static characteristics of a thyristor

Important Points About The V-I Characteristics of SCR

Forward Characteristics
When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristics.

In fig.1, OABC is the forward characteristics of SCR at IG=0.

If the supply voltage is increased from zero, a point reached (point A) when the SCR starts conducting.

Under this condition,the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply voltage appears across the load resistance RL .

If proper gate current is made to flow, SCR can close at much smaller supply voltage.

Reverse Characteristics
When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristics.

The reverse voltage does come across SCR when it is operated with a.c. supply.

If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts conducting heavily in the reverse direction as shown by the curve DE.

This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.

SCR in Normal Operation
In order to operate the SCR in normal operation, the following points are kept in view:

The supply voltage is generally much less than breakover voltage.
The SCR is turned on by passing appropriate amount of gate current ( a few mA) and not by breakover voltage.
When SCR is operated from a.c.  supply, the peak reverse voltage which comes during negative half-cycle should not exceed the reverse breakdown voltage.
When SCR is to be turned OFF from the ON state, anode current should be reduced to holding current.
If gate current is increased above the required value, the SCR will close at much reduced supply voltage.
Important Terms In The V-I Characteristics of SCR
The following terms are much used in the study of SCR :

Breakover voltage
Peak reverse voltage
Holding current
Forward current rating
Circuit fusing rating
1.  Breakover Voltage
It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on.

Thus, if the breakover voltage of an SCR is 200 V, it means that it can block a forward voltage  (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on.

In practice, the SCR is operated with supply voltage less than breakover voltage and it is then turned on by means of a small voltage applied to the gate.

Commercially available SCRs have breakover voltages from about 50 V to 500 V.

2.  Peak Reverse Voltage (PRV)
It is the maximum reverse voltage  (cathode positive w.r.t. anode) that can be applied to an SCR without conducting in the reverse direction.

PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of a.c. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external ciruit does not limit the current.

Commercially available SCRS have PRV ratings upto 2.5 kV.

3.  Holding Current
It is the maximum anode current, gate being open, at which SCR is turned OFF from ON condition.

When SCR is in the conducting state, it can not be turned OFF even if gate voltage is removed.

The only way to turn off or open the SCR is to reduce the supply voltage to almost zero at which point the internal transistor comes out of saturation and opens the SCR.

The anode current under this condition is very small (a few mA) and is called holding current.

Thus, if an SCR has a holding current of 5mA, it means that if anode current is made less than 5 mA, then SCR will be turned off.

4.  Forward Current Rating
It is the maximum anode current that an SCR is capable of passing without destruction.

Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR

SCR Thyristor summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.

Static Characteristics of a Thyristor SCR

✔Thyristors are semiconductor devices that can operate only in the switching mode.
✔Thyristor are current operated devices, a small Gate current controls a larger Anode current.
✔Conducts current only when forward biased and triggering current applied to the Gate.
✔The thyristor acts like a rectifying diode once it is triggered “ON”.
✔Anode current must be greater than holding current to maintain conduction.
✔Blocks current flow when reverse biased, no matter if Gate current is applied.
✔Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer applied providing Anode current is above latching current.

What is silica gel and its purpose

Function of Silica Gel Breather

Most of the power generation companies use silica gel breathers fitted to the conservator of oil filled transformers. The purpose of these silica gel breathers is to absorb the moisture in the air sucked in by the transformer during the breathing process.

What is Transformer Breathing?

When load on transformer increases or when the transformer under full load, the insulating oil of the transformer gets heated up, expands and gets expel out in to the conservator tank present at the top of the power transformer and subsequently pushes the dry air out of the conservator tank through the silica gel breather. This process is called breathing out of the transformer.

When the oil cools down, air from the atmosphere is drawn in to the transformer. This is called breathing in of the transformer.

Use of Silica gel breather

During the breathing process, the incoming air may consist of moisture and dirt which should be removed in order to prevent any damage. Hence the air is made to pass through the silica gel breather, which will absorb the moisture in the air and ensures that only dry air enters in to the transformer. Silica gel in the breather will be blue when installed and they turn to pink colour when they absorb moisture which indicates the crystals should be replaced. These breathers also have an oil cup fitted with, so that the dust particles get settled in the cup.

Thus Silica gel breathers provide an economic and efficient means of controlling the level of moisture entering the conservator tank during the breathing process.

AC bridge circuit

The bridge circuit :
                                  These are used for measuring component values, such as resistance inductance , capacitance etc. the bridge are compare the value of an unknown
components its measured accuracy is very high

Wheatstone Bridge :
                                      It is used for the measurement of d.c. resistance and a simple version the bridge has four resistor arms together with source of emf and null dector . The bridge is said to be balanced when the potential difference across the galvanometer is zero volts so there is no current through the galvanometer hence the bridge is balanced when R1 R4=R2R3

the Wheatstone widely used for precision measurement of resistance from approximately 1ohm to the low mega-ohm range

AC Bridge :
                     AC bridge are consist four arms a source of excitation and a null detector . The power source supplies an a.c. voltage to the bridge at the desired frequency . At higher frequencies. an oscillator generally supplies the excitation voltage . The null detector respond to an ac unbalance current and in its cheapest form consist of a repair headphone . other null dector maybe ac amplifier with an output meter or an electron ray tube indicator  the condition for bridge balances is 

ż1ż4=ż2ż3

Maxwell Bridge:
                          The Maxwell bridge is used to measures an unknown inductance in term of known capacitance . Once of the ratio arms has a resistance and capacitance in parallel. 

Maxwell bridge is limited to the measurement of medium Q-Coils (1<Q<10).  It is unsuit for the meaurement of coils with very low Q value (Q<1)

Hey Bridge circuit:
                                   It is different from Maxwell bridge is having R1 in series with standard capacitor C1 instead of being in parallel.
the value or Rs and Ls are given

Hey  Bridge is suited for the measurement  of high Q inductors having Q more then 10

Schering Bridge :
                                 It is used for measurement of capacitors particularly for measuring insulating properties .In other word for phase angle very nearly 90° . arm I has a parallel combination of a capacitor and combination of a resistor and a ca0acitor and standard arm contain only a capacitor

Wein Bridge :
                         The wein bridge has a series RC combination in one arm and a parallel RC combination in the adjoining arm. The frequecy of the applied voltage is

Wein bridge is used as a notch filter in harmonic distortion analyzer . 

It is used in audio and HF oscillator as frequency determining element
   

what is Tertiary winding

What is Tertiary Winding? What is Three Winding Transformer?
In some high rating transformer, one winding in addition to its primary and secondary winding is used. This additional winding, apart from primary and secondary windings, is known as Tertiary winding of transformer. Because of this third winding, the transformer is called three winding transformer or 3 winding transformer.

Advantages of Using Tertiary Winding in Transformer
Tertiary winding is provided in electrical power transformer to meet one or more of the following requirements-

a.) It reduces the unbalancing in the primary due ato unbalancing in three phase load.

b.) It redistributes the flow of fault current.

c.) Sometime it is required to supply an auxiliary load in different voltage level in addition to its main secondary load. This secondary load can be taken from tertiary winding of three winding transformer.

d.) As the tertiary winding is connected in delta formation in 3 winding transformer, it assists in limitation of fault current in the event of a short circuit from line to neutral.

Rating of Tertiary Winding of Transformer:
Rating of tertiary winding of transformer depends upon its use. If it has to supply additional load, its winding cross - section and design philosophy is decided as per load, and three phase dead short circuit on its terminal with power flow from both sides of HV and MV.
In case it is to be provided for stabilizing purpose only, its cross-section and design has to be decided from thermal and mechanical consideration for the short duration fault currents during various fault conditions single line to ground fault being the most onerous.

Purpose of Transformer Core

In an electrical power transformer, there are primary, secondary and may be tertiary windings. The performance of a transformer mainly depends upon the flux linkages between these windings. For efficient flux linking between these windings, one low reluctance magnetic path common to all windings should be provided in the transformer. This low reluctance magnetic path in transformer is known as core of transformer.