From Liability to Asset: A Three-Mode Grid-Forming Control Framework for Centralized Data Center UPS Systems

Overview

This paper, by Mohamed Shamseldein, was published in IET Generation, Transmission & Distribution in 2026 (volume 20, issue 1, e70369). The full text was read from the arXiv preprint (v1, December 2025); the published version was not checked and may differ. The paper proposes a supervisory control scheme for a centralised medium-voltage UPS serving a large AI data center on a weak grid. The motivation is NERC's July 2025 white paper on large loads.

NERC's warning is that large data center loads dropping off during faults, or pulsing with AI workloads, can destabilise the grid. In that context the paper cites an aggregate load drop of about 1.5 GW in the Eastern Interconnection, caused by voltage-sensitive load reduction during faults. The proposed scheme has three modes: normal operation with a stiff DC bus and smoothed grid draw; a fault mode with current-limited reactive-power priority, a minimum grid draw, UPS-battery buffering and a rate-limited 'soft return'; and an optional droop-based fast frequency response.

The test case is a 50 MW block at 13.8 kV on an ultra-weak grid (short-circuit ratio SCR = 1.5, X/R = 5), with a balanced 0.5 p.u. three-phase dip for 150 ms and inverter current limited to 1.0 p.u. Unserved IT energy was 0.00000 MWh with the proposed controller versus 0.00208 MWh for a momentary-cessation benchmark. Peak inverter current was 0.57 p.u. versus 1.02 p.u. for an SRF-PLL LVRT baseline, and the settled PCC voltage minimum after one cycle was 0.79 p.u. versus 0.66 p.u. Mean fault-window grid draw was kept at 0.20 p.u. rather than about zero.

When many facilities ride through together, this minimum draw limits the frequency disturbance: with N = 40 blocks and H = 3 s, the frequency proxy reached about 60.159 Hz for momentary cessation versus about 59.953-59.961 Hz with the proposed controller. The UPS battery also absorbs a 1 Hz pulsed AI load so the grid sees a smoother draw. As a limit, the results rest on averaged dq simulations, not on field or hardware-in-the-loop validation.

Key points

1. Motivation is NERC's July 2025 large-load white paper, including a ~1.5 GW aggregate load drop in the Eastern Interconnection caused by voltage-sensitive load reduction during faults.
2. Test: 50 MW block at 13.8 kV on an ultra-weak grid (SCR = 1.5, X/R = 5), balanced 0.5 p.u. three-phase dip for 150 ms, inverter current limited to 1.0 p.u.
3. Unserved IT energy was 0.00000 MWh with the proposed controller versus 0.00208 MWh for a momentary-cessation benchmark.
4. Peak inverter current was 0.57 p.u. versus 1.02 p.u. for an SRF-PLL LVRT baseline; settled PCC voltage minimum after one cycle was 0.79 p.u. versus 0.66 p.u.
5. Mean fault-window grid draw was kept at 0.20 p.u. rather than about zero, limiting the frequency disturbance when many facilities ride through together.
6. With N = 40 blocks and H = 3 s, the frequency proxy reached about 60.159 Hz for momentary cessation versus about 59.953-59.961 Hz with the proposed controller.
7. Three modes: normal, fault (current-limited reactive-power priority, minimum draw, battery buffering, soft return) and optional droop-based fast frequency response.

Using it in design

• For large or AI facilities, record the UPS's fault ride-through behaviour (momentary cessation versus ride-through with minimum grid draw), the inverter current limit and the post-fault ramp rate, alongside the point-of-interconnection short-circuit ratio.
• Flag sites with SCR below about 3 as weak-grid cases that need UPS control studies.
• Check that the UPS battery power rating covers pulsed-load buffering plus backup.
• Note in interconnection deliverables that grid operators increasingly require large loads to ride through faults rather than disconnect.

Who should read it

Electrical engineers and power electronics specialists doing UPS and grid-interconnection studies for large and AI data centers.

Related standards

• IEC 62040-3
• IEEE 2800
• IEEE 1547

Publication details

Authors

Mohamed Shamseldein

Publisher

IET Generation, Transmission & Distribution (Institution of Engineering and Technology (IET))

Year

2026

Document number

arXiv:2512.16497

Volume

20

Issue

1

Pages

e70369

Document language

English

Source type

Scientific

Access

Free to read

DOI

10.1049/gtd2.70369

Licence

arXiv distribution permission — reuse conditions remain with the rights holder

Citation