Frequency Control_bb (2)

*FREQUENCY CONTROL-- Bhanu Bhushan --

(April, 2011)

* What is frequency ?

How would you explain it to a layman ?

* Number of times the voltage reverses in half a second.

* From where does it come ?

What governs the frequency ?

* 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 )

* Is it same all over a system ?

* Yes. But, why ?

* Because polarity of voltages produced by all generators must be same at a particular time, and must reverse together.

* Is the frequency constant ?

* 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 ?

* 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 outputTransformers : --- / over-fluxingCapacitors : --- / effectiveness fallsReactors : effectiveness falls / ---Transmission lines : parameters changeSteam turbines : resonance of LP bladesGenerating stations : --- / MW capability fallsPower system : effect on voltage profile.

*Permissible band of frequency ? Obligation to consumers

Requirement of power plants

Requirement of system operation

Electric clocks.

*Indian Electricity Rules, 1956 : +/- 3%

IEC for Steam turbines : + 1%, - 2%

IEGC ?

England :

*The basic function of governor on a prime moverTo 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.

*(G1+G2+G3) = (L1+L2+L3)

GIL1G2L2xREGIONyG3REGIONL3x=L1-G1y=L3-G3

*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 G3x may not be = x; y may not be = ySuch 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.

*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.

*Steps in frequency controlChange 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.

*Frequency and all tie-line flows automatically return to the previous level, without any corrective action in other control areas. The latters 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?

*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 dayThen, (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.

* 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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