Waveform Peak Value for Transmission Voltage

[111R]

When determining the peak value of a voltage waveform, the oscillography from relays and waveform analyzers seems to always show the L-G voltage value as the "peak" value (peak of the sine wave). For a 345 kV system, the peak instantaneous value is given as 199 kV. The software also shows the RMS value as 199 kV divided by sqrt(2).

In reality, isn't the 199 kV an RMS value? The peak of the waveform, if actually plotted, would be 199 kV * sqrt(2)? For example, a wall receptacle in the U.S. is referred to as 120 V as read on an RMS meter, but we've all been in classes where the peak of the sine wave is shown as 169.7 V.

Based on the turns ratios of instrument transformers, the 199 kV makes sense as being an RMS value. But, why does the oscillography not plot the sine wave scaled correctly?

 

[davidbeach]

For the convenience of the engineer, so you don't have to continuously multiply and divide by sqrt(2). Seems intuitive, but I've been working with it that way for years.

 

[waross]

You are assuming a sine wave. Not all of our power is a clean sign wave. Harmonics mess up the sine wave and the root 2 ratio is no longer accurate.
Also on an unloaded wye secondary, the waveform may be distorted. In the days of d'Arsonval analogue Volt-meters the ratio between line to line voltage and line to neutral voltage would be incorrectly reported as other than the expected root 3 ratio.
The meter movements responded to average voltage which was scaled to RMS by a factor known as the form-factor. Form factor is the ratio of average to RMS for a given wave form. If the waveform is not a sine wave the form factor will be different and the reading will be in error.
Transformer hysteresis badly distorts some waveforms in an unloaded wye transformer bank.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Relay waveforms are likely to have already been filtered to remove everything except the 60Hz (or 50Hz) component. Lots already gone at that point but still very useful as that's what the relay operates on. Raw and filtered can be very different.