Sunday, June 15, 2014

Instrument transformers working principle; Current Transformer ; Potential Transformers

Instrument transformers means for stepping down voltages and currents of the system for metering and protection function voltage transformer & current transformer are used. Really meters and relays used for metering and protection, aren't designed for voltages and high currents.

Voltages or high currents of electrical power system cannot be right fed to meters and relays. CT measures down rated system current or 5 Amp likewise voltage transformer measures down system voltages Meters and the relays are typically designed for 5 Amp, 1 Amp and 110 V.

Definition (CT)

A CT is an instrument transformer where the secondary current is considerably proportional and differs by degree that is ideally zero in phase.

CT Current Transformer Category or Precision Class

A CT is not dissimilar to an electrical power transformer somewhat, but there are a lot of difference in operation and building principle. For sign and metering function, correctness between secondary and primary currents are not dispensable within standard working range. Usually precision of current transformer needed of rated current up to 125%; as because system that was allowable current must be rated current. So within operating range truth is primary standards of a CT used for metering function. Truth of a metering CT's level is conveyed by CT precision category or just current transformer type or CT category.
current transformer

But of the same quality, the CT may not possess the precision amount in the event of protection as principal is passed through by metering CT although it's wanted to not be saturated during high fault current. So core is designed that it could not be saturated for long range. If saturation of the center comes at lower degree of primary current secondary will not be come to by the appropriate expression of primary current, thus relays linked to the secondary may not work correctly and protection system declines its dependability.

Imagine one CT with current ratio 400/1 A's protection center is situated at and you've it 500 A.

Truth of a protection CT's level might not be as good as metering CT but it's also expressed by CT precision class or just current transformer type or CT group as in the instance of of metering current transformer but in small bit distinct way.
Theory of CT or Current Transformer

If other general purpose transformer or an electrical power transformer, primary current changes with secondary current or load.

Have you been mistaken? OK let you clear.

The electricity transformer's primary takes current in the source proportional to the load linked with secondary. But with power line, the primary is connected in case of CT. Current through its primary is just the current flows. The CT's primary current, thus will not depend upon what's the impedance value of weight or not or whether the load or weight is connected to the secondary. Say Ip is the current and Np is amount of turns. Thus the primary AT is not unlike to NpIp AT.

To secondary AT the primary AT is just is equivalent in a perfect CT.

So in the preceding statement it's clear that if a CT has 400 A current if it has one turn and 400 turns in secondary winding afterward it'll have 1 An in weight that is secondary.

So the turn ratio is 400/1 A

Error in CT or Current Transformer

current transformer
Is - Secondary current.
Es - Secondary voltage
Ip - Primary current.
Ep -Primary voltage
KT - Turns ratio = Amounts of secondary turns/amount of primary turns.
I0 - Excitation current.
Im - Magnetizing component.
Iw - Core loss part of I0.
phim - Primary flux.

Let's take flux as benchmark. The magnitude are proportional to primary and secondary turns. The excitation present Io that's composed of two parts and Iw.

The complete current flows is subsequently vector sum of I0 and KT Is.

The Present Blunder or Ratio Blunder in CT or Current Transformer

The error in current transformer added as a result of this difference is called some times ratio error or present error.

Phase Phase Angle or Mistake Mistake in Current Transformer

For a perfect CT turned secondary current vector and the angle between the main is zero. But for a real CT there's constantly a difference in phase because of how primary current must provide the part of the current that is departing. The angle between the preceding two phases in termed in CT or current transformer.
In the pharos diagram it's beta
Cause of Error

The complete primary current isn't really transformed in CT. One part is consumed for center excitation and remaining is really transformers with turns ratio of CT there's error means there are a phase angle error along with both ratio error.

The best way to Reduce Mistake in Current Transformer

It's desirable to reduce these mistakes, for functionality that is better. For attaining minimal error the following can be followed by one,

Using a center of low hysteresis decline magnetic substances and high permeability.
Ensuring minimal amount of flux course and raising cross sectional area of the center, minimizing joint of the center.