In normal power supply mode, energy is provided by the utility and delivered to the point of use. When its main source stops working, power from the second mains input or the backup generator used must be manually or automatically supplied to the loads, for which the ATS (automatic transfer of reserve) scheme serves. Its main task is to redistribute power from the power system to a backup power source.
III category of power supply reliability
As you know, energy supply companies divide all their consumers, i.e. those persons (legal and natural), with whom they enter into contracts for the supply of electricity, into three categories according to the degree of reliability of electricity supply. Category 3 has the lowest reliability. Such a client of the power industry is provided with only one three-phase voltage input of 6 or 10 kV (sometimes 400 V) or a single-phase input of 230 V from one supplysubstations, but the cost of connecting loads to the network in this category is minimal - it is enough to install a simple single-transformer package transformer substation and connect it to the nearest power transmission line.
Do I need an ATS scheme for Category III?
PUE allows the possibility of power supply according to such a scheme, if the power engineers guarantee the restoration of power after accidents in no more than a day. What if it's not? Then you need a backup power source, which is usually a gas-powered unit or a diesel generator. In the old days, consumers manually connected their loads to them and started them up. But as the automation of these products developed, it became possible to launch them without human intervention.
And since it is possible to start a diesel generator automatically, then in the same way it is possible to connect consumer loads to it. This is how the modern concept of a two-input ATS arose, the electrical circuit of which, given below, is already becoming the standard for power supply to a private house.
Category II: Does she need ATS
If a consumer orders two mains power inputs, then he goes to the next category - the second. In this case, power engineers, as a rule, require customers to pay for the construction of a two-transformer substation. In the simplest version, it contains two sections of busbars (these are just aluminum or, at best, copper strips) of high voltage with their input switches, each of which is connected to only one ofhigh voltage inputs (6 or 10 kV). Between the sections is the so-called sectional switch. If it is open, then each high-voltage input can feed 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 power failure at one of the inputs, the consumer's electrician can manually turn on the sectional switch and load the constantly working transformer from another high-voltage input.
These customers don't actually need an ATS. However, in the last decade, power engineers have often offered them to install them in typical two-transformer substations on the low voltage side. Such an ATS shield has two inputs from the low voltage windings of different transformers (both of them must 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-th category - ATS is mandatory
But if the consumer, in principle, is not satisfied with the time delay for manual switching of inputs, then he is forced to use ATS without fail and move to the next category of power supply reliability - the first. In the simplest version, the circuit diagram of the ATS may contain two inputs from the same two sections of the high-voltage buses of the substation and a block for switching on a sectional switch (usually a vacuum one). If the voltage disappears at the supply input, then the automation turns off its input switch andincludes sectional. After that, 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 the consumers of the 1st category, the PUE singles out the so-called special group, which includes not enough two network power inputs, but a third backup input is also required, usually performed from a diesel generator. In this case, an ATS for 3 inputs is required. Its circuit is performed at low voltage.
How Generator Input ATS works
Recently, many automatic redundancy devices have appeared on the market with a microprocessor controller. In this regard, control relay-controllers of the Easy series manufactured by Moeller are very popular. By analyzing the signals from the voltage sensors, the microcontroller detects a power failure and initiates the procedure for starting the generator motor (usually synchronous). As soon as it reaches the rated voltage and frequency, the control system switches the consumer loads to power from it. From the point of view of electrical engineering, the connection of ATS for critical and powerful loads is a rather difficult task, since the inevitable time delays and other technical difficulties make it difficult to obtain instant backup power.
Control frequency and voltage
One of the main functions of an ATS device is to detect a voltage drop or fullloss of the main power source. As a rule, all phases of the supply network are monitored externally by means of an undervoltage relay (phase monitoring relay). The point of failure is determined by the voltage drop below the minimum allowable level on any of the phases. Information about voltage and frequency is transmitted to the ATS shield, where it is determined whether it is possible to continue powering the loads. The permissible minimum voltage and frequency must be overcome before switching the loads to power from the standby generator, the power of which should be provided.
Main time delay
The ATS circuit usually has the ability to widely adjust the delay time of its operation. This is a necessary function to be able to stop unjustified disconnections from the main power supply sources in case of short-term disturbances. The most prevailing time delay overrides any momentary outages so as not to cause unnecessary starts of generator drive motors and load transfers to them. This delay ranges from 0 to 6 seconds, with one second being the most common. It should be short, but sufficient to connect consumer loads to standby power supplies. Many companies are now buying powerful, battery-powered uninterruptible power supplies that provide the lowest possible connection latency.
Additional time delays
After the restoration of the main power, some temporarythe delay is necessary to ensure that the load is stable enough to be disconnected from the standby power. As a rule, it is from zero to thirty minutes. The ATS for the generator should automatically bypass this time delay in returning to the main source if the backup fails and the main is working fine again.
The third most common time delay involves the engine cooling down period. During this period, the diesel generator control system controls the unloaded engine until it stops.
In most cases, it is usually desirable to transfer loads to a standby generator once the appropriate voltage and frequency levels have been reached. However, in some situations, end users want a sequence of transfers of different loads to the standby generator. When required, several ATS circuits for the generator are executed with individual time delays so that the loads can be connected to the generator in any desired order.
Executive units of reserve input schemes
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 one. As noted above, in electrical substations, the ATS circuit can be implemented both on the high and low voltage sides. In the first case, its executive elements are standard high-voltage circuit breakers. In the second case, which includes the switching of loads to the generator input, switching is carried out by low-voltagedevices.
They can either be part of the ATS shield (panel) equipment, or they can be external to it and be part of the overall load power supply circuit. In the first case, it is possible to use magnetic starters - it is used in backup devices for non-industrial consumers with their load power up to several tens of kW. At higher powers, AVR is used on contactors. The circuit diagram of the device is the same in both cases.
External low-voltage devices of reserve input circuits are power circuit breakers with electromagnetic drives. The function of the ATS device itself in this case is reduced to the formation and issuance of appropriate on / off signals to them.
Typical ATS block for 3 inputs. Scheme and algorithm of work
It is designed to implement continuous power supply of loads with a voltage of 0.4 kV from three power sources: two three-phase network inputs and a three-phase input of a diesel generator. The executive devices are regular circuit breakers Q1, Q2 and Q3 of each of the inputs, protecting loads of the 1st category of power supply reliability.
The block operation algorithm is as follows:
1. There is voltage on the main input. Then Q1 is enabled and Q2 and Q3 are disabled.
2. There is no voltage at the main input, but it is at the reserve input. Then Q2 is enabled and Q1 and Q3 are disabled.
3. On the main and backup inputsno tension. Then Q3 is enabled and Q1 and Q2 are disabled.
