Working Principle of Conventional Power Plant

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What is the Working Principle of Conventional Power Plant?

Principle of Conventional Power Plant:

Classification of power plants (3 pieces)

-Power plant comprises 3 parts

-Float Rectifier

-Battery Charger

-Switching Cubicle.

2. FLOAT RECTIFIER

2.1 The function of the Float Rectifier is to receive three phases 440 V AC and to give a constant 51.5 Volts D.C without AC ripples.

-The steps involved to achieve the function are:

1. a) Step-down
2. b) Rectification
3. c) Filtering of A.C. ripples.
4. d) Regulation.

2.2 Stepdown

The transformer steps down the 3 phases A.C voltage from 440V to around 80 volts.

2.3 Rectification

Any unidirectional device rectifies the AC to DC.

Here Diodes & SCRs are used for rectification.

2.4 Filtering:

Here multi-stage L.C. Filters are used for filtering the A.C. Ripples.

2.5 Regulation:

2.5.1 What is Regulation?

-As far as Float Rectifier is concerned,” Regulation is the mechanism by which it kept the output of a float rectifier constant at 51.5 _+0.5V irrespective of input voltage variations of  12%. Output load variations of 5% to 105% and input frequency variations of  4% or 48-52 Hz”.

2.5.2  Why Regulation is required?

The float rectifier should not only supply power to the load but also take care of its battery sets, which are floated across its output.

If the float rectifier output voltage is 51.5v, the cells are floated at 2.15v/cell and hence they are continuously trickle charged and this compensates losses due to “self-discharge or local action”. If FR output is 49.2V, the battery set is not trickle charged, hence trickle charging is to be given once in a fortnight.

• If FR output is <49.0V, the battery discharges.
• If FR output >51.5, the floating voltage of each cell will be > 2.15V and the battery will be over charged.

Hence the regulation is required.

2.5.3 How Regulation is done?

1. By “Transductor or saturable reactor or magnetic amplifier” method.
2. By varying the secondary of the main transformer automatically depending on output voltage.
3. By SCR method.
4. SMPS method.

-Second method was used in the olden days but not used nowadays due to mechanical involvement in regulation.

-Any of the other three methods. Controls the portion of the input A.C cycle to feed to the rectifier so that output voltage gets regulated.

2.5.3.1 Transductor Method:

-Normally this principle is used in small exchange power plants.

– In this, a transductor is placed in series with the rectifier and uses the principle that it can vary the impedance of an iron-cored coil by varying the degree of saturation of the core.”

-By varying the series impedance to the rectifier, we can vary the portion of the input cycle that is fed to the Rectifier.

2.5.3.2 SCR Method:

-In this the SCR is used as rectifying element.

-Let us recapitulate the working of SCR.

-SCR can be switched on by applying the positive pulse to the gate. Once if the SCR is switched on, it will be in ‘ON’ condition as long as the current flowing through SCR is above a threshold value called “Holding current”.

-In a Float rectifier, across each half cycle one SCR is connected. Hence for 3 phases i.e. R,Y,B totally of 6 SCRs are connected.

-Let an SCR be connected across the positive half cycle of a phase. The total time period of a half-cycle is 10 ms. Within this half-cycle triggering pulses can be given. Assume that triggering pulse is given to SCR at Point’A’ after 4 ms of starting of the half-cycle, the SCR will be on. Even though the triggering pulse is removed, the SCR will remain on. But the current flowing through SCR depends on the amplitude applied across its terminals. At 9 ms say at point “B” let the current flowing through the SCR is just below the holding current. The SCR will be switched off. That means “switching on” of SCR is in our hands, but “swg off” of  SCR is not in our hands, it is automatic. In this case, the portion of half-cycle between points A and B is rectified.

-The output voltage of the FR depends on both the input AC voltage and output DC load.

-Look at the above table. Whenever the input voltage increases or output load decreases the output DC voltage increases and vice versa. Hence if we monitor output voltage, it is sufficient to regulate it.

-If the output voltage is increased, then the triggering pulse to the SCR will be delayed or retarded, point A moves towards B, hence the portion of AC cycle rectified will be reduced, hence output voltage will be automatically reduced and brought to the specified value.

-If the output voltage is decreased, then the triggering pulse to the SCR will be advanced, hence output voltage will be automatically increased and brought to the specified value. -This is how regulation is achieved by using SCR.

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