Knee Point Voltage of Current Transformer PS Class

Current Transformer PS Class

The Knee Point Voltage of Current Transformer is the secondary voltage at which a 10% rise produces a 50% rise in exciting current. A current transformer PS class specifies that knee-point voltage and the exciting current at half of it. The related ratios are instrument security factor of CT and accuracy limit factor.

Instrument Security Factor or ISF of Current Transformer

Instrument security factor is the ratio of instrument-limit primary current to rated primary current. Instrument-limit current of a metering current transformer is the maximum primary current beyond which the current transformer core saturates. ISF of a CT is the factor used when choosing metering instruments for the CT secondary. Safety of the measuring unit is better if ISF is low. The example below shows that.

Suppose one current transformer is rated 100/1 A with ISF 1.5 and another has the same rating with ISF 2. In the first CT the metering core saturates at 1.5 × 100 or 150 A. In the second CT the core saturates at 2 × 100 or 200 A. Whatever the primary current of both CTs, secondary current will not increase further after 150 A and 200 A primary respectively. Maximum secondary current of the CTs would be 1.5 A and 2.0 A.
The first CT allows a maximum current of 1.5 A through the connected instrument, while the second CT allows up to 2 A. The first CT therefore gives better safety for the connected instruments.
ISF also matters during a heavy electrical fault: short-circuit current in the CT primary does not damage the measuring instrument attached to it, because secondary current will not rise above rated secondary current multiplied by ISF.

Accuracy Limit Factor or ALF of Current Transformer

For a protection current transformer, ALF is the ratio of accuracy-limit primary current to rated primary current.
Accuracy-limit primary current is the maximum primary current beyond which the protection CT core, or the protection core of a CT, starts to saturate. Rated accuracy-limit primary current is always many times the instrument-limit primary current. A CT transforms fault current of the electrical power system so that protection relays on the CT secondary can operate. If the CT core saturates at a low primary current, as a metering CT does, the system fault will not appear properly on the secondary, and the relays may stay inoperative even when the fault level is large.

The protection CT core is designed to have a high saturation level. It cannot have an infinitely high saturation point because such a core does not exist. If the saturation level is too high, protection relays could be damaged by large fault currents.
Rated accuracy-limit primary current should not be so low that relays never operate, and it must not be so high that it can damage the relays. So accuracy limit factor or ALF should not be near unity and at the same time it should not be as high as 100. The standard values of ALF as per IS-2705 are 5, 10, 15, 20 and 30.

Knee Point Voltage of Current Transformer

This marks the saturation level of a CT core used mainly for protection. The sinusoidal voltage of rated frequency applied to the secondary terminals of the current transformer, with the other winding open-circuited, which when increased by 10% causes the exciting current to increase 50%. The CT core is made of CRGO steel.

It has its own saturation level.
The EMF induced in the CT secondary windings is
E₂ = 4.44φfT₂
Where, f is the system frequency, φ is the maximum magnetic flux in Wb. T₂ is the number of turns of the secondary winding. The flux in the core is produced by excitation current I_e. We have a non-linear relationship between excitation current and magnetizing flux. After a certain excitation current, flux will not increase so rapidly with further excitation current. This non-linear relation curve is also called the B – H curve. From the equation above, secondary voltage of a current transformer is directly proportional to flux φ. Hence a typical curve can be drawn from this relation between secondary voltage and excitation current as shown below.
From the curve, linear relation between V and I_e is maintained from point A and K. The point ′A′ is known as ′ankle point′ and point ′K′ is known as ′Knee Point′.

In differential and restricted earth fault (REF) protection scheme, accuracy class and ALF of the CT may not ensure the reliability of the operation. Differential and REF relays should not operate when fault occurs outside the protected transformer. When a fault occurs outside the differential protection zone, the fault current flows through the CTs of both sides of the electrical power transformer. Both LV and HV CTs have magnetizing characteristics. Beyond the knee point, a slight increase in secondary emf needs a large increase in excitation current. After this knee point, excitation current of both current transformers will be extremely high, which may cause mismatch between secondary current of LV & HV current transformers. That may cause unexpected tripping of the power transformer. So the magnetizing characteristics of both LV and HV side CTs should be the same: they should have the same knee point voltage V_k as well as the same excitation current I_e at V_k/2. If knee point voltage of the current transformers and magnetizing characteristics of the CTs on both sides of the power transformer differ, there will be a mismatch in high excitation currents during a fault, unbalance between secondary current of both groups of CTs, and the transformer trips.

So for choosing a CT for differential protection of transformer, one should consider current transformer PS class rather than its conventional protection class. PS stands for protection special which is defined by knee point voltage of current transformer V_k and excitation current I_e at V_k/2.

Why CT Secondary Should Not Be Kept Open?

The electrical power system load current always flows through current transformer primary, whether the current transformer is open circuited or connected to burden at its secondary.

If the CT secondary is open-circuited, all the primary current becomes excitation current, producing a huge voltage. Each CT has a non-linear magnetizing curve, limiting open-circuit voltage by core saturation. Measuring the RMS voltage across the secondary might not show dangerous levels. Because the CT primary current is sinusoidal (50 Hz), the flux change rate at each current zero is very high, causing extreme voltage peaks. Those peaks might not be detected by a standard voltmeter but can damage the CT insulation and pose a risk to personnel. The CT secondary should never be left open.