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arxiv: 2606.21536 · v1 · pith:4GJG3C3Cnew · submitted 2026-06-19 · 📡 eess.SY · cs.SY

Stability Enhancement of Centralized UPS Data Center Systems Under Weak-Grid Conditions

Jesus D. Vasquez Plaza , Yonghao Gui , Jin Dong , Jamie Lian This is my paper

Pith reviewed 2026-06-26 13:21 UTC · model grok-4.3

classification 📡 eess.SY cs.SY
keywords UPSweak-gridpower-based controlPLLSCRdata centerstabilityrectifier
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0 comments X

The pith

Direct power regulation without phase-locked loops keeps centralized UPS systems stable when grids are weak.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

Centralized UPS systems in data centers rely on front-end rectifiers to deliver reliable power, but standard control methods based on phase-locked loops and proportional-integral controllers lose stability when the grid is weak, as measured by low short-circuit ratio. The paper uses simulations to show that these controls fail at a short-circuit ratio of 2. It then applies a different control method that directly sets active and reactive power flows instead of relying on synchronization, and shows that this restores stable operation with better damping. Because modern power systems are trending toward weaker grids from more renewable integration and dynamic loads, finding controls that work without assuming strong grids matters for keeping data centers online.

Core claim

Conventional PLL-based PI control strategies for the front-end rectifier in centralized UPS architectures exhibit degraded performance or instability under low short-circuit ratio conditions. Electromagnetic transient simulations demonstrate that PI-controlled rectifiers can become unstable at SCR=2. A power-based control approach applied to the three-phase rectifier enables direct regulation of active and reactive power without relying on PLL synchronization. This strategy improves system damping and restores stable operation under weak-grid conditions.

What carries the argument

Power-based control of the three-phase rectifier that directly regulates active and reactive power flows without PLL synchronization.

If this is right

  • System damping is improved under weak-grid conditions.
  • Stable operation is restored for the UPS system at low SCR values.
  • Reliable power delivery to IT equipment is maintained in low-strength grid environments.
  • The importance of control design for data center power systems in emerging weak-grid settings is highlighted.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar power-based methods could be tested on other types of grid-connected inverters facing weak-grid challenges.
  • Hardware-in-the-loop validation would strengthen the case beyond simulations.
  • The approach may reduce the need for grid-strengthening infrastructure in data center deployments.

Load-bearing premise

Electromagnetic transient simulations capture the real dynamics and interactions of the centralized UPS system under weak-grid conditions.

What would settle it

A physical experiment or field measurement showing that the power-based control does not improve damping or fails to stabilize the system at SCR=2 would disprove the central claim.

Figures

Figures reproduced from arXiv: 2606.21536 by Jamie Lian, Jesus D. Vasquez Plaza, Jin Dong, Yonghao Gui.

Figure 1
Figure 1. Figure 1: Simplified centralized UPS data center architecture. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 3
Figure 3. Figure 3: Conventional PLL-based PI control structure of the front-end rectifier. [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 2
Figure 2. Figure 2: Aggregated rack-level active power waveform and corresponding fre [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: Baseline response of the centralized UPS data center system under [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comparison between conventional PI control and VM-DPC under [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Effect of reactive power support on VM-DPC stability under weak [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
read the original abstract

Data center power systems are increasingly exposed to weak-grid conditions due to the evolution of modern power systems and the integration of large and dynamic loads. In centralized uninterruptible power supply (UPS) architectures, the front-end rectifier plays a critical role in maintaining stable operation and ensuring reliable power delivery to information technology (IT) equipment. However, conventional phase-locked loop (PLL)-based proportional-integral (PI) control strategies may exhibit degraded performance or instability under low short-circuit ratio (SCR) conditions. This paper investigates the behavior of centralized UPS systems under weak-grid conditions and demonstrates, through electromagnetic transient simulations, that PI-controlled rectifiers can become unstable at SCR=2. To address this issue, a power-based control approach is applied to the three-phase rectifier, enabling direct regulation of active and reactive power without relying on PLL synchronization. Simulation results show that the proposed control strategy improves system damping and restores stable operation under weak-grid conditions. The findings highlight the importance of control design for maintaining reliable operation of data center power systems in emerging low-strength grid environments.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 0 minor

Summary. The paper claims that conventional PLL-based PI control in the front-end rectifiers of centralized UPS data center systems can become unstable at SCR=2 under weak-grid conditions, while a proposed power-based control strategy (direct active/reactive power regulation without PLL) improves damping and restores stable operation; both the instability and the improvement are demonstrated exclusively via electromagnetic transient simulations.

Significance. If the EMT simulation results hold under real-world conditions, the work would usefully illustrate a practical control alternative for data-center rectifiers facing low-SCR grids. The simulation-based comparison itself is a clear strength in showing the concept, but the complete absence of analytical support or validation reduces the result to an unverified demonstration rather than a substantiated stability enhancement.

major comments (2)
  1. [Abstract] Abstract: the assertion that 'PI-controlled rectifiers can become unstable at SCR=2' and that the power-based strategy 'restores stable operation' is supported only by EMT simulations; no small-signal model, eigenvalue analysis, hardware/HIL validation, error bars, or sensitivity study is referenced, rendering the instability threshold and damping improvement dependent on unverified model fidelity.
  2. The manuscript provides no derivation or independent analytical confirmation of the SCR=2 instability point; the central claim therefore rests entirely on the simulation setup (grid equivalent, rectifier dynamics, and control implementation), which is not shown to reproduce real-system behavior.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the detailed review. Our manuscript is a simulation-based study using EMT to illustrate instability of PLL-PI control at low SCR and the improvement with power-based control. We respond to the major comments below.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the assertion that 'PI-controlled rectifiers can become unstable at SCR=2' and that the power-based strategy 'restores stable operation' is supported only by EMT simulations; no small-signal model, eigenvalue analysis, hardware/HIL validation, error bars, or sensitivity study is referenced, rendering the instability threshold and damping improvement dependent on unverified model fidelity.

    Authors: We agree the results rely on EMT simulations, as stated in the abstract and throughout the paper. The contribution is a practical demonstration of the issue and the alternative control in a realistic UPS system model rather than a full analytical treatment. To address the concern on model fidelity, we will add a sensitivity study varying PLL gains, power references, and SCR around the threshold in the revised manuscript. revision: partial

  2. Referee: The manuscript provides no derivation or independent analytical confirmation of the SCR=2 instability point; the central claim therefore rests entirely on the simulation setup (grid equivalent, rectifier dynamics, and control implementation), which is not shown to reproduce real-system behavior.

    Authors: The SCR=2 point emerges from parametric EMT sweeps in which SCR is reduced while holding other parameters fixed; instability manifests as growing oscillations in the DC voltage and currents. The grid is modeled as a standard Thevenin equivalent with specified SCR, and the rectifier and controls use industry-typical parameters for centralized UPS units. We will expand the model-description section with additional justification of these parameters drawn from data-center UPS literature to better substantiate the setup. revision: partial

Circularity Check

0 steps flagged

No circularity; simulation-only comparison with no derivations or fitted inputs

full rationale

The paper presents no mathematical derivations, equations, or parameter-fitting steps. Claims rest on EMT simulation comparisons of PLL-PI vs. power-based control at low SCR, with no self-definitional loops, fitted inputs renamed as predictions, or load-bearing self-citations. The reader's assessment of score 0.0 is confirmed by the absence of any analytical chain that could reduce to its own inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Abstract provides no information on free parameters, axioms, or invented entities; the ledger is therefore empty.

pith-pipeline@v0.9.1-grok · 5720 in / 1174 out tokens · 25612 ms · 2026-06-26T13:21:49.075139+00:00 · methodology

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Reference graph

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