Current Transformer: A deeper dive


Installation and polarity of a current transformer.

Since current transformers are used on AC circuits and the direction of the current in an AC system is constantly changing, the polarity of the transformer terminals also changes on each cycle. One might easily think that current transformers like most AC devices have no polarity.

If you follow this assumption, you would be wrong because transformers have polarity. The Polarity of a CT is very important when installing them for relay protection and metering. The polarity of a CT is determined, by the direction in which the coils are wound, around the CT Core, and how the secondary leads are brought out of the transformer case.

However, whenever we talk about the polarity of a transformer, we are referring to the instantaneous polarity. In essence, the polarity markings on the current transformer denote the instantaneous polarity of the transformer.

The easiest way to understand this is by using the dot convention. From the diagram below, we can simply understand it as.

At any instant, if current flows into the dot on the primary side of the CT, then current must flow out of the dot on the secondary side of the CT.

For a given direction of primary current entering the CT, there should be a known direction of the secondary current leaving the CT same is true for the numbering notation. At any instant if current flows into H1 on the primary side of the CT, then current must flow out of X1 on the secondary side of the CT.


 

There are two types of polarity namely  

Additive Polarity: 
Here, the primary and secondary side voltage is in phase, therefore the primary current will also be in phase with the secondary current meaning that they will have the same direction of the flow.


Subtractive Polarity:

Here, the primary and secondary side voltages are 180 ° out of phase, therefore the primary and secondary side current will also be 180 ° out of phase and they will flow opposite to each other.


 

 All current transformers are subtractive polarity. Finally, window type CTs are always installed with H2 facing the load. The conductor is passed through the window from H1 to H2.

Interpreting Current Transformer Nameplate Data

The nameplate of a current transformer contains the following details as per standard, then additional information could be provided varying from manufacturer to manufacturer. The following details like manufacturers name, frequency, and CT weight are pretty standard and need no explanation. Most transformers also have an information that specifies the standard as per which the transformer was manufactured it could be IEC 618569-2 or ANSI/IEEE. So now that we have gotten that out of the way, let us begin.

CT Voltage Level - This shows the rated operating voltage of the transformer or the maximum voltage that the CT is expected to come in contact with.

CT Burden – Rated in VA, refers to any load connected across the secondary of the CT, this includes the cables, relay coil, and meter. For example, a CT with a rating of 5VA would mean that 5VA is the maximum apparent Power that the CT can deliver, and loads connected to the CT should not demand more than 5VA because it would cause a metering error.

CT Ratio – The CT ratio is the ratio of primary current input to secondary current output at full load. For example, a CT with a ratio of 300:5 is rated for 300 primary amps at full load, and it would produce 5 amps when 300 amps flows through the primary.

Accuracy Class – No transformer is ideal there are electrical losses that occur as current flows through the transformer windings and magnetizes the core. The losses on a transformer vary based on different conditions like secondary loading, power factor, etc. In these situations, the secondary current that is produced is not truly a scaled-down replica of the current flowing through the primary.

The extent to which the secondary current expected from the CT by virtue of the CT ratio differs from the actual current delivered by the CT is shown by the accuracy class of the CT. The ratio error is generally lower than 3%

There are 2 types of accuracy class in a Current transformer. One is protection accuracy and the other is metering accuracy. A CT can have ratings for both groups. The accuracy class could be defined according to ANSI/IEEE or IEC depending on the manufacturer of the CT.

In this article, we are showing Standard accuracy class rating according to IEC.

Protection Accuracy Class –

-      PX/PS          usually used for differential protection

-      5P & 10P usually used for overcurrent protection, 5P stands for 5% of composite error allowed, 10P stands for the same thing.

Metering Accuracy Class –   are designed to be highly accurate from very low current to the maximum current rating these numbers below typically represent the % of composite error at rated burden.

-      0.1

-      0.2

-      0.2s

-      0.5

-      0.5s

-      1

Instrument Safety Factor (ISF) – It is applicable for metering class CT. In some cases, due to short circuit, the primary current flowing can be 5 to 10 times higher and the current that flows in the secondary could damage the meter. It could be rated ISF <5 meaning that if the rated primary current goes up to 5 times, the meter is still safe. However, if the primary current rises beyond this point, 5 times for our example then the current transformer core becomes saturated. 

So, suppose a current transformer has a rating of 100:5 and an ISF of 3 it means that the CT core would be saturated at 300A. In essence no matter how much the primary current rises above 300A during this short circuit, the secondary the current would not increase further and in this case, the maximum value if secondary current the meter would receive during the short is 15A.

Accuracy Limit Factor (ALF) – Applicable for protection class CT, this is the highest value of primary current to which the composite error remains within the limit. It could be something like 5P10 which means that the percentage of composite error will be limited to 5% even if the primary current rises up to 10 times the rated current. For Current Transformers used to operate protective relays, the CT is expected to transform the fault current on the primary to a high level of current on the secondary for proper operation of the relays in a fault condition. So, suppose a current transformer has a rating of 100:5 and an ALF of 5P10 it means that the CT will continue to transform the fault current even when it rises to 10 times above the rated current with a composite error not greater than 5%. 

So, in this case, the maximum current that can be reflected on the secondary side during fault conditions is 50A. Composite Error is the difference between the ideal secondary current and the actual secondary current and it is made up if the ratio error and the phase displacement error

Knee Point Voltage (Vk) – If a voltage is applied across the secondary terminals the magnetizing current will increase in proportion to the applied voltage, until the knee point voltage is reached. The voltage level at which a 10% increase in voltage applied across the secondary of the CT in which causes 50% increase in secondary exciting current is the knee point voltage. Above knee point, the CT gets saturated.

Rated short-time current (STC) - When system short circuits flow in the circuit, the CT must be able to withstand the short circuit for a particular amount of time. This time ranges from 0.25seconds to 3 seconds. STC is expressed in kA, this is the maximum current (Ith) that the CT can withstand.

Exciting Current (Iexe) – The current level at which a ratio error is introduced in the CT.


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