What happens if a ct saturates

The core saturation is a physical phenomenon. It happens when the coupled magnetic flux is so intense that all magnetic domains on a ferromagnetic material are already aligned and thus does not respond to any further increase in the flux. The implications on the current transformer secondary current may be diverse.

Having a saturated core does not mean the current on the current transformer secondary will be high or even constant as the flux increases. Once saturated, the inductance of the circuit drops dramatically.

Taking account other phenomena like hysteresis, the resultant waveform on the secondary circuit coupled by a saturated core is highly distorted and full of harmonics. Depending on the level of saturation and the capability of the measuring device to process the distorted waveform harmonics the measured current on the secondary is much smaller than the corresponding RMS value present in the primary.

But things can be worse: A basic principle of electromechanical conversion states that the output on the secondary is related to the variation of the coupled magnetic flux. It explains why a regular current transformer and transformers can't operate on DC.

The calculated flux density is then compared to the capability of the steel used in the core of the CT and a determination is made whether the core will saturate or not for that fault current. Obtaining the cross-sectional area and maximum flux density of the CT core is not easy and this method is only applicable in certain rare situations.

This method is not discussed further. Excitation method. Excitation curve represents a curve of CT secondary rms voltage plotted against rms current with the primary open circuited.

The reasonably accurate secondary excitation curve for a given CT can be obtained by open circuiting the primary and applying AC voltage of appropriate frequency to the secondary. The current that flows in to the CT needs to be measured. The curve of applied rms terminal voltage vs rms secondary current is approximately the secondary excitation curve. Example1: For a given CT, the excitation curve is provided below.

The relay connected to the CT should operate for 60A of symmetrical primary current. Calculate the actual primary current need to trip the relay.

When 6A flows in the secondary circuit the voltage drop Vs can be calculated as. Look at the excitation graph below the excitation current I e for Thus, the relay will only operate if the primary current is A instead of the desired 60A. This can be improved by selecting a higher ratio CT. For example, by choosing a CT with internal resistance of 0.

When 3A flows in the secondary circuit the voltage drop Vs can be calculated as. Look at the excitation graph not shown the excitation current I e for Thus, the relay will operate if the primary current is The relay pickup current will be For calculations using excitation curve method it is important that the CT internal winding resistance is used in the calculation.

Example2: Excitation curve can be used to determine if a CT is adequate for a particular relay accuracy level from the excitation curve. This can be demonstrated using an example. Consider a C CT. Assume the total burden including the CT winding resistance is 0. Solution: To check whether the relay is adequate, first calculate the secondary voltage at 20 times the rated secondary current.

From the excitation curve find the excitation voltage corresponding to 10A of excitation current. From the excitation curve below, for 10A of excitation current, the secondary voltage is 90V. Since 90V is greater than the calculated 50V the CT is adequate for the relay accuracy. ANSI Standards. ANSI C relaying standard is described by two symbols- letter designation and a voltage rating.

Examples are C50, C, C etc. The letters stand for the following:. C: Transformer ratio can be calculated. T: Transformer ratio must be determined by test.

Important Note: CT saturation causes poor operation of devices. For example, metering, measurement, and protective devices can read invalid values. Incorrect CT settings can cause electrical devices to blow up due to miscoordination. Not only will you damage expensive equipment, but people can get hurt. You can use CTs for monitoring current, and protection. Each CT use case has a unique design though. Metering accuracy CTs have ratings for standard burdens, loads.

Due to their accuracy, utility companies use these CTs for customer billing purposes. For example, to detect a high fault current. To point out, metering CTs will have large errors during fault conditions. The currents can be several times greater than normal values for short durations. For example, whether to choose C, C, or C Selecting the proper CT accuracy class will ensure a CT can supply a linear output.

This is critical when a CT needs to respond to a high fault current. To choose the correct accuracy class, you need to know the fault current and burden on the CT. For example, assume the secondary of a CT connects to a protective relay. The burden then includes the relay, CT wiring, and anything else between the CT and the relay. This can be test switches and terminal blocks. This is because of the increased resistance.