Understanding Current Transformers
Introduction
The Current Transformer (CT), is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary. Current transformers reduce high voltage currents to a much lower value and provide a convenient way to safely monitor the actual electrical current flowing in an AC transmission line using a standard ammeter. The principal of operation of a basic current transformer is slightly different from that of an ordinary voltage transformer.
Physical Description
A “CT” is a single-winding transformer with a copper wire wound through its center opening around the entire circumference of its toroidal-shaped laminated iron core. The primary conductor is then fed through the center. (see graphic diagram and schematic symbol shown below). The two ends of this winding connects the toroid coil to a pair of wires which travel through a shielded cable and connect to a high-precision instrument (ammeter) input which displays the electrical current through the primary in amps.
Unlike a traditional voltage transformer, the primary winding of a current transformer consists of only one or very few turns. This primary winding can be of either a single flat turn, a coil of heavy duty wire wrapped around the core, or just a conductor or bus bar fed through the center hole as shown. The current transformer is often referred to as a series transformer since the primary winding is in series with the current carrying conductor supplying a load.
CT’s are available in many sizes, shapes and ratings,…they come in solid core, split core, and clamp-on style for both low-voltage and medium-voltage applications. They look especially different when it comes to the high-voltage, oil-cooled types (Live Tank & Dead Tank types shown below).
Operation
The conductor containing the current being measured is inserted through the center opening of the toroidal core and connected to its normal destination. When current flows through the single conductor or few conductors (called the primary), it generates a magnetic flux field around itself and in the toroidal core, which induces (or generates) a voltage in the CT winding (called the secondary).
Before going any further, let’s take the time to get a basic idea how a CT works. Even though a CT looks nothing like a normal transformer at first glance, it works the same way as the secondary side of a normal transformer…it’s just not as obvious. Let’s make an analogy by considering a simple single-phase transformer (shown below) with a primary winding and a secondary winding insulated from a common ferro-magnetic core. There is NO physical connection between the two windings.
When voltage is applied to the primary winding, it acts like an electro-magnet with a north and south pole and causes lines of magnetic flux to flow through the core. When that same flux flows through the core in the secondary winding, the secondary winding acts like a generator…which “generates” AC voltage proportional to the number of turns in the primary and secondary windings. If you eliminate the primary shown and continue wrapping the secondary winding completely around the entire core, it looks like a CT. The primary conductor (winding) is simply threaded (and sometimes looped multiple times) through the center of the core. Additional looping increases the output current on the secondary winding by reducing the amount of step-down magnitude by a factor of the number of loops through the core.
Primary to Secondary Current Ratio
Generally, current transformers are expressed in their primary to secondary current ratio. A 100:5 CT would mean the secondary current of 5 amperes when primary current is 100 amperes. The secondary current rating is generally 5 amperes or 1 ampere, which is compatible with standard measuring instruments.
CT’s operate on the principle of a known accurate turns ratio which converts extremely high current measurement values into low-level AC voltage signals which are proportional to the amount of flux induced in the primary conductor.
The rating of a CT secondary is typically either 5 Amps or 1 Amp. For example, with a rating of 1,000 to 5 or a turns ratio of 200 to 1, that means 1,000 Amps of current on the primary will produce 5 Amps of current in the secondary winding.
CT Frequency Response
The typical frequency response of a CT is generally between 3KHz and 5KHz. This is usually OK since most power system harmonics fall within this range. However, when higher frequency measurements in the 100’s of KHz or even MHz range are required, a Pearson CT is available (see below).
Where CT’s are Used
Solid-core CT’s are usually found in more permanent installations and are used for metering and protection in switchboards, panelboards, and switchgear. Split-core and clamp-on CT’s are generally used for more temporary application, such as power quality instrumentation.
For permanent applications of protection and metering, CT’s can be used anywhere from generators, transformers, connected loads, or anywhere we want to monitor current flowing in the system.
For example, utilities use CT’s at their customer’s incoming service to monitor the current and power usage for billing purposes. These CT’s must be extremely accurate and revenue grade since they are used for billing.
Permanent CT’s are also used to monitor power and power factor in order to optimize real power and reactive power when using a capacitor bank.
For protection, CT’s are used with trip units on low-voltage circuit breakers and on relays for medium-voltage breakers to trip the breakers when there are overloads or faults on the system. Many circuit breakers have built-in CT’s to monitor current. A single CT is required on each phase and neutral to monitor current.
For ground fault protection, a special type of CT is used. All of the phase and neutral conductors go through the ground-fault CT and if any residual current exists…in other words, current comes in on one of the phases but doesn’t return on the other phases or neutral,…then there is a ground-fault.
In residential applications, GFCI’s (Ground Fault Circuit Interrupters) are triggered at 5mA. In Industrial applications ground-fault protection devices and relays trip at 30mA or even a couple hundred amps.
Ground-fault protection is generally for personal protection in homes, and equipment protection in industrial applications.
CT Installation Characteristics
A common misconception is that the CT insulation needs to be rated for the line voltage where being used such as in 13.8kV systems. However, this is NOT the case because the CT is installed around an already insulated and typically shielded conductor. Most CT’s and their secondary winding insulation is rated at 600VAC. In medium-voltage switch gear, the CT’s are mounted around an insulating material or use physical separation to insulate the CT from the energized bus with an air-gap.
For all protection applications, CT’s must be rated to accurately measure the high currents that are possible during fault conditions…usually 20 times the full-load amps so that the breakers can trip in the proper sequence without saturating which yields an incorrect result.
CT’s come in many different ratios such as 100:5, 300:5, 5,000:5, 60:1, etc. And some have multiple taps which can be field-selected or for a specific application.
Example:
Considering a CT rated at 300:5 amps,…meaning that if 300 amps are flowing through the opening in the core, 5 amps of current will be generated in the secondary winding of the CT. Most CT’s have a 5A output, but others have a 1A output. So, when interfacing with a meter or circuit breaker, the correct multiplier must be used to convert the 5A or the 1A to the actual measured value.
Even though the current is stepped down from 300 amps to 5 amps, the voltage will step up on the secondary winding. An open circuit on the secondary winding can have dangerously high voltages of thousands of volts. When CT’s are not in use, they should always have their secondary winding shorted for safety by using a shorting block or a temporary jumper.
Temporary clamp-on CT’s have a built-in terminating resistor to protect from high-voltage spikes while opening the jaws of the core and the outputs of these instruments are often too low to accurately measure low currents. So, to increase the output of these devices, the primary wire is typically wrapped several times through the CT to increase the current through the core (shown below). For example, for 500:5 current rating (which is a 100:1 turns ratio), 5 loops through the core makes it a 100:5 current rating (which is a 20:1 turns ratio).
By checking the excitation curve for the CT you are using, you can see that the CT secondary output is fairly linear between 10% and 90% of its rated current output. The point at which the output reaches saturation and can no longer be relied upon for metering is known as the knee point (see below). Paying close attention to your CT’s excitation curve will reveal the upper and lower secondary output voltage limits and how many primary loops will offer the best performance in your application.
CT’s have a polarity dot and an arrow to indicate the correct orientation to the source and load. The arrow should always point toward the load. If the secondary output produces a negative output, the CT is likely installed backwards. CT’s can also sometimes experience a 0.3 to 6-degree phase-shift causing erroneous measurements and making it look like current is incorrectly flowing out of the load.
CT Burden
CT’s have VA capacity ratings and also limits to the number of devices and length of wire that can be connected to its secondary side. This is called the burden. Items that contribute to burden are relays, meters, and wiring. The best way to reduce burden is to keep the wiring between the CT’s and the metering devices as short as possible or increase the wire size to reduce resistance.
Due to the extremely high input resistance and low power consumption in today’s metering devices, the burden is very small. However, care must be taken not to overload CT’s, causing inaccurate measurements and poor protection.
Other Ways to Measure Current
Current Shunts - Precision -ground resistors,…Shunts can be used to measure low-frequency AC, but are mainly designed to measure DC current. Electrical noise can be an issue at higher frequencies, but usually work ok up to about 1khz. This is very simple current measurement by using Ohm’s Law. The differential voltage drop divided by the known shunt resistance is directly proportional to current. Although shunts will work on AC, they are not recommended. It can also be difficult to source an AC meter with low voltage AC inputs. The CT is normally preferred for AC current measurement applications.
Some people will also try to use CT’s for DC current measurement in large DC rectifiers by measuring the proportional AC ripple voltage. However, CT’s are sluggish to respond to transient DC voltage spikes, which makes them a poor choice in most cases. I recommend using CT’s for AC, and Shunts for DC…period.
Hall Effect Sensors - Requires an additional DC power supply to provide a constant current flow and create a magnetic field,…the sensor detects the strength of another magnetic field which is proportional to current flow. Used in UPS, Solar, and Microgrid applications for monitoring DC currents. Requires a battery or DC power source.
Rogowski Coils - Capable of measuring very fast transient currents. Low signal output requires amplification using a hall-effect sensor. Also used for high-frequency current measurements…such as in precision welders, arc furnaces, and other electronic equipment for high-frequency measurements. Also requires an additional DC power source.
How to Size a CT for an Application
CT’s should be sized for 20 times the normal full load current to allow for the high-level fault current. This is one reason why short circuit calculations are so useful. If you can calculate the fault current for an application, the CT can be more properly sized.
Safety Considerations
DANGER: All CT’s are dangerous if disconnected while energized!!!!!!!
NOTE: Never assume that just because the secondary AC signal output appears small that the CT is safe to handle while energized. Even small CT’s are hazardous if the secondary terminals are disconnected while energized.
If the primary circuit has current flowing, the secondary circuit should never be opened. This can cause very high voltages to occur due to the Ampere-Turns of the primary that start magnetizing the core. While it is acting as a transformer, it will cause very high voltage peaks.
An open-circuit condition in a current transformer (CT) can result in a dangerous over-voltage condition at the secondary terminals. A CT with an open secondary, especially those with a high ratio and carrying high currents, can produce secondary open-circuit voltage in the range of a few kilovolts.
NOTE: When CT’s are not in use, the secondary windings should always be shorted using shorting bars or jumper wires.
I hope you’ve enjoyed reading this post on Current Transformers (CT’s). I suggest reading this post more than once to grow your electrical skill set and commit the basic concepts to memory. Please feel free to visit often and share this resource with others. 😁
