Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected transmission lines that facilitate this movement form a transmission network, known as a “grid” in North America. In the United Kingdom, India, Tanzania, Myanmar, Malaysia and New Zealand, it is known as a “grid”; in Australia, it is known as a “power system”.
Electricity is most often transmitted using three-phase alternating current (AC), because this allows the use of transformers which make it possible to change the voltage (potential difference). Thus, power can be transmitted at high voltage across the grid, and then reduced to a lower and safer voltage for use by customers. The advantages of AC transmission are:
Advantages of AC transmission
- AC voltages can be increased or decreased easily using transformers.
- AC generators, motors and other equipment are readily available.
- Losses are lower compared to DC transmission.
- Reactive power flow can be controlled leading to increased system stability.
- Short circuits are more easily interrupted by circuit breakers.
- Harmonic and interference problems are minimized.
So why is DC not commonly used for transmission despite some advantages it has over AC?
Disadvantages of DC transmission
While DC transmission has some advantages like controllable power flow and no constraints on distance, it also has some significant disadvantages compared to AC transmission:
- Converting between AC and DC is complex and expensive due to requiring conversion equipment like rectifiers and inverters.
- DC does not allow transformers to be used, so it is not possible to easily change DC voltage levels.
- DC transmission requires both conductors to be insulated from ground, increasing cost.
- Control of DC power flow is complex, requiring expensive thyristor controls.
- DC lines have higher capacitive charges requiring additional current to control this leading to increased transmission losses.
- Short circuits are harder to clear on DC lines.
- DC has difficulty supplying inductive loads like motors.
So in summary, the key disadvantages of using DC for transmission are:
- Expensive conversion between AC and DC required.
- Impossible to use transformers to change DC voltage levels.
- More complex and expensive to control power flow.
- Higher capacitive charging current increases losses.
- Equipment like AC motors cannot be easily supplied.
Comparison of AC and DC transmission
| Parameter | AC Transmission | DC Transmission |
|---|---|---|
| Voltage level change | Easy, using transformers | Not possible, requires converter stations |
| Losses | Lower losses | Higher losses due to cable charging current |
| Power control | Relatively easy | Complex and expensive |
| Harmonics | Can be filtered out | Not applicable |
| Equipment availability | Motors, generators etc. easily available | Very few DC machines available |
| Short circuit interruption | Circuit breakers can interrupt AC faults | DC breakers are complex and expensive |
| Cost | Lower cost equipment | Higher cost due to converters required |
As the comparison shows, AC transmission has significant technical and economic advantages over DC transmission. The only applications where DC transmission is preferrable is for very long undersea cables or for connecting unsynchronized AC networks.
Reasons for not using DC transmission
Based on the disadvantages and comparative analysis covered so far, the main reasons why DC is not used for transmission are:
- Transformers cannot be used – The big advantage of AC is that voltage levels can be easily changed using transformers. This is not possible with DC.
- Higher costs – The converter stations required to convert AC to DC and back are very expensive. The cost of DC transmission lines is also higher.
- Technical challenges – Control of power flow, protection systems, insulation requirements etc. are more complex with DC transmission making it less feasible.
- Lower efficiency – The increased cable charging currents and converter losses make DC transmission less efficient than AC.
- Equipment compatibility – The AC power grid is completely designed for AC equipment like motors, generators etc. DC transmission is not compatible with these.
- Lack of standard DC voltages – Internationally AC transmission systems have standardized voltage levels. No such standardization exists for DC levels.
So in summary, the properties of AC power system components, economics and technical challenges have resulted in AC becoming the standard for electric power transmission worldwide.
Applications of DC transmission
Although AC is predominantly used for transmission, DC transmission does have some applications:
- Long undersea cables – DC transmission allows longer underwater cables due to lower capacitive charging currents.
- Interconnecting asynchronous networks – DC links can connect AC networks that are not synchronized with each other.
- Increasing transmission capacity – Existing AC corridors sometimes get DC links on them to increase total transfer capacity.
- Connecting remote renewable sources – DC links can connect offshore wind farms to the grid.
So while AC transmission dominates, DC links are often used to complement them in specific applications.
Conclusion
In conclusion, the predominant use of AC rather than DC for electric power transmission is due to the various technical and economic advantages of AC systems. The ability to transform voltages, use of standard equipment, lower losses, easier control of power flows etc. have resulted in AC becoming the worldwide standard. DC transmission has very few advantages that make it preferred only in some niche applications like very long distance underwater links. But for the vast majority of power transmission systems, AC is clearly the standard choice.
