Unit
- Power system structure: Generation plants produce electricity at voltages from 11 kV to 25 kV. Transmission systems step voltage up to 132 kV to 765 kV for long-distance transport. Distribution systems reduce voltage to primary levels (4 kV to 35 kV) and then to secondary levels (120 V to 480 V) for end users.
- Distribution system function: The distribution network delivers electricity from substations to residential, commercial, and industrial customers. It includes primary feeders, distribution transformers, secondary circuits, and service drops.
- Voltage classification: Primary distribution operates between 2.4 kV and 34.5 kV. Secondary distribution is below 1 kV, typically 120/240 V for residential and 277/480 V for commercial.
- System components: Key elements include distribution substations that step down transmission voltage, overhead or underground feeders, sectionalizing switches, transformers, fuses, and protective relays.
- Load types: Residential loads peak in evening hours with high diversity. Commercial loads peak during business hours with lower diversity. Industrial loads have steady demand with occasional high starting currents.
- Reliability criteria: System average interruption frequency index (SAIFI) and system average interruption duration index (SAIDI) measure performance. Utilities target SAIFI below 1.5 interruptions per customer per year and SAIDI below 150 minutes.
- Regulatory context: Public utility commissions set reliability standards and rate structures. Distribution system managers must balance cost, reliability, and safety while meeting regulatory requirements.
- Emerging trends: Distributed energy resources, electric vehicle charging, and smart grid technologies are transforming distribution planning. Managers must integrate these while maintaining power quality and stability.
- Feeder configuration: Distribution feeders are arranged in radial, loop, or network configurations. Radial systems are simple and economical but offer lower reliability. Loop and networked designs provide alternate paths, improving service continuity during faults or maintenance.
- Protection coordination: Overcurrent protection devices such as fuses, reclosers, and relays must be coordinated to isolate faults selectively. Proper coordination minimizes outage area by ensuring the device closest to the fault operates first while upstream devices remain closed.
- Power quality issues: Distribution systems face voltage sags, harmonics, and flicker caused by large motor starts or nonlinear loads. Mitigation includes capacitor banks, active filters, and voltage regulators to meet IEEE 519 standards for harmonic distortion.
- Distribution automation: Advanced sensors, remote-controlled switches, and SCADA systems enable real-time monitoring and fault isolation. Automation reduces outage times, improves voltage regulation, and supports integration of distributed generation and demand response programs.
Process Flow
- Step 1 - Generation: Electricity is produced at power plants using coal, gas, nuclear, hydro, or renewable sources.
- Step 2 - Transmission: Step-up transformers raise voltage for efficient long-distance transfer over high-voltage lines.
- Step 3 - Primary substation: Voltage is reduced to primary distribution levels (typically 69 kV to 34.5 kV) at transmission substations.
- Step 4 - Distribution substation: Further reduction to primary feeder voltage, with circuit breakers and switchgear for protection.
- Step 5 - Primary feeders: Three-phase circuits distribute power along streets or through underground conduits.
- Step 6 - Distribution transformers: Voltage is stepped down to utilization levels near customer premises.
- Step 7 - Secondary service: Single-phase or three-phase service drops connect individual customers.
Quick Revision
- Distribution systems form the final link between the bulk power system and end users.
- Voltage levels are classified as primary (up to 34.5 kV) and secondary (below 1 kV).
- Key components include substations, feeders, transformers, switches, and protection devices.
- Reliability indices like SAIFI and SAIDI are standard performance metrics.