REVIEW 2 major objections 5 minor 31 references
Revisiting the Effect of Grid-Following Converter on Frequency Dynamics -- Part I: Center of Inertia
T0 review · 2 major / 5 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Grid-following converters do not enter the center of inertia; their equivalent frequency couples to it through a weak, adjustable virtual tie line.
desk verdict Careful GFL swing-equation modeling, but the central no-COI-aggregation claim rests on a sign error in Eq. (21d) that likely does not survive contact with the paper's own Eq. (20). read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing object is the tying power between generators: for two synchronous generators the exchange term is $E_iE_jB_{ij}\sin(\delta_i-\delta_j)$ on one side and its exact negative on the other, so the pair cancels and all SGs can be merged into one COI. For an SG and a GFL, the corresponding expression built from the transfer parameter $V^{eq}$ reads $-E_iI_fV^{eq}\cos(\delta_i-\theta_{f})$ on both sides, giving equal magnitudes but the same direction, so the pair does not cancel and the GFL must remain outside the COI. The second piece of machinery is the GFL interface state variable $I^F\angle\theta^{F,I}$, which replaces the synchronous machine's internal EMF as the quantity that couples converter dynamics to the network, and whose dynamics are split into a continuous part governed by equivalent inertia and a discontinuous part governed by the proportional coefficient $L^F$.
What would settle it
Recompute the SG-side and GFL-side tie powers by substituting Eq. (20) into the active-power formula $P=\mathrm{Re}(\dot U\,\overline{\dot I})$ and compare the resulting signs; if the two expressions are negatives of each other, the SG-GFL pair cancels like an SG-SG pair and the paper's central claim fails, while if they are equal in sign the claim is confirmed.
Extended reading notes
Core claim
The paper's central claim is that grid-following converters and synchronous generators cannot be aggregated into one center of inertia because the tying power between them has equal magnitudes but the same direction, not opposite directions. It develops a multi-generator model in which each GFL's interface state variable is the output-current phasor $I^F\angle\theta^{F,I}$, whose dynamics are shaped by the d-axis power control, the DC boost, and the phase-locked loop; this variable obeys a swing-equation-like relation with an equivalent inertia $H^F$, an equivalent governor transfer function $J^F(s)$, and a proportional step coefficient $L^F$. On the network side, the admittance matrix is rearranged so that active power between SGs and GFLs flows through transfer parameters $V^{eq}$ and $W^{eq}$; because these terms do not cancel pairwise, only synchronous generators form the COI, and the GFL equivalent frequency interacts with the COI frequency through a virtual tie line. The paper further claims that this coupling is weak because the transfer parameters are small, and that the GFL-side equivalent inertia and governor, although they do not add directly to $H^{COI}$, still shape COI dynamics through that tie line and are adjustable via control setpoints.
Load-bearing premise
The central conclusion that a GFL cannot be aggregated into the COI rests entirely on the signs of the two tie-power terms in Eqs. (21d) and (22d): the block-matrix derivation in Eq. (20) yields the GFL-side term as the negative of the SG-side term, yet the printed formulas add to $-2EIV\cos(\delta-\theta)$, and the 'same direction' property comes from that printed sign.
Editorial extensions
If this is right
- Only synchronous generators define the system center of inertia; a GFL's equivalent inertia and governor reach the COI indirectly through the virtual tie line, not by direct addition to $H^{COI}$.
- In the paper's test system, the proposed model reduces COI-frequency error to 0.56% and GFL-power error to 4.41%, compared with 6.39% and 11.48% for the SFR-based treatment that ignores converter dynamics.
- A GFL's equivalent parameters are not fixed machine constants: they vary with active power, reactive power, voltage setpoint, PLL gains, and DC-control gains, so the same converter can look like different inertia values at different operating points.
- Reducing the PLL integral gain can raise the COI frequency nadir, so slower converter synchronization can actually improve the system frequency response in a low-inertia grid.
- The GFL equivalent governor provides no steady-state damping; it only contributes an oscillation at frequency $\omega_{Osc}$ that can be tuned through DC-side control gains and is reflected in COI dynamics.
Reading between the lines
- Because the paper's conclusion turns on the sign of the tie-power terms in Eqs. (21d) and (22d), a direct algebraic check of those equations against Eq. (20) would settle whether the cancellation argument extends to SG-GFL pairs; the matrix derivation already supplies a relative negative sign that the printed tie-power formulas do not carry through.
- The same construction likely transfers to grid-forming converters, whose internal synchronization differs from both SGs and GFLs; the critical test is whether their tying power with synchronous generators cancels pairwise.
- The paper's finding that the coupling is controlled by $V^{eq}$ and $W^{eq}$ suggests a design extension: frequency support could be shaped by adjusting converter output impedance or network impedance rather than only by tuning converter inertia.
- Since the case study shows a negative component ($H^{F,Id}$) and a positive component ($H^{F,PLL}$) combining into a positive total equivalent inertia, interpreting a single fitted 'inertia' value could be misleading; an experimental separation of the continuous and discontinuous frequency components would clarify what is actually being measured.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper develops a multi-generator model of a power system containing synchronous generators (SGs) and grid-following converters (GFLs). It derives a swing-equation-like model for the GFL equivalent frequency, including equivalent inertia, equivalent governor, and a proportional coefficient, all expressed in terms of control parameters and operating points. On the network side, it defines tying power between SG and GFL interface state variables and claims that SG-GFL tying powers have equal magnitudes but the same direction, so GFLs cannot be aggregated into the SG center of inertia (COI); instead, GFLs interact with the SG COI through a virtual tie line. Case studies on a modified WECC 9-bus system compare the proposed model with EMT simulations and two existing methods, reporting smaller errors for the proposed model.
Significance. If the central claim were correct, the paper would provide a principled answer to a long-debated question: whether GFL equivalent inertia and governor dynamics can be lumped into a system-wide COI. The paper's derivation of GFL equivalent parameters directly from control parameters and operating points, without fitting to simulation output, and its validation against EMT simulations with small error indices, are genuine strengths. However, the central no-aggregation result rests on a sign error in the tie-power equations. Once the sign is corrected, the SG-GFL tie powers cancel in exactly the same way as SG-SG tie powers, so the proposed virtual-tie-line interpretation and the entire COI aggregation structure collapse. The contribution, as a proof of the stated claim, is therefore not reliable.
major comments (2)
- [Section IV-A, Eqs. (21d), (22d), (24)] The sign of the V_eq cos term in Eq. (21d) is inconsistent with the block matrix (20). Expanding (20), the SG-side tie power is Re(E_G conj(T_eq I_F)) = E_G I_F [V_eq cos(δ_G - θ_F) + W_eq sin(δ_G - θ_F)], while the GFL-side tie power is Re(U_F conj(I_F)) = -E_G I_F [V_eq cos(δ_G - θ_F) + W_eq sin(δ_G - θ_F)]. The printed Eq. (21d) uses -V_eq cos(...) + W_eq sin(...), which is the negative of the correct first term, and Eq. (22d) also uses -V_eq cos(...) + W_eq sin(...). Consequently Eq. (24) prints -2 E_G I_F V_eq cos(...), whereas the correctly derived terms sum to zero. Since equation (24) is the sole stated basis for the claim that SG-GFL tie powers are 'equal in magnitude but same direction', the paper's central conclusion that GFLs cannot be aggregated into the SG COI is unsupported. This is a load-bearing error: correcting the sign reverses the conclusion and invalidates the virtual-tie-line model of Eqs. (26e) and (27d).
- [Section IV-B.1 and Table III] The claim that the connection between the SG COI and GFLs is 'relatively weak' is not derived in the paper but is inferred from a single numerical case (V_eq' = -0.84 pu versus G_eq' = 2.67 pu in Table III) together with a qualitative statement about the admittance-matrix inversion. No bound, scaling law, or parametric study establishes that V_eq and W_eq are generally small, so the abstract's statement that the impact of GFLs on COI frequency is 'relatively weak' is not supported beyond the specific test system.
minor comments (5)
- [Eq. (22c) and Eq. (27c)] The index in the cosine term is repeated: 'θ_{F,I}^{i_F} - θ_{F,I}^{i_F}' should read 'θ_{F,I}^{i_F} - θ_{F,I}^{j_F}'; the same typo appears in Eq. (27c).
- [Heading of Section IV-B] The heading contains a typo: 'Frequecny' should be 'Frequency'.
- [Section IV-B.2] The notation 'P_{F,M ac}' is inconsistent with 'P_{F,M ec}' used earlier in Eq. (16c); the same symbol should be used throughout.
- [Reference [30]] The derivations for the transfer functions J_Id(s), J_Pll(s), the coefficients c_Ei, c_Ep, c_Pll, c_Pi, and the network reduction are relegated to an external URL. The manuscript is not self-contained, which makes it difficult to verify the derivations.
- [Eq. (26e), Eq. (27d), and Table III] The coefficients V_eq' and W_eq' appear with a single GFL subscript i_F, whereas the earlier full-network model uses double subscripts i_G, i_F; the definition of these aggregated coefficients is deferred to the appendix and should be stated in the main text.
Circularity Check
No significant circularity: the derivation is self-contained and benchmarked against independent EMT simulation.
full rationale
The paper's core derivation chain is not circular. The GFL interface-state model (Eqs. 7-16) is obtained by linearizing the converter controls (DC boost, d-axis power control, PLL) under stated assumptions A1-A4; equivalent inertia H_F, equivalent governor J_F(s), and proportional coefficient L_F are closed-form functions of control gains and operating points, not parameters fitted to the EMT output. The network-side tying powers (Eqs. 21-22) are computed from the admittance matrix via the standard relation P = Re(U conj(I)), and the SG-SG cancellation (Eq. 23) and the SG-GFL non-cancellation (Eq. 24) are presented as derived results rather than assumed conclusions. The model is then tested against detailed EMT simulation on a modified WECC 9-bus system (Sec. V-A, Table II), which is an external benchmark independent of the model's fitted parameters. The appendix citation [30] supplies derivations and coefficient expressions rather than invoking an unverified self-cited uniqueness theorem, so it is not load-bearing circularity. The possible sign inconsistency in Eqs. (21d)/(22d) is a correctness or consistency concern, not a circularity defect: even if Eq. (24) is algebraically wrong, the paper's conclusion would be unsupported but not equivalent to its inputs by construction. No fitted parameter is renamed as a prediction, and no known result is repackaged as the claimed derivation. Hence no circular step is present.
Assumptions & free parameters
assumptions (4)
- domain assumption Synchronous generators are represented by a second-order swing equation with constant EMF amplitude behind a reactance, and SG tie susceptances dominate conductances.
- ad hoc to paper A1-A4: constant DC input power, constant control setpoints, ideal inner current loops, and constant q-axis voltage, reactive power, and q-axis current during frequency dynamics.
- domain assumption The power system can be linearized around the pre-disturbance operating point, and node elimination with constant-impedance loads yields the block matrices in Eqs. (17)-(20).
- ad hoc to paper The sign convention for tie power in Eqs. (21d) and (22d) is the same as for physical branch power.
invented entities (2)
-
GFL equivalent frequency with continuous and discontinuous components
-
Virtual tie line and virtual tying power between SG COI and GFLs
Cite this review
Pith. "Pith review of Revisiting the Effect of Grid-Following Converter on Frequency Dynamics -- Part I: Center of Inertia." pith.science (2026). https://pith.science/paper/ZM7JALK5
@misc{pith2026250715358,
author = {Pith},
title = {Pith review of: Revisiting the Effect of Grid-Following Converter on Frequency Dynamics -- Part I: Center of Inertia},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZM7JALK5}},
note = {Machine review of arXiv:2507.15358}
}
read the original abstract
Understanding the impact of grid-following (GFL) converters on system frequency dynamics is crucial, from both the center of inertia (COI) and frequency spatial variation perspectives. Part I of this series clarifies the mechanisms by which GFLs influence COI frequency dynamics. A multi-generator model of the power system with GFLs is developed, incorporating the local dynamics of GFLs and their interaction with synchronous generators via virtual tie lines. By aggregating the multi-generator model into the COI frame, the interaction between the COI frequency and the equivalent frequency of GFLs is revealed. The equivalent inertia and other components at the GFL side, determined by control parameters and operating conditions, support the COI through virtual tying power. Simulation validates the accuracy of the proposed modeling and demonstrates that the impact of GFLs on COI frequency is relatively weak. The equivalent inertia and other components of GFLs still significantly influence COI frequency dynamics, with their effects being both time-variable and adjustable.
Figures
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