Silicon Controlled Rectifiers - An Overview

Silicon Controlled Rectifier

A silicon controlled rectifier is a three terminal electronic component.  It consists of an anode, a cathode and a gate.

The device is similar to a diode, except that it needs to be switched on with an external voltage applied to the gate during the positive half cycle.  Once, the SCR has been trigerred, it "fires" and conducts as long as it is positively biased.

In AC circuits, during the negative half cycle, the SCR switches off automatically.

SCRS find application in applications where the current needs to be controlled.  They are used in many electronic equipments, such as inverters, converters, speed controller, etc.

Gate Triggering methods SCRs

There are different methods of trigerring the gate of a Silicon Controlled Rectifiers (SCRs).  They are

Using DC Voltage
By applying a positive voltage to the gate with respect to the cathode, the junction J2 can be forward biased.  This will switch on the SCR.  This process requires a constant dc voltage to be applied between the Gate and the Cathode, which is a disadvantage.  Besides, there is no isolation between this triggering dc voltage and the main dc supply

Using AC voltage
In AC applications, the trigger voltage can be obtained from the AC voltage suitable reduced.  The phase of the AC voltage is modified and applied to trigger the SCR at the desired instant.  The SCR will continue to conduct till the negative half cycle.

A separate transformer is required for the trigger circuit which increases the cost

Pulse triggering
This is the most widely used form of triggering.  In this method, a pulse of a small duration is applied to the gate to switch it ON.  Sometimes, a series of pulses are applied.  The pulse need not be continuous.  This reduces the losses in the gate.

Natural and Forced Commutation in SCRs

Commutation in dc machines refers to the changing of current flow from one circuit to another.  In SCRs ( Silicon controlled Rectifiers) and thyristors, it refers switching off a conducting electronic component.

In AC circuits, SCRs are commutated by the negative half cycle which reverse biases the anode and cathode terminals.  This is known as natural commutation.

However, in DC circuits, special circuits should be designed to switch off the SCRs once, they have been switched on.  The current is reduced to zero by means of external circuits.  This is known as Forced Commutation

Methods of turning on an SCR

The SCR cannot switch on on its own once its anode and cathode are connected to the positive and negative terminals respectively.

The following are some of the methods.

Gate Triggering

This is the most popular method.  A single pulse or a train or pulses are applied to the gate terminal of the SCR.  This creates a forward bias across junction J2 and switches on the SCR.

Thermal Triggering
When the SCR is heated above a certain value, more hole-electron pairs are produced this increases the charge carriers and can cause the SCR to switch on.

Light Triggering

When light is made to fall on the junction in reverse bias, hole-electron pairs are created due to the energy of the incident light.  This can cause the SCR to fire.  Special components such as LASCR (Light activated Silicon controlled Rectifiers) and LASCS ( Light activated Silicon controlled Rectifiers) work on this principle.  This method of triggering is cheaper when designing components of higher ratings.  The light is conducted to the junction by means of optical cables.

dv/dt triggering
In this method of triggering, a rapid change in the voltage current to flow through the junction J2 which acts as a dielectric between two conductive junctions (J1 and J3).  The SCR will switch on even if the voltage is low provided the rate of change of the voltage is high.

Advantages and disadvantages of grounding the neutral

The advantages of grounding (earthing) the neutral are as follows

1. Sensitive current protection schemes can be used to quickly identify an earth fault.
2. The external surges and overvoltages caused by lightning or switching are discharged to the ground.  If the neutral is not grounded, these waves will get reflected and cause overvoltages in the system.
3. The phase voltages are within limit and the value is the voltage between the phase to ground.
4. Arcing grounds, which occur at the location of an earth fault are avoided.

The disadvantages of grounding the neutral are

1. The system will trip even for a minor earth fault.  This affects the reliability of the power supply.
2. The zero sequence currents which flow through the neutral can cause interference to telecommunication lines.

Generator Neutral Breaker

The Generator Neutral Breaker is used in systems, which are grounded through low resistances or solidly grounded (without a resistance).  In such systems, the fault current in the line due to an earth fault will be high.

The current flowing through the equipment due to an earth fault can be limited if a breaker is connected in series with the neutral.  This breaker is opened simultaneously with the armature and the field breaker.  This will bring the fault current to zero quickly.

Pre Insertion Resistors in Circuit Breakers

Circuit breakers used in switching of long transmission lines have a resistors which is pre-inserted between the contacts before the contacts are closed. This resistor is called the Pre-insertion resistor. The function of this resistor is to limit the initial charging current of the line. The resistance of the line is around 500 ohms.

Once the closing command is given to the breaker, the resistor is first connected across the contacts. This resistance in series limits the line current. A few milliseconds later, the contacts are closed. 

While opening the breaker, the pre-insertion resistor is first disconnected before the contacts are opened by the circuit breaker. Pre-insertion resistors are also used in lines which have transformers to limit the high inrush current.

Capacitance Switching of Equipment

Capacitance is the phenomenon of holding electrostatic charge. In electrical systems, long transmission lines, power cables and capacitor banks can have large amounts of capacitance. In a circuit containing capacitance, the current will lead the voltage by 90 degrees. This means that at the instant of the current zero crossing, the voltage across the breaker contacts will be the maximum. If a circuit is isolated at this instant, the high system voltage will be retained by the line capacitances. If the breaker is opened when the current is zero and the voltage is maximum, half a cycle later when the supply voltage reaches maximum in the opposite direction, the voltage across the breaker contacts will be 2V. This can result in a restriking voltage being developed and a flashover occurring across the circuit breaker. Once the flashover due to the restrike occurs, oscillations are set up in the line between the system inductance and the capacitance. These oscillations and the restrikes they cause can result in the line voltage reaching up to 4 times the voltage (4V). Hence, in lines with high capacitances, air blast circuit breakers or multi break circuit breakers are used for isolation.

Resistor switching of an SCR

A Silicon controlled Rectifier (SCR) is a semiconductor device which conducts in only one direction.  It has three terminals.  An anode, a cathode and a gate.  Unlike a diode, however, it needs to be switched on by a pulse applied to the gate.  

The circuit below shows the method of switching on an SCR using a resistor.  The power source is connected across the SCR.  The gate voltage is provided by the voltage divider circuit.  The variable resistor, R4 is used to control the firing angle.  

The Diode D1 prevents negative voltage from reaching the gate during the negative half cycle.  The SCR will be switched off during the negative half cycle by the supply voltage

Circuit Diagram - Resistor switching of an SCR

Transient Stability Analysis

The Transient Stability Analysis is done to determine the behaviour of a system during transients or sudden changes in a power system.

Transients occur when there are power flows from one source to another.  They also occur during faults when a load or a generating point is cut off.  This can cause oscillations in the voltage or power.  Many of these oscillations will quickly get resolved and the system will return to its steady state operation.

However, in certain situations, the oscillations may can increase in severity and can cause fluctuations in the voltage or power which can affect the system and can cause trippings.  Hence, it is necessary to ensure that the system will be stable in the event of a transient.

The stability of a system is classified into

Steady State Stability and
Transient Stability

Steady state stability is the ability of the system to respond to small oscillations in the voltage or slow changes in the load.

Transient stability is the ability of the system to respond to sudden, unexpected changes such as the tripping of a power source or a fault in a tranmission line.

Transient stability analysis is used in relay setting and in determining the clearing time of breakers.  They are also used to determine the voltage level of a power system and the power transfer capability between different systems.

Fault Analysis

A Fault Analysis is a study which describes the fault currents and the behaviour of a power system during an electric fault.  Faults may be line to line faults or line to ground faults. The fault analysis provides information about the  The Fault Analysis is used to determine the ratings of fuses and circuit breakers.

Using the fault analysis, we can determine the maximum current which will be developed during a fault.  The bus bars, breakers and other transmission equipment should be equipped to withstand the heavy current which flow during a fault.

The protective relays are set based on the current calculated during the fault analysis.

Load Flow Analysis

The Load Flow Analysis is done to determine the voltage, current, real and reactive power in a particular point in a power system as well as the flow from one point to another. 

The Load Flow Analysis helps understand the operation and behaviour of the system when a generator trips or when a big load is suddenly cut off.  Load Flow Analysis also helps identify routes to transfer power when a transmission line has to be isolated due to a fault.  This ensures reliable power supply and ensures quick restoration in the event of blackouts. 

Load Flow Analysis is done during the design of the power system.  It should also be done before any modification of the power system such as the addition of loads or generating units.

Components and Structure of a Power System

A Power system has three main components
They are

• The Generating System
• The Transmission System 
• The Distribution System

Generating System
The Generating System is the source of the power.   The generation can be from generators, solar panels, etc.  Power can be generated from different sources such as hydropower, wind turbines, nuclear plants,etc.

Components: Synchronous Generators, induction generators, solar panels,

Transmission System
The transmission system transmits the generated power over large distances to the distribution centres such as industries and cities.  The distribution areas can be thousands of kilometres away from the generating stations.  The voltage is stepped up to high values to minimize the losses using transformers.  The power is then transmitted through the power lines to the distribution areas.

Transmission systems can be categorized into

Primary Transmission Systems, which transfer power at voltage of 110 kV and above.  These lines are hundreds of miles long.  They are connected to secondary receiving substations

Secondary Tranmission Systems, which receive the power from the primary transmission system send it to the distribution systems.  The voltage levels in the secondary transmission systems are about 33kv to 66kV

Components: Transformers, Circuit Breakers, Overhead Transmission Lines, Underground Cables.

Distribution Systems
The distribution system receives power from the transmission system and distributes the power to the individual customers at the required voltage.  The industrial supply voltage can be 33kV or 11kV.  The domestic supply voltage is 440 or 220V

Components: Transformers, underground and overhead transmission lines.

Power System Analysis

A Power System Analysis is a very important exercise in the design and operation of a power system.  The Power System Analysis is used to evaluate the performance of a power system. 

Power System Analysis deals, chiefly, with three important parts

They are

1. Load Flow Analysis
2. Short Circuit Analysis and 
3. Stability Analysis

A Power System Analysis helps the following aspects.

1. Study the ability of the system to respond to small disturbances caused by the application/removal of small or large loads.
2. Design of the breakers and isolating equipments.
3. Plan for future expansion of the power system
4. Study the response of the system to different fault conditions.
5. Observe and monitor the voltage, real and reactive power betwee different buses.
6. Calculate the setting of the relays and the design of the protection system.

Power Balance Equation

The power balance equation describes the relation between Power Demand and Power Generation in a power system.

The equation is

Where

P_D is the Total Power Demand
P_G is the output of individual generating stations

The sum of the power generated should equal the demand for power.