Selection and main technical requirements of automatic transfer switch

Automatic transfer switch ATSE (AutomaticTransfer Switch Equipment) is a commonly used electrical switching device. Its main function is to select a safe and reliable power supply to supply power to the load between two available power supplies to ensure the continuity of power consumption for the load. Since the emergence of a power distribution system with dual power supplies supplying power to loads, there has been the application of dual power automatic transfer switches. Relevant departments in my country promulgated ATSE's national standard GB/T14048.11 on October 8, 2002, and implemented it on April 1, 2003. This standard is equivalent to the International Electrotechnical Commission standard IEC60947-6-1. This article will closely combine the GB/T14048.11 standard, comprehensively analyze the existing problems in design and use, especially the requirements and limitations of ATSE in the bidding technical requirements, and make the following preliminary elaboration and discussion.

1. Definition of automatic transfer switch
 GB/T14048.11 definition: Automatic transfer switch electrical appliance ATSE is composed of one (or several) transfer switch electrical appliances and other necessary electrical appliances. It is used to monitor the power supply circuit and automatically transfer one or several load circuits from one power supply to Another power source for electrical appliances. From this definition we can understand as follows:
(1) ATSE is an independent electrical switch
 It is parallel to electrical switches such as circuit breakers, load switches, fuses, and contactors, and has corresponding electrical parameter requirements.
(2) ATSE is composed of a transfer switch and corresponding monitoring, control and other electronic circuits
 In other words, only a complete ATS main switch and matching controller can constitute ATSE.
(3) The control mechanism provided by ATSE monitors the status of the two power supplies on its own
 According to general understanding, the main parameters of the power supply to be monitored should be voltage and frequency, but the standard does not clearly indicate whether the monitored voltage/frequency should be three-phase or single-phase. For designers, if ATSE is required to have a phase loss conversion function, the ATSE controller should have a dual-channel three-phase detection function.
(4) This conversion process is completed automatically
 ATSE's controller will compare the actual status of the detected dual-channel power supply with the pre-stored settings of the controller and make a logical judgment, and then make a control command whether to drive the conversion.
(5) This conversion is completed between two power supplies
 In the current power supply system, most of the emergency loads have two power supplies, one in use and one in standby. However, there are also three-power supply systems in which one is in use and two are in standby. For this kind of system, one ATSE alone cannot complete the conversion. The solution is to use two ATSEs to complete it, or use three circuit breakers (pay attention to whether there is a mechanical interlock) external control and electrical interlock to complete it.

2. ATSE classification
 According to national standards, ATSE has only one classification method: PC level and CB level. At present, the "two-stage" and "three-stage" ATSE commonly referred to in the market are irregular titles.
 PC-level ATSE only completes the function of automatic conversion of dual power supplies, but does not have the function of short-circuit current breaking; CB-level ATSE can complete dual power supply conversion and has the function of short-circuit current protection.
 PC-level and CB-level ATSE each have their own advantages and disadvantages, and it is impossible to simply measure which one is better between PC-level and CB-level. Designers and users can choose the ATSE level according to the usage habits and system requirements of the region, and combine it with other electrical components to build an emergency power supply system.

3. Basic technical parameters of ATSE
 As an electrical switch, ATSE has some basic technical parameters and requirements. These are also the details that should be paid attention to in the design and the technical points that should be included in the preparation of bidding technical documents. The main ones are:
(1) Level
 To choose and use ATSE, you must first determine its level, whether it is PC level or CB level. This will affect how to choose the short-circuit characteristics of ATSE in the future.
(2) Rated voltage, rated operating current, rated frequency, number of switching poles
 These electrical technical parameters are the same as those of the circuit breaker we are familiar with, so we will not go into details here.
(3) Usage category
 Usage categories are classifications of load types. According to national standards, the specific classification is shown in Table 1.
Table l ATSE usage categories
 In the national standards, at least two important ATSE indicators, rated making and breaking capacity and operating performance, are closely related to the use category.
 The rated making and breaking capacity is specified by the manufacturer and refers to the current value that ATSE can make and break under specified conditions. For purely resistive loads, the making and breaking current is the rated current. However, in actual use, there are rarely pure resistive loads, and most of them are mixed loads of inductive, capacitive, and resistive loads. For example, for the conversion of motor-type loads, for most ATSEs, it is a "break first and then connect" conversion, that is, during the conversion process, for the motor load, it is actually a process of re-powering and restarting. The ATSE here must have sufficient making and breaking capabilities to meet the inrush current at the starting moment. The national standards have specific limitations and experimental conditions for this parameter, see Table 2.
Table 2 ATSE making and breaking experimental conditions
 As can be seen from the above table, the making capacity of AC-31 with non-inductive or slightly inductive loads is very small, and its rated making and breaking capabilities are only 1.5 times the rated current. Basically, the restart impact of inductive or capacitive loads is not considered. . For most loads, AC-33 should be considered, which means that it can meet the requirements of 6 times the rated current for making and breaking conditions (because the starting current of most motors is 6 to 8 times the rated current, so in the country In the standard, for the AC-33 load type, 6 times the rated current is used for experiments).
 During the design and use process, you must clearly distinguish the concept of use category II. Even if you choose to meet the requirements of IEC60947-6-1 (IEC standard equivalent to GBl4048.11), it is only suitable for AC-3l load use category. ATSE, for motor-type loads, the impact current generated by instantaneous contact will still cause the ATS contacts to melt and stick.
 Therefore, it is recommended that designers and users choose ATSE based on AC-33 A or 33 B type loads when they are unsure of the load type.
 Another ATSE technical indicator closely related to the usage category is operational performance. See Table 3 for relevant national standard requirements.
Table 3 Verification of the operating performance of the ATSE corresponding to the making and breaking conditions for various usage categories
 As explained above, due to the different load usage categories, the experimental conditions of the AC-33 type load are more stringent than the AC-3l type load, and it is more suitable for complex types of actual loads.
(4) Short circuit characteristics
 1) Rated short-time withstand current ICW
 For PC-level ATSE, it only completes conversion and does not have short-circuit protection capabilities. Its own short-circuit protection relies on the front-end circuit breaker (or fuse). This circuit breaker or fuse is not set up to protect ATSE alone, but protects the entire circuit.  ATSE is also an electrical component in the circuit and is also within the scope of protection.
 When PC-level ATSE encounters a short-circuit current, it must be able to withstand a certain short-circuit current impact within a certain period of time until the protective circuit breaker takes effect. This withstand capability is the rated short-time withstand current, which has two concepts: "withstand time" and "withstand current value". The provisions on rated short-time withstand current in national standards are shown in Table 4.
 Table 4 Rated electricity withstand during paving.Regulations related to flow
Note: Consider AC 50Hz
 ATSE currently on the Chinese market basically meets this requirement. There are two points that need to be explained:
 (1) The rated short-time withstand current is specified by the manufacturer. ICW, ICU and ICS are different concepts.
 The rated ultimate short-circuit breaking capacity ICU is also given by the manufacturer. It is the standard technical parameter of the circuit breaker, indicating the maximum short-circuit current that the circuit breaker can carry and break.
 The rated operating short-circuit breaking capacity ICS is also the basic technical indicator of the circuit breaker, indicating how much short-circuit current the circuit breaker can break during actual operation. Expressed by the percentage of the ultimate short-circuit breaking capacity ICU, generally: 50%, 75%, 100%.
 The rated short-time withstand current ICW is, firstly, the basic technical parameters for electrical components that do not have protection capabilities, such as load switches, contactors, PC-level ATSE, etc.  There is no conversion relationship between ICW and ICU, but the ICW value of the circuit breaker must be smaller than the ICU value. For example, the NS series switch has an 800A rated ICU value as high as 75kA, but the ICW value is only 25Ka (1s). They are all numerical values ​​given by manufacturers' experiments and are used to measure the parameters of different electrical equipment. In actual engineering design and use, these two numerical requirements should be calculated by the designer and met by the manufacturer.
 Simply put, the core component of ATSE should be the contacts of the switch. There are many ways to drive the contacts and lock the contact positions. The ICW value mainly depends on the quality of the contact. We know that the heat generated by a conductor when energized is proportional to the square of the current flowing and time. If you want to increase the ICW value so that the contacts do not melt due to overheating, you must increase the melting point of the switch contacts and use high melting point alloy contacts and increased Large contact area to reduce conductor resistance.
 (2) The ICW value that meets national requirements may not necessarily meet the usage requirements of actual projects. For circuit breakers with the same rated current, their short-circuit breaking capacity is divided into high, medium and low. Therefore, designers must specify the short-circuit withstand current at ATES based on their own calculations. This value is often higher than the national The standards require a lot. For example, a circuit with a rated capacity of 150A is directly drawn from the low-voltage bus of the substation for use by the substation itself. The selected ATSE rated capacity is 150A, but the short-circuit withstand capacity may be more than 40kA. The 150A ATSE 10kA corresponding to the national standard is sufficient.
2) Short circuit making ability
 Similar to the rated making capacity discussed earlier, if the transfer occurs during a short circuit fault, such as a circuit breaker tripping the normal power supply due to a short circuit and de-energizing the normal power supply side of the ATSE, the ATS contacts will withstand a maximum current when making contact. , ATSE's endurance is measured by its short-circuit connection capability.
 3) The technical indicator of the rated short-circuit making and breaking capacity of CB-level ATSE is easy to understand. It is actually the rated short-circuit making and breaking capacity of the circuit breaker, so I won’t go into details here.

4 Conclusion
 National standard GB/T14048.11 is only a general standard and minimum requirement for the development, production, design and use of ATSE. Other problems will be encountered in actual design and use, as well as ATSE's controller and logic judgment problems. We hope that the above introduction can give readers a general understanding of ATSE's technical indicators, and allow readers to more conveniently and flexibly select and use ATSE equipment based on their actual engineering conditions, while also proposing technical specification requirements that are consistent with national standards.