What are the technical parameters of the on-site testing device for current transformers?

Here are some of the parameters that need to be verified: Rated capacity: The apparent power consumed when the rated secondary current flows through the rated secondary load. The rated capacity can be expressed in apparent power V.A., or in the secondary rated load impedance Ω. Rated primary current: The current of the electrical load that is allowed to flow through the primary winding of the current transformer. The primary rated current of current transformers used in power systems ranges from 5 to 25,000 A, while those used in testing equipment are 0.1 to 50,000 A. A current transformer can operate for an extended period at its rated primary current. When the load current exceeds this rated value, it is considered an overload condition. Prolonged overload operation of a current transformer can cause its windings to burn out or reduce its service life. Secondary rated current: The primary induced current allowed to pass through the secondary winding of the current transformer. Rated current ratio (turns ratio): The ratio of the primary rated current to the secondary rated current. Rated voltage: The maximum voltage that the primary winding can sustain continuously with respect to ground (in RMS value, expressed in kV), which must be no lower than the rated phase voltage of the connected circuit. The rated voltages of current transformers include several voltage levels such as 0.5, 3, 6, 10, 35, 110, 220, 330, and 500 kV. 10% multiple: The multiple of the primary current relative to its rated value, at the specified secondary load and any power factor, when the current error of the current transformer is –10%. The 10% multiple is a technical parameter related to relay protection. Accuracy class: Indicates the class of the transformer’s own errors (ratio error and phase error). Currently, the accuracy classes of current transformers range from 0.001 to 1 and above, representing a significant improvement in accuracy compared to before. Electrical instruments used in power plants, substations, and the distribution control panels of electricity-consuming units generally adopt a precision class of 0.5 or 0.2 ； The relay protection for equipment and lines is generally not lower than level 1 ； When used for electric energy metering, the selection is made in accordance with regulatory requirements based on the capacity of the load being measured or the amount of electricity consumed. Ratio error: The error of a current transformer consists of two components: ratio error and angle error. The ratio error, also referred to as the ratio difference, is generally denoted by the symbol f. It equals the difference between the actual secondary current and the primary current converted to the secondary side, divided by the primary current converted to the secondary side, expressed as a percentage. Angle difference: The phase angle error, also referred to as the angle difference, is generally denoted by the symbol δ. It represents the phase difference between the secondary current vector after a 180° rotation and the primary current vector. It is specified that the lead angle δ of the secondary current vector with respect to the primary current vector is positive when it is ahead, and negative otherwise; the degree (’) is used as the unit of measurement. Thermal and dynamic stability factors: In the event of a power system failure, current transformers are subjected to the thermal effects and electromagnetic forces resulting from high currents caused by short circuits. They must have the capacity to withstand these conditions without being damaged, and this capacity is expressed in terms of thermal and dynamic stability factors. The thermal stability factor refers to the ratio of the current that allows the current transformer to remain within its allowable temperature limits over a period of 1 second, to the rated current of the current transformer. The dynamic stability factor is the ratio of the maximum instantaneous current that a current transformer can withstand to its rated current. Verification method: Before conducting error tests on current transformers, it is usually necessary to check main characteristics such as polarity and demagnetization. 1. Inspection of current transformer polarity: The primary windings of the current transformer are marked as P1 and P2, while the secondary windings are marked as S1 and S2. If P1 and S1 are identical terminals, this flag is called reduced polarity. The primary current enters at P1, and the secondary current exits at S1. Polarity checking is simple; in addition to being able to be done using a transformer tester, the DC testing method can also be employed. 2. Current transformer error test: The error test of a current transformer is generally carried out by comparing the transformer under test with a standard transformer; the difference in the secondary currents of the two transformers represents the error of the transformer under test. This testing method is called the comparison method. The standard transformer is required to be two grades higher than the transformer under test, at which point the error of the standard transformer can be considered negligible. If the standard transformer is only one grade higher than the transformer under test, the error in the test results should take into account the error of the standard transformer. The difference in secondary current between the transformer under test and the standard transformer is generally measured using a transformer calibration instrument. The ratio difference fx(%) and phase difference δx(’) are directly read from the transformer tester. Since a transformer tester measures the ratio of the current difference between the transformer under test and a standard transformer to the secondary current, it does not require high specifications. The level of current transformers that can be verified is primarily determined by the standard current transformers. Standard current transformers are the key core of the current transformer calibration system. To calibrate the transformer under test, in addition to a standard transformer and a transformer calibration instrument, a current booster is required to supply primary current to the transformer, along with a voltage regulator capable of adjusting the current from the booster, as well as a load. The principle of a current transformer is based on the law of electromagnetic induction. A current transformer consists of a closed core and windings. It has a very small number of turns in its primary winding, which is connected in series with the circuit where the current needs to be measured; as a result, the entire current flowing through the circuit passes through it. The secondary winding has a larger number of turns and is connected in series with the measuring instruments and protection circuits. When a current transformer is in operation, its secondary circuit remains closed at all times, so the impedance of the coils in the measuring instruments and protection circuits is very low. The operating condition of the current transformer is thus similar to that of a short circuit. A current transformer is used to convert the high current on the primary side into a lower current on the secondary side; the secondary side must not be open-circuited.