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Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
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At secondary transmission substation, electrical power is transmitted by
phase, 3-wire system
phase, 4-wire system
phase, 3-wire system
phase with midpoint earthed system
phase, 3-wire system
At a secondary transmission substation, electrical power is typically distributed via a 3-phase, 3-wire system. This configuration is widely used for transmission voltages ranging from 33 kV to 66 kV, providing a balance between economy and efficiency in power delivery.
At a secondary transmission substation, electrical power is typically distributed via a 3-phase, 3-wire system. This configuration is widely used for transmission voltages ranging from 33 kV to 66 kV, providing a balance between economy and efficiency in power delivery.
P=3тАЛVLтАЛILтАЛcos╧Х тАФ Total power in a balanced 3-phase system
VLтАЛ=3тАЛVphтАЛ тАФ Relationship between line and phase voltage in star connection
The 3-phase, 3-wire system eliminates the need for a neutral conductor in high-voltage transmission, as the load is inherently balanced across the three phases. By utilizing only three lines, the system minimizes conductor material usage and insulation costs while maintaining a constant power flow. This configuration is ideal for long-distance transport before the voltage is stepped down further for local distribution.
Secondary transmission typically operates at voltages like 33 kV, 66 kV, or 132 kV.
A 3-wire system is used because the balanced 3-phase load does not require a neutral return path.
Transmission lines are usually connected in Delta or Star without neutral grounding at the substation stage.
Reduced cost of line conductors.
Lower structural weight of supporting towers.
Reduced insulation requirements for transmission lines.
Not suitable for single-phase low-voltage loads directly.
Requires balancing of load to prevent phase voltage variations.
Interconnecting sub-transmission substations.
Feeding large industrial loads connected at higher voltage levels.
The 3-phase, 4-wire system is primarily used in secondary distribution (e.g., 415V/230V) to provide both phase-to-phase and phase-to-neutral voltages.
Option A is correct because secondary transmission does not require a neutral wire for distribution.
A is correct тАФ Secondary transmission utilizes a 3-phase, 3-wire system to transmit power efficiently over medium distances without the need for a neutral conductor.
Always remember that the neutral wire is introduced at the distribution level (below 33kV) to facilitate single-phase household loads.