Direction detection of alternating current
Every house with a PV system that draws electrical energy from the grid and can feed surplus energy back into the grid must be able to reliably distinguish between these two operating states. How does this work with alternating current? This short article explains this with the help of a simple transformer circuit that can be replicated without much effort.
Determining the direction of current flow with a current transformer coil
When my PV system was installed, a device was also installed that can detect whether the mains electricity is flowing into the house or being fed back into the grid as surplus energy. For this purpose, three hinged coils were placed on the supply lines of the three phases in the fuse box.
But how does this work with alternating current? After all, mains electricity changes direction 100 times per second at 50 Hz. Could the phase relationship between current and voltage be the solution?
To simulate this on a small scale, I built a relatively simple circuit. It consists of a transformer with two symmetrical secondary coils with the same output voltage. The coils should be separate and connectable.
Measurements taken at the low voltages of the transformer avoid the risk associated with measurements taken at 230V mains voltage.
Other components include a current transformer coil and a load resistor R1, which can be switched using switch S1. A dual-beam oscilloscope and a universal measuring device are used to measure phase angle and AC voltages. The current transformer coil also requires a terminating resistor R2.
But how does this work with alternating current? After all, mains electricity changes direction 100 times per second at 50 Hz. Could the phase relationship between current and voltage be the solution?
To simulate this on a small scale, I built a relatively simple circuit. It consists of a transformer with two symmetrical secondary coils with the same output voltage. The coils should be separate and connectable.
Measurements taken at the low voltages of the transformer avoid the risk associated with measurements taken at 230V mains voltage.
Other components include a current transformer coil and a load resistor R1, which can be switched using switch S1. A dual-beam oscilloscope and a universal measuring device are used to measure phase angle and AC voltages. The current transformer coil also requires a terminating resistor R2.
Circuit
Figure 1: Test circuit
Measurements
Figure 1: Test circuit
Measurements
An oscilloscope is used to measure the phase positions of the voltage and current signals at test points TP1 and TP2 against TP3 (ground).
The voltage of the interconnected transformer windings Sec. 1 and 2 is applied to TP1 as a reference signal. It is displayed on the red channel of the oscilloscope.
Figure 2
Figure 3
Figure 4
Results
The voltage of the interconnected transformer windings Sec. 1 and 2 is applied to TP1 as a reference signal. It is displayed on the red channel of the oscilloscope.
Figure 2
Figure 3
Figure 4
Results
The direction of the alternating current can therefore be reliably determined by comparing the phase of the mains voltage and the output voltage of an inductive current transformer.
It is important to align the current transformer correctly when mounting it on the wire that passes through the ring opening. Current transformers used in a measuring system to measure surplus energy from a PV system therefore have an arrow printed on them that points in the direction of the ‘household grid’.
To measure the current flowing through the current transformer, an analogue amplifier is first connected downstream, which can be calibrated according to the expected current. Further processing is usually digital, as is the detection of the phase relationship.
It is important to align the current transformer correctly when mounting it on the wire that passes through the ring opening. Current transformers used in a measuring system to measure surplus energy from a PV system therefore have an arrow printed on them that points in the direction of the ‘household grid’.
To measure the current flowing through the current transformer, an analogue amplifier is first connected downstream, which can be calibrated according to the expected current. Further processing is usually digital, as is the detection of the phase relationship.
The structure and function of an inductive current transformer are described in detail on Wikipedia under the keyword ‘current transformer’.
Figure 5
Figure 5
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