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.

Classification of Transformer

TYPE OF TRANSFORMER:

Constructionally Transformer are generally two types, distinguished from each other merely by manner in which primary and secondary coils are placed around the laminated core.

1.   CORE TYPE

2.   SHELL TYPE

Recent development is spiral-core or wound core type.

Another mean of classifying the Transformer according to the type of cooling employed are 3 types:

1.   oil-filled self cooled

2.   oil-filled water cooled

3.   air-blast type

The Transformer are classified as the voltage ratios from primary to secondary are two types. These are widely used transformer types for all the applications.

1.   STEP-UP

2.   STEP-DOWN

ONE ANOTHER TYPE OF TRANSFORMER PRESENCE THAT IS- ISOLATION

Based on usage Transformer are many types as follows:

1.   POWER TRANSFORMER

2.   DISTRIBUTION TRANSFORMER

3.   MEASUREMENT / INSTUMENT TRANSFORMER

4.   PROTECTION TRANSFORMER

Based on Rated voltage and current Transformer are two types:

1.   CURRENT TRANSFORMER

2.   VOLTAGE TRANSFORMER (POTENTIAL TRANSFORMER)

Based on Place of Use Transformer are two types:

1.   INDOOR TRANSFORMER

2.   OUTDOOR TRANSFORMER

Based on Phase Transformer are two types;

1.   Single Phase (1 Phase)

2.   Poly Phase (3 Phase) 

3 Phase transformer are 2 in types- a) Delta Type - Δ            b) Star Type- Y

ONE ANOTHER TYPE OFTRANSFORMER WIDELY USED THAT IS AUTO TRANSFORMER- IT IS A SINGLE WINDING TRANSFORMER

Autotransformer

WHAT IS AUTO TRANSFORMER?

It is an Transformer with one winding only, part of this being common to both primary and secondary.

In This Transformer Primary and Secondary coil are not electrically isolated from each other as in case of two winding Transformer. But its theory and operation are similar to those transformers.

Benefit of Auto Transformer: 

As this Transformer has only one winding for that less copper required and its cheaper than others two winding transformer.

Light in weight and smaller in size.

Voltage regulation is also superior.

Power Transformer

Power Transformer are used for stepping-up the voltage for transmission at generating stations and for stepping-down for further distribution at substation. Usually naturally cooled, oil immersed, On type, two winding. three phase transformer are used up to the rating of 10 MVA. The transformers of rating higher than 10 MVA are usally Air-blast cooled.  For very high rating transformer the forced oil, water cooling or air blast cooling may used.

For regulating the voltage the transformers used are provided with on load tap changer. They are put in operation during load hours and disconnect during light load hour, that is they are usually operated at approximately full load condition. This is possible because they are arranged in bank and parallel with other  unit or disconnected at will.

Design: Power Transformer

So power transformer are designed to have maximum efficiency at or near full load condition. that is iron loss to full load copper loss ration is 1:1.

Power Transformer are designed to have consider leakage reactance than permissible in distribution transformers because in power transformers inherent voltage regulation is not as much important as current limiting effect of the higher leakage reactance.

Power Transformer usually make use of flux density of 1.5 to 1.77, have percentage impedance ranging from 6-18% and regulation 6-10%.

SPECIFICATION OF TRANSFORMER :

1. KVA rating
2. Rated voltage
3. Number of phase (single phase or three phase)
4. Type of Core ( shell type or core type )
5. Tapping - if any
6. Frequency (in INDIA 50hz, USA- 60hz)
7. Construction ( Delta type or Star Type in case of three phase)
8. Type of cooling ( air cooling, oil cooling)
9. Ambient Temperature ( Average 40° C)
10. Voltage Regulation
11. No-load Current in ampere or percentage of rated current at at rated voltage and rated frequency.
12.  Efficiency in percent at full load, half load, 3/4 load at unity or 0.8 pf (power factor).

INSTALLATION:

Power Transformer are generally installed upon lengths of rail fixed on concrete slab having foundation 1 to 1½ meter deep.