Practical applications of zero-sequence current transformers
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I would like to ask how a zero-sequence current transformer works What happens when the three-phase currents are unbalanced?
A zero-sequence current transformer is used to detect imbalances in three-phase currents. In other words, when there is current in the zero-sequence current transformer, it indicates that the three-phase currents are unbalanced.
In a normal and ideal operating condition, the vector sum of the currents in phases A, B, and C is zero. The zero-sequence current transformer measures this vector sum of the phase currents; therefore, there is no output on its secondary side under normal conditions (in reality, there is a small output, but it is not sufficient to reach the threshold value). When an asymmetric fault occurs, a signal is generated on the secondary side of the zero-sequence current transformer, and once the threshold value is reached, the relay protection circuit will send a signal or trigger a trip, depending on how the protection system is configured
The A, B, and C phase cables all pass through the current transformer, and they are all on the primary side. When the three phases are in absolute balance, there is no output on the secondary side of the transformer ； Under normal conditions, there is a slight output on the secondary side, but it is far from reaching the value set for triggering an alarm ； When there is three-phase imbalance, output is generated on the secondary side; upon detection by protection devices or meters, a tripping or alarm action is initiated, and the magnitude of the value is displayed (or indicated). This not only protects the equipment but also facilitates the identification of the cause of the fault.
The basic principle of zero-sequence current protection is based on Kirchhoff’s current law: the algebraic sum of the complex currents entering any node in a circuit is equal to zero. When the lines and electrical equipment are in normal condition, the vector sum of the currents in each phase is zero; therefore, the secondary winding of the zero-sequence current transformer produces no signal, and the actuating element does not operate. When a ground fault occurs, the vector sum of the currents in each phase is not zero. The fault current generates flux in the toroidal core of the zero-sequence current transformer, and the induced voltage on the secondary side of this transformer causes the actuating element to operate, thereby activating the trip device and switching the power supply network, thus achieving the purpose of protecting against ground faults. Zero-sequence current and residual current are not the same. Zero-sequence current is the value of Ia+Ib+Ic; under symmetric load conditions, this value is 0. However, in reality, it can never be 0, as there is always some unbalanced current present. Residual current, on the other hand, is calculated as Ia+Ib+Ic+N. In this case, regardless of the load conditions, its value is normally 0. Therefore, residual current is more sensitive than zero-sequence current. The working principle of a zero-sequence current transformer for detecting low-voltage leakage is as follows: When a current transformer is connected in a three-phase four-wire system, the induced current is zero. But when there is an electric shock or leakage fault in the circuit, a leakage current flows through the circuit. At that time, the vector sum of the three-phase currents passing through the transformer is not zero; this vector sum is given by Ia+Ib+Ic = I (leakage current). As a result, an induced voltage is generated in the secondary coil of the transformer. This voltage is fed into the electronic amplification circuit in the detection unit, and it is compared with the preset operating current value of the protection device. If this value exceeds the operating current, the relay is activated, which in turn causes the switching element to operate and cut off the power supply. The transformer used here is called a zero-sequence current transformer. Since the vector sum of the three-phase currents is not zero, the current generated is known as zero-sequence current.
The zero-sequence current transformer requires all three wires to pass through it, right?
The zero-sequence current transformer in high-voltage switchgear is unable to detect any imbalance in the three-phase currents; values of 110A for phase A, 0A for phase B, and 0A for phase C still allow normal operation! ! I wonder if it only activates when there is a leakage current ? ? ? Is the zero-sequence current transformer only suitable for low voltage in cases of three-phase imbalance? ?