What is an electrical substation?
1. What is a substation?
Electrical substations are the installations responsible for transforming voltage, frequency, number of phases, or the connection of two or more circuits.
They can be located next to generating stations and on the periphery of consumption areas, outside or inside buildings. In cities today, substations are located inside buildings to save space and pollution. On the other hand, outdoor installations are located on the city's outskirts.
Substations can be of two types:
- Transformer substations: these substations transform electrical energy by means of one or more transformers. These substations can be either step-up or step-down substations.
- Switching substations: these substations connect two or more circuits and carry out their switching operations. Therefore, in this type of substation, the voltage is not transformed.
2. Step-up transformer substations
They raise the generated voltage from medium to high or very high to transport it. They are located outdoors and are located next to the electricity generating stations.
The primary voltage of transformers is usually between 3 and 36kV. While the secondary voltage of the transformers is conditioned by the voltage of the transmission or interconnection line (66, 110, 220, or 380 kV).
3. Step-down transformer substations
These are substations whose function is to reduce high or very high voltage to medium voltage for subsequent distribution.
The primary voltage of the transformers depends on the voltage of the transmission line (66, 110, 220, or 380 kV). While the secondary voltage of the transformers is conditioned by the voltage of the distribution lines (between 6 and 30 kV).
4. Main types of failures and their protection systems.
The most frequent faults occurring in electrical circuits are as follows:
- Short-circuit: the voluntary or accidental connection of two points of a circuit between which there is a potential difference. These faults must be eliminated in less than 5 seconds.
The protection systems used are:
o Fuses.
o Disconnectors.
o Electromagnetic switches.
- Overcurrent: a current that exceeds the rated current and can eventually cause an overload or a short circuit. Overcurrent means an increase in current that exceeds the rated current.
The protection systems used are:
o Fuses
o Electromagnetic and magneto-thermal circuit breakers.
- Direct contact: it is the contact between people and active parts of the installation. The protection systems used are:
o Isolate the active parts of the installation.
o Enable a safety distance by means of obstacles.
- Indirect contact: contact of people with masses that are accidentally live, for example often happens with the casings of electrical machines.
The most commonly used protection against indirect contacts is the one that combines the residual current circuit breaker with the grounding earths.
- Disturbances:
o Overvoltages: voltages higher than the maximum value that can exist between two points of an electrical installation. Surge protection relays are used to prevent overvoltages.
o Undervoltages: voltage lower than the rated operating voltage of the circuit. To prevent undervoltages, undervoltage protection relays are installed.
5. Definition of protection systems
It is necessary to have protection systems for the different electrical installations, such as:
Fuse cutout
They are devices designed to automatically cut the electric circuit when the electric current that crosses them is very high.
The fuse is the part of a circuit that melts if it exceeds the current for which it was built.
The fuse is only the sheet or conductor wire destined to melt and, therefore, to cut the circuit, while the fuse cutter includes, in addition, the casing, the supporting materials, etc., and the fuse.
Thermal relay
A protection device capable of detecting inadmissible currents.
On its own, it cannot eliminate the fault and requires another element to disconnect the receivers. A signal lamp is usually used when closing the circuit to indicate that the thermal relay has tripped due to an impermissible overcurrent.
Magneto-thermal circuit breaker
An electromechanical device with the capacity to cut off, by itself, impermissible overcurrents and short circuits that may occur.
- Short-circuit disconnection: operates on the magnetic operating principle. A magnetic coil creates a force that, by means of a lever system, opens the moving contact (current input).
If the electric current through the circuit breaker exceeds the rated current by several times, the circuit breaker opens in less than 5 ms.
- Disconnection due to overload: in this case, it acts on the principle of thermal operation.
A bimetal is bent when it is crossed by an impermissible overcurrent and causes a force that is transmitted by means of levers and disconnects the moving contact.
The tripping time is determined by the current passing through it: the higher the current, the shorter the tripping time.
Differential circuit breaker
Protection device that detects and eliminates insulation defects.
This device is very important in electrical installations and needs to be protected from overcurrents and short circuits by placing a circuit breaker before it.
During the operation of this device in normal situations, the current entering a receiver has the same value as the current leaving it.
However, in the event of an insulation fault, there will be an imbalance between the input and output current; the current variation will not be zero. The differential switch acts by opening the circuit when it detects that this current variation is not zero.
Electromagnetic switch or relay
They protect electrical installations subject to strong current peaks (e.g. when starting motors on lifting equipment), against major overloads.
Disconnectors
A mechanical connection and disconnection device for switching circuit connections to isolate an element of the electrical network or a part of it from the rest of the network.
Before the disconnector can be used, the electrical current in the circuit must be cut off.
