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

Power Electronic Control in Electrical Systems

Electrical power systems

The major elements of an electrical power system are generators, transformers, transmission and distribution lines, loads and protection and control equipment. These elements are interconnected to be able to allow the generation of electrical energy in the optimal locations and in sufficient quantity to meet the customers' demand, to transmit it to the load centres and to supply high quality electrical energy at competitive prices.

The quality of the electricity supply may be measured in terms of:

constant voltage magnitude, e.g. no voltage sags
constant frequency
constant power factor
balanced phases
sinusoidal waveforms, e.g. no harmonic content
lack of interruptions
ability to withstand faults and to recover quickly.

Background

The last quarter of the nineteenth century saw the development of the electricity supply industry as a new, promising and fast-growing activity. Since that time electrical power networks have undergone immense transformations (Hingorani and Gyugyi, 2000; Kundur, 1994). Owing to the relative 'safety' and 'cleanliness' of electricity, it quickly became established as a means of delivering light, heat and motive power. Nowadays it is closely linked to primary activities such as industrial production, transport, communications and agriculture. Population growth, technological innovations and higher capital gains are just a few of the factors that have maintained the momentum of the power industry.

Clearly it has not been easy for the power industry to reach its present status. Throughout its development innumerable technical and economic problems have been overcome, enabling the supply industry to meet the ever increasing demand for energy with electricity at competitive prices. The generator, the incandescent lamp and the industrial motor were the basis for the success of the earliest schemes. Soon the transformer provided a means for improved efficiency of distribution so that generation and transmission of alternating current over considerable distances provided a major source of power in industry and also in domestic applications.

For many decades the trend in electric power production has been towards an interconnected network of transmission lines linking generators and loads into large integrated systems, some of which span entire continents. The main motivation has been to take advantage of load diversity, enabling a better utilization of primary energy resources. It may be argued that interconnection provides an alternative to a limited amount of generation thus enhancing the security of supply (Anderson and Fouad, 1977).

Interconnection was further enhanced, in no small measure, by early breakthroughs in high-current, high-power semiconductor valve technology. Thyristor based high voltage direct current (HVDC) converter installations provided a means for interconnecting power systems with different operating frequencies, e.g. 50/60 Hz, for interconnecting power systems separated by the sea, e.g. the cross-Channel link between England and France, and for interconnecting weak and strong power systems (Hingorani, 1996). The rectifier and inverter may be housed within the same converter station (back-to-back) or they may be located several hundred kilometres apart, for bulk-power, extra-long-distance transmission. The most recent development in HVDC technology is the HVDC system based on solid state voltage source converters (VSCs), which enables independent, fast control of active and reactive powers (McMurray, 1987). This equipment uses insulated gate bipolar transistors (IGBTs) or gate turn-off thyristors (GTOs) 'valves' and pulse width modulation (PWM) control techniques (Mohan et al., 1995). It should be pointed out that this technology was first developed for applications in industrial drive systems for improved motor speed control. In power transmission applications this technology has been termed HVDC Light (Asplund et al., 1998) to differentiate it from the well-established HVDC links based on thyristors and phase control (Arrillaga, 1999). Throughout this book, the terms HVDC Light and HVDC based on VSCs are used interchangeably.

Based on current and projected installations, a pattern is emerging as to where this equipment will find widespread application: deregulated market applications in primary distribution networks, e.g. the 138 kV link at Eagle Pass, interconnecting the Mexican and Texas networks (Asplund, 2000). The 180 MVA Directlink in Australia, interconnecting the Queensland and New South Wales networks, is another example.

Power electronics technology has affected every aspect of electrical power networks; not just HVDC transmission but also generation, AC transmission, distribution and utilization. At the generation level, thyristor-based automatic voltage regulators (AVRs) have been introduced to enable large synchronous generators to respond quickly and accurately to the demands of interconnected environments. Power system stabilizers (PSSs) have been introduced to prevent power oscillations from building up as a result of sympathetic interactions between generators. For instance, several of the large generators in Scotland are fitted with PSSs to ensure trouble-free operation between the Scottish power system and its larger neighbour, the English power system (Fairnley et al., 1982). Deregulated markets are imposing further demands on generating plant, increasing their wear and tear and the likelihood of generator instabilities of various kinds, e.g. transient, dynamic, sub-synchronous resonance (SSR) and sub-synchronous torsional interactions (SSTI). New power electronic controllers are being developed to help generators operate reliably in the new market place. The thyristor-controlled series compensator (TCSC) is being used to mitigate SSR, SSTI and to damp power systems' oscillations (Larsen et al., 1992). Examples of where TCSCs have been used to mitigate SSR are the TCSCs installed in the 500 kV Boneville Power Administration's Slatt substation and in the 400kV Swedish power network. However, it should be noted that the primary function of the TCSC, like that of its mechanically controlled counterpart, the series capacitor bank, is to reduce the electrical length of the compensated transmission line. The aim is still to increase power transfers significantly, but with increased transient stability margins.

Power Electronics Chip-Power electronics technology

A welcome result of deregulation of the electricity supply industry and open access markets for electricity worldwide, is the opportunity for incorporating all forms of renewable generation into the electrical power network. The signatories of the Kyoto agreement in 1997 set themselves a target to lower emission levels by 20% by 2010. As a result of this, legislation has been enacted and, in many cases, tax incentives have been provided to enable the connection of micro-hydro, wind, photovoltaic, wave, tidal, biomass and fuel cell generators. The power generated by some of these sources of electricity is suitable for direct input, via a step-up transformer, into the AC distribution system. This is the case with micro-hydro and biomass generators. Other sources generate electricity in DC form or in AC form but with large, random variations which prevent direct connection to the grid; for example fuel cells and asynchronous wind generators. In both cases, power electronic converters such as VSCs provide a suitable means for connection to the grid.

In theory, the thyristor-based static var compensator (SVC) (Miller, 1982) could be used to perform the functions of the PSS, while providing fast-acting voltage support at the generating substation. In practice, owing to the effectiveness of the PSS and its relative low cost, this has not happened. Instead, the high speed of response of the SVC and its low maintenance cost have made it the preferred choice to provide reactive power support at key points of the transmission system, far away from the generators. For most practical purposes they have made the rotating synchronous compensator redundant, except where an increase in the short-circuit level is required along with fast-acting reactive power support. Even this niche application of rotating synchronous compensators may soon disappear since a thyristor-controlled series reactor (TCSR) could perform the role of providing adaptive short-circuit compensation and, alongside, an SVC could provide the necessary reactive power support. Another possibility is the displacement of not just the rotating synchronous compensator but also the SVC by a new breed of static compensators (STATCOMs) based on the use of VSCs. The STATCOM provides all the functions that the SVC can provide but at a higher speed and, when the technology reaches full maturity, its cost will be lower. It is more compact and requires only a fraction of the land required by an SVC installation. The VSC is the basic building block of the new generation of power controllers emerging from flexible alternating current transmission systems (FACTS) and Custom Power research (Hingorani and Gyugyi, 2000). In high-voltage transmission, the most promising equipment is: the STATCOM, the unified power flow controller (UPFC) and the HVDC Light. At the low-voltage distribution level, the VSC provides the basis for the distribution STATCOM (D-STATCOM), the dynamic voltage restorer (DVR), the power factor corrector (PFC) and active filters.

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Power Quality

Introduction

It is necessary to prevent the power line disturbances from disrupting the operation of critical loads such as computers used in process control, medical equipments, operation theatre, large computer center, R & D laboratories and the like.

The voltage supplied by the a.c. main should ideally be perfect sine wave without any harmonics and voltage fluctuations
The voltages in a three phase system should be balanced, with each phase displaced by 120 degrees with respect to the others.

However, this does not happen in real practice due to the presence of disturbances such as over-voltage, under-voltage, voltage spikes, chopped voltage waveform, harmonics and electromagnetic interference.

The over-voltages (Swell) may be caused by sudden decrease in load or unbalance caused due to faults in the supply system.
Under-voltage (Sag) may be caused by over-load conditions such as starting of big induction motors and compressors or due to power line faults.
Spikes of voltages may be caused by switching- in and switching-out of power factor correction condensers or big motors and welding sets working in the vicinity.
The chopped voltages may be due to a.c. to d.c. line commutated thyristor converters.
The voltage harmonics may be caused by variety of sources such as thyristor converters, magnetic saturation of power system transformers etc.
Electromagnetic interference (EMI) is produced due to rapid switching of voltages and currents in power converters. In case of critical applications, where shut down due to the above disturbances is not acceptable, the back-up of the a.c. main is provided by means of UPS. The power conditioners provide an effective way of suppressing some or all of the electrical disturbances other than the power outages and frequency deviations.

The wave shapes of typical disturbances are shown in Fig.

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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.

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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

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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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