🔦Electrical Circuits and Systems II Unit 6 – Three-Phase Circuits and Power Systems

Key Concepts and Terminology

• Three-phase systems generate, transmit, and distribute alternating current (AC) power using three separate conductors, each carrying a sinusoidal voltage waveform displaced by 120 degrees
• Balanced three-phase circuits have equal impedances and voltages across all three phases, resulting in equal currents and a net zero current in the neutral conductor
• Unbalanced three-phase circuits have unequal impedances or voltages across the phases, leading to unequal currents and a non-zero current in the neutral conductor
• Power calculations in three-phase systems involve measuring voltage, current, and power factor to determine active power (watts), reactive power (volt-amperes reactive or VAR), and apparent power (volt-amperes or VA)
  • Active power represents the useful work done by the system
  • Reactive power represents the energy stored and released by inductive or capacitive components
  • Apparent power is the vector sum of active and reactive power
• Three-phase transformers convert voltage levels and provide electrical isolation between different parts of the power system (generation, transmission, and distribution)
• Power system components include generators, transformers, transmission lines, switchgear, and protective devices (relays, fuses, and circuit breakers)
• Power system infrastructure encompasses the physical facilities and equipment required to generate, transmit, and distribute electrical energy to end-users (residential, commercial, and industrial customers)

Three-Phase System Basics

• Three-phase systems consist of three sinusoidal voltage waveforms displaced by 120 degrees, each connected to a separate conductor or phase (typically labeled A, B, and C or R, Y, and B)
• The 120-degree phase shift between the voltages allows for a more constant power delivery compared to single-phase systems
• Three-phase systems can be connected in a wye (Y) or delta (Δ) configuration
  • In a wye configuration, each phase is connected to a common neutral point, forming a Y-shape
  • In a delta configuration, each phase is connected to the other two phases, forming a triangular shape
• The phase voltage ($V_p$) is the voltage between a phase conductor and the neutral point, while the line voltage ($V_L$) is the voltage between any two phase conductors
  • In a balanced wye-connected system, $V_L = \sqrt{3} \times V_p$
  • In a balanced delta-connected system, $V_L = V_p$
• Three-phase power is generated by synchronous generators in power plants, with each phase winding displaced by 120 degrees around the stator
• Three-phase motors, such as induction motors and synchronous motors, are widely used in industrial applications due to their high efficiency, smooth torque, and compact size

Balanced Three-Phase Circuits

• Balanced three-phase circuits have equal impedances and voltages across all three phases, resulting in equal currents and a net zero current in the neutral conductor
• In a balanced wye-connected system, the phase current ($I_p$) is equal to the line current ($I_L$)
  • $I_p = I_L$
• In a balanced delta-connected system, the phase current ($I_p$) is equal to the line current ($I_L$) divided by $\sqrt{3}$
  • $I_p = \frac{I_L}{\sqrt{3}}$
• The total power in a balanced three-phase system can be calculated using the formula:
  • $P = \sqrt{3} \times V_L \times I_L \times \cos{\phi}$, where $\cos{\phi}$ is the power factor
• Balanced three-phase circuits simplify power calculations and analysis, as the system can be treated as three identical single-phase circuits
• In a balanced system, the power is evenly distributed among the three phases, leading to efficient power transmission and utilization

Unbalanced Three-Phase Circuits

• Unbalanced three-phase circuits have unequal impedances or voltages across the phases, leading to unequal currents and a non-zero current in the neutral conductor
• Causes of unbalanced conditions include:
  • Uneven distribution of single-phase loads across the three phases
  • Faults or short circuits in one or more phases
  • Unequal impedances in the transmission or distribution lines
• Unbalanced currents can cause overheating in motors, transformers, and other equipment, reducing their efficiency and lifespan
• Analyzing unbalanced circuits requires using symmetrical component theory, which decomposes the unbalanced system into three balanced components: positive sequence, negative sequence, and zero sequence
  • Positive sequence represents the balanced portion of the system, with equal magnitudes and 120-degree phase shifts
  • Negative sequence represents the unbalanced portion, with equal magnitudes but opposite 120-degree phase shifts
  • Zero sequence represents the common-mode component, with equal magnitudes and no phase shift
• Unbalanced conditions can be mitigated by redistributing loads, using phase balancing equipment, or installing voltage regulators and uninterruptible power supplies (UPS)

Power Calculations in Three-Phase Systems

• Power calculations in three-phase systems involve measuring voltage, current, and power factor to determine active power (watts), reactive power (volt-amperes reactive or VAR), and apparent power (volt-amperes or VA)
• In a balanced three-phase system, the total active power can be calculated using the formula:
  • $P = \sqrt{3} \times V_L \times I_L \times \cos{\phi}$, where $\cos{\phi}$ is the power factor
• The total reactive power can be calculated using the formula:
  • $Q = \sqrt{3} \times V_L \times I_L \times \sin{\phi}$, where $\sin{\phi}$ is the sine of the phase angle between voltage and current
• The total apparent power can be calculated using the formula:
  • $S = \sqrt{3} \times V_L \times I_L$
• The power factor ($\cos{\phi}$) is the ratio of active power to apparent power, indicating the efficiency of power utilization
  • A power factor of 1 represents a purely resistive load, with all apparent power converted to active power
  • A power factor less than 1 indicates the presence of reactive power, which can be caused by inductive or capacitive loads
• Improving the power factor can be achieved by adding power factor correction capacitors or using synchronous motors to generate reactive power
• In unbalanced systems, power calculations must be performed separately for each phase, considering the unequal voltages, currents, and phase angles

Three-Phase Transformers

• Three-phase transformers are used to convert voltage levels and provide electrical isolation between different parts of the power system (generation, transmission, and distribution)
• Three-phase transformers can have various winding configurations, such as wye-wye, delta-delta, wye-delta, and delta-wye
  • The choice of winding configuration depends on factors such as voltage level, grounding requirements, and load characteristics
• Wye-wye transformers are commonly used in high-voltage transmission systems, as they provide a neutral point for grounding and allow for the use of smaller insulation levels
• Delta-delta transformers are used in distribution systems to supply power to industrial loads, as they can handle high currents and provide a stable voltage source
• Wye-delta and delta-wye transformers are used to interface between transmission and distribution systems, as they can convert between the two winding configurations and provide a phase shift to reduce harmonics
• Three-phase transformers are designed to handle the power capacity and voltage levels required for each application, with ratings ranging from a few kilovolt-amperes (kVA) to hundreds of megavolt-amperes (MVA)
• Proper maintenance and monitoring of three-phase transformers are essential to ensure reliable operation and prevent failures due to overloading, insulation breakdown, or other factors

Power System Components and Infrastructure

• Power system components include generators, transformers, transmission lines, switchgear, and protective devices (relays, fuses, and circuit breakers)
• Generators convert mechanical energy from prime movers (such as steam turbines, gas turbines, or hydroelectric turbines) into electrical energy
  • Most generators used in power systems are synchronous generators, which maintain a constant frequency and voltage output
• Transformers are used to step up the voltage for long-distance transmission and step down the voltage for distribution to end-users
  • High-voltage transmission (typically 110 kV to 765 kV) minimizes power losses and allows for efficient power transfer over long distances
• Transmission lines carry high-voltage electricity from power plants to substations and between substations
  • Overhead transmission lines are supported by towers or poles and use bare conductors (typically aluminum or copper) with air insulation
  • Underground transmission lines use insulated cables and are used in urban areas or environmentally sensitive regions
• Switchgear consists of switches, disconnectors, and circuit breakers used to control, protect, and isolate electrical equipment
  • Gas-insulated switchgear (GIS) is commonly used in high-voltage applications due to its compact size and reliability
• Protective devices, such as relays, fuses, and circuit breakers, detect and isolate faults to minimize damage and ensure the safety of personnel and equipment
• Power system infrastructure encompasses the physical facilities and equipment required to generate, transmit, and distribute electrical energy to end-users (residential, commercial, and industrial customers)
  • This includes power plants, substations, transmission and distribution lines, control centers, and communication networks
• Proper planning, design, and maintenance of power system infrastructure are crucial to ensure a reliable, efficient, and sustainable supply of electricity

Practical Applications and Real-World Examples

• Three-phase power is used in a wide range of applications, from industrial processes to commercial buildings and residential homes
• Industrial applications, such as manufacturing plants, refineries, and data centers, rely on three-phase power to operate large motors, pumps, compressors, and other equipment
  • Example: A chemical processing plant uses three-phase induction motors to drive pumps and mixers, ensuring a continuous and efficient production process
• Commercial buildings, such as office towers, shopping malls, and hospitals, use three-phase power for lighting, heating, ventilation, and air conditioning (HVAC) systems, and elevators
  • Example: A high-rise office building uses three-phase power to operate its HVAC system, maintaining a comfortable and productive work environment for tenants
• Residential homes typically use single-phase power for lighting and small appliances, but may also use three-phase power for larger loads such as electric vehicle charging or home energy storage systems
  • Example: A homeowner installs a three-phase solar inverter and battery storage system to generate and store renewable energy, reducing their reliance on the grid
• Renewable energy systems, such as wind turbines and solar farms, generate three-phase power that is fed into the electrical grid
  • Example: An offshore wind farm uses three-phase generators in each wind turbine to produce electricity, which is then transmitted to shore using high-voltage submarine cables
• Transportation systems, such as electric trains and ships, use three-phase power for propulsion and auxiliary systems
  • Example: A high-speed electric train uses three-phase traction motors to achieve smooth acceleration and efficient operation, powered by overhead catenary lines or third rails
• Understanding the principles of three-phase circuits and power systems is essential for electrical engineers, technicians, and managers working in various industries, as well as for policymakers and regulators involved in energy planning and grid modernization efforts