Discuss the configuration, installation and maintenance of UPS power systems

UPS is not only an uninterruptible power supply device, but also has the function of power purification. This article will focus on the selection and configuration of UPS, the selection and configuration of battery pack capacity, and the installation and maintenance of UPS power supply.

introduction

UPS stands for uninterruptible power supply. With the popularization of various electronic devices, UPS power supply has been more and more widely used.

1 UPS selection and configuration

1.1 Understand the performance indicators of UPS power supply: ① Input voltage: 220V or 380V (three-phase four-wire system), -15%~+10%. ② Output current: Select the wire cross-section and input fuse according to this value. ③ Output voltage: One is the output voltage stability, which is generally ±5%. Some are around ±3%. Another is the voltage regulation accuracy: steady state ≤±1%, transient ≤±5%. ④ Transient voltage recovery time: ≤±50ms. ⑤ Output capacity: that is, apparent power S, S=UI⑥ Backup time: refers to the time that UPS can continue to work after input interruption, which is the key indicator of UPS. ⑦ Power factor: 0.8 (lagging) ⑧ Efficiency: ≥90% (at full load) ⑨ Overload function: 10min (125% rated current); 10S (150% rated current). ⑩ Current limiting: 100%~110% rated current adjustable.

1.2 UPS selection:

1.2.1 UPS models①Online sine wave UPS power supply. Whether the mains power is normal or not, the power supply to the load is provided by the UPS power inverter. As long as the internal battery can provide energy to the UPS power inverter, when the mains power is interrupted, the online UPS power supply can achieve true uninterrupted power supply to the load. Its sine wave waveform distortion coefficient is the smallest, the load power supply conversion time is zero, high reliability, low failure rate but high price.②Backup sine wave UPS power supply. Anti-interference graded voltage regulation and voltage stabilization technology is adopted. Only when it is powered by batteries can it provide high-quality sine waves to the load. When switching from mains power supply to inverter power supply, there is about 4ms of power interruption for the load (mainly from the conversion time of the relay). When the backup sine wave UPS power supply is powered by the mains, the noise is small because the mains directly supplies power to the load through the anti-interference filter. However, when the UPS power supply is in inverter operation, the noise is large because the PWM pulse width modulation frequency is generally around 8KHZ. ③ Backup square wave output UPS power supply. The AC power provided to the load is a square wave instead of a sine wave. This type of UPS power supply can only be connected to micro-capacitive or pure group devices. The heavier the load, the wider the square wave pulse width, and the smaller the peak value of the square wave pulse. The conversion time of this type of UPS power supply is not fixed. Its range is 4 to 9ms and cannot be controlled by the user. This type of UPS cannot be shut down and started at a certain frequency, but the cost is relatively low.

1.2.2 Load capacity, load power factor and UPS crest factor When purchasing a UPS, you must first know the total capacity of the load, and also consider the load power factor to determine the standard power capacity of the UPS. Since the load power factor is difficult to calculate, the crest factor is given in the UPS technical specifications. The higher the crest factor, the stronger the UPS's ability to withstand nonlinearity. Generally, the crest factor ratio should be greater than 3:1.

1.2.3 Battery backup time In general, when selecting the backup time, it is usually sufficient to select a full load working time of 10 minutes, 15 minutes or 30 minutes. Since batteries are expensive, long-delay UPS is generally only used in situations where the power outage lasts for a long time. In this case, it is best to choose a UPS with an external large-capacity battery function to ensure that it can provide power for a long time after the city power outage.

1.2.4 UPS neutral line cross section Since UPS loads are mostly non-linear loads, the current flowing through the neutral line is not zero. Even when the three-phase load is completely balanced, the neutral line current can reach 1.8 times the three-phase current. The smaller the load power factor, the greater the multiple. Therefore, in the UPS power supply, the neutral line cross section must not be smaller than the phase line cross section. Otherwise, it is easy to cause the neutral line to heat up, and even burn the cable and cause a fire, resulting in serious consequences.

2. Battery selection and configuration

2.1 Basic technical indicators of batteries: ① Valve-regulated sealed lead-acid batteries: one group for each UPS. ② Float charge voltage tolerance: 1%. ③ Float charge voltage: 2.23~2.27V/cell. ④ Equalizing charge voltage: 2.3~2.4V/cell. ⑤ Discharge end voltage: 1.67~1.70V/cell. ⑥ Effect of temperature on battery life: Under floating charge operation at 25℃, the theoretical life is not less than 10 years.

2.2 Calculation of UPS battery capacity

2.2.1 Maximum battery discharge current I

I＝S×COSφ/η×Ei

Where: S is the nominal output power of the UPS power supply;

COSφ is the load power factor, generally 0.8;

η is the efficiency of the inverter, which is generally taken as 0.8;

Ei is the battery discharge end voltage.

2.2.2 Battery backup time t The battery backup time t is determined according to the needs of the user. Small and medium-sized UPS mostly use valve-controlled lead-acid batteries. The price is relatively expensive, and the full-load working time is generally selected as 10 minutes, 15 minutes or 30 minutes.

2.2.3 Battery Capacity C After calculating the maximum discharge current, the standard battery capacity is calculated based on the load properties and the UPS backup time required by the user: (C = It).

3 UPS Installation

3.1 UPS installation location requirements

3.1.1 To extend the battery life, the battery should be installed in an environment with an ambient temperature between 15℃ and 25℃, and the indoor temperature should not be too high.

3.1.2 There should be 50mm of space on the left and right sides of the UPS, 100mm of space at the back, and sufficient operating space at the front.

3.1.3 The external battery cabinet should be placed together with the UPS.

3.2 Connection between UPS, AC power and load The connection between UPS, AC power and load is very simple, but before connection, you should check: ①Whether the UPS input parameters are consistent with the voltage and frequency of the AC power. ②Whether the live wire and neutral wire connected to the UPS are consistent with the manufacturer's regulations. ③Check whether the load power is less than the UPS output power. ④When connecting the UPS to the battery, be sure to distinguish the positive and negative poles.

3.3 Selection of cable cross-section When selecting the conductor cross-section, the following should be considered:

3.3.1 Comply with safety standards for cable use.

3.3.2 Meet the cable temperature rise requirements.

3.3.3 Meet the voltage drop requirements. ① AC input current I phase. Because P = 3 × U phase × I phase × COSφ (for single-phase output: P = UICOSφ). So I phase = P/(3 × U phase × COSφ) = S/(3 × U phase). ② DC output current I = P/U (U should take the minimum value). After finding the AC input I phase and the DC output current I, look up the table to determine the cross-sectional area of ​​the wire.

4 UPS Maintenance

Based on years of work experience, I recommend the following aspects for maintaining the UPS power system:

4.1 Master the basic knowledge of UPS, read the equipment manual carefully, understand the meaning of various warning information, warning codes, indicator lights, as well as the causes and countermeasures. Be familiar with the functions of various switches and buttons on the equipment. Be familiar with and master the various operations of UPS, understand the connection relationship, and understand the method of substitution.

4.2 Strengthen daily inspections and maintenance, check whether the equipment has any alarms, odors, or abnormal noises, check whether the joints are loose or hot, whether the cooling fan is operating normally, whether the various indications of the equipment are normal, and solve problems in a timely manner if found.

4.3 Develop a regular maintenance plan: Measure the output voltage, current, power, battery internal resistance and terminal voltage of the equipment on a monthly basis. Perform a verification discharge test on the battery once a quarter or half a year, and generally discharge 30% to 40% of the rated capacity. Clean the inside of the UPS once a year and check whether all connectors are in good contact.

4.4 Battery discharge: In the battery discharge operation, if the battery is discharged offline using a dummy load, it is not only cumbersome to disassemble and unsafe, but also needs to be disassembled and installed for charging afterwards. In order to ensure the safety and effectiveness of the battery discharge test, it is necessary to find problems (lag battery, reverse polarity battery, etc.) and ensure safe and reliable power supply (no over-discharge, short circuit, power supply interruption, etc.). Here is a method of directly using the load to discharge the battery: that is, turn off the UPS AC input switch to discharge the battery. Since UPS power supplies are mostly used in important network communication systems, the load changes little, so the discharge current is basically unchanged when the load is directly discharged. In this way, the discharge termination voltage can be determined according to the battery voltage and discharge conditions, and the discharge time can be calculated. The discharge in each quarter will be compared with this record, and problems can be found and eliminated in time.