AC vs. DC Power – Why the AC system is better than a DC system

Power systems use either DC (Direct Current) or AC (Alternating Current). Let’s delve on these technologies.

AC versus DC Comparision

Consider the following scenario:

  • A power plant is feeding a house located over 1000feet away.
  • The house demands 100Amps current at 480V.
  • The plant generates 100Amp at 480V
  • Assume a DC system and an AC system with the AC system employing a transformer rated 480/4800V near the generating station and a 4800/480V transformer near the house. See the figure below.
AC vs. DC System
Figure 1: AC vs. DC System

Let’s see how a DC system stacks up against an AC system.

DC SYSTEM AC SYSTEM
1. To carry 100Amps over the line, a larger cable (in diameter) will be required for the DC system. 1. After transformation, the current on the power line will be 10Amps. A smaller cable will be required.

2. Larger cable means lower conductor resistance. Typically, 0.15 ohms per 1000feet can be used for a 100Amp conductor (per AWG). In which case,

Voltage Drop (VD) across the line = 0.15*100 = 15V.

2. Smaller cable (in diameter) means higher resistance. Typically, a 1.5 ohms per 1000feet can be used for a 10Amp conductor. In which case,

Voltage Drop (VD) = 1.5*10 = 15V.

Same as a DC system.

3. The DC generator must generate 480V plus 15V to deliver power to the house. At the house, the voltage, therefore, will go from 495V at no-load to 480V at full-load. A 15V variation.

3. Allow taps on the transformer to raise the voltage by 15V to obtain 4815V. At the house, this is equivalent to 481.5V. A 1.5V variation from no-load to full-load.

Engineers call this variation of voltage as the Voltage Regulation (VR). An important factor in the power system. The less the VR the better the system.

4. Losses in transmission system = VD*Current (in watts) = 15*100 = 1500 watts

4. Losses in transmission system (in watts) = 15*10 = 150 watts.

Ten times less than DC transmission.

5. Transformers cannot operate with DC supply wired to it. The only way to step down the voltage for distribution is through a motor-generator set or a rotary converter – an inefficient process.

5. Transformers operate at 99% efficiency at full load. Used throughout the AC system.

DC Systems

A DC system cannot be applied to all areas of the power system. They have critical issues in distributing power to customers who have loads with different voltage requirements. The current technology renders it inefficient. At high voltages and long-distance transmission, DC systems are favorable. With fewer conductors and cost savings from transmission infrastructure, the High Voltage Direct Current (HVDC) system can be implemented as a highway for bulk power transmission.

DC Circuit
Figure 2: DC Circuit

In DC systems, the power delivered to the load is given by:

P=V*I  (watts)

Where, V = R*I  (Ohm’s law)

Losses incurred in a DC system are purely resistive. They are emitted as heat, given by I²R (Joules).

Advantage of a DC system:

  1. Simple system. Easy to understand. No abstract concepts involved, unlike AC systems.
  2. Suitable for HVDC transmission. Fewer t-line conductors required to transmit DC power.
  3. It can be used to link two asynchronous AC systems.
  4. Undersea power transmission is feasible using DC lines. It does not have a capacitive effect as AC lines have under seawater.
  5. DC currents do not fibrillate your heart as AC currents do. It just stops it. Heart fibrillation is dangerous than a heart that has stopped beating momentarily.

Disadvantage of a DC System:

  1. DC system is not suitable for distributing power.
  2. HVDC systems currently in operation are derived from AC systems using expensive converter stations. Cost savings from reduced transmission lines (especially long-distance ones) in an HVDC system go to building the expensive converter stations.

AC Systems

Click on the image below if you fancy AC power equations.

AC Circuit
Figure 3: AC Circuit

AC Current, unlike DC current, is a timevarying quantity. This has major implications. AC currents now not only have to deal with resistances but also with opposition offered by the inductive nature of transmission lines.

The real power described in the equation (to the left) does the actual work in the power system. It’s what drives the motors, lights the bulbs, and so forth. The reactive power, on the other hand, does not do actual work. But it is essential, none-the-less. It’s primarily used to magnetize the transformers, motors, any coil product, transmission lines, etc. In other words, it facilitates the transfer of real power by addressing the need for each equipment. Still confused? Check the video below that best describes this phenomenon.

Without reactive power support to long transmission lines (from the generators, capacitor banks, etc.) there will be a significant voltage drop at the end of the lines.

Why three phase AC systems and not four, five, or six phases?

AC systems are mostly designed as three-phase systems. You can deliver more power with a three-phase system than a single or two-phase system but there is no advantage in using more than three phases. It is the break-even point. Employing more lines equates to higher infrastructure costs.

AC currents oscillate 60 times a second (in the USA). This is in the electrical domain. In the mechanical domain, this equates to 1800 rpm for a 4-pole generator. If more than one 4-pole generator is connected to the power grid then all these generators must “swing” at 1800 rpm to produce AC power at 60Hz. Failure to do so will cause the generators to trip and shutdown leading to a system blackout. More on this in another article.

Advantage of an AC System

  1. Very flexible system. It can deliver power to loads over vast distances using transformers.
  2. AC generators are sturdier and easier to build than DC generators. DC generators need brushes and commutators to generate DC current.

Disadvantage of an AC System

  1. Very hazardous. Susceptible to voltage surges.
  2. Complex system. Computer with power system analysis software has saved engineers.
  3. System stability is crucial. The system goes down if interconnected generators do not swing at the same frequency (i.e. not synchronized)

Summary

DC systems are great for moving bulk power at really high voltages. They are just not feasible for power distribution, however. AC systems provide easy means to deliver power to remote users from remote generating stations. A mix of both technologies is viable for the future power system.

Aleen Mohammed

Aleen Mohammed

Registered Professional Engineer | MSEE
Skillset: Substation Design | Power System Studies | Arc Flash Study
DISCLAIMER: ALL VIEWS EXPRESSED IN THIS ARTICLE ARE MINE ALONE.

7 Comments
Inline Feedbacks
View all comments

Nice comparison… This is exactly the reason we ended up with AC instead of DC about 100 years ago!
However, you are rather comparing a low voltage network with a high voltage network and not AC with DC. If you consider also an AC system without the transformers, you will see that it is not the DC itself that gives the increased losses… Today there are efficient ways to do the same increase in voltage with DC/DC converters that will make sure you will have lower losses in your DC system than in the AC-system!

Thanks, for a home solar PV system the latest technology gives DC output. Is it still efficient to have for example two strings of 21a DC over 200feet using #8 THHN cable? or is it better to run (post inverter) AC on one string with 42a on #4?

Great explanation-especially for someone like me who is in the industry but not an Engineer. Looking forward to reading more!

Hi,

Good, simple explanation and illustratration of difference in ac and DC systems.

Very helpful and informative even for a non-professional. Thanks a lot for the post.

Very much helpful and easily understandable thanks pe guru

Hi,
Thanks for this posting. It is really helpful.

Scroll to Top