FREQUENCY CONTROL
-- Bhanu Bhushan --
(April, 2011)
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What is frequency ?
How would you explain it to a layman ?
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Number of times the voltage reverses
in half a second.
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From where does it come ?
What governs the frequency ?
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Speed at which the turbine - generators
are running at that time.
3000 rpm = 50 rps = 50.0 Hz
2940 rpm = 49 rps = 49.0 Hz
( for 2 - pole generators )
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Is it same all over a system ?
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Yes.
But, why ?
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Because polarity of voltages produced
by all generators must be same at a
particular time, and must reverse together.
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Is the frequency constant ?
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No.
Why not ?
Why does frequency keep changing
all the time ?
(Transparencies)
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• Because total consumer load keeps
changing from time to time, and
• Because total generation is not changing
in step with the change in total load.
• Situation is seriously aggravated when
governors are not in normal operation.
• How is the frequency stabilized and
controlled ?
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What happens if the frequency differs from
the rated value of 50.0 Hz ?
What are the adverse effects of high and
low frequency ?
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• Motors & driven equipment :
over-loading / reduced output
• Transformers :
--- / over-fluxing
• Capacitors : --- / effectiveness falls
• Reactors : effectiveness falls / --• Transmission lines : parameters change
• Steam turbines : resonance of LP blades
• Generating stations : --- / MW capability
falls
• Power system : effect on voltage profile.
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Permissible band of frequency ?
• Obligation to consumers
• Requirement of power plants
• Requirement of system operation
• Electric clocks.
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• Indian Electricity Rules, 1956 : +/- 3%
• IEC for Steam turbines : + 1%, - 2%
• IEGC ?
• England :
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The basic function of governor
on a prime mover
• To vary the power input to match the
power output, and thereby contain the
speed change.
• Additionally, in case of grid-connected
generating units, to enable stable parallel
operation by equitable sharing of system
load changes.
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(G1+G2+G3) = (L1+L2+L3)
L1
L2
x
GI
G2
REGION
x=L1-G1
L3
y
G3
REGION
y=L3-G3
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• Suppose L1 increases to L1’. Frequency would
decline to a new level at which (G1’+G2’+G3’) =
(L1’+L2’+L3’)
• Then x’ = L1’ – G1’; y’ = L3’ – G3’
• x’ may not be = x; y’ may not be = y
• Such fluctuations keep happening all the time.
• Let us assume that L2 & L3 remain unchanged
when frequency declines.
• Then (G1’+G2’+G3’-G1-G2-G3) = L1’-L1.
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• If all generating units are on FGMO with an equal
droop, (G1’-G1)/G1 = (G2’-G2)/G2 =
(G3’-G3)/G3 = (L1’-L1)/(L1+L2+L3).
• In Western countries, it is mandatory to
continuously regulate G1 such that x’ = x within
10-15 minutes, assuming x was = interchange
schedule for the State to begin with.
• This is possible only if (G1’’-G1) = (L1’-L1).
• This requires that every constituent has adequate
spinning reserves and AFC. (Self-sufficiency)
• No concept of region-wide merit-order.
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Steps in frequency control
• Change of total generation in the system to
match the new total system load, through
FGMO, without any time delay.
• Change the total generation in affected area to
absorb the change in area load.
• Change the generation in affected area to
restore merit-order. May be combined with the
previous step.
• Additional sale / purchase for economy.
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• Frequency and all tie-line flows automatically
return to the previous level, without any
corrective action in other control areas. The
latter’s generation also returns to previous level,
without any change in governor settings.
• Can this conventional scheme work after vertical
unbundling of power utilities? How would a
Discom keep to its import schedule?
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In our case, x generally is above / below schedule.
• Each State has notified entitlements in Central
generating stations
• Suppose it is ‘a’ MW on a day
• Then, (x – a) = over-drawal, for which the State
has to pay UI charges into the regional pool
account. ‘x’ is metered in 15 minute wise energy.
• UI rate is determined for each 15-minute period
according to average frequency during the period.
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When frequency is low (because of a general
deficit), UI rate would be high and the State should
plan to curtail its over-drawal, by
-- Harnessing all generation within the State
-- Procuring additional power.
Load - shedding only when no additional power is
available at a price that the State utilities can pay,
and UI rate is not affordable or frequency has
come down to a level which endangers the grid.
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