UPS is the most commonly used power supply security equipment for electronic equipment. How to choose the appropriate power supply scheme framework is the most important link to ensure the power supply security level of the data center. If there are defects in the structure, it is difficult to optimize and improve in the later operation and maintenance services. 

The structure of the UPS system

1. Stand-alone system architecture
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As shown below, this architecture scheme is suitable for the power supply of load equipment that can be planned to be interrupted or stopped during operation. The system should be equipped with a built-in or external manual maintenance bypass switch to improve the load availability as much as possible.
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Schematic diagram of stand-alone system

The single-machine system is the most widely used solution at present. The load is independently protected by a UPS. The maintenance without power failure is manually forced to switch from the inverter to the static bypass for power supply, and then close the manual maintenance bypass switch. The load is directly supplied by the mains. The UPS host that is out of operation can be maintained without interruption. Manual maintenance bypass switching can be performed without interruption "first on and then off" switching process, which will not cause the problem of load interruption of power supply.

2. Parallel system architecture

This framework scheme, also commonly referred to as the N+X framework, is shown As shown below. (a) shows the electrical schematic diagram of the modular machine N+X redundant system; (b) shows the electrical schematic diagram of the tower machine N+X redundant system. N is the basic capacity of the UPS selected according to the load, X is the number of system backups, the system can accept the failure of "X" number of equipment or modules without affecting the system operation. In principle, the larger the number of X, the higher the safety factor of the system. Personally, it is recommended to set X to 1-2, which can not only save energy consumption, but also have corresponding fault tolerance.
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Schematic diagram of parallel redundant structure system of UPS

When using the N+1 parallel system, one UPS host can be allowed to carry out active maintenance without affecting the load operation, which has great advantages compared to the application scheme of the single-machine system. It is suitable for application in important single power supply load power supply systems that cannot be powered off. As shown in the figure above, the N+X system can be divided into two types: tower parallel and modular UPS. The basic working principle is the same, but the modular UPS has the hot-swap function, which is more convenient for maintenance, operation and system availability. .

3. 2N System Architecture

As shown below, the electrical schematic diagram of the 2N structure UPS power supply system, which is also the palace structure commonly used in banking systems.
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Electrical schematic diagram of 2N structure UPS power supply system

The 2N power supply system is a power supply structure developed on the basis of the popularization of the load dual power supply technology, also known as the dual bus power supply scheme. The two-way UPS system in the system is composed of completely independent power supply and distribution units. Both systems can meet the power needs of all loads, and the two systems work at the same time as backup for each other. During normal operation, each system supplies 50% of the electrical energy to the load, and when one system stops running due to failure or maintenance, another unit supplies 100% of the electrical energy to the load. Because the two power sources are independent of each other without any connection, this solution can effectively solve the hidden danger of single point failure between the UPS output and the terminal load, and further improve the reliability and availability of the system.
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In practical application cases, if there are some terminal equipments that do not have a dual power supply structure, it can be considered to add STS equipment to the front end for optimization. The electrical principle structure diagram of the 2N structure power distribution system shown below can solve the problem.
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2N structure power distribution system principle structure diagram

"Data Center Design Specification" stipulates that A-level data centers should meet fault tolerance requirements, and can use 2N systems or other system configurations that avoid single point failures, mainly 2N, DR, and RR power supply systems. In the choice of the system solution architecture, if economic conditions permit, the author personally prefers the 2N architecture, because the system architecture is concise and clear, it is easy to achieve physical isolation, and the advantages of simple daily operation and maintenance are incomparable to the other two system architectures.

The choice of circuit breaker switch

The main function of the circuit breaker switch is that when a fault occurs at a certain point in the power distribution system, it can be eliminated by the upper-level circuit breaker located at the fault point, and only eliminated by it. If we can't coordinate the switch configuration, there may be adverse effects on production safety such as the air switch jumping and tripping. The result of this problem will be very bad, and the author has suffered from it.

1. Power distribution structure of UPS system

The mainstream UPS in the current market can be divided into single input and dual input from the perspective of input mode. Among them, the UPS using single input is generally concentrated in the small power segment; Figure 5 shows the general circuit principle of medium and high power UPS Structure diagram.

Compared with high-power UPS, due to its large back-end load, high importance, and wide influence, the structure of dual power input is generally standard. The main purpose is to avoid UPS in rectifier, inverter and other circuits. When abnormal damage occurs, it may cause the risk of tripping of the "UPS upper-level input power distribution" switch, such as the two red current lines shown below. If a UPS input circuit breaker is shared for protection, the load may have a large interruption. electrical risk.
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UPS circuit schematic diagram

Therefore, in the construction of the UPS system, it is very important to install the UPS in the way of dual power input, and it is absolutely necessary to reduce the switch configuration in order to reduce the construction cost.

2. Selection of circuit breaker switch

2.2.1 Common parameter names of circuit breakers
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The circuit breaker is one of the main protection appliances in the power distribution system, and it is also the most complete protection appliance. Its main function is to protect against short circuit, overload, ground fault, voltage loss and undervoltage. According to different needs, the circuit breaker can be equipped with different releases to realize its protection function. The release types of circuit breakers include overcurrent release, undervoltage release, shunt release and so on. Overcurrent release can also be divided into overload release and short-circuit (electromagnetic) release, and there are long-delay, short-delay, and instantaneous points. Overcurrent releases are the most commonly used in current market applications.
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There are many parameters of the current characteristics of low-voltage circuit breakers, which are easy to be confused. In the design documents, when the current value of the circuit breaker is often marked, the significance of the current value is not specified, which brings us some troubles when purchasing and using products. Therefore, if we need to choose a circuit breaker completely and accurately, we must Clearly grasp the meaning of each current parameter of the circuit breaker.
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The current indicating the overcurrent release has the following parameters: - The rated current In of the release, refers to the maximum current that the release can pass through for a long time.
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——The setting value Ir or Irth of the action current of the long-time overload release, the fixed release has 1r=In, and the adjustable release Ir is a multiple of the rated current 1n of the release, such as 1r=0.4～1 ×1n.
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—— Short-time delay electromagnetic release action current setting value Im, which is a multiple of overload release action current setting value Ir, the multiple is fixed or adjustable, such as Im=2～10×Ir.
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——The rated value of the action current Im' of the instantaneous electromagnetic release is a multiple of the rated current In of the release, and the multiple is fixed or adjustable, such as Im'=1.5～11×In.
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——The rated short-circuit breaking capacity Icn is often expressed by Icu and Ics, and the specific product identification shall prevail.
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- Rated ultimate short-circuit breaking capacity Icu, is the limit short-circuit breaking current value of the circuit breaker under the specified test voltage and other specified conditions, which can be expressed by the expected short-circuit current.

——The rated operating short-circuit breaking capacity Ics is the breaking current value of the circuit breaker under the specified test voltage and other specified conditions, which is smaller than the limit short-circuit breaking current, and Ics is a percentage of Icu.
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For small circuit breakers with rated short-circuit breaking capacity greater than 1500A, the national standard "Circuit Breakers for Household and Similar Places" GB10963 (equivalent to IECB98) stipulates that the rated ultimate short-circuit breaking capacity Icu and rated operating short-circuit breaking capacity Ics should be tested. When Icu≤6000A, Icu=Ics, so I only need to do the Ics test, so the small circuit breaker marked with a short-circuit breaking capacity of 4500A and 6000A, its Icu=Ics=Icn, so generally only the rated short-circuit breaking capacity Icn value is mentioned.
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- The rated short-time withstand current Icw refers to the current value that the circuit breaker can withstand in a short time under the specified test conditions.

3. Switch selection principle

When choosing a switch, in addition to the basic parameters such as the basic rated voltage, it is also necessary to follow two basic principles: the setting threshold of the circuit breaker Ir goes online > Ir goes offline, and Im goes online > Im goes offline.
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As shown below, the circuit requires design considerations for implementing selective protection, otherwise a leapfrog trip fault will occur:
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Circuit diagram of selective protection Principle of circuit breaker switch

If IcuD2≤IrmDl, only the D2 switch is turned off.

Precautions for the use of battery monitoring system

The battery is the most important backup energy in the UPS power system, so the author does not need to repeat the importance. In recent years, various battery monitoring systems have also blossomed. These systems can help operation and maintenance personnel to monitor the current and terminal voltage changes of each battery in real time during discharge and charging, and detect the internal resistance performance of each battery. It is an important indicator used in the industry to determine whether the battery performance is aging.
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The main working principles of these systems are basically the same. The control system uses AC and DC methods to discharge the battery to a specific load, and then uses Ohm's law to calculate the internal resistance parameters of the corresponding battery. When the resistance is reached, the monitoring system will discharge the battery once.

Concluding

In the actual application process, the author found that when the battery internal resistance monitoring system is used for a long time, the probability of the UPS battery pack showing the uniformity of the battery internal resistance will become relatively high. This situation is reflected in some UPS equipment using intermittent charging technology. It will be more significant. The author thinks that this should be related to the frequent discharge detection of the battery by the monitoring system. Therefore, when using the battery monitoring system, it is best to ask the manufacturer to adjust the charging of the UPS to the floating charge mode. 1 or 2 times of equalizing maintenance to ensure that the uniformity of the battery system tends to be good.