What is Zero Sequence Current What does Zero Sequence Current mean

In a three-phase four-wire circuit, the sum of the phasors of the three-phase current is equal to zero, i.e., ia+ib+ic=0 If a current transformer is connected to a three-phase four-wire, the induced current is zero. When an electric shock or leakage fault occurs in the circuit, there is a leakage current flowing through the circuit, and then the three-phase current phasor and unequal zero through the transformer are: ia+ib+ic=i (leakage current) so that there is an induced voltage in the secondary coil of the transformer, which is added to the electronic amplification circuit of the detection part, compared with the predetermined action current value of the protection area device, if it is greater than the action current, even if the sensitive relay acts, it acts on the actuator tripping.

The transformer connected here is called a zero-sequence current transformer, and the phasor sum of the three-phase current is not equal to zero, and the generated current is the zero-sequence current.

Asymmetrical operation and single-phase operation are the main causes of zero-sequence currents. In a three-phase four-wire circuit, the vector sum of the three-phase currents is equal to zero, i.e., ia+ib+ic=0. If a current transformer is connected to a three-phase three-wire system, the induced current is zero.

When an electric shock or leakage fault occurs in the circuit, there is a leakage current flowing through the circuit, and the three-phase current vector sum passing through the transformer is unequal to zero, and the sum of its phasors is: ia+ib+ic=i (leakage current, that is, zero sequence current). In this way, there is an induced current in the secondary coil of the transformer, and this current is added to the electronic amplification circuit of the detection part, and compared with the predetermined action current value of the protection area device, if it is greater than the action current, the sensitive relay is made to act, and the actuator is tripped.

The transformer connected here becomes a zero-sequence current transformer, and the phasor sum of the three-phase current is not equal to zero, and the generated current is the zero-sequence current.

When a single-phase or two-phase ground short-circuit fault occurs at any point in the power system, a zero-sequence current is generated in the system.

At this time, there will be a zero sequence voltage at the ground fault point, and a zero sequence current will be generated under the action of this voltage, and the zero sequence current is a unique current that reflects the ground fault from the fault point after returning to the fault point after being grounded at the neutral point of the electrical equipment through the earth to the neutral point of the electrical equipment.

Generating conditions. 1. Whether it is a longitudinal fault, a transverse fault, or an asymmetry between normal and abnormal times, there is a zero sequence voltage;

2. There is a path for zero sequence current.

The above two conditions are indispensable. For without the first, there is no source; The second one is missing, which is the question of "whether there is voltage and current" that we usually discuss.

Zero-order formula: 3u0=ua+ub+uc, 3i0=ia+ib+ic

The occurrence of positive sequence, negative sequence and zero sequence is to analyze the decomposition of the asymmetric components of the three phases into symmetrical components (positive and negative sequences) and zero sequence components in the same direction when the voltage and current of the system are asymmetrical.

The above content refers to Encyclopedia - Zero Sequence Current.

Zero current and zero line current are two different concepts that have different meanings and roles in the power system.

1.Zero-sequence current refers to the symmetrical fault current in the power system. When there is a fault in the power system, such as a ground fault or a phase-to-phase short-circuit fault, current flows through the wires of the power system to form a fault loop.

If the fault current is symmetrically distributed, i.e., the amplitude and phase of the three-phase current are the same, it is called the zero-sequence current. Zero-sequence currents can lead to unbalanced power equipment and systems, which can cause equipment damage or safety issues.

2.Neutral current refers to the current flowing through a neutral conductor (also known as neutral or neutral) in the power system. In a three-phase, four-wire power system, a neutral conductor is connected to a power supply negative or grounding point and provides a symmetrical loop to balance the current of the three-phase load.

The magnitude of the neutral line current depends on the imbalance between the three-phase loads. In the case of perfect equilibrium, the neutral line current should be zero. However, in the case of unbalanced loads, due to the uneven distribution of the load, it will result in a non-zero zero line current.

Therefore, there is a clear difference between the zero-sequence current and the zero-line current in terms of properties and generation mechanisms. Zero-sequence currents are usually associated with faults and imbalances in the power system, while zero-line currents are associated with three-phase load imbalances in the power system.

Different meanings, different names.

1. The meaning is different. The zero-sequence current is an asymmetrical circuit, and the symmetrical component analysis method is used to determine the symmetrical component of the same phase as the same in the calculation of the first charge group. The neutral line current refers to the current flowing through the neutral line in the three-phase four-(five) wire distribution circuit due to the unbalanced current of the three-phase fire balance wide line.

2. The name is different. One name is zero sequence current, and the other name is zero line current.

Prerequisites for the generation of zero-sequence currents:

1. There is a path for the zero sequence current.

2. Whether it is a longitudinal fault, a transverse fault, or an asymmetry between normal and abnormal times, there is a zero sequence voltage.

The above two conditions are indispensable. For without the first, there is no source; The second missing question is "whether there is voltage and current".

The formula for zero sequence current is 3u0=ua+ub+uc, 3i0=ia+ib+ic. The occurrence of positive sequence, negative sequence and zero sequence is to analyze the decomposition of the asymmetric components of the three phases into symmetrical components and zero sequence components in the same direction when the voltage and current of the system are asymmetrical. As long as it is a three-phase system, the above three components can be decomposed.

1. Whether it is a longitudinal fault, a transverse fault, or an asymmetry between normal and abnormal times, as long as there is a zero sequence voltage.

2. There is a path for zero sequence current.

The above two conditions are indispensable.

Hazards of excessive zero-sequence current:

1. Cause the neutral line current to be too large, so that the neutral cable is now overloaded, causing the insulation aging to accelerate, breaking down the insulation and causing a short circuit, increasing the hidden danger of fire.

2. The third harmonic generates zero sequence circulation and zero sequence magnetic field in the transformer coil, which reduces the output of the transformer, increases the loss of the transformer, and makes the transformer heat up seriously.

3. Current and voltage distortion increases the loss, heat, accelerated insulation aging of other equipment and materials in the power supply system, and reduces the service life.

4. The neutral line current is too large, which makes the switch trip frequently.

5. The zero drift of the neutral point causes the voltage of the zero line to the ground, which causes hidden dangers to the equipment and personal safety.

6. Current and voltage distortion increases the vibration and noise of equipment and materials in the power supply system.

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Zero-sequence currents and negative-sequence currents are actually just artificial. For a three-phase unbalanced (asymmetric) system (voltage or current), for the sake of analysis and calculation, the three-phase asymmetric system is artificially decomposed into "positive sequence components", "negative sequence components" and "zero sequence components". This method is called the "symmetrical component method".

1. The characteristics of the negative sequence component (voltage or current) are: the three phases are the same size, the difference is 120 degrees, and the phase sequence and the positive sequence are opposite.

2. The characteristics of the zero sequence component (voltage or current) are: the three-phase size is the same, and the three-phase phase is the same.

Conclusion: For the neutral non-grounding system, there is no zero sequence current in any fault of the system when the distributed capacitance current is ignored, only positive sequence and negative sequence current. (In the case of distributed capacitive current, the capacitive current is the property of the zero-sequence current).

For a neutral direct grounding system, there is no zero-sequence component for phase-to-phase shorts, and only for ground faults.

The PT opening triangle shows the sum of the three-phase zero-sequence voltages, i.e., 3UO.

Zero-sequence CT shows the sum of the three-phase zero-sequence currents, i.e., 3IO.

Conditions under which zero sequence currents are generated:

1. Whether it is a longitudinal fault, a transverse fault, or an asymmetry between normal and abnormal times, as long as there is a zero sequence voltage.

2. There is a path for zero sequence current.

in the power system.

When a single-phase or more ground short-circuit fault occurs at any of these points, a zero-sequence current is generated in the system. This is due to the fact that there is a zero-sequence voltage u0 at the ground fault point, and under the action of this voltage u0, the zero-sequence electric silver ant car flow i0 is generated.

The losses of transformers include no-load losses and load losses. Under normal circumstances, the operating voltage of the transformer is basically unchanged, that is, the no-load loss is a constant quantity. The load loss varies with the change of the transformer's operating load and is proportional to the square of the load current.

When the three-phase load is unbalanced, the load loss of the transformer can be regarded as the sum of the load losses of the three single-phase transformers.

When the system fails, the three phases become asymmetrical, and the magnitude of the negative and zero sequence components can be decomposed (sometimes only one of them), so by detecting these two components that should not occur normally, it is possible to know that something is wrong with the system (especially the zero sequence component when the single phase is grounded).

The zero-sequence current is also known as the abandonment shortage: the zero-line current. There are two reasons for this.

1. The three-phase imbalance produces a zero-sequence asymmetric current flowing back to the zero line.

Second, the load generates a clear flow of "third harmonics" superimposed on the zero line.

When the three phases are unbalanced, the reed balance can be assigned to reduce the neutral current. The zero sequence current generated by the load "third harmonic" can be reduced by adding a neutral current eliminator.

Zero-sequence current: In a three-phase four-wire circuit, the phasor sum of the three-phase current is equal to zero, i.e., ia+ib+ic=0.

1. Generation of zero sequence current:

When a phase or multiple phases have a problem that causes three imbalances, and its vector sum is not zero, the zero sequence appears, and in the actual project, you can simplify the occurrence of the zero order of the circle brigade and recognize that the occurrence of the zero order is the fault between the phase electricity and the "zero line (ground wire, middle line)" to produce the "zero order".

2. Zero sequence: Zero sequence is a ground fault detection method, which uses a sensor (CT) to connect all phase conductors and neutral conductors together, and the sensor will generate an output in equal proportion to the unbalance of the ground fault current in the line, and then a relay measures this output, and disconnects the circuit breaker or releases the orange finger ground fault alarm.

Zero-sequence current: In a three-phase four-wire circuit, the phasor sum of the three-phase current is equal to zero, i.e., ia+ib+ic=0.

1. Generation of zero sequence current:

When a phase or multiple phases have a problem that causes three imbalances, and its vector sum is not zero, the zero sequence appears, and in the actual project, you can simplify the occurrence of the zero order of the circle brigade and recognize that the occurrence of the zero order is the fault between the phase electricity and the "zero line (ground wire, middle line)" to produce the "zero order".

2. Zero sequence: Zero sequence is a ground fault detection method, which uses a sensor (CT) to connect all phase conductors and neutral conductors together, and the sensor will generate an output in equal proportion to the unbalance of the ground fault current in the line, and then a relay measures this output, and disconnects the circuit breaker or releases the orange finger ground fault alarm.