Figure 1 – Current transformer nameplate. Example: In = 1,154 A; selected transformer ratio = 1,250/5. The turns ratio of a current transformer is N P / N S = I S / I P.A CT of turns ratio 100:5 would mean the primary current of 100 amperes would produce a secondary current of … turn. The CT ratio is the inverse of the voltage ratio. SELECTION OF CURRENT TRANSFORMERS & WIRE SIZING IN SUBSTATIONS Sethuraman Ganesan ABB Inc. Allentown, PA ABSTRACT More and more sub-stations are retrofitted with numerical relays, meters and monitoring devices. Recommended is a ratio slightly beyond the measured / defined maximum load current (In). The ratio of primary current to the secondary current is known as the current transformation ratio of the CT. Usually the current transformation ratio of the CT is high. HOW TO SELECT A CURRENT TRANSFORMER The steps to select a transformer from our catalogue are as follows: O Selection of Highest Equipment Voltage: Rated highest equipment voltage shall be given V or kV. In theoretical expression, current is normally represented by the symbol “

Current transformers, or CTs, are transformers used to step down the current level of high-power transmission systems to measure or monitor the current levels. Calculate the CT ratio. a) V 1 = 4000 V, V 2 = 400 V, Transformer Rating = 50 kVA = V 1 × I 1 = V 2 × I 2 Normally the secondary ratings are of the order 5 A, 1 A, 0.1 A, whereas the primary ratings vary from 10 A to 3000 A or more.
1.1 (next transformer size) As long as the current to be measured are small and at low voltage, the direct method of connecting the instruments and relays in the circuits is convenient and adequate. Accuray Class, rating factor and phase shift

This means the current level is stepped down 5 times where, if the primary current is 200 amps, the CT output is 40 amps. To Subtract - loop X1 opposite H1) Np = Number of primary turns • 3 Primary Loops + 2 Secondary Loops added Example: An application requires a 20:5 CT ratio, but only a 50:5 CT is available. In this example, the voltage ratio is 1:5, so the CT ratio is 5:1. Example: 12kV, 17.5 kV, 24 kV, 36 kV etc. The nominal current can also be defined on the basis of the following considerations: Dependent on the mains supply transformer nominal current times approx. In simple words, we can say that the ratio of the secondary current to the primary current is known as the current ratio. This note will look at several methods, with particular attention being paid to protection class CTs sized in IEC 60044, which is adopted internationally. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current. It is used for auxiliary current transformers and for many low or moderate ratio current transformers used in switchgear of up to 11kV rating. The current ratio can be defined as the ratio of the current flowing through the secondary winding of a transformer to the current flowing through the primary winding of the same transformer. Current transformers, or CTs, are transformers used to step down the current level of high-power transmission systems to measure or monitor the current levels. First, the calculation of the actual burden of the CT, including connection wires and protection relay impedance, is presented. Kn = Nameplate transformation ratio Transformation ratio = primary current / secondary current Nsa = Number of secondary turns added or subtracted (To Add - loop X1 lead through H1. Several methods exist to size current transformers. Assuming an ideal transformer, determine (a) the primary and secondary full-load currents, (b) the transformer turns ratio. Current Transformer Basics: Understanding Ratio, Polarity, and Class. Current transformers, or CTs, are transformers used to step down the current level of high-power transmission systems to measure or monitor the current levels. Go back to Content Table ↑ 1.4 Rated thermal short-time withstand current: I th (kA). The CT handles much less power. testguy. current, Ira, essentially subtracts from the metered current and effects the true ratio and phase angle of the output current. The current ratio can be defined as the ratio of the current flowing through the secondary winding of a transformer to the current flowing through the primary winding of the same transformer. Most often, the older relay gets replaced with a newer relay with the rest of the installation such as Current Transformer (CT) and lead remaining undisturbed. I 1 = primary current, I 2 = secondary current. Figure 5 – Wound Primary Current Transformer (Low ratio, wound primary current transformer, suitable for primary currents from 2.5A-100A with 5A outputs. The correct sizing of current transformers is required to ensure satisfactory operation of measuring instruments and protection relays. Current Transformer (C.T) and Theory and operation of current transformer. In theoretical expression, current is normally represented by the symbol “


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