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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)

GI

L1

G2

L2x

REGION

y

G3

REGION

L3

x=L1-G1 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.