Key Points of Current Transformer Selection

The selection of voltage transformer shall be based on the primary wiring mode (including Y-type connection and V-type connection), the power consumption grade of primary voltage, the capacity requirements of secondary line and the requirements for transformation accuracy.Current transformers are mainly assembled in different switchgear. Due to different models of current transformers, current transformers often have great differences in structure (including copper bar lapping form, iron core, shape, etc. and dynamic and thermal stability tolerance). For example, the current transformers equipped with the central handcart cabinet are mostly lzzbj9 or AS12, but there are many models equipped with fixed cabinets.There are many differences between voltage transformers of the same model and specification. Generally, there are many combinations mainly due to different transformation ratio, capacity of secondary coil, accuracy of protection coil and metering coil. When selecting the transformation ratio of the current transformer, the actual load rated current should be obtained first. This current is preferably 65% - 85% of the measurement range of the current transformer.

Key points of current transformer selection1. The rated current (primary side) shall be 1.0 1.3 times of the load current during normal operation of the line;2. The rated voltage shall be 0.5kV or 0.66kv;

3. Pay attention to the accuracy level. If it is used for measurement, the accuracy level shall be 0.5 or 0.2; If the load current changes greatly, or the load current is 30% lower than the rated current at the primary side of the current transformer during normal operation, class 0.5 shall be selected;4. Determine the transformation ratio and turns as required;5. Model and specification selection: determine the transformation ratio according to the primary load current of the power supply line, and then determine the model according to the actual installation situation;

6. For the selection of rated capacity, the secondary rated capacity of current transformer shall be greater than the actual secondary load, and the actual secondary load shall be 25 100% of the secondary rated capacity. The capacity determines the load impedance at the secondary side, and the load impedance affects the measurement or control accuracy. The load impedance is mainly affected by the coil resistance, reactance and wiring contact resistance of measuring instrument and relay Influence of secondary connection conductor resistance.The key points of current transformer selection are as follows: Standard value of rated primary and secondary current

The standard value of rated primary current of current transformer for power system is shown in the following table, and the value in brackets is only limited to old products:The rated secondary current standard is 1a and 5A.In addition to the numbers listed in the above table, the standard values of rated primary current of instrument current transformer include: 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.5, 0.75, 1.5, 2, 2.5, 3, 4, 7.5, 8, 25, 80, 120 and 250. The standard value of rated secondary current is 0.5A in addition to 1a and 5A.

Accuracy levelThe accuracy levels of domestic current transformers include: 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, 1, 3 and 5. Near the rated current, the error of current transformers at all levels can also be roughly considered as corresponding to level 0.01... Level 5, and the allowable error is 0.01%... 5%.The accuracy level of instrument current transformer is generally higher than 0.5. Current transformer for power system: used for measuring

The amount of electric energy shall not be lower than grade 0.5. For large electric energy, grade 0.2 or 0.1 is required; For measuring current and power, level 1 is generally used; Levels 3 and 5 are only used to monitor the current, that is, roughly observe the current. Level 5 (there are 10 for old products) is only used on the bushing of high voltage circuit breaker.The accuracy level of current transformer for protection is represented by 5p and 10p, which is also equivalent to its allowable error of 5% and10%

Rated capacity / rated loadThe load of current transformer refers to the total impedance of electrical instruments and connecting wires connected to the secondary of current transformer,It includes the impedance of these instruments or relays, as well as the resistance of connecting wires and the contact resistance of connecting points.

The relationship between secondary rated capacity Sn and rated load Zn can be expressed by the following formula: SN = i2n2 * ZnAccording to the standard, the standard value of rated secondary load of current transformer shall be 5, 10, 15, 20, 30, 40, 50, 60, 80 or 100VA. For current transformers with rated secondary current of 5A, the impedance of rated load is: 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.6, 2.0, 2.4, 3.2 or 4.0 . Rated voltage

The rated voltage of current transformer refers to the line voltage on the line connected to the primary winding. Current transformer primaryThe winding is connected in series on the line, so the rated voltage of the current transformer is not the voltage at both ends of the primary winding of the current transformer, but the insulation voltage of the primary winding of the current transformer to the secondary winding and ground. Therefore, the rated voltage of the current transformer only shows the insulation strength of the current transformer, and has no direct relationship with the capacity of the current transformer.