While voltage measurements up to 1kV are really simple, things are getting more complicated with measuring voltages over 1600 VDC because voltage probes/dividers or voltage transducers are necessary which adjust and reduce the voltage to a level which is suitable for the amplifie.
Please be especially careful using voltage probes or voltage transducers. There are several things which have to be taken into consideration when using voltage probes or voltage transducers.
Voltages Probes (Voltage Dividers)
There are two different types of voltage probes: The pure resistor voltage probe (for AC and DC measurement) and the resistor-capacitive voltage probe (only for AC measurement). The input impedance of the voltage probe should be as high as possible, therefore the input resistance should be as high as possible and the input capacitance as low as possible.
There are active, passive and differential voltage probes available.
Passive voltage probes are simple, cheap and robust but have a high input capacitance and problems measuring low voltages.
Active voltage probes have a high input resistance and low input capacitance but need an external power supply. They are a lot more sensitive and more expensive than passive ones.
Differential Voltage Probes: Passive and active voltage probes are single-ended amplifiers with reference to earth. If you want to measure differential signals you have to choose a differential voltage probe.
Voltage probes have no galvanic isolation. If the GND connection is interrupted the full voltage potential is on the measurement device and can destroy the device and can be hazardous for people around.
The function of the voltage probe can be easily explained using a simple resistor voltage probe with the serial connection of two resistors with high resistance.
If we use the voltage divider with the same resistance value, the voltage drop on one resistor will be half of the connected voltage. So if we want to measure a high voltage in the range up to 2000V, this simple transducer reduces the voltage to 1000V. This is low enough to measure it using a Sirius high voltage module with an input range up to 1200V.
If we have resistors with too low impedance, compared to a circuit in which we are measuring voltage, we will cause a substantial current. This current affects the measured circuit and reduces the accuracy of the measurement. We also must ensure that the input impedance of the measurement device is 100 to 1000 times higher than the value of R1, otherwise the ratio will change and the impedance of the measurement device also has to be considered. We must also bear in mind that R1+R2 is the short-circuit resistance – if this sum is too low it will cause a short circuit.
If you use a voltage probe, the ratio between the input and the output voltage has to be calculated and adapted to in the channel setup of the software. This ratio is important because the measured voltage of the measurement device has to be multiplied by this ratio to become the real voltage (Vin in the picture).
Voltage transducers are mainly used to monitor the voltage on the public grid. A voltage transducer is easily explained as a transformer in a no-load operation. On the input (primary) side, a high voltage signal is connected. On the output side of the voltage transducer, we get a low voltage signal which is directly proportional to the input voltage. In public grid operation the secondary voltage is standardized with a level of 100V respective 100V/sqrt(3). The level of 100 V/sqrt(3) is used in unipolar isolated voltage transducers in star connection. The level of 100V is used in bipolar isolated transducers (line-line voltage).
There are different measurement classes of voltage transducers which describe the accuracy and phase shift of the transducers. The classes span from 0.1 to 3. Class 0,1 means that the accuracy of the measured amplitude is 0,1% and the phase shift is ± 5 minutes. At class 3 the accuracy is 3% and the phase shift ± 120 minutes.
Attention: Never operate voltage transducers with a short-circuit on the secondary side. This creates high currents which will destroy the transducer.
Tip: Voltage dividers and transducers always have a frequency dependent behaviour concerning amplitude and phase. Using the Dewesoft sensor editor allows to correct this behaviour and increase the accuracy of measurement. See details in the "Current Pro Training" in the chapter "Sensor Editor".