Measurement of Electrical Parameters in Steam Turbine

As the generator is coupled with a steam turbine, we shall discuss the measurement of some Electrical parameters such as Voltage, Current, Power, Frequency, and Energy are the major parameters measured.


  • The other names for voltage are Potential Difference, Electromotive Force (EMF), and pressure of an electrical circuit.
  • This voltage is measured with the help of a voltmeter.
  • Voltmeters are standard items on switchboards and control panels.
  • They are basically single-circuit elements therefore when three phases are to be indicated, either three instruments are used or one with a multipoint switch.
  • The voltmeter is connected across the line and is a high-impedance one.
  • The voltmeter ranges can be varied with high-value resistors connected in series; such resistors are called voltmeter multipliers.
  • For high voltages, the Potential Transformer (PT) is connected to the main line called the primary, and the voltmeter is connected to the secondary.
  • Voltmeters are normally of moving iron types.
  • Moving coil meters can also be used but in conjunction with rectifiers.
Measurement of Electrical Parameters in Steam Turbine


  • The quantity of electrical current flowing through a circuit is measured by an ammeter.
  • The low-impedance ammeter is connected in series with the line.
  • Ammeters connected in parallel to the instruments are accommodated to various ranges with small value resistances.
  • For the measurement of a very large current, these current transformers are connected to the main lines called primary, and the instruments are connected to their secondary.
  • The Current Transformer (CT), is used to measure large currents.
  • Normally the power line itself is considered as primary windings of the current transformer.
  • The secondary windings are practically short-circuited by the measuring ammeters.
  • Current Transformers convert the current to be measured into conveniently measurable secondary current preserving values and phases in proportion.
  • Ammeters are almost identical to voltmeters, but for their input impedances and mode of connections.

Power (Active)

Single Phase Power Measurement
  • The power of a direct current (always active) can be found in one ammeter and one voltmeter.
  • Both voltage and current readings can be multiplied to get the required power.
  • D.C. power can be measured by an Electro-dynamic wattmeter with a voltage coil and current coil.
  • Instrument transformers are connected such that the voltage path is connected ahead of the current path.
  • For safety reasons, the secondary of both transformers are connected to a common earth.
  • Cosφ is the phase angle between the voltage and the current.
  • For an alternating current circuit, the active power is given by P= V*I*Cosφ
  • The active power for a 3φ phase system is the total of the three components.

For star connection, it is given by.

P = 3 Vp I cos φ

For delta connection, it is given by.

P= √3 VL I cos φ


V = RMS values of voltage

I = RMS values of current

P = Total power

Cosφ = Power Factor

Vp = Phase to neutral Voltage

VL= Line Voltage

Some of the common methods used in power plants are

Three-wattmeter is the most accurate method with indications and mechanical adder having a common shaft.

Three-wattmeter Method (Three-phase three-wire system)

WR+WY+WB = (VRC * IR) + (VYC * IY) + (VBC * IB)

WR+WY+WB = (VRN * IR) + (VYN * IY) + (VBN * IB)

WR+WY+WB = PR +PY+ PB = Total Power

It is observed that the sum of the three individual wattmeter readings (WR+WY+WB) indicates the total power consumed by the load (PR +PY+ PB).

Three–Wattmeter Method (3φ 4-wire System)

WR+WY+WB   = (VRC * IR) + (VYC * IY) + (VBC * IB)

WR+WY+WB = (VRN * IR) + (VYN * IY) + (VBN * IB)


The total power consumed by load is given by the summation of three wattmeters.

Two Wattmeter Methods

  • This is the most commonly used method for measuring three-phase power.
  • It is useful for both balanced and unbalanced loads.
  • The sum of two watt-meter readings will be equal to the total power consumed as given below.
  • Power consumed by the load

P= VRN * IR + VYN * IY + VBN * IB

Summation of wattmeter readings

W = W1 + W2

W = (VRN -VYN)* IR + (VBN – VYN) * IB

W = (VRN * IR) +(VBN* IB) + VYN (-IR – IB)

By Kirchhoff’s law, we know that,  IR+IB+IY=0, or  IR -IB = IY

W1+W2= VRN * IR + VYN * IY  + VBN * IB = P

For delta connection load

P = VRB * IR + VYR * IY + VBY * IB

W₁ + W₂ = -VYR * (IR-IY) + VBY *(IB-IR)

P= VRB * IR + VYR * IY + VBY * IB

Where (VYR + VBY +VRB = 0)

It can be proved that the total power

W1 + W2= 3*V*I*cos φ

W1 – W2 =√3*V*I*sin φ


tanφ = √3*[(W1 – W2)/(W1 + W2)]

Power factor Cosφ = Cos [tan-1 {√3 ((W1 – W2)/ (W1 + W2))}]


Single Phase Energy Meter
  • In terms of watt-hour or kilowatt-hour (kWh).
  • The kWh is defined as the energy supplied or consumed at an average rate of 1 kilowatt for one hour.
  • In commercial metering, 1 kWh is called 1 unit of energy.
  • Energy meters are used for the measurement of energy.
  • Energy meters have moving systems that revolve continuously, it deflects only through a fraction of a revolution
  • In energy meters, the speed of revolution is proportional to the power and the total number of revolutions made by the meter moving system over a given period is proportional to the energy.
  • The most commonly used energy meters are Induction type instruments.
  • Such meters have lower friction and a higher torque/weight ratio.
  • These meters are inexpensive but accurate.
  • Constructional details of an induction type single phase energy meter are shown schematically in the Figure above
  • The operating system consists of two electromagnets.
  • The current coil is wound on the series magnet and the pressure coil is wound on the shunt magnet.
  • Shading bands made of copper are used to bring the flux produced by the shunt magnet exactly in quadrature with the applied voltage.
  • The moving system consists of a light aluminum disc mounted on a spindle.
  • The disc is placed in the space between the series and shunt magnets.
  • The disc is positioned such that it intersects the flux produced by both magnets.
  • The deflecting torque on the disc is produced by the interaction between these fluxes and the eddy current they induce in the disc.
  • As there is no control spring, continuous rotation is possible.
  • The braking system consists of a permanent magnet positioned near the edge of the aluminum disc whose position is adjustable.
  • When the braking torque becomes equal to the operating torque, the disc rotates at a steady speed.
  • A registering system consisting of a train of reduction gears makes counting in numerical value possible.


  • Frequency meters are more convenient for most practical purposes.
  • These indicate the generated power frequency.
  • a vibrating reed frequency meter mainly consists of more thin steel strips called reeds arranged close electromagnet.
  • The electromagnet is laminated and its winding is connected, in series with a resistance, across the supply whose frequency is to be measured.
  • The external connection is the same as the voltmeter.
  • The reeds are made with slight differences in that their natural frequencies are different.
  • These reeds are arranged in ascending order of frequency from 47 cycles per second up to 53 cycles per second.
  • The natural frequency of the first reed is 47 cycles per second, and the next reed is 47½ cycles per second, and so on.
  • The magnetism of the electromagnet alternates with the supply frequency and exerts an attracting force when the meter is in use.
  • The one whose natural frequency is matching with the supply frequency will vibrate appreciably showing the supply frequency.
  • The flags at the top of the reeds are painted with the corresponding frequency so that one can read directly the frequency by observing the reed which is vibrating most.
  • A great advantage of this type of meter is that its indications are independent of the applied voltage and waveforms.
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