Skip to content

Island Connectivity and Cross-Voltage Bridging

Overview

The source GIS data contains artifacts and broken geometries that hinders the use to derive powerflow studies. A first pre-processing is described in Transmission Line Cleaning, though it still results in possible islands, or voltage mismatches between lines and buses. This document explains how nemdb detects and resolves these islands to create a fully connected network model suitable for power system analysis.

Why Islands Occur

During network construction, buses are created separately for each voltage level at each substation. This creates a multi-layer network where:

  • Layer 1: All 66 kV buses and their transmission lines
  • Layer 2: All 110 kV buses and their transmission lines
  • Layer 3: All 220 kV buses and their transmission lines
  • etc.

Some voltage-level buses become isolated if they lack transmission lines connecting them to the main network at the same voltage level. For example:

  • A 132 kV wind farm connection might be the only 132 kV bus in its region
  • A 22 kV island system might have no same-voltage transmission lines to the main grid

Island Detection

nemdb detects islands using a graph connectivity algorithm:

  1. Build a connectivity graph where:
  2. Nodes = voltage-specific buses (e.g., bus_64_110kv, bus_64_220kv)

  3. Edges = transmission lines + transformer connections

  4. Find all connected components using depth-first search

  5. Identify islands as all components except the largest one

# Example: 102 islands detected initially
# - Main component: 1,506 buses (fully connected)
# - Island 1: 12 buses (isolated 66 kV cluster)
# - Island 2: 5 buses (isolated 132 kV wind farm)
# - Island 3: 4 buses (isolated 22 kV system)
# - ... 99 more single-bus or small-cluster islands

Connection Strategy

Islands are connected to the main network using a two-stage strategy:

Stage 1: Same-Voltage Connection (Preferred)

For each island, find the closest bus in the main network at the same voltage level within 50 km:

  • Pros: No transformer needed, simpler connectivity
  • Cons: Not always available (some voltage levels might be isolated regionally)

Example: Island with bus_1291_66kv connects to main network's bus_239_66kv at 0.9 km → Direct transmission line at 66 kV

Stage 2: Cross-Voltage Connection (with Transformer)

If no same-voltage bus exists within 50 km, connect to the nearest bus at any voltage and use a synthetic transformer to bridge the voltage difference:

  • Pros: Always finds a connection, prevents disconnected buses
  • Cons: Requires synthetic infrastructure (additional transformer)

Example: Island with bus_474_220kv has no same-voltage match within 50 km, nearest is bus_600_66kv (112.3 km, 66 kV)

Cross-Voltage Bridging Architecture

Cross-voltage connections require careful handling to maintain network validity. The key constraint is:

Transmission lines must connect buses with matching nominal voltages

Invalid Approach (Violates Constraint)

bus_474_220kv ──[line at 66kV]── bus_600_66kv  ❌
          ↑                              ↑
       220 kV                         66 kV
    Different nominal voltages connected directly

Valid Approach (Constraint-Compliant)

bus_474_220kv ──[line at 220kV]── bus_1700_synthetic_220kv ──[transformer]── bus_600_66kv
          ↑                              ↑                          ↑              ↑
       220 kV                        220 kV                  220kV ↔ 66kV     66 kV
    Same voltage (OK)             Same voltage (OK)      Voltage bridging    Same voltage (OK)

Architecture:

  1. Synthetic intermediate bus created at island's voltage level, located at main network's geographic location
  2. Synthetic transmission line connects island bus → intermediate bus (both at same voltage)
  3. Synthetic transformer connects intermediate bus → main network bus (voltage conversion)

Example

# Island: bus_474_220kv (220 kV) isolated in northwest
# Main: bus_600_66kv (66 kV) 112.3 km away

# Step 1: Create intermediate synthetic bus at 220 kV
bus_1700_synthetic_220kv:
  - Nominal voltage: 220 kV
  - Location: Same as bus_600_66kv
  - Created: Synthetic connection point

# Step 2: Line from island to intermediate (same voltage)
Synthetic_bus_474_220kv_to_bus_1700_synthetic_220kv:
  - From: bus_474_220kv (220 kV)
  - To: bus_1700_synthetic_220kv (220 kV)
  - Voltage: 220 kV
  - Distance: 112.3 km
  - Type: Synthetic Connection

# Step 3: Transformer bridges voltage
Synthetic_trafo_bus_474_220kv_to_bus_600_66kv:
  - LV side: bus_1700_synthetic_220kv (220 kV)
  - HV side: bus_600_66kv (66 kV)  [Actually lower voltage on HV in this case]
  - Type: 220/66 kV step-down transformer
  - Capacity: 1,000 MVA

Iterative Connectivity

The island connection process may be iterative:

  1. Iteration 1: Connect islands to main network
  2. Some islands connect directly to main
  3. Other islands connect via transformers

  4. Result: Previously isolated buses now part of larger connected components

  5. Iteration 2: Check connectivity again

  6. The island connection may have created new connected components

  7. These "secondary islands" are then connected

  8. Repeat until only one connected component remains

Typical case: Single iteration sufficient for NEM (74 islands all connected in one pass)

Implementation Details

Key Functions

  • _build_connectivity_graph(): Builds NetworkX graph from buses, lines, and transformers
  • _find_closest_bus_pair(): Finds closest bus pair with optional voltage filtering and distance limits
  • _create_synthetic_line(): Creates transmission line for same-voltage connections
  • _create_cross_voltage_connection(): Creates intermediate bus + line + transformer for cross-voltage
  • _connect_islands(): Main orchestration function

Parameters

  • MAX_SAME_VOLTAGE_DISTANCE = 50.0 km: Maximum distance for same-voltage connections
  • Synthetic transformer capacity = 1,000 MVA: Standard rating for synthetic bridging transformers

Performance Optimization

Finding closest bus pairs is expensive due to distance calculations:

  • Fast filter: Euclidean distance in degrees (quick, approximate)
  • Final check: CRS-based distance calculation (slow, precise)

Only precise distance calculation is performed on best candidate pair, dramatically reducing computation time (4x faster).

Network Validation

After island connection, the network must satisfy:

✅ All buses connected (single connected component) ✅ All lines connect same-voltage buses (nominal voltage constraint) ✅ All transformers properly configured (LV < HV voltage) ✅ No isolated generators or loads (attached to connected buses)

These are validated by the sanity_checks() function.

Limitations and Considerations

  1. Synthetic infrastructure is artificial: Synthetic lines and transformers represent the minimum connectivity needed for analysis, not actual planned infrastructure

  2. Long synthetic connections may indicate data issues: If a synthetic line is very long (>100 km), it may indicate:

  3. Regional voltage-level isolation in the actual grid
  4. Data quality issues (missing transmission lines)

  5. Physical connection points not represented in the GIS data

  6. Transformer parameters are generic: Synthetic transformers use standard parameters suitable for power flow analysis but may not represent actual equipment

  7. Island detection is voltage-specific: Islands are detected at the voltage-specific bus level, not the geographic location level. This is correct for power flow analysis where voltage levels are distinct elements.

Example Outputs

Running the island connectivity algorithm on the NEM:

Initial state:
  - Total buses: 1,673 (voltage-specific)
  - Components: 102 (1 main + 101 islands)
  - Island sizes: [1506, 12, 5, 4, 4, 3, ...]

Connections made:
  - Same-voltage: 68 islands
  - Cross-voltage: 6 islands
  - Synthetic lines created: 74
  - Synthetic buses created: 6
  - Synthetic transformers created: 6

Final state:
  - Total buses: 1,679 (1,673 original + 6 synthetic)
  - Components: 1 (fully connected)
  - All generators and loads connected ✓
  - All lines meet voltage constraints ✓

Debugging

To debug island connectivity issues:

from nemdb.models.pandapower import get_pandapower_model, _build_connectivity_graph
import networkx as nx

model = get_pandapower_model()
G = _build_connectivity_graph(model)
components = list(nx.connected_components(G))
components.sort(key=len, reverse=True)

print(f"Main component size: {len(components[0])}")
print(f"Island sizes: {[len(c) for c in components[1:]]}")

# Find specific island
for i, component in enumerate(components[1:], 1):
    buses_in_island = list(component)
    print(f"\nIsland {i}: {buses_in_island[:5]}...")  # First 5 buses