CT Secondary Test Current Injection Methods

I have noticed in the field various ways of injecting test current in a current transformer (CT) circuit during pre-energization, commissioning checks.  The purpose of injecting test current is to prove the integrity of the entire CT circuit that is connected to a CT, once all the burden elements have been individually tested.  This article covers the methods of each.
  
Before doing any current injection, the circuit should be checked for only one ground connection on the neutral circuit.  If there is more than one ground connection on a CT circuit, the neutral could have unintentional ground current flow and cause relay schemes and metering to function improperly while the equipment is in service.  This check can be done by "ringing" out the circuit to a grounding point, near the location of the CT termination board.  Then, you would remove the designed ground connection on your neutral circuit and attempt to ring out the circuit to ground again.  If you lost the ground connection, then you know you only had one ground.  If you still have the connection, then you have another ground you need to go look for.
   
The preferred method to inject CT secondary test current is to break away the circuit leads at the CT terminal board and connect them to a test set.  Then, the open CT would be shorted down and grounded.  The designed ground connection on the neutral circuit would be removed, since a test set would provide a ground point on the common leads.  It is typical to have the test set inject .5A, 1.0A and 1.5A on the phases and then going to each of the circuit elements to check that the current magnitudes are reading correctly for each phase and the neutral.  The drawback of this method is that the technician needs to make sure that all the leads are connected back properly.
   
An alternate method, similar to the preferred method, would leave the CT circuit intact and "piggyback" the test leads onto the circuit, thus creating a parallel path for test current to flow.  This is demonstrated in the first picture.  Here, a 69/13.2kV delta-wye distribution transformer uses a SEL 551 relay to provide overcurrent protection.  The 69kV phase CT's and the transformer ground CT provide inputs to the relay.  The current in the neutral circuit for the phase CT's is calculated and called IG by the relay.  A Doble F6150 test set is used to provide test currents.  When pushing current the piggyback way, almost all of the test current will travel through the relays since the high inductive impedance of the CT causes it to act as an open circuit.  The small current that goes through the CT is excitation current and depends on the secondary voltage across the CT.  The reason why this is not a preferred method is that when doing testing, you want to isolate any unknown variables that might interfere with your test results.
   
Last, an old-school method would require you to only have a 100Watt light bulb with some leads attached to it.  A test set would not be needed.  I don't recommend this but I have come across technicians who utilize this method because of its versatility and ease of use.  I believe old-school methods are the best way to learn how tests are run.  With modernized test equipment these days, a technician could lose sight of the principles behind a test.  My Father-In-Law, a retired 40-year relay technician, shared a story with me that hits home on this.  When he was training an apprentice, they were well on their way to a job site to perform CT testing, when the apprentice frantically realized that he forgot his CT test set.  The trainer ignored this and continued to travel on.  When the apprentice asked why they weren't turning around, the trainer asked the apprentice if he had his Variac, voltmeter and ammeter on him.  Those items were obviously on hand and the apprentice learned how these individual items represent the different functions of a CT ratio/excitation/polarity test set.
   
To use the light bulb method, the neutral ground connection must be left on.  One lead from the light bulb would clip onto the hot leg connection of a 120VAC source and the other lead would piggyback onto one phase of a CT circuit.  Once this connection is made, a circuit loop is created in which AC current travels from the outlet to the light bulb, through the phase CT circuit and then to the designed neutral ground connection.  This is demonstrated in the second picture.  I added test switches in my drawing for one of the relays and these would be closed.  The 100Watt light bulb serves three purposes: 1) limit current flow 2) indicate current flow and 3) demonstrate a set current magnitude.
   
Since typical push currents are around 1A as explained above, the light bulb would limit the current to .83A (100Watts / 120V = .83A).  To indicate that current is actually travelling through the circuit, the light bulb would light up.  To demonstrate a set magnitude for the relays to read, each element in the circuit would be checked to see if it is reading .83A.  This way you know the relay is in the circuit and is metering properly.
   
Although the first method is preferred, it is always a good idea to learn all methods and understand the benefits and drawbacks of each.  This allows the technician to have a better understanding of the nature of the test, have multiple means to perform a particular task and to have a better ability to troubleshoot.
   
Thanks to Edler Power Services and Relay Protection Group on LinkedIn for shared discussion on this.
   
Article by D Scrobe III