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Here is some information I have discovered on CTs that might be of interest
to PV folks.
There are several good references on the web that can be consulted for more details.
Several interesting and important features of a true current transform.
The secondary winding looks like a high impedance when no primary current is flowing.
All CT devices that need to monitor a primary current are all wired with the primary sides
in series so each sees the same current flow. Likewise, all measurement circuits
(Sensing or A/D converter) on the secondary side need to be wired in series so
all see the same current.
If the secondary of a true,current output, CT, is left open when there is primary current
very high voltages can result as the secondary tries to push the correct current thru the open
wires.
That means if the output of true current output CT is not used it needs to
be shorted.
To avoid these problems many CTs have internal terminating resistors so they
are voltage output rather that current output devices. They can be shorted
or left open when not in use.
The way these two styles are used is very different.
Loads on a true, current output, CT should be very low resistances and
should be wired in series.
Loads on a voltage, self terminated, CT should have high a resistance as
practical and be wired in parallel so they don't load the resistor in the CT.
Notice that the typical industrial or electric system CT usually has a
5A secondary and will typically only put out a few VA or Watts.
Interesting things happen when you try to parallel CTs.
If you parallel true Current Output CTs the secondary current is
the vector sum of the two currents.
If you parallel voltage CTs the Volts/Amp rating drops due to the
parallel combination of the two terminating resistors in the CTs.
To protect Current CTs some have internal, or require external
"CT Savers" These are typically diodes that conduct and limit
the secondary voltage to a safe level.
Consider a typical 5A 5VA CT secondary. IF the device is not used
for overload sensing and breaker tripping the most the voltage should
be at full load is 1V. So, a full wave diode bridge with the
AC terminals in parallel with the CT output and one additional diode
with Anode on bridge plus and the Cathode on bridge minus effectively
puts 3 diode drops in series in each direction across the output
and limits the voltage to about 2V which should be safe for the CT.
NOTE: This is not a solution for a CT in protection circuit. That secondary current can
be many times nameplate during a fault so the circuit protective
devices can pick up and clear the circuit.
Enphase CT-200
Some basic measurements on the Enphase CT-200 split CTs appears to
reveal the following.
They are 2400:1 CTs and the Envoy inputs appear to be about 2.4ohms.
That makes the Enovy about 1mV per primary Amp.
The seem to parallel like true current CTs.
In testing, paralleling
the outputs of two devices while leaving one device with no current
resulted in no change in output as measured from just the active CT.
In phase currents in the two devices added as would be expected for a current output CT
i.e. 50A in a single CT produced about 21mA in the secondary.
Paralleling a second CT with no primary current did not change the reading.
50A in each of the two parallelled CTs (currents inphase) resulted in about 42mA.
CT Testing:
A common way to test is a variac and supplying power to the 120V side of
a 120 to 6 or 12V transformer capable of 5 to 10A. Connect the secondary
thru the primary (window) of the CT. and thru a reference Ammeter
and back to the other secondary winding. Basically you have shorted the
secondary. Turn up the Variac slowly until you the reference current
you desire. Adding a small, high power, resistor in the series string
makes the adjustment of the Variac less touchy.
If you need more current for testing a window style primary just wrap
turns thru the window. 20 turns thru the window turns 5A reference
current into 100A to the device.
Good luck, and be careful!
to PV folks.
There are several good references on the web that can be consulted for more details.
Several interesting and important features of a true current transform.
The secondary winding looks like a high impedance when no primary current is flowing.
All CT devices that need to monitor a primary current are all wired with the primary sides
in series so each sees the same current flow. Likewise, all measurement circuits
(Sensing or A/D converter) on the secondary side need to be wired in series so
all see the same current.
If the secondary of a true,current output, CT, is left open when there is primary current
very high voltages can result as the secondary tries to push the correct current thru the open
wires.
That means if the output of true current output CT is not used it needs to
be shorted.
To avoid these problems many CTs have internal terminating resistors so they
are voltage output rather that current output devices. They can be shorted
or left open when not in use.
The way these two styles are used is very different.
Loads on a true, current output, CT should be very low resistances and
should be wired in series.
Loads on a voltage, self terminated, CT should have high a resistance as
practical and be wired in parallel so they don't load the resistor in the CT.
Notice that the typical industrial or electric system CT usually has a
5A secondary and will typically only put out a few VA or Watts.
Interesting things happen when you try to parallel CTs.
If you parallel true Current Output CTs the secondary current is
the vector sum of the two currents.
If you parallel voltage CTs the Volts/Amp rating drops due to the
parallel combination of the two terminating resistors in the CTs.
To protect Current CTs some have internal, or require external
"CT Savers" These are typically diodes that conduct and limit
the secondary voltage to a safe level.
Consider a typical 5A 5VA CT secondary. IF the device is not used
for overload sensing and breaker tripping the most the voltage should
be at full load is 1V. So, a full wave diode bridge with the
AC terminals in parallel with the CT output and one additional diode
with Anode on bridge plus and the Cathode on bridge minus effectively
puts 3 diode drops in series in each direction across the output
and limits the voltage to about 2V which should be safe for the CT.
NOTE: This is not a solution for a CT in protection circuit. That secondary current can
be many times nameplate during a fault so the circuit protective
devices can pick up and clear the circuit.
Enphase CT-200
Some basic measurements on the Enphase CT-200 split CTs appears to
reveal the following.
They are 2400:1 CTs and the Envoy inputs appear to be about 2.4ohms.
That makes the Enovy about 1mV per primary Amp.
The seem to parallel like true current CTs.
In testing, paralleling
the outputs of two devices while leaving one device with no current
resulted in no change in output as measured from just the active CT.
In phase currents in the two devices added as would be expected for a current output CT
i.e. 50A in a single CT produced about 21mA in the secondary.
Paralleling a second CT with no primary current did not change the reading.
50A in each of the two parallelled CTs (currents inphase) resulted in about 42mA.
CT Testing:
A common way to test is a variac and supplying power to the 120V side of
a 120 to 6 or 12V transformer capable of 5 to 10A. Connect the secondary
thru the primary (window) of the CT. and thru a reference Ammeter
and back to the other secondary winding. Basically you have shorted the
secondary. Turn up the Variac slowly until you the reference current
you desire. Adding a small, high power, resistor in the series string
makes the adjustment of the Variac less touchy.
If you need more current for testing a window style primary just wrap
turns thru the window. 20 turns thru the window turns 5A reference
current into 100A to the device.
Good luck, and be careful!