The neutral mode is a point in the zero sequence of the transformer or generator windings, which is connected to an earthing switch, specialized equipment, or isolated from external terminals. Her right choice determines the protective mechanisms of the network, introduces significant features in the performance. What varieties are found and the advantages of each option, read on in the article.
General view
The neutral modes of electrical installations are selected from generally accepted, established world practice. Some changes and adjustments are made from the features of state energy systems, which is associated with the financial capabilities of associations, the length of the network and other parameters.
To determine the neutral and the mode of its operation, it is enough to navigate in visual schemes of electrical installations. Particular attention must be paid to power transformers and their windings. The latter can be performed by a star or a triangle. More details - below.
A triangle assumes zero point isolation. Star - the presence of a ground electrode that connects to:
- ground loop;
- resistor;
- an extinguishing reactor.
What determines the choice of the connection zero point?
The choice of neutral mode depends on a number of characteristics, among which are:
- Network reliability. The first criterion is associated with building protection against a single-phase earth fault. An isolated neutral is often used to operate a 10-35 kV network, which does not disconnect the line due to a fallen branch or even a wire to the ground. And for a network of 110 kV and higher, an instantaneous shutdown is required, for which it is effectively grounded.
- Cost. An important criterion that determines the choice. It is much cheaper to implement an isolated network, which is associated with the lack of need for a fourth wire, cost savings on traverses, insulation and other nuances.
- Well-established practice. As noted above, transformer neutral modes are selected based on global and state statistics. This suggests that most manufacturing enterprises that create power equipment adhere to these standards. Because of this, the choice is predetermined by the manufacturer of the transformer or generator.
Next, we consider each variation individually and find out the advantages and disadvantages. Note that there are five main modes.
Isolated
The neutral operating mode, in which there is no zero point, is called isolated. On the diagrams it is depicted in the form of a triangle, which indicates the presence of only a three-phase wire. Its use is limited to a network of 10-35 kV, and the choice is determined by a number of advantages:
- When a single-phase earth fault occurs, consumers do not feel an in-phase mode. Disabling the line does not occur. At the moment of a single-phase fault on the damaged phase, the voltage becomes equal to 0, on the remaining two rises to linear.
- The second advantage is associated with cost. Running such a network is much cheaper. For example, there is no need for a neutral wire.
The main disadvantage of this option is security. When the wire falls, the network does not turn off, the latter remains energized. When approaching a distance closer than eight meters, you can get under the step voltage.
Effectively grounded
Modes of operation of neutrals in electrical installations above 110 kV are implemented in the presented way, which provides the required conditions for network protection and safety. The zero point of the transformer is grounded to the circuit or through a special device called "ZON-110 kV". The latter affects the sensitivity of the protection.
When the wire falls, a potential is created between the ground electrode and the break point. Because of this, relay protection is triggered. The shutdown is performed with a minimum delay time, after which it is switched on again. This is due to the fact that a tree branch or a bird could have affected performance. Re-inclusion (AR) allows you to identify the reality of damage. The benefits include the following points:
- Relatively low cost, which allows cheaper to build high-voltage networks. It should be noted that power lines also have three wires instead of four, which is a distinctive feature.
- Increased reliability combined with safety. This is considered an important criterion that determines the choice of the presented type of neutral.
There are practically no drawbacks. In practice, it is considered that this is ideal for high voltage networks.
Grounded through DHA (DGR)
The neutral mode is called resonant-grounded when its point passes through an arcing coil or reactor. A similar system is mainly applicable for cable distribution networks. It allows you to compensate for the inductance and protect the system from larger and more complex damage.
When a single-phase earth fault occurs, a coil or reactor starts to work, which compensates for the current strength, reducing it at the point of breakdown. It should be noted that the difference between the DHA and the GDR is associated with the presence of automatic adjustment when the inductance in the network changes.
The main advantage is energy compensation, which prevents damage to the cable line from developing from single-phase to interphase. As for the shortcomings, this is the appearance of other damage in weak places of the insulation of cable lines.
Grounded through a low impedance, high impedance resistor
The neutral mode, in which the grounding of the zero sequence point is performed through a high-impedance or low-impedance resistor, is also considered resonantly-grounded and is used in 10-35 kV networks. Features of the presented system are associated with disconnecting the network without a time delay.
This is convenient in terms of network protection, but negatively affects the supply of electric energy. Such a system is not suitable for the work of responsible consumers, although it is an excellent option for cable lines. The use of power lines on overhead lines is unsuitable, since the appearance of earth in the network leads to the disconnection of the feeder.
Another nuance of a relatively grounded neutral through a resistor is the appearance of high currents when a short circuit occurs on the resistor itself. There were cases that led to the ignition of the substation because of this moment.
Deaf earthed
The operation mode of the transformer neutral for the consumer network is called dead-earthed. Features are as follows. The presented variation assumes grounding of the zero point on the circuit of the substation, regarding which the protection works. Such a system is used in distribution networks where direct electricity consumption is carried out.
The 0.4 kV output has four wires: three phase and one neutral. With a single-phase circuit, a potential is created with respect to the grounded point. This will turn off the machine or cause fuses to blow. It should be noted that the operation of the protections is largely determined by the correct choice of fuse-links or the rating of the machine.
Conclusion
Neutral mode is a way of grounding the zero point of a transformer or generator. The choice of one or another option depends on a number of criteria, the main of which is generally accepted practice. You can determine the neutral by circuits where it is enough to consider the transformer windings. This should be taken into account during course projects, when it is necessary to depict the scheme of substations.
Each option has several advantages and disadvantages. Based on the use of a particular neutral, the working and protection conditions are determined. Ideal for a high-voltage network is considered to be effectively grounded, for distribution - resonant grounding. For the consumer, it is applied earthed. We recommend that you consider the main types of protection that are used in modern electric power industry.