Metering Transformers Theory of Operation

In electric power installations metering transformers are used extensively for monitoring voltages and currents. A POTENTIAL transformer, sometimes called PT, is used to monitor a power line voltage. A CURRENT transformers, sometimes called CT, is used to monitor a power line current delivered to a load. Transformers are also used in many other non-metering applications.

An electrical transformer is a device for stepping down, stepping up or isolating AC voltages or AC currents. A transformer usually consists of a primary coil and one or more secondary coils wound around a common core. The core is generally iron or directional silicon steel for low frequencies. Ferrite cores are used in high frequency applications, and air cores are used in very high frequency transformers. Recent technology using air cores for low and medium frequency resulted with the development of current transformers known as Rogowsky probes.

The output signal obtained from an AC transformer generally has the same wave-shape as the voltage or current being measured. However, the transformers introduce measurement errors of amplitude, phase and wave-shape due to frequency range limitations.

Symbols of Voltage Transformer Types
Universal Transformer Symbol		European Transformer Symbol

POTENTIAL TRANSFORMERS

When an alternating (AC) voltage is applied to the primary winding of a potential transformer, an alternating magnetic field is generated that is sensed by the secondary coil. The secondary coil then generates an AC voltage whose waveform is the same as the waveform of the primary voltage. The amplitude of the AC voltage generated by the secondary coil depends on the ratio of primary to secondary turns, often known as the “turns ratio”. It also depends on the core material, the driving frequency and coupling.

Step-down Voltage transformers are used to reduce a high voltage to a lower lever. In the electric power industry such Potential Transformers have an output that is generally 110 Volts, 120 Volts, 240 Volts or 480 Volts. AYA potential transformers are available for use on power line voltages as high as 36,500 Volts AC. They are used to scale down the line-to-neutral voltage of a Wye system or the line-to-line voltage of a Delta system to the rated input scale of a monitoring meter, which is typically 120 V in the United States.

Potential transformers are designed in many sizes, shapes and configurations. A few are shown on the right.

The BLACK potential transformers are rated for use on power lines up to 600 Volts AC. The RED potential transformers have ratings up to 36,500 Volts AC

The VOLTAGE ratio of a standard magnetic-core transformer, ignoring losses, is defined by the equation:

E2 = (N2/N1) x E1

Where E1 = Input Voltage, E2 = Output Voltage

N1 = Number of turns of primary coil

N2 = Number of turns of secondary coil

CURRENT TRANSFORMERS

An AC current transformer is a special kind of transformer. Most current transformers do not have a primary coil. The conductor, whose current is to be measured, acts as the primary coil when it is placed inside the magnetic path of the core.

A current transformer converts the primary current of the conductor to a current output whose value depends on N2. The output current can be computed if N2 is known. If N2 is 1000 turns, the output current is 1/1000 of the primary current, which can be expressed as 1 Milliampere per Ampere. Such a current transformer has a turns-ratio of 1000:1. The output of this CT can be read by any AC ammeter whose input impedance is compatible with the specifications of the current transformer.

Current transformers are also designed in many sizes, shapes and configurations. A few are shown on the right.

The GREEN current transformer is a CLOSED-CORE type that meets the ROHS standard. The BLACK transformer is a SPLIT-CORE type. The RED current transformers have voltage ratings up to 36,500 Volts.

The CURRENT ratio of a standard magnetic-core transformer, ignoring losses, is defined by the equation:

I2 = (N1/N2) x I1

Where I1 = Input current, I2 = Output current

N1 = Number of turns of primary coil

N2 = Number of turns of secondary coil

A current transformer which has a window, converts the primary current of the conductor to a current output whose value depends on N2, because N1 is one, the conductor itself acts as the primary coil. The output current can be computed if N2 is known. If N2 is 1000 turns, the output current is 1/1000 of the primary current, which can be expressed as 1 Milliampere per Ampere. Such a current transformer has a turns-ratio of 1000:1. The output of this CT can be read by any AC ammeter whose input impedance is compatible with the specifications of the current transformer.

APPLICATIONS

Properties of A Transformer

Transformers have many properties due to construction. The Ideal transformer is lossless. An ideal transformer is shown below. N₁ and N₂ are the number of turns of each winding, e₁ and e₂ are the input and output voltages, and Φ is the flux.

Metering Transformer Properties

When a load is attached to the output, the currents are represented by i₁ and i₂.

Primary Induced Electro Magnetic Force (E.M.F.) is ${e_1}={N_1}\frac{d\Phi}{dt}$ ,

and the Secondary E.M.F is ${e_2}={N_2}\frac{d\Phi}{dt}$

The transformation ratio (a) of the Transformer is defined by the turns ratio:
$\frac{v_1}{v_2}=\frac{e_1}{e_2}=\frac{N_1}{N_2}=\frac{i_2}{i_1}=a$

The Voltage and current relationship is: ${\textbf{v}_1}{i_1}={\textbf{v}_2}{i_2}$

The changing Flux relationship $\Phi = \Phi_m\sin\omega\textbf{t}$ , where $\omega= \textbf{2}\pi{f}$ which is the angular frequency in radians/second, is the power line frequency, $\textbf{t}$ is the time of the measurement, and $\Phi_{\textbf{m}}$ is the magnetizing Flux density.

The load Load Impedances is defined as: ${Z_2}=\frac{V_2}{I_2}=\frac{1}{a^2}\frac{V_1}{I_1}=\frac{1}{a^2}{Z_1}$