In the normal mode of power supply, energy is provided by the energy company and supplied to the place of its use. When its main source stops working, the power from the second network input or the used backup generator must be manually or automatically supplied to the loads, for which the ABP (automatic reserve transfer) circuit serves . Its main task is to redistribute power from the power system to a backup power source.
III category of reliability of power supply
As you know, energy supply companies divide all their consumers, that is, those individuals (legal and physical) with whom they conclude contracts for the supply of electricity, into three categories according to the degree of reliability of electricity supply. The lowest reliability in the 3rd category. Such an energy client is provided with only one three-phase input of voltage of 6 or 10 kV (sometimes even 400 V) or a single-phase input of 230 V from one supply substation, but the cost of connecting loads to the network in this category is minimal - it is enough to install a simple single-transformer KTP and connect it to the nearest overhead transmission line.
Do I need an ABP scheme for category III?
The PUE allows the possibility of power supply according to this scheme, if the power engineers guarantee the restoration of power after accidents in no more than a day. And if this is not so? Then you need a backup power source, which is usually a gas-electric unit or diesel generator. In the old days, consumers manually connected their loads to them and put them into operation. But as the automation of these products developed, it became possible to launch them without human intervention.
And since it is possible to start the diesel generator automatically, it is also possible to connect consumer loads to it. So the modern concept of a two-wire automatic transfer switch came into existence, the electrical circuit of which, given below, is already becoming the standard of power supply for a private house.
II category: does she need ABP
If the consumer orders two power supply network inputs, then he goes into the next category - the second. In this case, energy companies usually require customers to pay for the construction of a two-transformer substation. In the simplest version, it contains two bus sections (these are just aluminum or, at best, copper strips) high voltage with its input switches, each of which is connected to only one of the high voltage inputs (6 or 10 kV). Between sections there is a so-called section switch. If it is open, then each high-voltage input can supply only one transformer (as a rule, only one of the two is in operation, the second is in reserve - and this is also a typical requirement of power engineers). In the event of a voltage failure at one of the inputs, the consumer electrician can manually turn on the sectional switch and apply a load to the constantly working transformer from another high-voltage input.
Such consumers, in fact, do not need an ABP. However, in the last decade, power engineers often offer them to install them in typical two-transformer substations on the low-voltage side. Such an ABP shield has two inputs from the low voltage windings of different transformers (they must both be energized, but only one of them is loaded at any time) and one output to the low voltage buses, to which all loads are connected.
I category - ABP is required
But if the consumer, in principle, is not satisfied with the time delay for manual switching of inputs, then he is forced to use ABP without fail and move to the next category of power supply reliability - the first one. In the simplest version, the circuit breaker circuit breaker circuit can contain two inputs from the same two sections of the high-voltage substation buses and a block switching unit for a section switch (usually vacuum). If the voltage disappears at the supply inlet, then the automation switches off its input switch and turns on the section switch. After that, the voltage is supplied to the combined buses from the second input. ATS for two inputs in this case can also be performed on the low voltage side of the substation, as described above.
But among consumers of the 1st category, the PUE distinguishes the so-called special group, which is not enough for two network power supply inputs, and a third backup input, usually performed from a diesel generator, is also necessary. In this case, an ABP for 3 inputs is required. Its circuit is performed at low voltage.
How the ABP device with generator input works
Recently, many automatic backup devices with a microprocessor control controller have appeared on the market. Very popular in this regard is the control relay controllers of the Easy series manufactured by Moeller. By analyzing the signals of the voltage sensors, the microcontroller detects a power failure and initiates the procedure for starting the generator engine (usually synchronous). As soon as it reaches the rated voltage and frequency, the control system switches the consumer load to power from it. From the point of view of electrical engineering, the connection circuit of the ATS for critical and powerful loads is a rather difficult task, since the inevitable time delays and other technical difficulties make it difficult to instantly obtain backup power.
Frequency and voltage control
One of the main functions of the ATS device is the detection of a voltage drop or the complete disappearance of the main power source. As a rule, all phases of the supply network are monitored externally by means of a minimum voltage relay (phase monitoring relay). The point of failure is determined by the voltage drop below the minimum allowable level at any of the phases. Information about the voltage and frequency is transmitted to the ATS switchboard, where the possibility of continuing the supply of loads is determined. The permissible minimum voltage and frequency must be overcome before switching the loads to power from the backup generator, the power of which should provide it.
Main time delay
The ABP circuit usually has the ability to widely adjust the delay time of its operation. This is a necessary function for the possibility of stopping unjustified shutdowns from the main power sources in case of short-term violations. The most prevailing time delay covers any short-term shutdowns so as not to cause unnecessary starts of the drive motors of the generators and switching loads on them. This delay is in the range of 0 to 6 seconds, with one second being the most common option. It should be short, but sufficient to connect consumer loads to redundant power supplies. Many companies today buy powerful uninterruptible power supplies with rechargeable batteries, which provide the minimum connection delay time.
Additional time delays
After restoring the main power, some time delay is necessary to make sure that the load is stable enough to disconnect it from the backup power. As a rule, it is from zero to thirty minutes. ATS for the generator should automatically bypass this time delay in returning to the main source if the backup fails, and the main one is working normally again.
The third most common time delay includes the cooling period of the engine. During its duration, the control system of the diesel generator controls the unloaded engine until it stops.
In most cases, it is usually desirable to transfer loads to the backup generator as soon as the appropriate voltage and frequency levels are reached. However, in some situations, end users want a sequence of switching different loads to the backup generator. When required, several ABP circuits for the generator are executed, triggered with individual time delays, so that the loads can be connected to the generator in any desired order.
Executive devices of reserve input circuits
The end result of the work of the considered class of devices is the switching of electrical circuits, their switching from the main input to the backup. As noted above, in power substations, the ABP circuit can be implemented on both the higher and lower voltage side. In the first case, standard high-voltage switches serve as its executive elements. In the second case, which includes switching loads to the generator input, switching is carried out by low-voltage devices.
They can either be part of the equipment of the switchboard (panel) ABP, or they can be external to it and form part of the general power supply circuit of the loads. In the first case, the use of magnetic starters is possible - it is used in backup devices for non-industrial consumers with a load of up to several tens of kW. At higher powers, ABP is used on contactors. The schematic diagram of the device in both cases is the same.
External low-voltage devices of reserve input circuits are power circuit breakers with electromagnetic drives. The function of the ATS device itself is reduced in this case to the formation and generation of corresponding on / off signals to them.
Type ABP block for 3 inputs. Scheme and algorithm of work
It is designed to provide continuous supply of 0.4 kV loads from three power sources: two three-phase mains inputs and a three-phase diesel generator input. Executive devices are standard circuit breakers Q1, Q2 and Q3 of each of the inputs, protecting the loads of the 1st category of reliability of power supply.
The block operation algorithm is as follows:
1. There is voltage on the main input. Then Q1 is on, and Q2 and Q3 are off.
2. There is no voltage at the main input, but it is at the backup. Then Q2 is on, and Q1 and Q3 are off.
3. There is no voltage at the main and backup inputs. Then Q3 is on, and Q1 and Q2 are off.