Neutral mode is the zero-sequence point of the windings of a transformer or generator, which is connected to an earth electrode, specialized equipment, or isolated from external clamps. Its correct choice determines the protective mechanisms of the network, makes significant features in the performance. What varieties are found and the advantages of each option, read further in the article.
General view
The neutral modes of electrical installations are selected from the generally accepted, well-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 its mode of operation, it is enough to navigate in the visual diagrams of electrical installations. Special attention should be paid to power transformers and theirwindings. The latter can be performed by a star or a triangle. More details below.
Triangle implies the isolation of the zero point. Star - the presence of a ground electrode, which is connected to:
- ground loop;
- resistor;
- arc reactor.
What determines the choice of the zero connection 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. For the operation of a 10-35 kV network, an isolated neutral is often used, which does not turn off the line due to a fallen branch and even a wire to the ground. And for a network of 110 kV and above, an instant shutdown is required, for which an effectively grounded one is used.
- Cost. An important criterion that determines the choice. It is much cheaper to implement an isolated network, which is associated with the absence of the need for a fourth wire, savings on traverses, insulation and other nuances.
- Established practice. As noted above, the neutral modes of transformers are selected based on global and national statistics. This suggests that the majority of manufacturing enterprises that create power equipment adhere to these standards. Because of this, the choice is predetermined by the transformer or generator manufacturer.
Let's consider further each variation separately and find out the advantages and disadvantages. Note that there are five mainmodes.
Insulated
The mode of operation of the neutral, in which there is no zero point, is called isolated. On the diagrams, it is depicted as a triangle, which indicates the presence of only a three-phase wire. Its use is limited to the 10-35 kV network, and the choice is determined by a number of advantages:
- When a single-phase earth fault occurs, consumers do not feel open-phase operation. The line is not disconnected. At the moment of a single-phase short circuit, the voltage on the damaged phase becomes 0, on the remaining two it rises to linear.
- The second benefit is related to cost. It is much cheaper to make such a network. 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. If you approach closer than eight meters, you may be exposed to step voltage.
Effectively Grounded
The 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 operation.
When a wire falls, a potential is created between the ground electrode and the break point. Because of this, the relay protection is activated. Shutdownis carried out with a minimum time delay, after which it is switched on again. This is due to the fact that a tree branch or a bird could affect performance. Reclosing (AR) allows you to identify the reality of damage. The benefits include the following points:
- Relatively low cost, which makes it 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 shortcomings. In practice, this is considered ideal for high-voltage networks.
Grounded through DHA (DGR)
The neutral mode is called resonantly grounded when its point passes through the arc quenching coil or reactor. Such a system is mainly applicable for cable distribution networks. It allows you to compensate for inductance and protect the system from larger and more complex damage.
When a single-phase ground fault occurs, a coil or reactor starts to work, which compensates for the current, reducing it at the breakdown site. It should be noted that the difference between DGK and GGD is associated with the presence of automatic adjustment when the inductance changes in the network.
The main advantage is energy compensation, which prevents damage to the cable line from developing from single-phase tointerfacial. As for the disadvantages, this is the appearance of other damage in the weak points of the insulation of cable lines.
Grounded through a low-resistance, high-resistance resistor
Neutral mode, in which the zero-sequence point is grounded through a high-resistance or low-resistance resistor, is also considered resonantly grounded and is used in 10-35 kV networks. The features of the presented system are associated with a network disconnection without time delay.
This is convenient in terms of protecting the network, but negatively affects the supply of electrical energy. Such a system is not suitable for responsible consumers, although it is an excellent option for cable lines. The use of power transmission lines on overhead lines is unsuitable, since the appearance of earth in the network leads to the disconnection of the feeder.
Another nuance regarding the grounded neutral through the resistor is the appearance of large currents when shorted on the resistor itself. There were incidents that caused the substation to catch fire because of this moment.
Deaf-Earthed
The operating mode of the transformer neutral for the consumer network is called dead-earthed. Features are as follows. The presented variation involves grounding the zero point to the substation circuit, relative to which the protections work. Such a system is used in distribution networks where electricity is directly consumed.
Output 0.4 kV has four wires: three phase and one zero. With a single-phase circuita potential is created with respect to a grounded point. This disables the machine or causes the fuses to blow. It should be noted that the operation of the protection is largely determined by the correct choice of fuses or the rating of the machine.
Conclusion
Neutral mode is a way to ground 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 according to the diagrams, where it is enough to consider the transformer windings. This should also be taken into account during course projects, when it is necessary to depict a substation diagram.
Each option has a number of advantages and disadvantages. Based on the use of one or another neutral, the working conditions and protection are determined. Effectively grounded is considered ideal for a high-voltage network, and resonant grounding is considered ideal for a distribution network. For consumer use deaf-earthed. We recommend considering the main types of protection that are used in the modern electric power industry.
