Home / Technical Articles / The Essentials Of Current Transformers In Power Circuits (Theory and Practice) 5SFT251 Current Transformer – 250:5A Current Ratio in addition to a full line of Current Voltage Transformer. The rated burden is the value of the burden on the nameplate of the CT. The rated burden is the product of the voltage and current on the secondary when the CT supplies the instrument or relay with its maximum rated value of current.
In a current transformer, the load’s impedance or burden on the secondary has slightly differed from the power transformers. The ratio of the primary current and the secondary current is known as a current transformer ratio of the circuit. Special configurations with secondary current x/1 A or as metering transformer with 4-20 mA for primary currents between 500 and 4000 A are also available.
Under normal operating conditions, voltages and currents are sinusoidal and transformation from the primary high voltage/current signals to secondary measurement-level signals is straightforward. These errors must be small and current transformers are classified (in descending order of accuracy) into types AL, AM, BM, C and D. Ratio errors in class AL must be within the limits of ±0.1 per cent for AL and ±5 per cent for D, while the phase error limit on class A1 is ±5 min to ±2 min in types CM and C. Ratio and phase angle errors also occur due to the need for a portion of the primary current to magnetize the core and the requirement for a finite voltage to drive the current through the burden.
The current transformers also implement galvanic separation between power and control circuit. Current transformers (CTs) are located in various parts of the installation, depending upon the protection scheme employed. Current transformers used on generator voltage busbar systems are also, in effect, bar-primary devices with features developed specifically to meet the requirements of the system.
The temperature rise of the current transformer’s winding when carrying a rated primary current, at rated frequency and with rated burden, should not exceed the approximate values given in Table 2. The product of current and voltage at the secondary side of the CT is called the rated burden. Accuracy class tells you how accurate the current transformer is. Accuracy class shall be 0.2, 0.5, 1, 3 or 5. For example, if the accuracy class of a current transformer is 1, then the ratio error will be ±1% at the rated primary value.
The disadvantage of making transformers with lower secondary current ratings is that they produce much higher voltage if they are ever accidentally left open circuited. The value of the rated secondary current is 5A. A secondary current rating of 2A and 1A may also be used in some cases if the number of secondary turns is low and the ratio cannot be adjusted within the required limits by the addition or removal of one turn, if the length of secondary connecting lead is such that the burden due to them at higher secondary current would be excessive.