Uninterruptible power supply is an important equipment in petrochemical systems, responsible for supplying power to instrument DCS, SCADA, SIS and other systems in case of power system failure. The “Design Specification for Instrument Power Supply in Petrochemical Industry” SH/T3082-2019 has some new regulations on UPS output and power supply system grounding methods. This article introduces and discusses the configuration and grounding of UPS power supply in petrochemical systems. It should be noted that the parameters of products from different brands vary, and this article is a reference for the designers.
Typical configuration and main parameters of UPS power supply
The Design Specification for Instrument Power Supply in Petrochemical Industry SH/T3082-2019 has been implemented on January 1, 2020. According to regulations, the output voltage of UPS is AC220 ± 22V, and an isolation transformer should be installed on the output side. The output terminal of the isolation transformer should adopt TN-S grounding method. Based on the requirements of regulations and the current situation of UPS power supply in petroleum and petrochemical systems, the typical configuration is briefly introduced as follows:
1) Input isolation transformer to provide power for rectifier module. Generally, 380V three-phase input is used. For small capacity UPS power supplies such as 5KVA and below, 380V single-phase or 220V single-phase can also be used:
2) Rectification modules, currently mainly including thyristor rectification or IGBT rectification;
3) DC system, including DC bus, filtering circuit, battery and other parts. The DC bus voltage after thyristor rectification is generally around 110V and 220V, while the DC bus voltage after IGBT rectification is generally around ± 384V and ± 395V. Different brands have slightly different rectified DC voltages;
4) Inverter modules currently use IGBT modules, and the AC voltage after inversion mainly includes AC110V, AC220V, and AC380V.
5) Output transformer. The “Design Specification for Instrument Power Supply in Petrochemical Industry” SH/T3082-2019 stipulates that “AC UPS output wiring should adopt single-phase 220V, AC three wire system”. According to the different UPS inverter voltages, the output transformer needs to step up, step down or isolate the inverter voltage to maintain stable output of AC220V and achieve the “single output” required by the specification. The secondary side of the output isolation transformer usually adopts a “floating output”, which means that the voltage on both output sides is 110V to ground, but their phases are 180 ° apart (similar to the analysis of the bypass transformer later). When one end is grounded, the voltage to ground at the other end is AC220V, which meets the regulatory requirements. This new grounding point will reconstruct the power grounding point and N line of the TN-S power supply system. In terms of the structure of UPS itself, AC/DC isolation and output transformer can suppress and absorb the impact of low-frequency harmonics on the load side of UPS, improving the reliability of UPS itself.
6) The bypass transformer is generally 380/220V or 220/220V, and one of its functions is to provide power to important loads from the mains when the UPS inverter fails; Another function is to isolate the mains and electrical loads, eliminating the impact of some harmonics. The secondary side N line of the general bypass transformer is directly connected to the N line of the UPS output transformer, forming a “one point grounding” grounding connection point that meets the power supply requirements of TN-S.
7) UPS comes out of the cabinet. According to the requirements of SH/T3082-2019 “Design Specification for Instrument Power Supply in Petrochemical Industry”, the distribution cabinet can be a main distribution cabinet with only one distribution switch, which is the “single output” scheme in the specification; Or a master switch with multiple outputs.