Measurement of mains voltage fluctuation and frequency over a week

Introduction

The purpose of this experiment is to monitor mains voltage and frequency fluctuations over a seven day period. This information should show the effects of loading of the National Grid.

The electricity generating companies have tolerances which they are required to meet.

The single phase mains tolerances in Europe are:

• Voltage 230 volts AC RMS +10% / - 6%
• Mains frequency 50.00 Hz +/- 0.2 Hz

Equipment required

• PC with PicoLog data logging software installed
• ADC-42 oscilloscope
• Mains AC adaptor
• Potential divider

Equipment setup

The ADC-42 reads voltages in the range of ±5 volts. The mains voltage has to be dropped to ±4 volts (leaves room for the voltage fluctuations).

One of the simplest and safest ways to measure mains voltages is to use a simple ‘plug in the wall’ power block to drop the mains voltage to a safer level. This has the advantage that it provides electrical isolation from the dangerous mains voltage.

Figure 1: equipment used

There are two types of basic power block available:

AC output types

These are most easily used since they simply give a scaled version of the mains voltage. They usually give an output in the range 6 Vac to 12 V AC, and therefore a simple potential divider circuit may still be required to attenuate the output to a suitable range (see below).

DC output types

These are similar to the AC types above but they also have a built in rectifier to convert the AC voltage to DC and also usually a capacitor to provide some smoothing of the DC signal - some types may also have some kind of voltage regulator. To use this type, you need to remove the rectifier and capacitor (and regulator if fitted) — this should only be attempted by a qualified electrician.

Warning: if you are in any doubt then seek advice from a qualified electrician.

The output from the power block may need attenuating further to suit the ADC-42 input range. This is best done with a simple resistor voltage divider as shown with the formula below.

Figure 2: circuit diagram

Potential divider calculation

Vo = Vs x Rb / (Ra+Rb)
Vo / Vs = Rb / Ra+Rb
Vs / Vo = Ra+Rb / Rb
(Vs / Vo)Rb = Ra+Rb
Ra = (Vs / Vo) Rb - Rb

Potential divider calculation example

Vs = 20 volts peak
Vo = 4 volts peak
So Ra = 5Rb - Rb
Therefore Ra = 4Rb

With the equation above and Rb = 10l, Ra = 40k. A value of 40k is not a standard resistor value, so a 30k resistor and a 10k resistor in series can be used.

Using these power blocks with PicoLog and PicoScope:

The PicoScope spectrum analyzer can be used to investigate the harmonic content of the mains supply.

PicoLog can be used to trend the longer term changes in supply voltage and also supply.

Carrying out the experiment

The experiment was carried out over one week using the equipment above and PicoLog software. Samples were taken every 30 seconds.

Questions

1. What happened to the mains voltage?
2. What happened the the mains frequency?
3. Did the mains stay within the tolerances specified for the electricity company?