Source - Från dödnätsstart till ödrift Uppsala Thesis (2025)
Uppsala University master’s thesis, 2025 — Från dödnätsstart till ödrift: Simulering av stabilitet, elkvalitet och pålastningsstrategier (From Black-Start to Island Operation: Simulating Stability, Power Quality and Loading Strategies). Author: Märta Strømme. Supervisor: Robert Eriksson; Subject reviewer: Göran Ericsson; Examiner: Elísabet Andrésdóttir. Teknisk-naturvetenskapliga fakulteten, Uppsala University.
Simulation study of a real (anonymized) municipality in central Sweden, modelling black-start and subsequent island operation from a hydropower plant and a CHP plant.
Summary
The thesis investigates how a waterpower plant and a CHP plant handle the transition from dead-grid start (dödnätsstart) to stable island operation supplying 15 prioritized buildings in the municipality. Four loading strategies (pålastningsstrategier) are evaluated using Simulink simulation models. Results show that both plant types maintain acceptable stability, but the CHP plant is more robust due to its higher capacity relative to the load. The choice of loading strategy significantly affects frequency and rotor angle transients.
Methodology
- Tool: MATLAB/Simulink
- Plant models: water turbine, steam turbine (CHP), excitation system, synchronous generator
- Grid model: transformers, cables, prioritized buildings as loads
- Data provided by the anonymous municipality
- 15 prioritized buildings modelled as composite loads
- Simulations cover the initial load connection phase only — not long-term operation or reconnection to the main grid
CHP plant rated capacity: 30.6 MVA
Loading strategies evaluated
Four strategies for connecting priority loads after island startup:
| Strategy | Description |
|---|---|
| Fastighetsordning | Connect buildings in a preset priority order, with fixed time gaps |
| Storleksordning (störst först) | Largest loads connected first |
| Storleksordning (minst först) | Smallest loads connected first |
| Kluster | Buildings grouped into clusters; all buildings in a cluster connected simultaneously, with inter-cluster time gaps |
| Alla samtidigt | All 15 buildings connected simultaneously |
Key results
Frequency and rotor angle (CHP plant)
- Maximum frequency dip: 49.87 Hz in the “Alla samtidigt” maximum load scenario — within the permissible range but near the lower limit
- Frequency and rotor angle stabilize approximately 80 seconds after the last load connection
- Strategies with staged loading (time gaps between connections) produce significantly smoother frequency and rotor angle transient responses
- Kluster: best compromise — better frequency performance than all-at-once, better rotor angle performance than pure sequential strategies
Voltage
- All scenarios: voltage remains within ±5% p.u. across all loading strategies and scenarios
- Voltage transients recover in approximately 1 ms
Hydropower plant
- Also maintains acceptable stability, but with larger relative perturbations than the CHP plant — the CHP plant has more available capacity relative to the 15 priority buildings’ demand, making it more tolerant of load steps
Power quality
- Impulse transients occur at each load connection step but remain within acceptable limits
- No sustained oscillatory instability observed in any scenario
Discussion
Model limitations: simplified models exclude higher-order turbine dynamics, stochastic load variation, and simultaneous faults. Results are indicative, not certifiable for production use.
Implication for loading strategy design: gradual, time-separated loading reduces stress on frequency and rotor angle stability. The Kluster strategy offers a practical middle ground between operational simplicity and stability performance.
Grid-forming capability: both plants simulated are synchronous machines with inherent grid-forming characteristics — they establish voltage and frequency references without external support. This contrasts with inverter-based sources (solar, wind, BESS with grid-following control) which cannot independently sustain island operation without grid-forming control.
Preparedness gap: MSB (2022) noted that Swedish municipalities are insufficiently prepared for large-scale outages and that there are too few black-start-capable generators. This thesis provides quantitative simulation support for the selection and operational design of island networks.
Relevance to other wiki pages
- Island Operation — loading strategies, simulation methodology, frequency/voltage results for island startup
- Energy Storage — grid-forming vs grid-following; role of synchronous generation as frequency anchor
- Electric Power Distribution — protection relay settings in weak (low kortslutningseffekt) island networks; transformer and cable modelling
Data gaps
- Municipality name and actual implementation status — the thesis uses an anonymized municipality; whether the island operation plan was subsequently realized is unknown
- Whether the study’s Simulink models or data have been made publicly available for other municipalities to adapt