CA2627 Building Science Instructor: Jiayu Chen Ph.D. Lecture 06 Transformers, three phase AC circuits, building power distribution

© Jiayu Chen, Ph.D. 2 Learning Objectives 1.Analyze the ideal transformer; compute primary and secondary currents and voltages and turns ratios. Calculate reflected sources and impedances across ideal transformers. 2.Understand maximum power transfer. 3.Learn three-phase AC power notation; compute load currents and voltages for balanced wye and delta loads. 4.Understand the basic principles of residential electrical wiring and of electrical safety.

© Jiayu Chen, Ph.D. 3 Electricity supply in Hong Kong Ref.: Supply rules from CLP s/Pages/Default.aspx?lang=en Supply rules from HK Electric F /0/SupplyRule_en_2008.pdf Power Distribution

© Jiayu Chen, Ph.D. 4 EMSD Code of Practice EMSDCode of Practice for Electricity (Wiring) Regulations, 2009 Edition ( Power Distribution

© Jiayu Chen, Ph.D. 5 Power Distribution

© Jiayu Chen, Ph.D. 6 Beyond the generation plant, an electric power network distributes energy to several substations. This network is usually referred to as the power grid. Power Distribution

© Jiayu Chen, Ph.D. 7 Transformers A transformer is a device that couples two AC circuits magnetically rather than through any direct conductive connection and permits a “transformation” of the voltage and current between one circuit and the other (for example, by matching a high- voltage, low-current AC output to a circuit requiring a low-voltage, high current source).

© Jiayu Chen, Ph.D. 8 Transformers The ideal transformer consists of two coils that are coupled to each other by some magnetic medium. There is no electrical connection between the coils. The coil on the input side is termed the primary, and that on the output side the secondary. An ideal transformer multiplies a sinusoidal input voltage by a factor of N and divides a sinusoidal input current by a factor of N.

© Jiayu Chen, Ph.D. 9 Transformers

© Jiayu Chen, Ph.D. 10 Transformers

© Jiayu Chen, Ph.D. 11 Transformers In many practical circuits, the secondary is tapped at two different points, giving rise to two separate output circuits. The most common configuration is the center-tapped transformer, which splits the secondary voltage into two equal voltages. The most common occurrence of this type of transformer is found at the entry of a power line into a household, where a high-voltage primary is transformed to 240 V, and split into two 120-V lines. 110 V vs 220 V ?

© Jiayu Chen, Ph.D. 12 Transformers Impedance Reflection A very common and rather general situation is that where an AC source, represented by its Thévenin equivalent, is connected to an equivalent load impedance by means of a transformer.

© Jiayu Chen, Ph.D. 13 Impedance Reflection Transformers

© Jiayu Chen, Ph.D. 14 Maximum power transfer Transformers When the load impedance is equal to the complex conjugate of the source impedance, the load and source impedances are matched and maximum power is transferred to the load.

© Jiayu Chen, Ph.D. 15 Transformers

© Jiayu Chen, Ph.D. 16 Transformers A substantial amount, 20% of the power from the source, is lost in the line if no transformers are used. Since power goes into heating up the line (line has resistance), and P ~ I 2 R, the loss in the line can be reduced by reducing the current in the line, by using transformers. Transformers at the source step up the voltage and step down the current, keeping the power constant, before transmission. At the load, we use a step-down transformer to step down the voltage and step up the current.

© Jiayu Chen, Ph.D. 17 Transformers Example: If the transformer delivers 50 A at 110 V rms with a certain resistive load, what is the power transfer efficiency between the source and the load? The transformer primary voltage The transformer primary current The current through the inductance The source current is (add phasor currents) The source power is Power transfer efficiency

© Jiayu Chen, Ph.D phase AC circuits Generation of Three Phase AC Electricity The generation of electrical power is more efficient in 3- phase systems. A 3-phase system employs three balanced voltages, equal in magnitude and differing in phase by 360°/3 = 120° se/threeph.htm

© Jiayu Chen, Ph.D. 19 As the rotor rotates counterclockwise at 50 r/sec, its magnetic field cuts the armature windings, thereby inducing in them the sinusoidal voltages. These voltages have peaks at one-third of a period apart, or 120° apart, because the armature windings are displaced by 120° in space. As a result, the alternator produces three voltages of the same rms value, but with phase differences of 120°. 3-phase AC circuits In HK: phase voltage = 220 V line voltage = 380 V

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits 3-phase circuits and machines possess some unique advantages: The power transmitted in a three-phase circuit is constant or independent of time rather than pulsating, as it is in a single-phase circuit. Three-phase motors start and run much better than do single-phase motors. The transmission of electrical power is more efficient in 3-phase systems employing three sinusoidal voltages. For example, for the addition of one more conductor, a three-phase supply provides 73% more power than a single-phase supply. With a three-phase supply the voltage between two line or phase cables is √3 = 1.73 times that between the neutral and any one of the line cables, i.e. 220 volts x 1.73 =380 volts.

© Jiayu Chen, Ph.D phase AC circuits Y and Δ connections Referring to the generator, there are six terminals and three voltages, v a, v b and v c. We use phasor notation and assume that each phase winding provides a source voltage in series with a negligible impedance. Under these assumptions, there are two ways of interconnecting the three sources. Y connection The Y connection selects terminals a', b', and c' and connects them together as neutral. The common terminal is called the neutral terminal and is labeled n. The neutral terminal may or may not be available for connection. Balanced loads result in no current in a neutral wire, and thus it is often not needed.

© Jiayu Chen, Ph.D phase AC circuits Δ connection The Δ source connection is seldom used in practice because any slight imbalance in magnitude or phase of the three-phase voltages will not result in a zero sum. The result will be a large circulating current in the generator coils that will heat the generator and depreciate the efficiency of the generator.

© Jiayu Chen, Ph.D phase AC circuits Y-connected source

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits Y-to-Y circuit This three-phase circuit consists of three parts: a three-phase source, a three-phase load, and a transmission line. 4-wire Y-to-Y The transmission line used to connect the source to the load consists of four wires, including a wire connecting the neutral node of the source to the neutral node of the load. 3-wire Y-to-Y The three-phase source is connected to the load using three wires, without a wire connecting the neutral node of the source to the neutral node of the load.

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits The per-phase equivalent circuit The Balanced Y-to-Y Circuit

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits Y-Δ transformations The Δ-to-Y and Y-to-Δ transformations convert Δ-connected loads to equivalent Y- connected loads and vice versa.

© Jiayu Chen, Ph.D. 34 Y-Δ Circuit 3-phase AC circuits

© Jiayu Chen, Ph.D phase AC circuits Balanced 3-phase circuit We have only two possible practical configurations for three-phase circuits, Y-to-Y and Y-to-Δ and we can convert the latter to a Y-to-Y form. Thus, a practical three-phase circuit can always be converted to the Y-to-Y circuit. Y-to-Δ circuit The per-phase equivalent circuit

© Jiayu Chen, Ph.D phase AC circuits The instantaneous power delivered to a balanced three-phase load is a constant.

© Jiayu Chen, Ph.D. 37 An electric connection to the general mass of earth, whose dimensions are very large in comparison to the electrical system being considered. The terms ‘Ground’ and ‘Grounding’ are synonymous with ‘Earth’ and ‘Earthing’ and are more prevalent in some countries like North America. Earthing System - Additional Earthing System:

© Jiayu Chen, Ph.D. 38 Earth Electrode 1 : Electrical connect to earth Independent earth electrode for each system/equipment Maintain sufficient distance, 3.5m or twice of driven length, from on another without significant affect the potential of the others Main Earthing Terminal 6 : Connect group of protective conductors to the earth electrode 1 through earth conductor 2 Protective / bonding Conductor 3 : Exposed conductive parts Conductive part of electrical installation - not a live part Extraneous conductive parts 4 Conductive part not forming part of the electrical installation Earthing System - Additional

© Jiayu Chen, Ph.D. 39 Earthing System - Additional Residual Current Devices (RCDs) The RCD is a circuit breaker which continuously compares the current in the phase with that in the neutral. The difference between the two will flow to earth, because it has left the supply through the phase and has not returned in the neutral. The purpose of the residual current device is to monitor the residual current and to switch off the circuit quickly if it rises to a preset level. When the amount of residual current, and hence of tripping current, reaches a pre- determined level, the circuit breaker trips, opening the main contacts and interrupting the circuit

© Jiayu Chen, Ph.D. 40 Lightning System - Additional Thunder Cloud Unstable upper atmosphere before lightning generation Positive charge and negative charge of thunder cloud separated Light weight positive charge at higher altitude, heavier negative charge at the base of the cloud Lightning Discharge May be cloud to cloud or cloud to ground Faintly luminescent leader channel from cloud toward to ground into many fingers Rapid escalating electric field at ground cause streamer move upward Discharge commenced the ionized channel completed at the junction of the leader and streamer Useful lighting projection system wiki:

© Jiayu Chen, Ph.D. 41 The Surge Protection Device (SPD) is a component of the electrical installation protection system. This device is connected in parallel on the power supply circuit of the loads that it has to protect It can also be used at all levels of the power supply network. This is the most commonly used and most efficient type of overvoltage protection. Lightning System - Additional Note: Surge Protection Devices (SPD) are used for electric power supply networks, telephone networks, and communication and automatic control buses.

© Jiayu Chen, Ph.D. 42 Thank You!