instruments transformer (electrical measurement notes)

In this article, you will find the study notes on Instrument Transformer & Digital Volt-Meter which will cover the topics such as Instrument Transformers,Circuit Diagram of Instrument Transformer, Advantages of Instrument Transformer, Different Ratios of Instrument Transformer, Different Parameter in Instrument Transformer, Current Transformer (CT) & Potential Transformer, Analog to Digital Converter & Ramp Type Digital Voltmeter.

Instrument Transformers: Instrument transformer used for over current, under voltage earth fault and for relay protection.

It might appear that the extension of range could be conveniently done by the use of shunts for currents and multiplier for voltage measurement, as in DC.But this method is suitable only for small values of current and voltage due to the following reason

• Difficult to achieve accuracy with a shunt on AC.
  
• Capability of having shunt of large range is not possible.
  
• The power consumed by multipliers become large as the voltage increases.
  
• The measuring circuit is not isolated electrically from the power circuit.

Circuit Diagram of Instrument Transformer

Advantages of Instrument Transformer

• Their reading do not depend upon circuit constant such as R, L & C as in the case of shunts and multipliers As in the case of shunts and multipliers.
• Possible to standardize the instrument around their ratings this makes the replacement of instrument transformer very easy.
• The measuring circuit is isolated from the Power  circuit.
• Low power consumption in the metering circuit.Several instrument can be operated from a single instrument transformer.

Ratios of Instrument Transformer

Transformation Ratio (K) It is the ratio of the magnitude of the primary phasor to the secondary phasor.

For current transformer,

For potential transformer,

Nominal Ratio (NR): It is the ratio of rated primary winding current (or voltage) to the rated secondary winding current (or voltage).

Where, CT = Current transformer

PT = Potential transformer

Turns Ratio (N)

For current transformer

For potential transformer

Ratio Correction Factor (RCF)

Different Parameter in Instrument Transformer

Burden of an Instrument Transformer

It is volt-ampere loading.

Secondary winding burden

Or Secondary winding burden

= (Secondary winding current)² × (Impedance of secondary winding circuit)

Current Transformer (CT)

Transformation Ratio

Phase angle  degree 

Where, 

Potential Transformer (PT)

Actual Transformation Ratio (K)

Analog to Digital Conversion

Voltmeter is an electrical measuring instrument which is used to measure potential difference between two points. The voltage to be measured may be AC or DC. Two types of voltmeters are available for the purpose of voltage measurement i.e. analog and digital. Analog voltmeters generally contain a dial with a needle moving over it according to the measure and hence displaying the value of the same.

• These are generally known as DVM that use digital formatting of the input fed to its input leads. Here, result of measurement is shown in form of discrete numbers for which they employ display devices for decimal number system.
• Digital voltmeter also attains an inherited greater speed of operation. Because output obtained from these instruments comes to be digital form so it becomes easier to use them directly as an input to many other devices like memory devices so that the result may be use further in future, this is called storage of data. 
  
• Because of high accuracy, high speed operation and greater reliability they are frequently used in laboratories and industries for the purpose of experimentation and obtaining highly accurate results.
• It can be accepted as an disadvantage of digital voltmeters that they always need some external power supply for its operation that make it less portable and also bulkier but with the advancements made in the field of integrated circuits it has becomes possible to make such digital device very compact, more efficient, low cost and having even greater accuracy.
• This advancements has led to the verge that now some digital devices are there having less cost than their competent analogue ones with the same extent of accuracy. 
• A/D converts an analog signal into the digital code which is proportional to the magnitude of the coming signal.

Vin ≈ k ×Digital output

Where k is step size or resolution.

Quantization error or Conversion error of a A/D

Conversion time,T_c: The time requires to convert an analog signal to the corresponding digital code.

On the basis of A/D conversion method used digital voltmeters can be classified as:

• Ramp type digital voltmeter
• Integrating type voltmeter
• Potentiometric type digital voltmeters
• Successive approximation type digital voltmeter
• Continuous balance type digital voltmeter

Now-a-days digital voltmeters are also replaced by digital millimeters due to its multitasking feature i.e. it can be used for measuring current, voltage and resistance. But still there are some fields where separated digital voltmeters are being used.

Ramp-type Digital Voltmeter

Its Operating principle is based on the measurement of the time it takes for a linear ramp voltage to rise from 0 V to the level of the input voltage, or to decrease from the level of the input voltage to zero. This time interval is measured with an electronic time-interval counter, and the count is displayed as a number of digits on electronic indicating tubes.

The working principle i.e., the Conversion from a voltage to a time interval is illustrated by the waveform 

• At the start of the measurement cycle, a ramp voltage is initiated; this voltage can be positive-going or negative-going. The negative-going ramp, is continuously compared with the unknown input voltage.

• At the instant that the ramp voltage equals the unknown voltage, a coincidence circuit, or compactor, generates a pulse which opens a gate.

• The ramp voltage continues to decrease with time until it finally reaches 0 V (or ground potential) and a second compactor generates an output pulse which closes the gate.

• An oscillator generates clock pulses which are allowed to pass through the gate to a number of decade counting units (DCUs) which totalize the number of pulses passed through the gate.

• The decimal number, displayed by the indicator tubes associated with the DCUs, is a measure of the magnitude of the input voltage.

   

• The sample-rate Multi-vibrator determines the rate at which the measurement cycles are initiated.

• The oscillation of this multivibrator can usually be adjusted by a front-panel control , marked rate , from a few cycles per second to as high 1,000 or more.

• The sample-rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the same time, a reset pulse is generated which returns all the DCUs to their 0 state, removing the display momentarily from the indicator tubes.