Transmission and Distribution of D.C. Power
By transmission and distribution of electric power is meant its conveyance from the central station where it is generated to places, where it is demanded by the consumers like mills, factories, residential and commercial buildings, pumping stations etc.
In the realm of electrical engineering, the transmission and distribution of power play pivotal roles in ensuring efficient energy delivery from generation sources to end-users. While AC (Alternating Current) systems have traditionally dominated the landscape, DC (Direct Current) transmission and distribution systems have gained significant attention due to their unique advantages. This article delves into the fundamentals of DC transmission and distribution power systems, elucidating their advantages, challenges, and emerging trends.
Understanding DC Transmission and Distribution
DC transmission and distribution systems convey electrical power using direct current, where the voltage polarity remains constant over time. Unlike AC systems, which periodically change direction, DC systems maintain a constant flow of electricity from the generation source to the load. DC systems employ converters to transform AC power from generators into DC for transmission and subsequently convert it back to AC for distribution to end-users.
Electric Power May Be Transmitted by 2 Methods
By overhead system or
By underground system
Underground system is especially suited for densely populated areas though it is somewhat costlier than the first method. In over-head system, power is conveyed by bare conductors of copper or aluminium which are strung between wooden or steel poles erected at convenient distances along a route. The bare copper or aluminium wire is fixed to an insulator which is itself fixed onto a cross-arm on the pole.
The number of cross-arms carried by a pole depends on the number of wires it has to carry. Line supports consist of (1) pole structures and (2) tower.
!!! Poles which are made of wood, reinforced concrete or steel are used up to 66 kV whereas steel towers are used for higher voltages. The underground system employs insulated cables which may be single, double or triple-core etc.
A good system whether overhead or underground should fulfil the following requirements :
The voltage at the consumer’s premises must be maintained within ± 4 or ± 6% of the declared voltage, the actual value depending on the type of load*.
The loss of power in the system itself should be a small percentage (about 10%) of the power transmitted.
The transmission cost should not be unduly excessive.
The maximum current passing through the conductor should be limited to such a value as not to overheat the conductor or damage its insulation.
The insulation resistance of the whole system should be very high so that there is no undue leakage or danger to human life.
It may, however, be mentioned here that these days all production of power is as a.c. power and nearly all d.c. power is obtained from large a.c. power systems by using converting machinery like synchronous or rotary converters, solid-state converters and motor-generator sets etc.
There are many sound reasons for producing power in the form of alternating current rather than direct current.
Typical Power System
Figure shows a typical power system for obtaining d.c. power from a.c. power. Other details such as instruments, switches and circuit breakers etc. have been omitted.
Two 13.8 kV alternators run in parallel and supply power to the station bus-bars. The voltage is stepped up by 3-phase transformers to 66 kV for transmission purposes** and is again stepped down to 13.8 kV at the sub-station for distribution purposes.
Figure shows only three methods commonly used for converting a.c. power to d.c. power at the substation.
Advantages of DC Transmission and Distribution
Reduced Transmission Losses: DC systems exhibit lower transmission losses compared to AC systems, especially over long distances. This is attributed to the absence of skin and proximity effects, reducing resistance losses and enhancing efficiency.
Enhanced Voltage Control: DC transmission allows for precise voltage control, mitigating voltage drops and improving the stability and reliability of the grid. This is particularly beneficial for interconnecting grids with varying voltage levels.
Increased Power Transfer Capability: DC transmission lines can carry higher power loads without encountering limitations posed by reactive power, enabling efficient utilization of existing infrastructure.
Integration of Renewable Energy: DC systems facilitate the integration of renewable energy sources such as solar and wind power into the grid by providing efficient long-distance transmission of power from remote generation sites to urban centers.
Challenges and Limitations
High Initial Costs: The installation and deployment of DC transmission infrastructure entail substantial initial investments, including converter stations and specialized equipment, which can pose financial challenges.
Converter Station Complexity: DC transmission requires converter stations at both ends to convert AC power to DC for transmission and vice versa for distribution. These stations involve complex control and protection systems, necessitating meticulous design and maintenance.
Limited Market Penetration: Despite its advantages, DC transmission and distribution technology has yet to achieve widespread adoption, primarily due to the entrenched infrastructure and standards associated with AC systems.
Future Trends and Developments
HVDC Grids: The development of High Voltage Direct Current (HVDC) grids offers promising prospects for enhancing grid resilience, integrating renewable energy sources, and facilitating cross-border power exchange on a global scale.
Solid-State Power Electronics: Advancements in solid-state power electronics are driving innovation in DC converter technologies, enabling higher efficiency, compact designs, and improved reliability of DC transmission systems.
Grid Modernization Initiatives: Governments and utilities are increasingly investing in grid modernization initiatives aimed at upgrading aging AC infrastructure and deploying advanced DC transmission technologies to meet evolving energy demands and environmental objectives.
DC transmission and distribution power systems represent a compelling paradigm shift in the domain of electrical engineering, offering a myriad of advantages including reduced losses, enhanced voltage control, and increased power transfer capability. While challenges such as high initial costs and infrastructure compatibility persist, ongoing research and development efforts are paving the way for widespread adoption and integration of DC technology into future energy grids, ushering in a new era of efficiency, reliability, and sustainability.
