Planned power outages are not bad, whereas unexpected ones may have disastrous operational and financial consequences in factories. A 3 Phase Automatic Transfer Switch (ATS) of the proper size will provide smooth power operation, in that it will safely switch electrical loads between the utility supply and a backup power supply. Wrong sizing, however, may cause overheating, irritating tripping, failure of equipment, or even disastrous system failure. The proper sizing of a 3-phase ATS is thus a very important procedure in any industrial power system design.
The Knowledge of the 3-phase ATS
A 3 Phase Automatic Transfer Switch is used to monitor a loss of power supply to the main source and automatically redirect the load to some other source, such as a generator. In industry, ATS units are expected to operate under high currents, under high switching frequency, and under heavy load conditions like motors, transformers, and variable frequency drives. The ATS should be in a position to sustain the full load continuously, as well as to resist the abnormal conditions such as short circuiting and inrush currents.
Calculation of the Total Load Current
Sizing of a 3-phase ATS would require, first of all, knowing the total load current that the switch would carry. This is determined by adding together the connected load, which is usually in kilowatts or kilovolt-amperes. Engineers put this value into full-load current by using the system voltage and power factor. The ATS continuous current rating must be just as much or more than this calculated current.
Industrial loads tend to increase with time. Due to this reason, the experienced designers usually have a margin on top of the calculated load so that the ATS can expand in the future without overheating.
Motor and Inductive Loads
Purely resistive loads are often not a common feature of industrial power systems. Motors, pumps, compressors, and conveyors cause a tremendous amount of inrush currents when starting up, which can range between five and seven times the rated currents. Although the ATS does not interrupt the starting of the motor when used in normal operation, it needs to be able to withstand these momentary bursts of current without mechanical or thermal damage.
In sizing a 3-phase ATS, consideration has to be made of the largest motor load and the mode of starting the motor. Direct online systems cause increased stress on the ATS than soft starters or variable frequency drives. In such a case, it is important to choose an ATS with sufficient short-time withstand current capacity.
ATS Rating Vs Generator Capacity
The ATS should also be compatible with the load and the backup generator. Generator output current, voltage regulation, and transient response are all factors to consider when selecting an ATS. An undersized ATS will limit the output of the generators, whereas an oversized switch will not be more cost-effective.
Another factor to note is to make sure that the ATS controller logic is the same as the start and stabilization time of the generator. The adequate coordination of loads would facilitate the smooth transfer and avoid loading connection until the generator stabilizes at both voltage and frequency.
Identifying the Right ATS Class
ATS devices are IEC 60947-6-1 divided into PC Class and CB Class. Pc Class ATS units are made only to transfer power with a high short-time withstand rating. They use fault-clearing devices that are upstream. CB Class ATS units have circuit breakers and offer transfer and protection services.
In the case of industrial systems that are highly faulted with an already implemented upstream protection, PC Class ATS units are commonly used because they have a high current withstand capability and a shorter transfer time. The selection of the appropriate class has a direct influence on the decision of size, particularly in those systems where the sizing of the system is of great concern due to the high prospective short-circuit currents.
Testing of Short-Circuit Withstand Requirements
In addition to a continuous current rating, an ATS of proper size should be able to survive short-circuit conditions until protective devices respond. This is characterized by a short-time withstand current rating, which is normally given in kiloamperes over a certain time. Facilities that have transformers of high size or utility connections of low impedance tend to be very high on fault levels.
A mismatch of ATS short-circuit to the fault current available in the system may cause serious equipment damage in the occurrence of a fault. The level of fault calculation is thus a critical component of sizing ATS.
Environment and Installation
ATS performance depends on ambient temperature, type of enclosure, and the method of installation. Ambient temperatures decrease current-carrying capacity, and this can necessitate a high-rated ATS. Pendant-mounted ATS units should also be designed with regard to ventilation and heat dissipation, especially at high current ratings.
The size of cable, the layout of busbar, and the switching requirements of a neutral also influence the final ATS specification and should be in compliance with local electrical codes and grounding.
Conclusion
The process of calculating the size of a 3-phase ATS to fit industrial loads is a technical exercise that extends way beyond matching the current rating. It involves a thorough knowledge of the load characteristics, motor behaviour, compatibility, fault level, and environmental conditions of the generators. An ATS of the right size will assure achievement of reliable power transmission, prolonged service life, and safe functioning both in normal and abnormal conditions.
