Source - Energiforsk 2023-948 Reliability Analysis Microgrid (2023)
Energiforsk Report 2023:948 — Reliability Analysis of Microgrid
Author: Ying He (Vattenfall AB)
Published: May 2023
Programme: Risk- och tillförlitlighetsanalys (Energiforsk)
File: raw/PDF extractions/2023-948-reliability-analysis-microgrid/
Document metadata
- Type: Energiforsk industry research report
- Programme chair: Jenny Paulinder (Göteborg Energi Nät)
- Funded by / stakeholders: Vattenfall Eldistribution, Ellevio, Svenska kraftnät, Göteborg Energi, Elinorr, Jämtkraft Elnät, Öresundskraft, Skellefteå Kraft, Umeå Energi, Kraftringen Nät, Jönköping Energi Nät, and ~10 additional DSOs; Ei and Elsäkerhetsverket as adjunct members
- Purpose: Develop a practical, reusable reliability analysis methodology for microgrids; demonstrate it via case study on a real Swedish microgrid
Summary
The report proposes an analytical (non-Monte-Carlo) reliability evaluation method for microgrids that can handle the unique complexity of:
- Intermittent/random output from wind and PV DGs
- Flexible switching between grid-connected and island (ö-drift) operation modes
- Battery energy storage system contribution in island mode
The method evaluates reliability from three angles: load point and customer reliability indices (SAIFI, SAIDI, AENS, ASAI), generating capacity adequacy indices (LOLE, LOEE, LOLP), and islanding operation indices (IOSR, MIOP).
The case study applies the method to the Arholma system using actual load data from Oct 2015–Jun 2019 supplied by Vattenfall Eldistribution AB. Note: the report was written while Arholma was still in the building phase — the BESS was commissioned in August 2023 after this report was completed.
Arholma case study — system description
At the time of the case study, Arholma had:
- 80 kWp PV system (measured output used directly)
- Two BESS units (ESS1 and ESS2): each 160 kW maximum active power; combined 320 kW discharge capacity
- Load data: 213 customers across 19 load points
- Pre-commissioning BESS assumed design target: 1 hour at 320 kW = 320 kWh per island cycle
Grid topology: 11 kV MV feeders and 0.4 kV LV feeders; mix of underground cables and overhead lines; overhead lines (L23–L25, L27, L28) have higher failure rates and longer repair times, dominating the worst-performing load points (16–19).
Key quantified results
Load point and customer reliability — three-mode comparison
| Mode | SAIFI (f/yr.cust) | SAIDI (h/yr.cust) | AENS (kWh/yr.cust) | ASAI |
|---|---|---|---|---|
| Grid-connected only | 2.263 | 5.249 | 4.617 | 99.9401% |
| Island mode (full) | 1.612 (−29%) | 2.888 (−45%) | 2.788 (−40%) | 99.9670% |
| Hybrid (realistic) | 2.003 (−11%) | 4.304 (−18%) | 3.886 (−16%) | 99.9509% |
Island mode represents the theoretical maximum improvement if the microgrid were always in island mode — not the operational reality. Hybrid mode is the practical result, combining grid-connected operation (most of the time) with island fallback during upstream faults.
Individual load point improvement in island vs grid-connected mode: unavailability reduced 27–53% across all 19 load points. Load points 16–19 (served by overhead lines) see proportionally the largest absolute improvement.
Generating capacity adequacy — island sufficiency
With the 2015–2019 load profile and the pre-commissioning generation spec (80 kWp PV + 320 kW BESS for 1 hour):
| Index | Value |
|---|---|
| LOLE | 130 h/yr |
| LOEE | 4,613 kWh/yr |
| LOLP | 1.5% |
Interpretation: The local generation is insufficient to meet island demand for approximately 130 hours per year on average. This means island operation fails (load must be shed or mainland connection restored) 1.5% of the time. The primary drivers are winter nights when solar output is zero and load peaks exceed battery discharge capacity.
This finding directly explains why the next phase of Arholma extends the system to include customer demand response (heat pumps, heating systems) — the battery alone cannot reliably serve winter peaks. It also validates Vattenfall’s design choice to add DSR alongside the BESS rather than simply upsizing the battery.
Component reliability data used
| Component | Failure rate (f/yr) | Repair time (h/f) |
|---|---|---|
| PCC (upstream system) | 2.1 | 2.0 |
| Underground cable 11 kV | 0.020 | 5.0 |
| Overhead line 11 kV | 0.120 | 20.0 |
| Power transformer 11/0.4 kV | 0.012 | 10.0 |
| ESS1, ESS2 | 1.1 | 5.0 |
| PV system | 0.9 | 5.0 |
| Isolation operation failure probability P | — | — |
The PCC failure rate of 2.1 f/yr (mainland cable faults) dominates system SAIFI — it is the most frequent failure event and the primary trigger for island mode activation.
Methodology overview
The recommended method follows four steps:
- Input data: power network data, customer/load data (hourly time series), DG specifications, component reliability data
- Load point indices: calculated separately for grid-connected, island, and hybrid modes using analytical equations; system indices aggregated via standard formulas (SAIFI = Σλᵢ·Nᵢ / ΣNᵢ; SAIDI = ΣUᵢ·Nᵢ / ΣNᵢ)
- Generating capacity adequacy: chronological hourly comparison of combined PV+BESS generation vs load; LOLE = hours/year where generation < load
- Operation indices: IOSR and MIOP from field performance data (not calculable for Arholma at time of publication — microgrid not yet operating)
For DG production modeling when measured data are unavailable: the report surveys four PV output methods (linear irradiance/temperature, quadratic irradiance, Beta distribution) and two wind speed models (Weibull, Rayleigh); recommends measured data or Renewables.ninja simulation as inputs.
Relevance to existing wiki pages
| Page | Relevance |
|---|---|
| Vattenfall Eldistribution | Quantifies the reliability improvement delivered by Arholma’s island capability; explains why DSR extension is needed (1.5% capacity shortfall) |
| Energy Storage | First quantification of reliability improvement from Swedish ö-drift BESS: SAIDI −45% in island mode, −18% in realistic hybrid mode |
| Demand Response | LOLP 1.5% provides the technical rationale for adding customer heat pump DSR to the Arholma system |
| Distribution System Operator | Multi-DSO funded methodology — reusable across Sweden’s ~170 DSOs for evaluating microgrid investments |
| Source - Vattenfall Arholma Microgrid (2025) | Pre-commissioning baseline against which the operational system (commissioned August 2023) can be assessed |
Companion reports
- Energiforsk 2023:957 — “Felbortkoppling i mikronät” (fault clearing in microgrids) — companion report on the protection system challenge. Referenced in the Lund University 2025 thesis; not yet ingested into this wiki.
- Source - Lund Arholma Microgrid Fault Detection (2025) — MSc thesis doing protection simulations of the same Arholma network; provides the engineering-level detail on why fault detection fails in inverter-dominated island operation.