The neutrals of electrical installations are called - general points of winding of generators or transformers connected to the star. The type of communication of neutrals of machinery and transformers with Earth largely determines the level of insulation of electrical installations and selection of switching equipment, overvoltage values \u200b\u200band methods for their limitations, currents with single-phase ground closures, working conditions for relay protection and safety in electrical networks, electromagnetic effect on communication lines and etc.
Depending on the neutral mode electricity of the net divided into four groups:
1) networks with ungrounded (isolated) neutrals;
2) networks with resonant-grounded (compensated
mi) neutrals;
3) networks with efficiently grounded neutrals;
4) networks with deaf-free neutrals.
The first and second groups include networks with a voltage of 3-35 kV, neutral transformers or generators of which are isolated from the ground or grounded through ground reactors.
Networks with efficiently grounded neutrals are applied to voltage above 1 kV. In them, the ground closure factor does not exceed 1.4. The coefficient of closure on the ground is the ratio of the potential difference between the intact phase and land
at the point of closure to the land of the damaged phase to the potential difference between the phase and the ground at this point to the closure. This group includes networks of 110 kV and higher. The fourth group includes networks of 220, 380 and 660 V. The mode of operation of neutral determines the short circuit current. Networks, in which a single-phase closure current on the ground is less than 500 A, is called networks with small closure currents to the Earth (mainly networks with ungrounded and resonant-grounded neutrals).
Currents more than 500 A correspond to networks with large currents to earth closures (these are networks with efficiently grounded neutrals).
In networks S. isolated neutral The neutral point of the source (generator or transformer) is not attached to the ground contour. IN distribution networks 6-10 kV winding of the supply transformers, as a rule, are connected to the triangle, so the neutral point is physically absent. In installations with an isolated neutral, the process of grounding equipment should not exceed 4 Ohm and with power supply power up to 100 kVA RZ<10 Ом.
In Eu up to 1 kV with an isolated neutral, when closing one phase to the ground, the entire chain is turned off to increase the email. security. Such installations are applied with elevated email requirements. security (mines, metropolitan, peatpooling, mobile installations, some chemical combines).
Advantages:
Lack of need to immediately disable the first single-phase earth closure;
Small current in the place of damage (with low network capacity to the ground).
Disadvantages:
The possibility of arc overvoltages with a mixed character of an arc with a small current (units-dozens of amps) at the site of a single-phase ground-to-ground location;
The possibility of multiple damage (failure of several electric motors, cables) due to insulation breakdowns on other connections associated with arc overvoltages;
The possibility of long-term exposure to the insulation of arc surge, which leads to the accumulation of defects in it and reduce the service life;
The need to perform the insulation of electrical equipment relative to the land on linear voltage;
Complexity of damage location detection;
The danger of electroconducting staff and unauthorized persons with a long existence of ground closure on the network;
The complexity of ensuring the proper operation of relay protection against single-phase closures, since the real current of the earth closure depends on the network operation mode (the number of connections included).
The network with an isolated neutral effectively works only if there is a reliable continuous Insulation Control Devices With the disconnection of the network with an invalid reduction of insulation resistance.
Since ungroundless networks of a small length, as well as compensated networks, can work for a long time with the installation of the phase to the ground, the relay protection against damage to this species is customary to perform with the action to the signal.
The alarm of the earth closure may be non-selective and selective. Non-selective alarm is notified by the service personnel about the occurrence of ground closure without an indication of the attachment on which the insulation disruption occurred. Selective alarm, on the contrary, indicates which attachment caused a closure.
Non-selective alarm, or, otherwise, the insulation control device is based on changing the stresses of wires relative to the Earth. For this purpose, measuring voltage transformers are usually used - three single-phase or one three-phase with a five-track magnetic system. The windings are connected according to the star-star-star-open triangle. The grounding of the zero point of the star of the primary winding of transformers is ensured by measuring the stresses of wires relative to the Earth.
For this purpose, there are three voltmeters included on the secondary phase voltages. The maximum voltage relay is attached to the auxiliary secondary winding connected to the open triangle. The voltage at the outputs of the open triangle is equal to the geometric sum of the secondary three phases relative to the Earth.
In normal mode, the voltmeters show phase voltages, and the voltage on the relay winding is zero. When the wires are closed on the ground on the winding of the relay there is a voltage proportional to the potential of neutral Un(or zero sequence voltage):
The relay is triggered and starts a warning alarm that attracts the attention of the service personnel to a violation of the normal mode. Insulation control voltmeters, showing wire voltage relative to the Earth, make it possible to determine the damaged wire and roughly estimate the resistance value R.. Since the voltage of the wire relative to the Earth in the entire electrically connected network is the same, then its dimension cannot specify the location of the ground.
Isolated neutral mode Used at a voltage of up to 1 kV only in electrical installations with increased safety requirements (explosive installations, etc.). At voltage of 6 ... 35 kV, this neutral mode is recommended Pue in all electrical installations.
The reason for the wide distribution of the mode of operation with an isolated neutral is that in such a network, the closure of one phase to the ground is not short. The network with an isolated neutral can be operated up to several hours with a phase closure on the ground. The earth closure current turns out many times less than the current of the inter-phase KZ. This is the main advantage of the network with an isolated neutral. In such a network, it is usually not necessary to apply special high-speed protection against land, i.e., additional costs do not require and protect protection.
However, when closed to Earth, such a lack of a network is found as arising overvoltages on damaged phases relative to the Earth.
In fig. 1, and a simplified diagram with an isolated neutral when the phase of the phase A. Inclusion passes the ground of closure on the ground í z.z. It is due to the capacities of the network phases with B, with from relative to the Earth. The value of this current is small and usually does not exceed 100 A.
Fig. 1. Simplified scheme with an isolated neutral when the phase land is closed
In normal mode (Fig. 1, b) phase voltage relative to the Earth is the same and constitute U F \u003d U L / √3, where u l is a linear voltage. When the phase is closed, the Phase A potential becomes zero, i.e., the potential of the Earth (Fig. 1, B). The voltage of the damaged phases B and C relative to the phase A will remain the same as in normal mode, because linear voltages do not change. Thus, the phase voltages in and with relative to the land increase to linear, u 'B \u003d u' c \u003d u l (increase in √3 times), i.e. the fault coefficient on the Earth is √3. At the same time, the possibility of transit of the earth closure into double, which is short circuit and is accompanied by a large current.
In the network with an isolated neutral, the insulation of the phases relative to the Earth is selected by linear voltage so that the network can work for a long time with the ground.
Rules for technical operation (PTE) of electric stations and networks of the Russian Federation allow air and cable lines With ground closure when the network neutral is isolated before eliminating damage. At the same time, to find the place of damage should be proceeding immediately and eliminate damage in the shortest possible time due to the danger of lesion by the current of people and animals.
The phase voltages in the network with an isolated neutral during ground closures may exceed linear stresses, which is due to the occurrence of the so-called intermittent electrical arc. The term "interspersed" means that the electric arc is unstable: lights up for a while, then goes out and, after an interval of time, lights up again. Transition processes arising in the electrical circuit of the network (Fig. 1, a) taking into account the intermittent arc leading to the appearance of overvoltages that can reach (3.0 + 3.5) 11, where u is the amplitude of the phase voltage in normal mode. This often leads to a breakdown of isolation, especially electric motors with a voltage above 1 kV.
The presence of overvoltages caused by a mixed electric arc is the main disadvantage of the network with an isolated neutral. This deficiency explains the abundance of proposals for optimizing the neutral regime of urban electrical networks.
The main method of reduced overvoltages during ground closure, according to PTE, is the compensation of the capacitive current of the earth closure, which is achieved using special extinguishing reactors (inductance coils), which include between the neutral network and the earthing. In accordance with PTE, compensation i 3 3 is made if its value is greater than below:
The noncompensated currents in the ground closure can provide unstable burning of the electric arc, i.e., as a result, lead to the appearance of overvoltages. It is especially dangerous to the ground to earth in networks with air lines on reinforced concrete and metal supports, since the current I z. may fail the grounding devices and bearing metal parts of the supports. Therefore, in networks with a voltage of 6-35 kV with air lines on the specified supports, the permissible value of the ground closure current is 10 A.
The current in the place of closure on the Earth is calculated by the approximate formula
where u is a linear voltage, kV; l in - the total length of the network of network lines, km; L K is the total length of the cable lines of the network, km.
Example. Calculate the closure current on the ground in the network with a voltage of 10 square meters. The network contains two air lines with 2 km long and 7 km and 5 cable lines with 1.5 km long; 2.5 km; 0.8 km; 1.2 km; 1.6 km.
Decision:
We determine the total lengths of the air and cable lines L B \u003d 2 + 7 \u003d 9 km; L K \u003d 1.5 + 2.5 + 0.8 + 1.2 + 1.6 \u003d 7.6 km.
Earth closure
Obviously, the compensation of the capacitive current of the earth closure in this network is not required.
