UPS Backfeed Protection

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1 This is a photographic template your photograph should fit precisely within this rectangle. UPS Backfeed Protection Enhanced safety and reliability

2 Back-feeding what is it? Back-feeding of power means that the power transfer within an electrical device is towards the input terminals This creates a safety hazard since, as a result, input terminals may be powered even if they have been disconnected from the mains supply Backfeed protection is implemented to shield the bypass line from thyristor short circuit failure in the static switch Backfeed protection is implemented to prevent leakage current via the static switch snubbers and SCRs from creating upstream voltages when mains is not present 2 2

3 Backfeed protection requirements The international standard IEC and AS :2003 require that the UPS device shall prevent all hazardous voltage and energy from being transferred to the UPS input terminals after the input power has been interrupted The standards allows for two alternative implementations of backfeed protection installing an internal backfeed isolation device within the UPS installing an external backfeed isolation device with only backfeed detection implemented within the UPS Note: (AS Uninterruptible power systems (UPS) General and safety requirements for UPS used in operator access areas, section Back feed protection.) 3 3

4 Backfeed protection requirements IEC and AS :2003 state the following: For permanently connected UPS without internal automatic backfeed isolation (see 5.1.4), the instructions shall require the fitting by the user of a warning label on all primary power isolators installed remote from the UPS area, to warn electrical maintenance personnel that the circuit feeds a UPS. The warning label shall carry the following wording or equivalent. ISOLATE UNINTERRUPTIBLE POWER SUPPLY (UPS) BEFORE WORKING ON THIS CIRCUIT The practical implications are that all isolators in the chain feeding the UPS must be so identified if the UPS does not have backfeed protection. 4 4

5 Backfeed protection in a single UPS system Eaton 3-phase UPS equipment include internal backfeed contactors as standard* Backfeed contactor is situated in the bypass line *model dependent 5 5

6 Benefits of an internal backfeed contactor In addition to the safety related advantages: Internal backfeed protection brings operational advantages The UPS is able to continue in doubleconversion mode even in the case of a static switch fault before and while maintenance work is carried out The bypass lines in a parallel system have redundancy Backfeed protection is tested during standard manufacturing testing 6 6

7 Backfeed protection in a single UPS system The internal backfeed contactor is used to automatically protect against fault situations in the static switch If a thyristor suffers a short-circuit failure The inverter output to the bypass input terminal become shorted Opening the backfeed contactor prevents the backfeeding of power The UPS can stay in double-conversion mode even in the case of a static switch failure 7 7

8 Backfeed protection in a single UPS system Internal backfeed contactors in Eaton UPS units are always tested as a part of standard manufacturing testing During the UPS burn-in tests The UPS unit runs at 80% of its nominal power in both doubleconversion and bypass modes Current measurement accuracy of the static switch is tested The backfeed contactor functionality is tested Accurate bypass current measurement enables backfeed detection 8 8

9 Backfeed protection in a parallel UPS system If a fault in one of the static switches were to occur, inverters would start feeding current back to the input transformer Backfeed contactor opens The static bypass lines have redundancy Failure of one static bypass does not prevent the other bypass lines from operating 9 9

10 Backfeed protection in a centralised bypass system In a centralised bypass topology the system bypass module (SBM) provides a common bypass line for the paralleled UPS modules The Eaton SBM module includes an internal backfeed disconnect device as standard 10 10

11 Problems using external backfeed contactors External backfeed contactor means: Space needs to be reserved in the input switchgear for the UPS backfeed isolation device Often only one backfeed contactor is installed per entire parallel UPS system Failure in one of the static switches results in the loss of all the static bypass lines connected to the common backfeed contactor If the UPS doesn t have a circuit for controlling an external contactor, then the only functionality that you can have is to protect against feedback through SCRs and snubbers during mains failure 11 11

12 Which Eaton UPS has backfeed protection? 12 12

13 Questions 13 13

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