The normal steady-state mode of operation of an electrical installation is considered to be such a mode, the parameters of which are within the normal range. Short-circuit current (short-circuit current) occurs in the event of an accident in the operation of an electrical installation. It most often appears due to damage to the insulation of live parts.
As a result of a short circuit, the uninterrupted power supply of consumers is disrupted and entails malfunctions and equipment failure. As a result, when selecting current-carrying elements and devices, it is necessary to calculate them not only for normal operation but also to check under the conditions of the expected emergency mode, which may be caused by a short circuit.
Types of short circuits
The meaning of a short circuit means an electrical connection that is not provided for by the operating conditions of the equipment between points of different phases, or a neutral conductor with a phase or earth with a phase (if there is a neutral ground loop of the power supply).
When operating consumers, the supply voltage can be connected in different ways:
- According to the scheme of a three-phase network of 0.4 kilovolts.
- Single-phase network (phase and zero) 220 V.
- A source of constant voltage with terminals of positive and negative potential.
In each case, an insulation fault may occur at some points, as a result of which a short-circuit current occurs.
For a 3-phase AC network, there are types of short circuits:
- Three-phase closure.
- Two-phase closure.
- Single phase earth fault.
- Single-phase earth fault (isolated neutral).
- Two-phase earth fault.
- Three-phase earth fault.
When implementing a project for the supply of electrical energy to an enterprise or equipment, such modes require certain calculations.
Causes of insulation damage
- Mechanical impact on insulation.
- Electrical breakdown of live parts due to excessive loads or overvoltage.
- Similar to a breach of insulation, the collision of bare wires of overhead lines from strong winds can be considered the cause of damage.
- Throwing metal objects onto the line.
- Impact of animals on live conductors.
- Errors in the work of maintenance personnel in electrical installations.
- Failure in the functioning of protections and automation.
- Technical aging of equipment.
- Deliberate action aimed at damaging the insulation.
Consequences of a short circuit
The short-circuit current is many times the current during the normal operation of the equipment. Possible consequences of such a closure can be:
- Overheating of live parts.
- Excessive dynamic loads.
- Cessation of supply of electrical energy to consumers.
- Disruption of the normal functioning of other interconnected receivers, which are connected to serviceable sections of the circuit, due to a sharp decrease in voltage.
- Disruption of the power supply system.
Short circuit principle
Before the onset of a short circuit, the current in the electrical circuit had a steady-state value of ip. With a sharp short circuit in this circuit, due to a strong decrease in the total resistance of the circuit, the electric current rises significantly to the value of ito. Initially, when the time t is equal to zero, the electric current cannot sharply change to another steady-state value, since in a closed circuit, in addition to the active resistance R, there is also an inductive resistance L. This increases in time the process of increasing the current when switching to a new mode.
As a result, in the initial period of a short circuit, the electric current retains its original value i K = i but. It takes some time for the current to change. In the first moments of this time, the current rises to its maximum value, then decreases slightly, and then after a certain period of time, it takes on a steady state.
The period of time from the beginning of the closure to the steady state is considered a transient process. The short-circuit current can be calculated for any moment during the transient.
Short-circuit current during the transition mode is best seen as the sum of the components: the periodic current i pt with the highest periodic component I pt and aperiodic current i AT (its largest value – I am ).
The aperiodic component of the short-circuit current during the short circuit gradually decays to zero. Moreover, its change occurs exponentially.
The possible maximum short-circuit current is considered a surge current i у. When there is no damping at the initial moment of the fault, the surge current is determined:
I y – i p m + i and t = 0 ‘, where i p m is the amplitude of the periodic current component.
Useful short circuit
It is believed that a short circuit is a negative and undesirable phenomenon, from which devastating consequences occur in electrical installations. It can create conditions for a fire, disconnection of protective equipment, de-energization of objects, and other consequences.
However, short-circuit current can be of real benefit in practice. There are many devices operating in the mode of increased current values. For example, consider a welding machine. The most striking example of this is electric arc welding, during which the welding electrode is short-circuited with a ground loop.
Such short-circuit modes are short-lived. The power of the welding transformer ensures operation with such significant overloads. During welding, a very high current is generated at the point of contact of the electrode. As a result, a significant amount of heat is released, sufficient to melt the metal at the point of contact, and the formation of a weld of sufficient strength.
Even at the beginning of the development of electrical engineering, the problem of protecting electrical devices from excessive current loads, including short circuits, arose. The simplest solution was to install fuses, which burned out from their heating due to exceeding a certain current value.
Such fuse-links are still in operation. Their main advantage is reliability, simplicity and low cost. However, there are also disadvantages. The simple design of the fuse prompts a person, after the combustion of the fusible element, to replace it with independently available materials in the form of paper clips, wires and even nails.
Such protection is not able to provide the necessary protection against short circuits, since it is not designed for a specific load. In production, electrical machines are used to disconnect circuits in which a short has occurred. They are much more convenient than conventional fuses and do not require replacing a burnt-out element. After eliminating the cause of the short circuit and cooling the thermal elements, the machine can simply be turned on, thereby supplying voltage to the circuit.
There are also more complex protection systems in the form of differential circuit breakers. They are expensive. Such devices disconnect the circuit voltage in the event of the smallest leakage current. Such leakage can occur if a worker is electrocuted.
Another method of short-circuit protection is a current-limiting reactor. It serves to protect circuits in high voltage networks, where the magnitude of the short-circuit current is capable of reaching such a size that it is impossible to select protective devices that can withstand large electrodynamic forces.
The reactor is a coil with an inductive reactance. It is connected in a daisy chain. During normal operation, the reactor has a voltage drop of about 4%. In the event of a short circuit, the main part of the voltage falls on the reactor. There are several types of reactors: concrete, oil. Each of them has its own characteristics.
Ohm’s law at short circuit
The calculation of circuit closures is based on the principle that determines the calculation of the current strength by voltage, by dividing it by the connected resistance. The same principle applies when determining rated loads. The difference is as follows:
- When an emergency mode occurs, the process proceeds in a random way, spontaneously. However, it lends itself to some calculations according to the methods developed by specialists.
- In the course of normal operation of the electrical circuit, resistance and voltage are in a balanced mode and may slightly vary within operating ranges within normal limits.
Power supply power
For this power, an assessment is made of the energetic power capability of a destructive action that a short-circuit current can carry out, an analysis of the flow time, size is carried out.
For example, consider that a piece of copper conductor with a cross-sectional area of 1.5 mm 2 and a length of 50 cm was first connected directly to the Krona battery. And in another case, the same piece of wire was inserted into a household outlet.
In the case of “Krona”, a short-circuit current will flow through the conductor, which will heat this battery until it fails, since the battery power is not enough to heat and melt the connected conductor to break the circuit.
In the case of a household outlet, the safety devices will work. Let’s imagine that these protections are out of order and do not work. In this case, the short-circuit current will flow through the household wiring, then along the wiring of the entire entrance, at home, and then along the overhead line or cable. So it will reach the power transformer at the substation.
As a result, a long circuit with many cables, wires, various connections is connected to the transformer. They will greatly increase the electrical resistance of our test piece of wire. However, even in this case, there is a high probability that this piece of wire will melt and burn.
The section of the power line from the power source to the short circuit has some electrical resistance. Its value affects the magnitude of the short-circuit current. The windings of transformers, coils, chokes, capacitor plates contribute to the total resistance of the circuit in the form of capacitive and inductive resistances. This creates aperiodic components that distort the symmetry of the fundamental forms of harmonic vibrations.
There are many different techniques used to calculate the short-circuit current. They make it possible to calculate the short-circuit current with the required accuracy from the available information. It is practically possible to measure the resistance of the existing circuit using the “phase-zero” method. This resistance makes the calculation more accurate, makes appropriate adjustments when selecting short-circuit protection.