What is the Working Principle of Conventional Power Plant?
Classification of power plants (3 piece)
-Power plant comprises 3 parts
- 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
- a) Step-down
- b) Rectification
- c) Filtering of A.C. ripples.
- d) Regulation.
2.2 Step down
Transformer steps down the 3 phases A.C voltage from 440V to around 80 volts.
Any unidirectional device rectifies the AC to DC.
Here Diodes & SCRs are used for rectification.
Here multi-stage L.C. Filters are used for filtering the A.C. Ripples.
2.5.1 What is Regulation?
-As far as Float Rectifier is concerned,” Regulation is the mechanism by which the output of a float rectifier is kept 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?
Float rectifier should not only supply power to the load but also takes 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 starts discharging.
- If FR output >51.5, the floating voltage of each cell will be > 2.15V and the battery will be over charged.
Hence regulation is required.
2.5.3 How Regulation is done?
- By “Transductor or saturable reactor or magnetic amplifier” method.
- By varying the secondary of the main transformer automatically depending on output voltage.
- By SCR method.
- SMPS method.
-Second method was used in 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 rectifier so that output voltage gets regulated.
220.127.116.11 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 the impedance of an iron cored coil can be varied by varying the degree of saturation of the core.”
-By varying the series impedance to rectifier, we can vary the portion of input cycle that is fed to Rectifier.
18.104.22.168 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 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 at any time. 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 remains 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 the 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.
2.6 The four main parts of a Float Rectifier are:
a) 3 phase step-down transformer
b) Rectifying circuit
c) Smoothening or Filtering circuit
d) Control circuit.
2.7 GENERAL REQUIREMENTS:
2.7.1 Ambient conditions
The float rectifiers are designed for continuous full load operation under ambient temp,(i.e. temp. Of air surrounding the cubicle) of upto 45 degree C averaged over a period of 24 hours and going to a peak of upto 50 degree C.
2.7.2 Input Conditions
For large exchange power plant, float rectifiers are designed to work from 3 phase 4 wire AC supply and are capable of giving the specified performance for nominal input voltage settings of 350,370,390,410, and 430 volts across phases and for frequency variation of ± 2 Hz from the nominal frequency of 50 Hz. For medium exchange power plant, the unit works of single phase 2 wire AC supply and is capable of giving the specified performance from nominal input voltage settings of 200,210,220,230,240v across phases and neutral. In small exchange units also the float rectifier works from single phase but no taps are used.
2.7.3 Cooling Arrangements
Natural air-cooling is employed for this purpose. The use of fans etc., for inducing an accelerated airflow is not permitted. Steel sheets with louvers or perforations are provided at the bottom and top panels. Oil cooled components are not used.