Instrument and Measurement Transformer

In dc circuit when large current are to be measured its normally use low range ammeter with suitable (low resistance) shunt connected in parallel. when large voltage are to be measured use low range voltmeter with high resistance  connected in series. But this method is not convenient for practical use for AC voltage and current instruments.

For this purpose specially constructed with high accuracy Instrument Transformer or Measurement Transformer are used.  Their purpose is to reduce line current or voltage to a value small to be easily measured with meters of moderate capacity.

Instrument Transformer are two in types:

1. CURRENT TRANSFORMER (CT)-    For measuring large alternating current (AC)
2. POTENTIAL TRANSFORMER OR VOLTAGE TRANSFORMER (PT/VT) -   For measuring large Voltage (AC)

CIRCUIT DIAGRAM OF CT-PT:

ADVANTAGE OF INSTRUMENT TRANSFORMER:

(1)  The cost of the instrument or meter together with transformer is less than the instrument alone if it installed for high voltage.

(2)  The instrument is installed from the line voltage, hence it can be grounded.

(3)  If necessary several instrument can be operated from a single transformer for that power consumption is to be low and cheap in cost.

(4) For extension on A.C circuit, instrument transformer are more desirable than shunt for current and multiplier for voltage measurements.

they are as follows:

(a) Range extension is limited by the current carrying capacity of the shunt. that is upto few hundred amperes at all.

(b) If current is high voltage, instrument insulation is a big problem but in case instrument transformer its solved out.

(c) Use of multipliers above 1000 become a impractical.

(d) Time constant of the shunt closely match the time constant of the instrument. hence, a different shunt is need for each instrument, but in this case  If necessary several instrument can be operated from a single transformer for that power consumption is to be low and cheap in cost.

SHOULD KNOW BEFORE USE INSTRUMENT TRANSFORMER:

In using Measurement or Instrument Transformer  for Current and Voltage measurement, firstly we have to know the ration of primary and secondary voltage or current (Transformer Ratio). this ratio gives us multiplying factor for primary values from the instrument reading on the secondary side.

For energy power measurement  we also have to know PHASE ANGLE between primary and secondary current or voltage. its necessary for correction to the meter reading.