Power, Electronic Systems, Applications and Resources on Electrical and Electronic Project-Thesis: Converter

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AC POWER SUPPLY

UPS

Basic Operating Principle of UPS

Standby Off-Line UPS:
The standby a.c. power supply for critical loads, where normal a.c. power supply is not available, is known as UPS. The critical loads are normally supplied from a.c. main supply and the rectifier maintains full charge of the battery. If the a.c. supply fails, load is switched to the output of the inverter, which then takes over the main supply. The solid state switch normally takes 4-5 msec for the switchover. The inverter runs only during the time supply failure.
Standby On-Line UPS:
The schematic arrangement is shown in Fig.1a.

In the alternate configuration, shown in Fig.1b, the inverter operates continuously and is connected to the critical load. In case of inverter failure, the load is switched to the main supply. In this case, conditions of static switches are reversed, however. In case of power failure, the battery supplies the inverter.

Figure 1: UPS configuration

The stand-by battery is normally either nickel-cadmium or lead-acid type. However, the former is preferred due to long life and its ability to withstand overheating and discharging. The cost of former is quite high in comparison, however. An alternative arrangement of an UPS system is shown in Fig.2. It consists of a battery, inverter and static switch. In case of power failure, the battery supplies the inverter. When the main supply is on, the inverter operates as a rectifier and charges the battery.

In this arrangement the inverter has to operate at the fundamental output frequency. Consequently, the high frequency capability of the inverter is not utilized in reducing the size of the transformer. Similar to the d.c. power supplies, the a.c. power supplies can be categorized into- Switched-mode, Resonant and Bidirectional a.c. power supplies.

Figure 2: Arrangement of UPS system.

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Flyback Converters

All the dc-dc converter mentioned previously do not has electrical isolation between the input stage & the output stage. If the input supply is grounded, the output stage will share the same ground point. A dc-dc converter that provides isolation between the input and output is the flyback converter. We begin the evolution of a flyback converter from a basic Buck-Boost converter as shown below:

A) Start with a basic Buck-Boost converter:

B) Inductor L may be produced by 2 windings in parallel on same core:

C) As there is tight magnetic coupling between both coils of the inductor, it is no longer necessary to have them linked electrically, also not really necessary to have same number of turns on each winding. Re-arranging winding polarity.

As there is tight magnetic coupling between both coils of the inductor, it is no longer necessary to have them linked electrically, also not really necessary to have same number of turns on each winding. Re-arranging winding polarity

D) Switch S could be put in the return side of the supply E in the primary windings to give the basic Flyback converter circuit.

E) The input & output relationship is given by

Fig : A basic flyback converter with its equivalent ideal transformer model incorporating the equivalent
magnetizing inductance L_m

The operation of the flyback converter is similar to that of the buck-boost converter. The output voltage of the converter can also be smaller or larger than the input voltage depending on the duty ratio of the switch as well as on the turn ratio of its transformer. When the switch is closed or on, the diode is reverse-biased & thus isolating the output stage from the input source and the transformer. The energy is first stored in the magnetizing inductance L_m (or the transformer core) while the capacitor which is previously charged up will provide energy to the output load. When the switch is opened or off, the diode is forward biased & the output stage receives energy from the inductance L_m via the transformer. The energy stored in the inductance is charging up the capacitor & transferred to the load at the same time. Most popular choice of converter for applications required less than 100W of output power at audio range frequency (a few tens of kHz). By adding additional secondary windings to the transformer core, multiple output terminals are also possible. Thus, MOSFET is normally a preferred choice of switch to BJT and IGBT.

previous DC-DC converters with electrical isolation

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DC-DC converters with electrical isolation

Transformer core characteristic:

Two basic categories of DC-DC converters with electrical isolation, based on the way of utilization of the transformer core:

Unidirectional core excitation: only the positive part (quadrant I) of the B-H loop is used.
Bidirectional core excitation: both the positive (quadrant I) and the negative (quadrant III) parts of the B-H loop are used alternatively.

Hence, DC-DC converters that constitute a switching DC power supply can be:
Transformer core characteristic

Unidirectional Core excitation DC-DC converters

Flyback converter (derived from buck-boost converter)
Forward converter (derived from buck converter)

Bidirectional Core excitation DC-DC converters

Push-pull converter (derived from buck converter)
Half-bridge converter (derived from buck converter)
Full-bridge converter (derived from buck converter)

Control of DC-DC converters with isolation

Flyback and forward converters
Push-pull, half-bridge and full-bridge DC-DC converters
Resonant DC-DC converters

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Switching Power Supplies-Overview

EMI filter: to prevent the conducted EMI into the 50Hz AC input

Input rectifier + filter: to convert 50Hz AC voltage to unregulated DC voltage

Switching converter: to convert DC voltage to high-frequency (HF) AC voltage

HF power transformer: to step-up or step-down the HF AC voltage and to be used for electrical isolation

Output rectifier + filter: to convert HF AC voltage to DC voltage

Error amplifier: to compare the output voltage V_o to a reference voltage V_o,ref and produce an output voltage

PWM controller: to control its output pulse-width according to the output voltage of the error amplifier

HF signal transformer: to be used for electrical isolation

Gate/base drive circuit: to drive the power switches in the switching converter switching power supplies

Note:

The power supply output voltage V_o is regulated by means of the feedback control circuit.
The HF signal transformer can be replaced by an optocoupler.
The power processor is a cascading of AC-DC, DC-AC and ACDC converters with a HF transformer for electrical isolation.

Advantages:

The switching devices in the switching converter operate as a switch (either completely on or completely off). Therefore, only switching losses are involved. This results the conversion efficiency is in 70 - 90% range. Furthermore, a switching element operating in switch-mode has a much larger power-handling capacity compared to its linear mode (refer to SOA).
Since a high-frequency isolation transformer is used, the size and weight of switching supplies can be significantly reduced.

Disadvantages:
Switching supply circuits are more complex
Switching supply circuits produce EMI due to high-frequency switching.
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Linear Power Supplies

50Hz transformer: to step-up or step-down 50Hz AC voltage and to be used for electrical isolation

BJT: operates in the active region and acts as an adjustable resistor in response to its base current, i_B

Feedback control: The error amplifier is used to compare V_o with a reference voltage V_o,ref and incorporated with the base control circuit (which adjusts i_B) to keep V_o = v_d(t) - v_CE(t) = V_o,ref = constant.

Possible sources of changes:

v_d ripple voltage
AC input rms voltage: This changes the vd average value.
load: This changes the output current io and consequently the vd average value due to the transformer load regulation

Disadvantages:

The low-frequency (50Hz) transformer is larger in size and weight compared to a high-frequency transformer which has the same ratings.
The power loss in the series BJT (v_CEi_C) is significant. This loss can be minimized by proper selection of the transformer turns ratio. The overall efficiencies of linear power supplies are usually in a range of 30% to 60%.

Advantages:

Linear power supply circuitry is simple. Therefore, the construction cost is less for small power ratings (<25W).
Linear power supplies do not produce large EMI (electromagnetic interference) with other equipment.

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Power Supply

Switching DC Power Supplies

Introduction

Most DC power supplies are designed to meet some or all of the following requirements:

Regulated output - constant output voltage Within a specified range in the input voltage and the output load current, the average output voltage changes within a specified tolerance and the output ripple voltage is below a specified value.
Isolation - electrical isolation between input and output The electrical isolation is referred to the DC isolation.
Multiple outputs - The multiple outputs can differ in voltage polarities, voltage ratings and current ratings.

Common targets for DC power supply design:

Small size (transformer, heatsink, filter capacitor)
Light weight (transformer)
High efficiency
Low construction cost

Unregulated DC power supplies: diode bridge rectifiers

Regulated DC power supplies can be divided into two types:

Linear power supplies - the power semiconductor devices act as adjustable series resistors (operated in active region)
Switching power supplies - the power semiconductor devices act as switches (either completely on or completely off)

Phase-controlled bridge rectifiers can be used as regulated DC power supplies but their output voltage response times (against input rms voltage or load change) are relatively slow.

Electrical isolation: use transformers

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THREE-PHASE DUAL CONVERTER

In many variable-speed drives, the four quadrant operation is generally required and three phase dual converters are extensively used in applications up to the 2000kW level.

THREEPHASE DUAL CONVERTER WAVE FORMS

previous Three-phase full-wave Controlled Rectifier

Three-phase full-wave Controlled Rectifier

Three-phase full-wave Controlled Rectifier with highly inductive load (Continuous load current)

Average Load/Output Voltage

${V_0} = \frac{{3\sqrt 3 {V_m}}}{\pi }\cos \alpha$

${V_0} = \frac{{3\sqrt 6 V}}{\pi }\cos \alpha$

V_m peak phase voltage

V rms phase voltage

Three-phase full-wave Controlled Rectifier with highly inductive load

Voltage and current waveforms of a three-phase full converter with a highly inductive load is shown in figure. This converter provides two quadrant operation and thyristors are fired at an interval of π/3 degrees. Since thyristors are fired every 60°, the frequency of the output ripple voltage is six times the frequency of the supply voltage. At ωt = π /6 + α, thyristor S₆ is already conducting and thyristor S₁ is turned on. For the interval ωt of π/6 to π/2 thyristors S₁ and S₆ conduct, and line to line voltage v_ab appears across the load. At ωt = π /2 + α, thyristor S₂ is turned on and thyristor S₆ is turned off due to natural commutation. This occurs because when thyristor S₂ is turned on, the line to line voltage across thyristor S₆ is the positive voltage v_bc from cathode to anode which reverse biases thyristor S₆. During the interval ωt of (π /2 + α) (5 π /6 + α), thyristors S₁ and S₂ conduct and line to line voltage appears across the load. The firing sequence of the thyristors is: 12, 23, 34, 45, 56 and 61.

The average output voltage is given by

${V_{dc}} = \frac{6}{{2\pi }}\int\limits_{\pi /6 + \alpha }^{\pi /2 + \alpha } {{V_{ab}}d\left( {\omega t} \right)}$

${V_{dc}} = \frac{3}{\pi }\int\limits_{\pi /6 + \alpha }^{\pi /2 + \alpha } {\sqrt 3 {V_m}\sin \left( {\omega t + \frac{\pi }{6}} \right)d\left( {\omega t} \right)}$

${V_{dc}} = \frac{{3\sqrt 3 {V_m}}}{\pi }\cos \alpha$

The maximum output dc voltage is given by

${V_{dm}} = \frac{{3\sqrt 3 {V_m}}}{\pi }$

The rms output voltage is given by

${V_{rms}} = {\left[ {\frac{3}{\pi }\int\limits_{\pi /6 + \alpha }^{\pi /2 + \alpha } {{{\left( {\sqrt 3 {V_m}\sin (\omega t + \frac{\pi }{6})} \right)}^2}d(\omega t)} } \right]^{\frac{1}{2}}}$

${V_{rms}} = \sqrt 3 {V_m}{\left( {\frac{1}{2} + \frac{{3\sqrt 3 }}{{4\pi }}\cos 2\alpha } \right)^{\frac{1}{2}}}$

Three-phase Converter Output Characteristics for continuous load current (Full Converter)

For fully controlled rectifier, The DC Motor operates in two modes.
Rectification [As Motoring]

V₀ = positive
E_a = Positive
I_o= positive
Power Flow (+ve) from input AC to DC machine

Inversion [As Regenerative Braking]

V₀ = negative
E_a = negative
I_o= positive
Power Flow (-ve) from DC machine to AC supply

Thyristor based Rectifiers (3-phase)

E_d becomes smaller as α increases, but still each thyristor conducts 120 deg. Power flow is from AC side to DC side. I_d=(E_d-E₀)/R

Thyristor based Line Commutated Inverter (3-phase)

I_d=(E_o-E_d)/R, real power flow is from DC to AC side, Polarity of E_d is reversed.
Triggering range:

Rectifier 15°-90°, inverter: 90°-165°. Thyristor may misfire for α less than 15° (def. 8°) for sudden change in line voltage and hence discontinuity in output current. If we go beyond 165°, the inverter may lose its ability to switch from one thyristor to the next. As a result currents build up very quickly until the CB trips. For safety margin max α is 150°.

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Three-phase half-wave Controlled Rectifier

Three-phase half-wave Controlled Rectifier circuit with R load

Average Load/Output Voltage
${V_{dc}} = \frac{3}{\pi }\int_{\pi /6 + \alpha }^\pi {\sqrt 2 V\sin \theta d\theta }$
$= \frac{{3\sqrt 2 }}{\pi }V\left( {1 + \cos \left( {\frac{\pi }{6} + \alpha } \right)} \right)$
Three-phase half-wave Controlled Rectifier circuit
The rms output voltage is obtained from with resistive load

${V_{rms}} = {\left[ {\frac{3}{{2\pi }}\int\limits_{\frac{\pi }{6} + \alpha }^\pi {V_m^2{{\sin }^2}\omega td\left( {\omega t} \right)} } \right]^{\frac{1}{2}}}$
${V_{rms}} = \sqrt 3 {V_m}{\left[ {\frac{5}{{24}} - \frac{\alpha }{{4\pi }} + \frac{1}{{8\pi }}\sin \left( {\frac{\pi }{3} + 2\alpha } \right)} \right]^{\frac{1}{2}}}$
For a continuous load current with highly inductive load, the average output voltage is given by
${V_{dc}} = \frac{3}{{2\pi }}\int\limits_{\pi /6 + \alpha }^{5\pi /6 + \alpha } {{V_m}} \sin \omega td\left( {\omega t} \right)$
${V_{dc}} = \frac{{3\sqrt 3 {V_m}}}{{2\pi }}\cos \alpha$
The rms output voltage is obtained from
${V_{rms}} = {\left[ {\frac{3}{{2\pi }}\int\limits_{\pi /6 + \alpha }^{5\pi /6 + \alpha } {V_m^2{{\sin }^2}\omega t\left( {\omega t} \right)} } \right]^{\frac{1}{2}}}$
${V_{rms}} = \sqrt 3 {V_m}{\left[ {\frac{1}{6} + \frac{{\sqrt 3 }}{{8\pi }}\cos 2\alpha } \right]^{\frac{1}{2}}}$
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THREE-PHASE CONTROLLED RECTIFIER

The majority of line-commutated rectifier/inverter used in industry operates on three-phase networks. Although their operation is more complex than single-phase rectifiers/inverters, they posses following advantages.

Greater power transfer capability
The output ripple current is reduced

The delay/firing angle is measured from the point where two line voltages are simultaneously at the same level.

THREE-PHASE WAVE FORMS

EQUATIONS FOR THREE-PHASE VOLATGE

The three line-to-neutral voltages are given by (V_m is the peak phase voltage):

${V_{an}} = {V_m}\sin \omega t$

${V_{bn}} = {V_m}\sin \left( {\omega t - \frac{{2\pi }}{3}} \right)$

${V_{cn}} = {V_m}\sin \left( {\omega t + \frac{{2\pi }}{3}} \right)$

The line to line voltages are given by:

${V_{ab}} = {V_{an}} - {V_{bn}} = \sqrt 3 {V_m}\sin \left( {\omega t + \frac{\pi }{6}} \right)$

${V_{ba}} = {V_{bn}} - {V_{an}} = \sqrt 3 {V_m}\sin \left( {\omega t - \frac{{5\pi }}{6}} \right)$

${V_{bc}} = {V_{bn}} - {V_{cn}} = \sqrt 3 {V_m}\sin \left( {\omega t - \frac{\pi }{2}} \right)$

${V_{cb}} = {V_{cn}} - {V_{bn}} = \sqrt 3 {V_m}\sin \left( {\omega t + \frac{\pi }{2}} \right)$

${V_{ca}} = {V_{cn}} - {V_{an}} = \sqrt 3 {V_m}\sin \left( {\omega t + \frac{\pi }{2}} \right)$

${V_{ac}} = {V_{an}} - {V_{cn}} = \sqrt 3 {V_m}\sin \left( {\omega t - \frac{\pi }{6}} \right)$

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