Why Does AC Voltage Require More Insulation than DC Voltage for the Same Working Voltage Level?
For the same working voltage, the potential stress on the insulation is lower in a DC system than in an AC system. Therefore, a DC line requires less insulation.
In other words, a DC system has less potential stress than an AC system for the same voltage level and power rating. This is why an AC system needs more insulation than a DC system.
The typical DC test voltage is approximately 2 to 3 times the AC breakdown voltage. Such high DC voltages are necessary to detect faults within the XLPE insulation. The DC breakdown voltage is significantly higher than the AC breakdown voltage. Therefore, DC voltage tests may be ineffective in detecting serious defects in the cable.
Let’s explain in detail.
When we talk about DC, let’s say 230 Volts DC, it means the maximum value (peak value) of the voltage is 230V. However, when we talk about 230 Volts AC (as in our home distribution system for single-phase AC supply, which is 230 volts AC in many countries following IEC or 120 or 240 volts AC in the US), it refers to the RMS (Root Mean Square) value. In this case, the RMS value of 230 volts AC is √2 times the peak value, which is approximately 230 volts AC.
The peak value of AC voltage is equal to √2 × VRMS = 1.414 × VRMS (where √2 = 1.414)
In other words. The peak value of AC voltage = VRMS ÷ 0.707
Now, in our case, the peak value of 230V AC = 325V AC peak (calculated as 230V ÷ 0.707 or 230V × 1.414, which equals 325V AC peak.
This means that 230V DC is equivalent to 325V AC peak. This is why AC voltage needs more insulation than DC voltage for the same rating of voltage and power.
Additionally, 230V AC can reach its peak value of 325V peak. This means that 230V AC RMS reaches 325V peak and cycles back to -325V peak in one hertz (Hz) of frequency, where frequency is measured in cycles per second.
That’s why AC needs more insulation than DC.
Click image to enlarge
As shown in the above diagram, the RMS value of AC voltage is below its peak value, and we have to provide insulation for the peak value or maximum voltage, not for the RMS value. In DC systems, the RMS value and peak values of voltages are equal.
Now, you have a clear understanding of the need for more insulation in an AC system compared to a DC system for the same working voltage and power rating.
Moreover, there are additional factors in AC voltage systems, such as the Corona effect, Skin effect, and Ferranti effect which we will discuss in detail in the next posts.
FAQ
Why is AC preferred over DC voltage?
AC power can easily be step-up or step down with the help of a transformer which is not possible in the case of DC power. Electricity that is coming to our homes is AC because it comes from a far away power generation plant so the loss of energy during transmission is very less in the case of AC power than DC power
How does voltage affect insulation?
One of the primary concerns when dealing with high voltage and wire insulation is the risk of dielectric breakdown. This phenomenon occurs when the insulating material can no longer withstand the applied electric field, leading to a sudden and catastrophic increase in conductivity
Why do we use AC more than DC?
Alternating current is easier to generate and transmitting alternating current leads to lower energy losses than direct current over distances more than a few metres
Why are AC voltages generally higher than DC voltages?
This is because generating and transporting AC across long distances is relatively easy. At high voltages (over 110kV), less energy is lost in electrical power transmission. Higher voltages mean lower currents, and lower currents mean less heat generated in the power line due to resistance
Why is high voltage DC better than AC?
Unlike AC, there is no technical limit on the length of cable or overhead line that can be used in HVDC connections, so HVDC has advantages for long transmission distances. On the DC side of the converter, a reactor is provided to smooth the DC current
Why AC needs more insulation than DC at same voltage level?
The peak value of AC voltage is equal to √2 x VRMS = 1.414 x V. Let us consider 220 V Dc voltage which may be equivalent to an AC voltage peak of 311 AC peak. AC systems are subject to corona loss skin effect which is absent in the DC systems.so AC Systems need more insulation than DC
Why do high voltage power lines need to be insulated?
All wires carrying dangerous levels of voltage are a serious hazard, and people need to be protected from them. One way of protecting against the hazards of high-voltage wires is to envelop them with an insulating covering that is rated for the wire’s voltage
Why is insulation important in high voltage?
Aside from resisting current flow from one body to another, insulators also provide physical support to electrical conductors, preventing them from sagging, falling or damaging. These insulators prevent thermal stress on the system especially when carrying high current producing heat
Why is AC so much better than DC?
AC’s primary benefit over DC is that it is easily modifiable by a transformer from extremely high voltages — transmitted through the utility grid from power plants over power lines — to low voltages for safe use. Get the savings of solar with the convenience of gas
