INSTRUMENT TRANSFORMERS SERVICE CONDITIONS FOR OPERATION

INSTRUMENT TRANSFORMERS SERVICE CONDITIONS FOR OPERATION
What are the conditions that are considered in choosing instrument transformers?

The standard ratings of instrument transformers are based on operation at the thermal rating of the instrument transformer for defined ambient temperature conditions provided the altitude does not exceed 1000 meters (3300 feet).

Instrument transformers may be used at higher ambient temperatures, at altitudes higher than 1000 meters, or for other unusual conditions if the effects on performance are considered. Consult the manufacturer for specific applications.

Altitude:
A higher standard BIL may be required at high altitudes in order to obtain the insulation required for the voltage used. The decreased air density at higher altitudes also affects heat dissipation and the permissible loading on instrument transformers. Current transformers may be operated at altitudes greater than 1000 meters if the current is reduced below rated current by 0.3 percent for each 100 meters the altitude exceeds 1000 meters. Voltage transformers may be operated at higher altitudes only after consultation with the manufacturer.

Temperature:
For 30ºC average ambient temperature conditions, the temperature of the cooling air (ambient temperature) does not exceed 40ºC (104ºF), and the average temperature of the cooling air for any 24-hour period does not exceed 30ºC. Instrument transformers may also be rated for 55ºC ambient temperature for use inside enclosed switchgear, provided the ambient temperature of the cooling air on the inside of enclosed switchgear does not exceed 55ºC. See ANSI Std. C37.20, “Switchgear Assemblies including Metal-Enclosed Bus,” and NEMA Std. SG5, “Power Switchgear Assemblies,” for further information. Current transformers designed for 55ºC temperature rise above 30ºC ambient temperatures are given a continuous-thermal-current rating factor (RF). The RF is multiplied by the rated current to indicate the current that can be carried continuously without exceeding the standard temperature limitations. Voltage transformers can be operated at higher ambient temperatures only after consultation with the manufacturer.

Accuracy:
To be a useful part of a measurement system, instrument transformers have to change the magnitude of the voltage or current that is being measured without introducing any unknown errors of measurement into the system. The accuracy of transformation should, therefore, be either a known value so that the errors can be included in the computation of the overall measurement, or the errors have to be within the limits of a specified small value so they may be disregarded.

The accuracy obtainable with an instrument transformer depends on its design, circuit conditions, and the burden imposed on the secondary. Accuracy is measured in terms of its true value and phase angle under specified operating conditions.

Accuracy Classes for Metering Service: Accuracy classes for metering service have been established that limit the transformer correction factor (TCF) to specified values when the metered load has a power factor of 0.6 lagging to 1.0.

Transformer Correction Factor: The transformer correction factor for a current or voltage transformer is the ratio correction factor (RCF) multiplied by the phase angle correction factor for a specified primary circuit power factor.

Ratio Correction Factor: The ratio correction factor is the ratio of the true ratio to the marked ratio.

Phase Angle Correction Factor: The phase angle correction factor is the ratio of the true power factor to the measured power factor. It is a function of both the phase angles of the instrument transformers and the power factor of the primary circuit being measured. The phase angle correction factor corrects for the phase displacement of the secondary current or voltage, or both, due to the instrument transformer phase angles. Phase angle of an instrument transformer is the phase displacement, in minutes, between the primary and secondary values.

Secondary Burdens:
As defined in ANSI/IEEE Std. C57.13, burden for an instrument transformer is “that property of the circuit connected to the secondary winding that determines the active and reactive power at the secondary terminals. The burden is expressed either as total ohms impedance with the effective resistance and reactive components, or as the total volt-amperes and power factor at the specified value of current or voltage, and frequency.”

The burden on the secondary circuit of an instrument transformer affects the accuracy of the device. Accordingly, the burdens of the various meters and other instruments on the secondary have to be known. This information is usually obtained from data sheets issued by the manufacturers.

Construction:
All instrument transformers have external terminals or leads to which the high-voltage or primary circuit and the secondary circuits are connected. These terminals are marked to indicate the polarity of the windings. When letters are used to indicate polarity, the letter H shall be used to distinguish the terminals of the primary winding. The letters X, Y, Z, W, V, U are used to identify the terminals of up to six secondary windings, respectively.

In addition to the letters, each terminal is numbered (e.g., H1, H2, X1, X2). Letters followed by the same number are of the same polarity. If multiple primary windings are provided, the H terminals are numbered with consecutive pairs of numbers (H1-H2, H3-H4, etc.). The odd-numbered terminals are of the same polarity. When taps are provided in the secondary windings, the terminals of each winding are numbered consecutively (X1, X2, X3, etc.). The lowest and highest numbered terminals indicate the full winding with intermediate numbers indicating the taps. When the X1 terminals are not in use, the lower number of the two terminals used is the polarity terminal.

source: RUS Bulletin 1724E-300