Neutral is Definition, device and purpose

2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16

Electric power industry is a complex industrial complex, which consists of many components. In order for each element to work correctly and perform its tasks, an accurate knowledge and understanding of the physical processes that occur in power equipment is necessary. Some of them are easy to explain, so we suggest getting acquainted with such a concept as "neutral".

General purpose of the neutral wire in transformer windings

Neutral is a common, zero point conductor connection in three-phase transformers or generators. At the moment, there are 4 main types of zero point attachment:

1. Isolated. This type is characterized by the absence of a neutral. The main connection scheme for the presented network is a triangle. With single-phase earth faults in the working phases, they do not feel changes in energy consumption. This type is used in distribution networks.6-35 kV.
2. Resonance grounded. This option involves the use of grounding the zero point of the windings of the transformer or generator through arc-suppressing coils or reactors (DGK, DGR). The presence of specialized equipment compensates for the rising current level, avoiding more complex phase-to-phase faults.
3. Deep-earthed. The most common type of neutral used in domestic networks. The winding of transformers on the low side is done in an open star connection and the neutral point is earthed through the earth loop of the transformer or transformer substation. In the event of line faults or a single-phase short circuit, a potential is created with respect to earth, which activates the protection that disconnects the line.
4. Effectively grounded. A kind of grounded neutral, which is used in high-voltage networks of 110 kV and above. The zero point of power transformers and the fault potential are brought to the ground. To improve the efficiency of the protections, additional equipment is used - a single-column neutral earthing switch (ZON). The position of the switching device is determined by the mode instructions. For distribution networks of 6-35 kV, grounding through a low-resistance resistor is used.

Types of connection of windings of power transformers

As noted above, the neutral is the connection of the neutral conductor of a three-phase power transformer or generator. To determine the type of grounding, it is enoughlook at the scheme of power equipment. For an isolated neutral, the circuit diagram is a triangle.

The rest of the options are implemented through the grounding of the neutral conductor to the ground, DHA, low-resistance resistor. The latter are mainly used in substations that convert high voltage electrical energy to low, consumer. Schematic diagram - star.

Insulated neutral in electrical networks

Used in distribution networks 6-35 kV. With regard to the physical manifestations of an isolated neutral, the voltage rises to linear. The main purpose of this type is associated with the following points:

1. The network does not turn off, it continues to work. Consumers on phases without a circuit use single-phase household appliances until the line is disconnected. There is no voltage imbalance in 0.4 kV networks, in networks 6-35 it increases to linear.
2. The implementation of such networks is many times cheaper to maintain, which allows you to save significant funds on the distribution of electrical energy.
3. High reliability, especially on overhead power lines. Falling branch will not turn off the feeder and ensure its performance.

The main disadvantages of isolated networks are:

1. With a single-phase short circuit, the network continues to work, the protections do not work, which sometimes leads to accidents with the population.
2. The presence of ferroresonant processes and the occurrence of reactive power, which degrades the qualityelectrical energy.

Resistor and voltage of 110 kV and above: how is the zero point executed?

Effective grounding is a special type of neutral conductor connected to specialized equipment, which is used in electrical installations above 1 kV. For distribution networks, a variant with earthing through low-resistance resistors is used, which provides line disconnection in case of a single-phase earth fault without time delay.

High voltage lines of 110 kV and above also use the presented type of neutral, which ensures quick response of protections. To increase the sensitivity of the “relay” operation, each power transformer has special ZON equipment. Single column neutral earthing also provides overload protection.

Grounding through low resistance resistors

The use of low-resistance resistors is considered an ideal solution for the safety of people in distribution networks, as well as for maintaining the insulation of cable lines. The implementation of protection involves bringing the zero point to specialized equipment, which has a lower ohmic resistance and gives a signal to turn off the line. The feeder is switched off with a minimum time delay, which is one of the advantages. Others include:

• First, it is a neutral, which, when the "earth" appears, accurately determines the damaged direction and turns off the requiredline.
• Second: there is no need for additional calculations and compilation of regime maps with limited possibilities for ringing distribution networks.

Important disadvantages of this type of grounding:

1. Not effective for high earth fault currents as it causes problems in substations where low resistance resistors are installed.
2. Low efficiency on overhead lines, as well as on long-distance lines. In the first case, the slightest approach of tree branches will cause the feeder to be turned off. Especially relevant with consumers of 1 special, 1 and 2 categories.
3. Extra shutdowns that occur due to improper operation of protections (lack of automatic reclosure), implies downtime in consumption, material losses of the power supply organization.

Blind grounding of power transformers to earth

Everything connected with the 0.4 kV distribution network is a neutral with a deaf ground to the ground. The presented type has a special place and role in terms of security. When a short circuit to the ground occurs, protection is triggered, in particular, PN-2 burns out or the machine turns off. Regarding such a network, protections are also being developed for wiring in houses and apartments. A striking example is the operation of the RCD, which ensures the detection of leakage currents.

The main advantages of this type of neutral are:

1. Ideal for distribution of electrical energy, keeps household and specializedsingle-phase/three-phase equipment.
2. The protection circuit does not require specialized and expensive equipment. Technical means such as fuses or circuit breakers can easily cope with a dead short to ground.

Disadvantages include:

1. Protections are insensitive to long-range short circuit. It is necessary to accurately calculate the ohmic resistance of the phase-zero loop and the correct choice of circuit breakers or fuses.
2. Trip does not occur if there is no earth fault. This poses a danger to humans, which is corrected through the use of insulated wires.

Resonantly grounded or compensated neutrals

Resonantly grounded neutrals are mainly used in distribution networks with a voltage of 6-35 kV, where the connection scheme is carried out by cable lines. The connection of the zero point is carried out through special plunger or adjustable transformers RUOM. Such a system allows you to determine the inductance in the network during a single-phase short circuit, which provides compensation for the current level.

This type of neutral reduces the risk of an accident, the transition of a single-phase short circuit to an interphase one. The advantages for voltage 6-35 kV are:

The main advantage is associated with the purpose of the equipment. High degree of insulation protection of cable lines with proper adjustment

The disadvantages of a network with this type of neutral are:

1. Difficult to set up. Undercompensation or overcompensation may occur,that will prevent proper use of the equipment. For alignment, it is necessary to calculate the inductance of the currents depending on the length of the line, the power of the transformers. In case of changing the scheme or adding power equipment, plunger transformers do not always cope with the tasks.
2. Incorrectly configured equipment and high wear of cable lines leads to a chain reaction that involves the failure of several weak sections of the network.
3. Increase in technical losses that occur during operation, as well as security issues. Current compensation in the substation is implemented with respect to ground.
4. Inability to determine the line where the short circuit occurred. The process of choosing a feeder with "ground" is carried out through a comparison of harmonic currents, which is not always considered an effective means of obtaining reliable information.

Neutral conductor and arc quenching coil, reactor

The difference in resonant grounded neutral is related to the equipment used. As noted above, the zero point can be located on a plunger-type arc quenching coil or on an adjustable reactor. The main differences are associated with the following points:

1. DGK assumes compensation through a tuned system of plunger transformers. The setting is implemented through calculations of a real network by the relay protection service. When an earth fault occurs, currents are compensated based on inductance. The process is not regulated or adjusted, whichis an unpleasant moment in the case of the appearance of "earth" at several points on different lines.
2. DGR - more modern equipment, which involves the use of automatic systems for determining the network inductance. Among the popular options are RUOM-type reactors with SAMUR tuning. Real-time polling implementation ensures operability even with multiple ground faults.

Whether solidly grounded or insulated, each type has its place in today's power industry. And knowledge of the features will allow you to deal with the physical essence of the issue.