Tests on the POD-P controller of INELFE Spain-France VSC-HVDC interconnector

1 INTRODUCTION,

• CE has North-South, East-West, and East-Center-West inter-area modes, and past oscillatory events highlighted the need to use available resources to damp them,,.Source: §1 · p. 3
• INELFE is a 2x1000 MW France–Spain VSC-HVDC link with POD-P and POD-Q; a parallel AC corridor limits POD-P effectiveness unless it is open.With the corridor open, electrical distance from the Iberian Peninsula to central Europe increases and inter-area damping tends to be lower.Source: §1 · p. 3
• ADC is INELFE's normal operating mode and has been associated with poorer damping,,,; slowing its time constant from τ=0.75 s to τ=50 s reduced controller-interaction risk.POD-P activation alongside ADC was originally forbidden to avoid interactions.Source: §1 · p. 3
• The French and Spanish TSOs sought to increase POD-P gain and enable its activation together with ADC to improve damping contribution.The modifications aim to improve POD-P effectiveness and increase its contribution to damping CE inter-area oscillations.Source: §1 · p. 3

2 DESCRIPTION OF THE SYSTEM

• INELFE comprises two 1000 MW symmetrical monopoles at 320 kV DC, each with reactive-power capability of [-600 Mvar, +400 Mvar]; about 70% of HVDC setpoint changes are assumed by Vic-Baixas.The remaining 30% is distributed among the other three AC interconnection lines; the interconnector is embedded in the CE system practically in parallel with Vic-Baixas 400 kV.Source: §2 · p. 4

2.1 Constant power control (CPC)

• Under CPC, each HVDC link controls active-power flow to a constant setpoint, with a supplementary active-power setpoint provided by POD-P.P0 is the constant active-power setpoint; ΔP_ref,POD is the supplementary POD-P setpoint.Source: §2.1 · p. 5

2.2 Angle difference control (ADC)

• ADC controls HVDC active power with a term proportional to the terminal-angle difference, emulating an AC line; for an HVDC embedded in an AC network, it is described as more practical than CPC.The active-power setpoint includes constant, ADC-supplementary, and POD-P-supplementary terms; ADC was made slower (τ=50 s) to decouple it from POD-P and avoid interaction.Source: §2.2 · p. 5, §2.3 · p. 6

2.3 POD-P control

• POD-P converts terminal-frequency difference through gain, low-pass and wash-out filters, and saturation into active-power setpoints; per-link gain rose from 500 to 2500 MW/Hz, and ADC-mode disabling was removed.The modifications were supported by power-system studies, hardware-in-the-loop studies in replica, and field tests.Source: §2.3 · p. 6

• Both TSOs’ tuned dynamic models reproduced the East-Center-West inter-area oscillation in frequency, damping ratio, and mode shape.Red Eléctrica used DSM, while RTE used DRM.Source: §3 · p. 7
• Red Eléctrica used Powerfactory for small-signal and PSS/E for transient stability studies; RTE used SMAS3 and Eurostag, respectively.Source: §3 · p. 7
• Only the results obtained by RTE are presented in the paper.Source: §3 · p. 7

Eigenvalue analysis

• In CPC mode with France importing 2,000 MW from Spain, POD-P increases inter-area-mode damping and is more effective with the Vic-Baixas 400 kV line open.Table 3 presents the eigenvalue analysis.Source: Eigenvalue analysis · p. 7, Table 3 · p. 7
• Reported eigenvalue cases include 0.20 Hz and 5.9% damping with POD-P on at 2500 MW/Hz/link; other reported pairs are 0.23 Hz and 2.9%, and 0.20 Hz and 15%.Source: Eigenvalue analysis · p. 7

Time domain simulations

• Figure 4 depicts opening of the Vic-Baixas 400 kV line, with South of Spain frequency and the POD-P controller’s supplementary active-power set point at a gain of 2500 MW/Hz/link.Source: Figure 4 · p. 8, Eigenvalue analysis · p. 8

Discussion

• At 2500 MW/Hz/link, POD-P remains stable, saturates only at Δω = 40 mHz during very severe events, and improves damping more when Vic-Baixas 400 kV is open than closed.The passage characterizes 2500 MW/Hz/link as a compromise based on frequency amplitudes in historical system events.Source: Discussion · p. 8
• Across all simulations, POD-P increased the inter-area mode damping ratio, leading the TSOs to implement the changes using a conservative gain of 2500 MW/Hz/link.Simulated gains of 2500–5000 MW/Hz/link produced good results; the lower value was selected to account for dynamic-model inaccuracies.Source: Discussion · p. 8
• The selected 2500 MW/Hz/link gain is consistent with typical POD-P controllers using frequency-error input signals,,.Source: Discussion · p. 8

4 PRACTICAL IMPLEMENTATION OF THE CHANGES MADE ON POD-P CONTROLLER

• RTE tested the changes in a hardware-in-the-loop setup, connecting physical INELFE C&P replicas to a real-time simulator and modeling the 1 December 2016 incident’s 0.155 Hz inter-area oscillation.The Hypersim benchmark represented the French and Spanish sides with two synchronous machines.Source: §4 · p. 9
• With K_P=2500 MW/Hz/link, POD-P activation under constant-power control or ADC increased inter-area-mode damping; gains around 2500–5000 MW/Hz/link were judged reasonable.POD-P activation with ADC and τ=50 s was also possible.Source: §4 · p. 9
• The tested situations varied active-power or AC-voltage/reactive-power control, inverter or rectifier operation, 0 MW crossing, and POD-P gain; the modifications triggered no control or protection warnings.A joint operator procedure covered onsite POD-P activation in ADC and changing the gain to K_P=2500 MW/Hz/link; Red Eléctrica and RTE implemented the changes in September 2022.Source: §4 · p. 9

5 TESTS ON POD-P CONTROLLER

• From September to December 2022, tests checked the modified POD-P controller’s behavior across operating conditions, with two representative tests presented.The representative tests covered POD-P in CPC and ADC on 5 October and INELFE-1 during the Vic-Baixas 400 kV line disconnection on 9 December; Spanish and French PMU data were analyzed.Source: §5 · p. 10

Test 1: Tests of POD-P control in CPC and ADC (5-Oct-2022)

• Test 1 evaluated POD-P in CPC and ADC against small system disturbances and checked its correct behavior during a planned Vic-Baixas 400 kV outage.The outage was selected because POD-P is more effective when the line is open; the two configurations were CPC with POD-P and ADC with POD-P.Source: §5 · p. 10
• Before Test 1, both HVDC links were in service in Multi-Link mode, France was PCONMASTER, power flowed from Spain to France, and Vic-Baixas 400 kV was disconnected.Source: §5 · p. 10

Test 1-a: CPC with POD-P

• In Test 1-a, HVDC active power increased when Llogaia frequency exceeded Baixas frequency and decreased when it was lower, while average power flow remained at its CPC set point plus POD-P input.The two links’ active-power behavior was symmetric under multi-link control; the Llogaia–Baixas frequency difference was non-negligible with the Vic-Baixas 400 kV line open.Source: Test 1-a: CPC with POD-P · p. 10, Figure 6 · p. 11

Damping ratio of the inter-area mode

• The average inter-area-mode damping ratio increased around 10% when POD-P was active in both CPC and ADC modes, although system conditions prevent isolating the controller’s effect.The reported averages use the time window for each case; the critical East-Center-West mode was measured by WAMS at around 0.15 Hz.Source: Test 1-a: CPC with POD-P · pp. 11–12, Table 4 · p. 12

Test 2

• Test 2 examined POD-P under disconnection of the Vic-Baixas 400 kV line in CPC and ADC modes; only Test 2-a results were presented.The tests aimed to evaluate POD-P response to a system disturbance, with POD-P on and POD-OFF as the comparison condition; both HVDC links were in service in multi-link control.Source: Test 1-a: CPC with POD-P · p. 12

Conclusions

• POD-P increases the inter-area-mode damping ratio, with greater improvement when the Vic-Baixas 400 kV line is open.The improvement is difficult to quantify exactly because multiple factors affect the damping ratio.Source: §6 · p. 14
• The POD-P modifications were implemented successfully, and tests confirmed correct behavior in different scenarios under CPC and ADC.The validated changes increase controller gain and allow POD-P activation during angle difference control.Source: §6 · p. 14
• The modified POD-P controller has been validated and used in normal operation in both CPC and ADC modes since 5 January 2023.Source: §6 · p. 14