Transformerless T-type three-level inverters are widely used in photovoltaic (PV) systems because of their high efficiency and simple structure. However, their performance is often limited by leakage current and neutral-point (NP) voltage imbalance. Existing modulation methods may restrict the common mode voltage (CMV) range, but cannot always coordinate CMV transitions, sequence-boundary continuity, and NP voltage regulation. To address these issues, a sequence-optimized large-medium-small vector modulation (LMSVM) method is proposed. A systematic switching sequence design procedure is developed to constrain both the CMV level range and its transition magnitude within and between switching cycles. By analyzing the effects of the voltage vectors on the NP voltage and coordinating small and medium vectors, the proposed method also provides effective bidirectional NP voltage regulation. As a result, leakage current root-mean-square (RMS) values and spikes are effectively reduced while satisfactory NP voltage balance is maintained. The effectiveness of the proposed method is verified by simulation and experimental results.
This paper investigates the mode-dependent magnetic characteristics of a Δ-Cr-Y three-phase LLC converter and develops a transient control strategy for seamless mode transition. The magnetic behavior of the integrated core is evaluated under three, two, and single-phase operation, with particular emphasis on flux distribution and associated core and winding losses. The analysis shows that reducing the number of active phases alters the flux distribution among the magnetic columns and introduces flux imbalance. At high load, three-phase operation provides lower magnetic losses, whereas phase shedding to two-phase operation improves efficiency under light-load conditions. Furthermore, the change in operating mode modifies the resonant gain characteristics, which can cause transient output-voltage deviation under closed-loop control. To address this issue, a frequency pre-conditioning (FPC) strategy is proposed to adjust the switching frequency toward the target operating condition before mode transition. A 2 kW prototype is developed to experimentally validate the proposed magnetic analysis and transient control strategy. The measured efficiency reaches 98.1% in three-phase operation at full load and 98% in two-phase operation under light-load conditions.
This paper presents an integrated engineering planning workflow for configuring ancillary services, namely bidirectional battery energy storage systems (BESS) and one-way fast response resources (FRR), to maintain frequency stability in an isolated power system with high photovoltaic (PV) penetration. The workflow links PSS/E time-domain simulations of the 2025 Taiwan system, saturation-aware linear screening relations, and a simultaneous multi-disturbance, multi-objective optimization solved with NSGA-II and verified against the exact ϵ-constraint front. Four disturbance classes are simulated with and without a 1000-MW BESS: 12-s PV ramp-down, 12-s PV ramp-up, instantaneous PV trip, and thermal-unit trip. Winter daytime operation dictates the reserve requirement (ramp-down nadir sensitivity −0.186 Hz/GW versus −0.109 Hz/GW in summer), and thermal-unit trips are the most severe under-frequency event (−0.415 Hz/GW). The predictive capability of the screening relations is quantified by leave-one-out cross-validation (nadir errors within 0.02 Hz for the ramp classes) and by out-of-sample simulations of BESS–FRR portfolios interpolated from the fitted security boundary, which meet the 59.2-Hz nadir target within 0.002 Hz. BESS saturation is defined as the converter rated power reached at the 59.5/60.5-Hz full-output points of the storage frequency–watt characteristic; the energy exchanged within the events is below 4% of a 15-min rating, so the boundary is a converter limit rather than a state-of-charge limit. Because FRR cannot absorb over-frequency events, the ramp-down, trip, and ramp-up constraints are enforced concurrently; the ramp-up constraint imposes a BESS floor of about 790 MW that limits the useful FRR share to 147 MW for the 50% ramp-down design event. The balanced portfolios are 74 MW FRR with 859 MW BESS (933 MW, NT$4.16 billion per year) for the 50% event and 150 MW FRR with 1309 MW BESS (1459 MW, NT$6.45 billion) for the 55% event, and a closed-form break-even analysis shows that FRR remains cost-reducing unless its effective unit cost exceeds 92–95% of the BESS unit cost.
Line current differential relays rely on communicated remote-end current words, making the 87L element vulnerable to false tripping or fault masking if these words are modified in transit. Existing LCDR cyber-resilience methods mainly rely on physical models, supervisory logic, trained detectors, or system-specific signatures; however, communication-facing relay-word admission under the native fixed-payload constraint remains less directly addressed. This paper proposes a fixed-width relay-word wrapper that preserves the native 48-bit three-phase payload, changes the on-wire representation in a frame-varying manner, embeds an in-band validity check, and releases recovered data to 87L only after receiver-side consistency admission. Evaluation on an IEEE 39-bus LCDR implementation shows preserved benign 87L trip/restraint behavior, an empirical blind-forgery admission rate of 0.0732% for B = 12 and Δ = 1, mutual information of 0.0536 bits between the original and on-wire word sequences, and sender-wrapping and benign receiver admission/recovery times of 7.19–7.21 μs/frame and 10.37 μs/frame, respectively, measured on a 180-MHz Cortex-M4 microcontroller. The results demonstrate lightweight pre-87L relay-word admission with reduced on-wire visibility, no payload expansion, and handshake-free operation within the provisioned epoch-key bank.
