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The understanding of zero sequence transformer

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The understanding of zero sequence transformer

2017/04/07 00:00
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Zero-sequence current transformers, also known as residual current transformers, grounding current transformers or leakage current transformers, are usually used in power fire protection equipment. When the power system generates zero-sequence grounding current, they should be used together with relay protection devices to protect or monitor in advance.
The basic principle of the zero sequence current transformer is based on kirchhoff's current law: the algebraic sum of the complex current flowing into any node in the circuit is equal to zero. When the line and electrical equipment are normal, the vector sum of the current in each phase is equal to zero, Ia+Ib+Ic=0. Therefore, the secondary side winding of the zero-sequence current transformer has no signal output and the actuator does not move. When earthing fault occurring in the vector of each phase current and is not zero, Ia + Ib + Ic = Io > 0, or called Io is zero sequence current leakage current and fault current, the fault current of zero sequence current transformer magnetic flux is formed in the annular core, zero sequence current transformer secondary side of the induced voltage causes a actuators, drives the tripping device, switching power supply network, achieve the goal of earth fault protection.
Zero sequence transformer is one of the current transformers. There are no specific provisions on zero sequence transformer in the national standard of GB20840. However, the current transformer for protection in GB20840 is defined as the current transformer that transmits information signals for protection and control devices. Therefore, from the definition point of view, zero sequence transformer is a special protection current transformer.
Is it the current transformer for protection in the standard? Obviously not completely. They are obviously different from each other.
First is the scope of protection current, GB20840 the protection with the protection range of the current transformer is up, it provides a standard accurate limit coefficient of ALF (see GB20840.2-2014), usually 5, 10, 15, 20, 30, for example: the protection current transformer with 100 a / 5 a, ALF = 10, then the protection current transformer in a * 10 = 1000 a, 100 transformer can not saturated, can more accurate current monitoring system. The zero sequence current transformer is downward. The zero sequence current is usually a small leakage current, usually at mA level, which requires the transformer to work normally at a small current. This is also an important parameter of the zero sequence current transformer. That is, the sensitivity of the zero-sequence current transformer. However, if the zero-sequence current is too large and your transformer fails to detect it due to insufficient sensitivity, it will cause a mistake. This sensitivity is often closely related to the magnetic core material of the ct. The core of the zero-sequence current transformer needs to be made, which has a high initial permeability and can be sensed at a small current. Current transformers are not as high as zero sequence current transformers.
Therefore, the measurement of protection current transformer or current transformer is generally upward test, while the zero sequence current transformer is downward test.
The second is the accuracy level. Gb20840.2-2014 "transformers part 2: supplementary technical requirements for current transformers" has two accuracy levels, 5P and 10P, for current transformers for protection, although the accuracy level is not high. But there are certain requirements. The zero sequence current transformer, on the other hand, should be completely protected or monitored. If it is completely protected, there is no need to have an accurate level. Just set a threshold for it and let the relay protect the system behind it. Example: zero sequence transformer, minimum zero sequence current 5mA is required to work, the output signal is greater than or equal to 100mV, so as long as the relay receives the signal greater than or equal to 100mV, it will be the default to generate 5mA zero sequence current in front, and cut off the line immediately. Monitoring requires a certain level of accuracy. In case of failure, the relay will cut off the line when receiving the signal and know how large the zero sequence current is. Of course, to play the role of monitoring, the zero sequence transformer requirements are higher, regardless of product design or iron core material or manufacturing process have higher requirements, and at the same time, all three are indispensable.
Therefore, on the accurate level, the ct has the requirement of accurate level for both protection and measurement.
In terms of appearance, the difference between a zero-sequence transformer and an ordinary one is that the inner hole of a zero-sequence transformer is generally larger than that of an ordinary one. This is also the first method for transformer workers to distinguish the transformer at first sight, because most zero-sequence transformers have three lines inside, while the common one has a single line.
Sensitivity is an important indicator of zero sequence transformer. Is there any other important indicator of zero sequence transformer? The answer is yes. The zero sequence current transformer also has an important indicator called the equilibrium characteristic. In GB 14048.2-2008 "low-voltage switchgear and controlgear -- part 2 circuit breakers", there are requirements for balancing characteristics. It is clearly stipulated that when the current flowing through the circuit is equal to 6In and the minimum value of 80% of the setting value of the maximum short-circuit tripping current, when the power factor is 0.5, the product shall not tripping. That is to say, when Ia+Ib+Ic=Io=0, the zero sequence transformer should not generate output signal twice. Otherwise, the relay will be misoperated. It is reasonable to say that zero sequence current should not generate a secondary signal when the transformer is in three-phase equilibrium. However, in the actual situation, there are several cases of zero sequence transformer error. 1. It is impossible for the magnetic permeability of the transformer core to be evenly distributed, which is related to the firing process of the core (furnace temperature, uniform shielding gas, etc.) and the winding process of the core; (in order to eliminate this effect, the manufacturers will use a punched core, but at the cost of reducing the magnetic permeability of the core.) 2. The coil winding process of the transformer can not be completely evenly wound on the magnetic core; 3. It is impossible for the primary conductor to pass through the zero sequence transformer evenly and generate tilt, leading to changes in the magnetic field; The above three possibilities may cause the poor balance characteristics of the zero sequence ct. The larger the secondary electromotive force, the worse the balance characteristic of the zero sequence transformer. In addition to the above three methods to control the balance characteristics, the zero sequence transformer can also add a shield made of high permeability to reduce the influence of unbalanced characteristics and prevent the influence of external signals on the zero sequence transformer. It can be said that zero sequence transformer has no absolute balance characteristics, so the general customers will give a threshold value of balance characteristics, which is convenient for manufacturers to test and produce.
This paper differentiates the difference between the zero sequence transformer and the current transformer for protection in GB20840 based on the sensitivity, accuracy and shape of the transformer, and introduces an important characteristic of the zero sequence transformer. Currently, the GB20840 series transformer standard has not standardized the zero-sequence transformer, and it is expected to be included in the transformer standard as soon as possible. (please indicate the source of reprint, jiangyin xinghuo electronic technology co., LTD.)