional grid used to split into four parts in the post fault scenario due to tripping of various lines in the South-West-East-North regional corridor due to power swings.
The Eastern part used to have surplus of generation over load resulting in frequency shooting up to 52 Hz and above leading to tripping of several generating units on high frequency. Another pattern observed was isolation of Tamilnadu grid from the Southern part followed by severe frequency decay and under frequency load shedding through df/dt relays which brings up frequency above 52 Hz once again leading to tripping of some generators on high frequency.
After interconnecting with WR and ER grids also, similar pattern continued in the post fault scenario with tripping of generating units on high frequency. With implementation of free governor mode of operation on generating units, tripping on high frequency could be avoided during grid disturbances as load generation balance can be attained at a faster rate.
Even during normal operation, tripping of a 500 MW unit leads to frequency drop of around one hertz due to low system stiffness as the frequency has to be controlled only by load damping effect in the first 20-seconds after the tripping. FGMO would increase system stiffness significantly and avoid large frequency dips in the event
of unit tripping.
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