Analysis of Common Problems in UPS Power Supply Systems for Large Generator Sets

Modern large-scale generator sets use complex computer monitoring and safety protection systems, and their AC 220V power supply must be stable and reliable, and must not stop for a second. The uninterruptible power supply (UPS) that came into being meets the strict requirements of this "harsh" load and eliminates the impact of transient fluctuations in grid voltage on important loads.

UPS power supply is crucial to the safe and stable operation of the unit. UPS power failure means shutdown of the unit and the furnace. How to improve the reliability of UPS is a question that field technicians often think about.

This article only analyzes and studies some of the problems encountered over the years in the planning, configuration, selection, installation, commissioning, operation and maintenance of peripheral equipment of the UPS system of the No. 5 and No. 6 300MW units of Huangpu Power Plant, and exchanges and discusses with peers.

1 System Overview

The UPS system is a low-voltage multi-terminal network with multiple power inputs. The core equipment of the network, such as inverters and static switches, is a set of power devices automatically controlled by electronic components. The UPS system of the 300MW unit of Huangpu Power Plant is shown in Figure 1.

Figure 1 UPS system for 300 MW units at Huangpu Power Plant

The technical specifications of the main equipment in the original design are:

a) Rectifier

Input parameters: AC 380V, three-phase, 50Hz, 126A;

Output parameter——DC 280V.

b) Inverter

Input parameters: DC 210~280V, 245A;

Output parameters: AC 220V, 50Hz, 227A, 50kVA;

c) Charger

Input parameters: AC 380V, three-phase, 50 Hz;

Output parameters: DC 160~310 V, 250 A, 65 kW.

2 System Power Supply Planning and Configuration

2.1 Power Configuration Analysis

The orientation of AC power is an important part of UPS system planning and design.

To explore the best solution, we may wish to further study the principle. The battery charger and the rectifier before the inverter are both three-phase half-controlled bridges, and their functions are similar. The rectifier bears the regular load of the UPS. The charger float charges the battery and is parallel to the battery as a backup for the rectifier. The bypass power supply is the backup for the inverter. The APS carries some thermal control secondary loads and serves as a backup for the UPS. Based on this, the general principles of UPS system power configuration can be derived:

a) The power supplies of the rectifier and charger should be connected to different busbars respectively;

b) The bypass and rectifier power supplies are connected to different busbars separately;

c) APS and bypass power supply should also be staggered and connected to different busbars. The 5 AC power supplies in Figure 1 are only taken from 3 busbars, 3 of which are from the safety b section. When the UPS device fails, the inverter is under maintenance, or an accident occurs in the factory power system, when the static switch of the UPS device has been switched to bypass operation, once the safety b section loses pressure, the UPS bus and APS2 bus lose power, and the WDPF and BMS control systems are paralyzed.

Analyzing the power connection of the 300 MW units in Huangpu Power Plant, the No. 2 and No. 0 low-voltage plant transformers are connected to the same 6 kV B section. When the 6 kV A section loses voltage, No. 0 transformer can also automatically supply power to the working a section and safety a section connected to No. 1 transformer. The 380 V working a section and safety a section are better than the working b section and safety b section.

In view of this, under the premise of reducing changes, the power supply configuration of Figure 1 can be further adjusted:

a) The charger is reconnected to working section b;

b)APS2 is reconnected to security section a.

The power plant units are equipped with working and standby high-voltage plant transformers and multiple low-voltage plant transformers, as well as diesel engines or safety standby transformers. It is possible to connect the multiple power supplies of UPS and APS to different transformers and different low-voltage busbars of the unit without duplication and in a more reasonable way. After the unit is disconnected due to an accident, the frequencies of different power systems may be different. In order to prevent the static switch from failing to switch due to asynchrony, or the switching between UPS and APS from failing due to asynchrony, these power supplies should be connected to the low-voltage system of the same network of the unit, and should not be connected to the public system or other unit systems.

2.2 Phase of bypass and APS power supply

The single-phase AC voltage output by the inverter and the single-phase AC voltage of the bypass power supply should be synchronized in order to be converted in parallel. Regardless of which phase of the AC the bypass is taken from, the inverter can adjust the output voltage to be the same frequency and phase as the bypass voltage.

The DPU cabinet of the unit's WDPF control system and electronic equipment such as computing, storage, and recording stations are powered by dual power supplies, as shown in Figure 2. This is a two-input and one-output three-terminal network formed by reverse parallel connection of thyristors, and the UPS is powered first. When the UPS bus loses voltage or is undervoltage to a certain value, the control circuit triggers the bidirectional thyristor on the APS side, causing it to conduct alternately and shut down the thyristor on the UPS side, and the APS continues to supply power. At this time, the switching is first connected and then cut, and the automatic return after the UPS voltage is normal is also first connected and then cut. Obviously, in terms of voltage phase and frequency, if the UPS bypass side is inconsistent with the APS side, the conversion moment will be short-circuited or a large impact current will be generated due to the large differential pressure, resulting in power failure and component damage in the station.

Figure 2 Dual power supply

In 1991, when the No. 6 unit of Huangpu Power Plant was conducting a switching test at the end of UPS installation, many switches tripped, and it was only then that the phase mismatch and switching short circuit problems were discovered for the first time. Other power plants also had similar problems of wrong phase connection.

It must be pointed out that if the phase is connected incorrectly during installation, it will not be detected unless the UPS switches or switches back under abnormal voltage conditions. Even if many switches trip, if power is supplied from the UPS side first (usually so) and then from the APS side, there will be no short circuit at this time, which still conceals this huge hidden danger.

We hope that the design and construction departments will pay attention to this problem, mark the phases on the drawings, and wire correctly to prevent similar incidents from happening again.