A brief discussion on the configuration scheme of UPS power supply system in communication room

Uninterruptible power supply (UPS) is widely used in the field of communication. It is becoming more and more common to install UPS (uninterruptible power supply) power supply systems in communication rooms. A well-designed UPS power supply system can provide high-quality power to the load. How to establish a reasonable and safe UPS power supply system has become a concern for everyone. The author discusses various aspects of UPS power supply system configuration for reference by peers.
1 Requirements for UPS front-end power supply system
UPS can provide high-quality power with high voltage stabilization accuracy, stable frequency, and low waveform distortion to the load, and can provide uninterrupted power supply when switching with static bypass. However, to achieve this, its front-end power supply quality cannot be ignored. When designing the front-end power supply system of the communication room, we should consider the following aspects:
(1) The power quality of the front-end power supply system should not be too poor, and the voltage and frequency should be stable within the normal range. Generally speaking, the input voltage range of a large-capacity UPS host should be 380V±15%. If the voltage is too low, the UPS backup battery will be discharged frequently, and eventually its service life will be greatly shortened due to long-term undervoltage charging; on the contrary, if the voltage is too high, it is easy to cause inverter damage. For bypass input, its voltage and frequency fluctuation also have a certain range, generally rated voltage ±10%, rated frequency ±15%. If the range of the front-stage power supply is too large, the switching between the inverter and the bypass power supply will be prohibited or interrupted. Therefore, if the voltage range of the front-stage power grid of the communication room does not meet the requirements, a suitable anti-interference AC voltage stabilizer should be configured at the front stage of the UPS, but it is not advisable to use an electronic tube AC ​​voltage stabilizer or a magnetic saturation voltage stabilizer, because these two types of voltage stabilizers can generate instantaneous high voltage when turned on, and the output waveform distortion is also large, which can easily cause UPS failure.
(2) The front-stage power supply system should not have frequent starting loads, such as frequently used elevators and frequently turned on air conditioners. The reason is that when these loads are turned on and off, instantaneous high and low voltages will occur, causing excessive voltage waveform distortion on the power supply line, causing the UPS mains bypass power supply and inverter power supply conversion control circuit to malfunction, and then causing synchronous control circuit failure. Therefore, if conditions permit, the UPS power supply should be placed at the front end of the power grid input.
(3) The capacity of the AC generator set in the front-stage power supply system should be appropriately enlarged. Most communication rooms are equipped with generator sets to solve the problem of power supply difficulties during long power outages. However, when configuring the generator set, its capacity should be considered to be no less than 1.5~2 times the rated output power of the UPS power supply to ensure that the generator output voltage and frequency are normal and improve its waveform distortion.
2 Determination of UPS capacity
According to the load capacity and nature, selecting an appropriate UPS can not only ensure the power supply quality of the UPS and reduce the failure rate, but also save investment and improve economic benefits. Generally speaking, the determination of UPS capacity is mainly to meet the needs of the current load. At the same time, several factors should also be considered:
(1) The impact of load nature on UPS output power. The output power given by most UPS manufacturers in the product manual refers to the value when the load power factor is -0.8 (lagging), while the actual load that the UPS power supply can carry is closely related to the load power factor. When the load is purely resistive or inductive, the active power of the inverter will decrease at the rated power. Therefore, when considering the UPS capacity, power conversion should be performed for different load power factors. Usually, the following estimate can be made: Assuming that the UPS rated power is 1 kVA when the load power factor is -0.8 (lagging), the output power is approximately 0.9~0.92 kVA and 0.74~0.77 kVA when the power factor is -0.9 and -1.0, respectively. For computer loads, as long as the peak factor of the load is within the range allowed by the UPS, the UPS can basically output the rated power. For inductive loads, the UPS capacity needs to be increased as appropriate.
(2) The UPS capacity should not be too large compared to the load to avoid excessive light load operation. Although excessive light load operation is beneficial to reduce the probability of inverter damage, it may cause the battery discharge current to be too small and the discharge time to be too long when the city power is cut off. When the battery protection device fails, the battery pack is deeply discharged and permanently damaged.
(3) The UPS capacity should not be too small to avoid long-term heavy load operation. Although this can save some investment, due to the heavy load operation of the inverter, its output waveform will be distorted and the output voltage amplitude will jitter too much. This will not only fail to provide high-quality power for the load, but also easily cause damage to the UPS inverter. Therefore, even from an economic point of view, it is not worth the loss. According to the recommendations of some UPS manufacturers, the UPS load should not exceed 80% of its rated capacity for a long time.
(4) For the situation where the communication room area is large and the load is continuously expanded in stages, when configuring the UPS capacity in the first phase, the medium and long-term development trend should be appropriately considered, and models that can be operated in parallel or multiple machines should be selected in the selection, so that when the load capacity increases in the medium and long term, the output capacity can be expanded through UPS parallel operation. Accordingly, when configuring the UPS input and output distribution panel, input switches for multiple UPSs and medium and long-term load shunt switches should be reserved to facilitate future expansion.
3 Correctly configure the UPS backup battery
In order to ensure that the UPS power supply can continue to provide high-quality power to the computer when the power grid is out of power, the configuration of the backup battery is particularly important. When the load is not allowed to be powered by the power supply, the UPS battery backup time in the communication room should be greater than the time from the interruption of the mains power to the recovery time or the time required for the generator set to supply power normally (the front-end power supply system is equipped with a generator set). If this period is long, an external long-delay battery pack should be configured, but at this time, it should be confirmed that the UPS internal rectifier has the ability to charge the external large-capacity battery pack, otherwise an external charger should be configured. The selection of battery capacity should follow the following principles: the battery must supply power to the inverter within the backup time, and under rated load, the battery pack voltage should not drop below the minimum voltage allowed by the inverter. When arranging the equipment in the computer room, the battery pack should be as close to the UPS host as possible, the length of the connection between the two should be shortened, and the cross-sectional area of ​​the connection should be increased to reduce the self-inductance of the connection and the line voltage drop. The battery pack can be installed in a battery cabinet or in an open battery rack. The former is beautiful and tidy, but has higher requirements for the load-bearing capacity of the floor. The latter can disperse the load and has good heat dissipation, but it occupies a large area and is prone to dust accumulation, which brings inconvenience to maintenance.
4 Increase power supply reliability through redundancy
In order to improve the reliability of UPS power supply, a variety of UPS redundant connection methods can be used. Each method has advantages and disadvantages. When considering the solution, you should choose the appropriate mode according to the actual load situation. There are three types of redundant connection:
(1) Dual-machine master-slave hot backup. The output of UPS1, which is the slave, is connected to the bypass input of UPS2, which is the master. During normal operation, UPS2 supplies power and UPS1 is in backup mode. When UPS2 fails, the load is switched to UPS2 bypass and UPS1 takes on the load supply task. This system has a simple structure and control, but has the following disadvantages: the master works for a long time while the slave is in a long-term standby state, and the aging degree of the components of the two machines is uneven; when the slave is powered, the static bypass of the master fails, which will cause the system power supply to fail; the system load cannot exceed the capacity of a single machine and cannot be expanded later.
(2) Power sharing parallel backup. This system connects two or more UPS inverter units in parallel. Under normal circumstances, two (or more) inverters will supply power to the load at the same time. When one of them fails, the UPS will be disconnected from the system, and the load current required by the user will be redistributed by the remaining inverters according to the new share. There are currently two structures for this method. One is that the UPS is connected in parallel through an external parallel cabinet. The parallel cabinet provides synchronization and multi-machine current sharing control, and also provides the total static bypass of the parallel system; the other is to install a set of logic control boards in each UPS to control the synchronization and current sharing output of each machine. The advantages of this solution are easy expansion (when using the parallel cabinet method, the parallel cabinet should be considered as the final period) and the power supply reliability is improved through redundant backup, but there are also disadvantages: (a) When the parallel cabinet method is adopted, the parallel cabinet becomes the common bottleneck of the system. Once it loses control or fails internally, the power supply of the entire system will fail. (b) Since it is difficult to keep the output parameters of each UPS completely consistent, each UPS will form a loop between the inverters inside the UPS while supplying power to the load. When the circulating current is too large, it will directly endanger the safety of the inverter. In addition, if the current difference between each UPS supplying power to the load is too large, the aging speed of the power amplifier components of the inverter will be unbalanced, which will also cause failures. Generally speaking, the more parallel machines there are in the power supply system, the greater the probability of UPS power system failure.
(3) Parallel hot backup. This system connects the battery input, rectifier output and inverter output of two UPS in parallel and shares the bypass. Under normal circumstances, the two rectifiers supply power to the two inverters at the same time and charge the two sets of batteries. The inverter output static switch selects one of the inverters to supply power to the load. The two rectifiers and inverters are backup for each other. Only when the two inverters fail at the same time, the system will switch the load to the common static bypass, and the AC power will continue to supply power to the load. This solution has no bottleneck failure point. If any UPS fails partially or completely, the system can still continue to supply power to the load. Since there is only one inverter for the actual output, there is no circulating current between the inverters. However, since this mode is similar to the single-machine operation mode, the load capacity is relatively poor and it is not easy to expand.
5. The power supply system should be intelligent
In order to ensure that the power supply system can operate uninterruptedly for a long time, the UPS must be intelligent, automatically detect the status of the UPS in operation, and promptly detect, diagnose and handle UPS faults, and reduce interruptions caused by faults or maintenance. At the same time, as part of the power system of the communication room, a communication protocol should be provided so that it can be included in the power centralized monitoring network. Therefore, we should take these factors into consideration when designing the system. Generally speaking, as an intelligent UPS, it should have the following functions:
(1) Real-time monitoring function. Monitor the status of each part of the circuit and obtain relevant parameters of the host at any time.
(2) Human-computer interaction function. According to the actual operating conditions, the various critical operating point thresholds inside the UPS can be reset through program modification, and various operating parameters of the UPS power supply can also be read.
(3) Fault diagnosis function. Timely analyze the abnormal parameters detected, discover the signs of faults as early as possible, display their nature and location, provide treatment methods, and automatically record relevant information.
(4) Remote monitoring function. Provide a remote computer interface, which can communicate with remote computer terminals through RS232 or RS485 interface via modem to achieve the purpose of telemetry and telesignaling.
6 Conclusion
When designing the UPS power supply system for the communication room, we must save investment and consider the reliability and flexibility of the system to provide effective power guarantee for communication equipment and computer loads.
Switch to facilitate future expansion.
3 Correctly configure the UPS backup battery
In order to ensure that the UPS power supply can continue to provide high-quality power to the computer when the power grid is out, the configuration of the backup battery is particularly important. When the load is not allowed to be powered by the power supply, the UPS battery backup time in the communication room should be greater than the time from the interruption of the mains power to the recovery or to the power outage.