Precautions for Use of Current Transformer_ Harm of Open Circuit on Secondary Side of Current Transformer
Precautions for use of current transformerPay attention to the following points when using current transformer(1) The configuration of current transformer shall meet the requirements of measuring meter, relay protection and automatic device, and shall be powered by separate secondary winding respectively. Only in this way can it meet the requirements of different equipment without mutual influence.
(2) When selecting current transformer, the rated voltage shall be equal to the voltage of the tested circuit, its primary rated current shall be greater than the maximum continuous working current of the tested circuit, and its structure, model and capacity shall meet the accuracy requirements.(3) Open circuit is not allowed for the secondary winding of current transformer in operation.(4) The secondary winding, iron core and shell of current transformer shall be reliably grounded, and only one grounding point is allowed.
(5) According to the development of the power grid, check whether the dynamic thermal stability current and rated continuous thermal current of the current transformer meet the requirements under the actual operating current ratio. If the short-circuit current of the substation exceeds the specified value on the nameplate of the current transformer, change the current ratio or replace the current transformer in time.(6) If there is doubt about the rated continuous thermal current, the secondary winding shall be short circuited for temperature rise test. If it does not meet the requirements, the current transformer shall be replaced.What is the harm of open circuit at the secondary side of current transformer
In normal operation, the impedance of the current transformer is very small and close to the short-circuit state. Most of the magnetic potential generated by the primary current is offset by the magnetic potential generated by the secondary current, so that the total magnetic flux in the iron core and the induced potential of the secondary winding are small. However, when the secondary side of the current transformer is open circuit, it is equivalent that the load impedance Z is infinite and the secondary current is zero. The secondary current cannot generate magnetic potential to offset the magnetic potential on the primary side. The total magnetic flux is equal to the magnetic flux potential of the primary winding, that is, the primary current completely becomes the excitation current. As a result, the magnetic flux density of the iron core increases, resulting in iron core saturation and iron loss increase, resulting in serious heating of TA iron core, It may even burn out the insulation; At the same time, the increase of the magnetic flux density of the iron core will also distort the magnetic flux waveform into a flat top wave, which will induce a high voltage on the secondary side when the magnetic flux passes through zero, and its peak value can reach thousands of volts, which not only endangers the insulation of the current transformer itself, but also is extremely unsafe for the insulation of human body and secondary equipment. Therefore, the open circuit on the secondary side of the current transformer is never allowed in operation. If the secondary side of the current transformer in operation is open circuit, relevant protection and automatic devices can be disabled during treatment to reduce the secondary load to zero.
