Currently, for the safe power supply of electrical equipment, deaf grounding is mainly used. At the same time, there are devices that are operated in a three-wire network with isolated neutral. This includes mobile equipment, devices for peat extraction and other mechanisms that operate in 380-660 V networks. In addition, this type of protection is used in electric mains with voltage from 2 to 35 kV.
Content
- Neutral operating modes
- Isolated device description
- Operating principle
- Advantages and disadvantages
Neutral operating modes
The neutral of electrical equipment is the common point of the winding of the generator or transformer, which is connected in a star. The insulation level of the electrical equipment depends on how the neutral is connected to earth.
In addition, such a connection determines the choice of switching devices, the value of overvoltage and methods of their elimination, the magnitude of the currents during a single phase earth fault, etc. From what mode the neutral is in, schemes of four types are known:
-
with isolated neutrals; - with resonant-grounded devices;
- with effectively grounded equipment;
- with deafly grounded neutrals.
Currently, the first two types are used in electrical networks with voltages from 3 to 35 kV. Effective grounding is most often found in power supplies with a voltage higher than 1 kV and a closing factor of no more than 1.4. This indicator means the difference between the potential of the phase and ground in the normal state and in case of phase failure.
The group with a solidly grounded neutral refers to networks with voltages up to 1 kV.
Isolated device description
Such a protection device is a system when the neutral conductor of the generator or transformer is not connected to the earth electrode. Connection to a solid ground is allowed through signaling, protection and measuring devices that have high resistance.
In this case, the isolated neutral is a three-phase network connected from the electrical equipment to earth via resistors.
In this case, a system with capacitors is connected in parallel. Such a neutral connection scheme has two components:
- active;
- reactive.
The active circuit is designed to prevent the leakage current by using resistors, which, due to the high resistance, reduce its value to a minimum. The reactive system has capacitors in which one plate is connected to the line, and the other to the ground.
Operating principle
In a working three-phase network, the load is distributed evenly. In the event of a breakdown of any phase in an isolated neutral circuit, a ground fault occurs. Usually, in this case, a breakdown to the body of an electrical consumer occurs.
It can be both electric motors and metal equipment. If there is no grounding, then voltage appears on the devices. This situation is very dangerous when a person touches the body of the structure.
When there is an isolated neutral in the network, the current will decrease to a minimum and become safe for the employee. Currently, such a protection system is used:
-
In two-wire DC networks. - In electrical equipment operating in a three-phase network with a voltage of up to 1 kV.
- In low voltage circuits with protective devices.
Protective devices include the use of isolation transformers or the use of additional insulation. The fact is that it is impossible to disconnect too little current with conventional fuses and circuit breakers.
Such equipment is simply not designed for such values. Therefore, it is required additional relay equipment, which will warn about an emergency.
Since these devices are difficult to operate, only highly qualified workers carry out their maintenance.
Advantages and disadvantages
One of the most important advantages of the mode of such networks is the presence of a small current in case of single-phase earth faults. This fact makes it possible to significantly increase the operation of circuit breakers. The fact is that a ground fault is in practice 90% of the total number of emergencies.
In addition, the presence of a low current allows you to reduce the requirements for grounding equipment. This neutral mode also has many disadvantages. For example, a single-phase earth fault can cause ferroresonance phenomena, which often lead to the failure of electrical equipment.
Arc overvoltages may occur, resulting in a single-phase fault in a two- and three-phase fault. In addition, the design of the short-circuit protection is rather complicated, which leads to its insufficient performance and efficiency. There is an opinion that with a single-phase short circuit, further operation of electrical equipment is possible.
But practice shows that almost immediately there are two- and three-phase short circuits, which ultimately lead to the shutdown of electrical equipment. If a wire falls at power line supports, when a short circuit persists, hazardous touch voltages appear. Most deaths occur in these situations.
Therefore, for the uninterrupted operation of power supply in networks with isolated neutrals, automatic switching on of reserve power supplies is used.
