The characteristics and requirements of the current transformer

by:Mings     2020-04-19
The role of the current transformer is converting a large current in power system and its secondary small current is proportional to the, then input to the measuring instrument, relay protection and automatic device. Its composition and working characteristics are as follows: 1, a number of turns, less secondary turns more used in the power system in current transformer, the usually is a primary winding equipment into and out of conductor, only turn 1 or 2 turns; The second time the number of turns is many. Such as: variable ratio of 1250/1 of the current transformer, the first time for 1 turn, secondary 1250 turn number of turns. 2 work, the core flux density is very low and normal operation of system failure flux density, current transformer core work in flux density is low, to the secondary balance of ampere turns one time. When system failure, due to the fault current is very big, high secondary voltage, excitation current, flux density in core rise sharply, even make the iron core is saturated. 3, high resistance, a current source under normal conditions, core of flux density is low, excitation impedance is large, and the second number of turns a lot. Look from the secondary side, the impedance is very large. Compared with current transformer internal resistance of the load impedance, negligible, therefore, the changes of load impedance on the influence of the secondary current is not big, can be called a current source. 4, need a second load is small ( Relative to the voltage transformer) Current transformer secondary responsible if is large, the runtime quadratieally high voltage, excitation current will increase, so as to make the error of the current transformation increases. Especially when system failure, current transformer primary current may amount to tens of times the rated current, the iron core saturation, current transformation error is very big, do not meet the requirements of relay protection, and even make the protection misoperation. 5, secondary loop shall not open current transformer secondary circuit will not be open. If the secondary circuit open circuit in the running, the secondary current disappear, demagnetization effect also disappear, the flux density is high in iron core; And due to the second circle number of Trinidad and Tobago, secondary voltage will be very high, sometimes up to several kilovolts, endanger the secondary equipment and personal safety.
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