What is a CT Clamp?
A Current Transformer (CT) Clamp is a sensor that allows the inverter to detect current passing through a cable and which direction this current is flowing. Single phase inverters will come with one CT clamp and three phase inverters will come with three CT clamps. These will be used to measure the GRID imports and Exports of your system.
Different sized inverters will have a different CT clamp rating supplied but they will look something like this:
When is a CT Clamp used?
As the CT clamp is used as an export controller if you are using just the GEN and LOAD terminals on the inverter you do not need to install a CT clamp.
When the inverter is set up as a string inverter and no export limitation is in place using a CT clamp is optional. With no CT you can set the inverter to ‘Selling-First’ this will use the PV to cover the property load and then export any excess to the grid. Without a CT you will only see the generated PV values recorded. With a CT installed and the inverter set as ‘Limit to Home’ the priority will be the same as above but property load will also be recorded by the inverter.
When the inverter is set up as a string inverter and an export limitation is in place a CT clamp must be used to control the amount of PV exported to the GRID.
When using the inverter as a hybrid with battery storage a CT clamp must be installed. This allows the property load to be recorded and the inverter to discharge any connected batteries to this load.
When using the system with a generator connected to the GRID terminals a CT clamp must be used so you are able to control the export and prevent electricity being fed back into the generator.
CT Clamp Connections
For single phase inverters the CT clamp tails should be connected to pin 3 and pin 4 in the inverter. With the white tail in pin 3 and the black tail in pin 4.
The clamp should be clipped onto the live tail coming from the property meter and the arrow should be pointing towards the inverter.
For three phase inverters the CT tails for phase one must be in pin 3 and pin 4, the CT tails for phase 2 must be in pin 5 and pin 6, and the CT tails for phase 3 must be in pin 7 and pin 8.
When installing on three phase systems the clamps should be clipped onto the live tails of their corresponding phase coming from the meter and the arrow must be facing towards the inverter on all 3 clamps.
Checking CT Clamp Direction
You can check the CT polarity is correct on the inverter screen. If you press onto the AC Load dial on the inverter home screen a sub menu will come up.
This menu will have a single CT reading for single phase inverters and 3 readings for three phase inverters. (This may sometimes be labelled as HM and not CT).
With the DC switch on the side of the inverter ‘OFF’ and ‘use timer’ disabled in the system mode settings, the CT reading on this screen should be positive. If you introduce a high load item such as a kettle this value should increase. If this value is negative there is a polarity issue with the CT, this can be resolved by changing the clamp direction.
CT Tail Extension
The cable length between the inverter and the CT clamp should ideally be no more than 10 meters, although in some circumstances this can be extended up to 15 meters. Anything longer than this and you will need to use an Eastron meter to obtain the power values instead.
How to use the single phase Eastron meter with a Sunsynk Hybrid Inverter
3 Phase Eastron Meter with 3 Phase Inverter
As the CT coil can be effected by a variety of different factors, it is impossible to say if you will be able to extend the full 15 meters or if an Eastron meter will be required on a specific site, so it is best to understand the cable run prior to installation.
To extend the CT tails the cable used must be made of copper and an equal thickness or thicker to the existing CT cables. If using CAT5/6 for example a twisted pair should be used to reduce the resistance and protect against interference.
CT Interference
When installing on a property that has other CTs being used by other devices there should be clearance form these to avoid interference. Depending on circumstances this may only need to be a small distance but a clearance of up to 50-100cm may be required.
The CT cables should also not be run closely paralleled to any AC cables and they should never be contained in the same trunking.
Interference on the CT clamp can cause no/incorrect readings and electricity may be imported/exported when it has not been set to do so.
CT Ratios
The CT ratio can be set in the inverter under ‘Advanced Settings’. Different sized inverters will be supplied with differently rated CTs, the CT ratio should be set to the correct size as default however if you need to calculate this value you will need to divide the primary current by the secondary current.
CT ratio = 100A/0.05A
CT Ratio = 2000:1
The High Voltage series of inverters have compatibility with 100A/50mA CTs or 300A/5A CTs. If using the 300A/5A CTs these must be connected on the left hand CT connections in the inverter. The CT ratio has a x100 multiplier so the ratio is calculated as follows:
CT ratio = (300A/5A)*100
CT ratio = 6000:1
The 100A/50mA CTs mut be connected in the right hand side. There is no multiplier on these connections so the standard 2000:1 CT ratio applies.
High Voltage CT connections:
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