Power Electronics & SCR

Power electronics & SCRThe thyristor is a solid-state semiconductor with four layers

of alternating P and N material. They act as a switch, conducting when their gate receives a current pulse, and

continue to conduct for as long as they are forward biased

• Power diode

• Transistors- BJT, FET

• IGBT

• Thyristors-power thysistor, GTO,FCT, TRIAC

Advantage and Disadvantage ofBJTAdvantages• Requires very low driving

voltages• Can operate at very high speed• Can be returned on and off from

the base terminal • Good power handling • Low forward conduction voltage

drop

Disadvantages • -Considered less robust and less

tolerant of overload s and spikes than thysristor

• Do not tolerate reverse voltage• Slow switching • Has complex current – controlled

gate driver requirement

FETAdvantages

• High speed switching capability• Relative simple protection circuit• Relative voltage controlled gate

driver with low gate current

Disadvantages

Relative low poer handling capability Relative high forward voltage drop

which result to high lossLimitation for use in HV system

FET Construction • The FET is a three terminal device like the

BJT, but operates by a different principle. The three terminals are called the source, drain, and gate.

• The voltage applied to the gate controls the current flowing in the source-drain channel. No current flows through the gate electrode, thus the gate is essentially insulated from the source-drain channel. Because no current flows through the gate, the input impedance of the FET is extremely large (in the range of 1010–1015 Ω). The large input impedance of the FET makes them an excellent choice for amplifier inputs.

• The two common families of FETs, the junction FET (JFET) and the metal oxide semiconductor FET (MOSFET) differ in the way the gate contact is made on the source-drain channel.

IGBT

• IGBT combined advantages of BJT and FET

• tThe insulated-gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners, and even stereo systems with digital amplifiers.

Advantages of IGBT

• Advantage of IGBT• Good power handling• Low forward conduction voltage drop- 2-3v , which is higher

than BJT but Lower than a MOSFET of similar rating• The voltage increase with temperature making the device easy

to operate in parallel without danger of thermal instability • High speed switching capability• Relatively simple voltage – controlled gate driver• There is no secondary breakdown, giving it safe operating area

and low switching losses

Thyristors

• Also call Silicon Controlled Rectifier (SCR).• Thyristor is used for requiring high speed & high

power switching.• Handle V & I up to 1000A & 1 kV.• Anode – high +ve voltage with relative to cathode

& gate at small +ve potential w.r.t cathode.

Circuit Symbol

Biasing

No current can flow through the SCR in both circuit (a) & (b). WHY?

(a) (b)

P

N

N

PJ2

J1

J3

P

N

N

PJ2

J1

J3

Working of SCR

When Gate is Open• J2 R.biased & J1, J3 F. biased.• No current thro RL• V increased till breaks down

& SCR ON-stroke.

Gate is +ve w.r.t cathode • J2 R.biased & J3 F. biased.• ON- V small Vb/over decreases. IG flowing & Ia

increases more at J2 – breaks up – SCR ON.• Gate loses control. Remove SCR VG still ON

J1J3

J2

Working of SCRSwitching SCR OFF

• Reduce apply voltage to almost zero which the anode current is reduced below certain value called holding current.

Advantage of SCR • No moving parts – noiseless – h. operating fre.• Very high switching speed ( 109/s ).• Higher control over IL (20-150A) – small IG (mA)• Small size – reliable – longer life.

I – V Characteristics

Important TermsBreak over voltage

• Min forward voltage, gate being open, SCR starts conducting to turn ON ( 50 – 500V ).Holding current

• Max anode current, gate being open, SCR is turned OFF from ON state.Peak Reverse Voltage PRV

• Max reverse voltage can applied to SCR without conducting in reverse direction.Forward current rating

• Max anode current that SCR is capable of passing without destruction.

Important TermsGate triggering voltage, VGT

• Min values of gate voltage at which SCR is turned ON.Gate triggering current, IGT

• Resulting gate current.

Firing and Triggering

Lamp load

Push button

V

DC firing signal

Load

V

Pulse signal

V-I Characteristic In a conventional thyristor, once it has been switched on by the gate terminal, the device remains latched in the on-state (i.e. do not need a continuous supply of gate current to conduct), providing the anode current has exceeded the latching current (IL). As long as the anode remains positively biased, it cannot be switched off until the anode current falls below the holding current (IH)

Turning OFFConsidered the SCR circuit with pulse signal at gate:

Once fired, SCR remains ON even when triggering pulse is removed

This is referred as LATCHING – ability to remain ON even when gate current is removed

Methods used to turn SCR OFF:

1. anode current interruption

2. reducing current through SCR below holding current

Phase Controlled Half Wave Rectification

OFF in :• -ve half cycle & +ve half

cycle, only if proper IGT is provided.

• Vin = Vmsin , when is applied, V1< Vm.

Phase Controlled Half Wave Rectification

OFF in +ve half cycle :

L

in

L

SCRinL

R

v

R

vvi

cos12

mav

vv

cos12

L

mav

R

vI

SCR Full Wave Rectifier

cos1L

mav

R

vI

Silicon Controlled Rectification• Three phase power supply on ship has a fixed voltage and frequency- This is

generally 440V at 60 Hz • But for high power demands it is likely to be 6.6 kV and 60 Hz.• • Speed control for a propulsion motor requires variable voltage for a d.c. drive and

variable frequency + voltage for an a.c. drive. • Therefore, it is necessary t have bus system with controlled rectification (a.c.-

>d.c.) and/or controlled inversion (d. c. -> a. c. )' to match the propulsion motor type.

• A basic rectifier uses semiconductor diodes which can only conduct current in the direction of anode (A) to cathode (K) - It automatic when A is more positive than K.

• The diode turns-off automatically when its current falls to zero.• • In a single-phase a.c. circuit, a single diode will conduct only on every other half-

cycle and this is called half-wave rectification.• Other single-phase circuits using a biased arrangement with two diodes and a

centre-tapped transformer will create full-wave rectification

• Also four diodes in a bridge formation will also produce a full-wave d.c. voltage output.

Single-phase controlled rectification.

Silicon Controlled Rectification

• An equivalent three phase bridge requires six diodes for full-wave operation.

• • A diode, having only two terminals, cannot control the size of

the d.c. output from the rectifier.

• For controlled rectification it is necessary to use a set of three-terminal devices such as thyristors (for high currents) or transistors (for low - medium currents).

• A basic a.c.-d.c. control circuit using a thyristor switch compared with a diode, a thyristor has an extra (control) terminal called the gate (G).

Three-phase controlled rectifier bridge circuit.

• The thyristor will only conduct when the anode is positive with respect to the cathode where a brief trigger voltage pulse is applied between gate and cathode (gate must be more positive than cathode).

• Gate voltage pulses are provided by separate electronic circuit and the pulse timing decides the switch-on point for the main (load) current. The load current is therefore rectified to d.c. (by diode action) and controlled by delayed switching.

• In this circuit an inductor coil (choke) smooth the d.c. load current even though the d.c. voltage is severely chopped by the thyristor switching action.

• • An alternative to the choke coil is to use a capacitor across the

rectifier output which smooths the d.c. voltage.

Controlled Rectifiers

AC Voltage Controller

Inverters

Three-phase controlled rectifier bridge circuit.• Full wave controlled rectification from a three-phase a.c. supply is

achieved in a bridge Circuit with six thyristors.• For a 440V A.C. line voltage, the peak voltage is 0.707 x 440V. The

equivalent maximum d.c. average voltage output is taken to be about 600 V as it has a six-pulse ripple effect due to the three-phase input waveform.

Controlled inversion process – • A d.c. voltage can be inverted (switched) repeatedly from positive to

negative to form an alternating (a.c.) voltage by using a set of thyristor (or transistor) switches. The inverter bridge circuit arrangement is exactly the same as that for the rectifier.

• The d.c. voltage is sequentially switched onto the three output lines. The rate of switching determines the output frequency.

• For a.c. motor control, the line currents are directed into (and out of) the windings to produce a rotating stator flux wave which interacts with the rotor to produce torque.

• The processes of controlled rectification and inversion are used in converters that are designed to match the drive motor.

• A controlled three-phase thyristor bridge inverter is shown

Three-phase ac voltage controller with a Y connected resistive load

Three-phase controlled rectifier bridge circuit.

3 - Phase Fully - Controlled Rectifier

Output Waveform

Three-phase inverter circuit and a.c. synchronous motor

Converter Types

The principal types of motor control converters are:

-> a.c.-d.c. (controlled rectifier for d.c. motors)

-> a.c.-d.c.-a.c. (PWM for induction motors)

-> a.c.- d.c.-a.c. (synchroconverter or synchronous motors) .

-> d.c.-a.c. (cycloconverter for synchronous motors)

These are examined below:

a.c.- d.c. converter • This is a three phase a.c. controlled rectification circuit for a

d.c. motor drive. • Two converters of different power ratings are generally used

for the separate control of the armature current and the field current which produces the magnetic flux .

• Some systems may have a fixed field current which means that the field supply only requires an uncontrolled diode bridge

• Shaft rotation can be achieved by reversing either the field current or the armature current direction.

• Ship applications for such a drive would include cable-laying, offshore drilling, diving and supply, ocean survey and submarines.

Controlled rectification converter and d.c. motor

a.c.- d.c.-a.c. PWM converter• This type of converter is used for induction motor drives and

uses transistors as the switching devices. • Unlike thyristors, a transistor can be turned on and off by a

control signal and at a high switching rate (e.g. at 20 kHz in a PWM converter).

• The input rectifier stage is not controlled so is simpler and cheaper but the converter will not be able to allow power from the motor load to be regenerated back into the mains supply during a braking operation.

• From a 440 V a.c. supply, the rectified d.c. (link) voltage will be smoothed by the capacitor to approximately 600 V.

PWM Converter and a.c. Induction Motor

PWM converter and a.c. induction motor• The d.c. voltage is chopped into variable width, but constant

level, voltage pulses in the computer controlled inverter section using IGBTs (Insulated Gate Bipolar Transistors).This process is called pulse width modulation or PWM.

• By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an averaged sinusoidal ac. output over a wide range of frequencies typically 0.5-120Hz.

• Due to the smoothing effect of the motor inductance, the motor currents appear to be nearly sinusoidal in shape.

• • By sequentially directing the currents into the three stator

windings, a reversible rotating magnetic field is produced with its speed set by the output frequency of the PWM converter.

Converter Types• Accurate control of shaft torque, acceleration time and resistive

braking are a few of the many operational parameters that can be programmed into the VSD, usually via a hand-held unit.

• The VSD can be closelv tuned to the connected motor drive to achieve optimum control and protection limits for the overall drive.

• • Speed regulation against load changes is very good and can be

made very precise by the addition of feedback from a shaft speed encoder.

• VSDs, being digitally controlled, can be easily networked to other computer devices e.g. programmable logic controllers (PLCs) for overall control of a complex process.

a.c.-d.c.-a.c. synchroconverter • This type of convert is used for large a.c. synchronous

motor drives (called a synchrodrive) and It is applied very successfully to marine electric propulsion.

• A synchroconverter has controlled rectifier and inverter stages which both rely on natural turn-off (line commutation) for the thyristors by the three phase a.c. voltages at either end of the converter.

• Between the rectification and inversion stages is a current-smoothing reactor coil forming the d.c. link.

• An operational similarity exists between a synchrodrive and a d.c. motor drive. DC link synchroconverter and a dc motor drive.

Synchroconverter circuit- Synchronous motor

Inverter Current Switching Sequence

Synchroconverter circuit- Synchronous motor

• This view considers the rectifier stage as a controlled d.c. supply and the inverter/synchronous motor combination as a d.c. motor. with the switching inverter acting as a static commutator.

• The combination of controlled rectifier and d.c. link is considered to be a current source for the inverter whose task is then to sequentially direct blocks of the current into the motor windings

• The size of the d.c. current is set by the controlled switching of the rectifier thyristors.

• Motor supply frequency (and hence its speed) is set by the rate of inverter switching.

• The six inverter thyristors provide six current pulses per cycle (known as a six-pulse converter)

Synchroconverter circuit- Synchronous motor• A simplified understanding of synchroconverter control is that

the current source (controlled rectification stage) provides the required motor torque and the inverter stage controls the required speed.

• To provide the motor e.m.f. which is necessary for natural commutation of the inverter thyristors, the synchronous motor must have rotation and magnetic flux in its rotor poles.

• During normal running, the synchronous motor is operated with a power factor of about 0.9 leading (by field excitation control) to assist the line commutation of the inverter thyristors.

• The d.c. rotor field excitation is obtained from a separate controlled thvristor rectification circuit.

Synchroconverter circuit- Synchronous motor

• As the supply (network) and machine bridges are identical and are both connected to a three-phase a.c. voltage source, there roles can be switched into reverse.

• This is useful to allow the regeneration motor power back into the mains power supply which provides an electric braking torque during a crash stop of the ship.

Cycloconverter circuit and output voltage waveform.

a.c.- a.c. cycloconverter• While a synchroconverter is able to provide an output

frequency range typically up to twice that of the mains input (e.g. up to 120 Hz), a cycloconverter is restricted to a much lower range.

• This is limited to less than one third of the supply frequency (e.g. up to 20 Hz) which is due to the way in which this type of converter produces the a.c. output voltage waveform.

• Ship ropulsion shaft speeds are typically in the range of 0-145 rev/min which can easily be achieved by the low frequency output range of a cycloconverter to a multi-pole synchronous motor.

• Power regeneration from the motor back into the main power supply is available. A conventional three phase converter from a.c. to d.c. can be controlled so that the average output voltage can be increased and decreased from zero to maximum within a half-cycle period of he sinusoidal a.c. input.

a.c.- a.c. cycloconverter• By connecting two similar converters back-to-back in each line an a.c.

output frequency is obtained.• The switching pattern for the thyristors varies over the frequency range

which requires a complex computer program for converter control. • The corresponding current waveform shape (not shown) will be more

sinusoidal due to the smoothing effect of motor and line inductance.• The output voltage has ripple content which gets as the output frequency it

is this feature that limits useful frequency. • There is no connection between the three motor windings because the line

converters have to be isolated from each other to operate correctly to obtain line commutation (natural) switching of the thvristors.

• The converters may be directly supplied from the HV line but it is more usual to interpose step-down transformers. This reduces the motor voltage and its required insulation level while also providing additional line impedance to limit the size of prospective fault current and harmonic voltage distortion at the main supply bus-bar.

HV Power System

Concept of Electrical Propulsion

Twin Shaft EL Propulsion

Azipod drive unit