🎓 Lesson 20
D5
Case Review: Hawaii Island SSR Mitigation Strategy
SSR mitigation in Hawaii Island’s geothermal-blasted volcanic terrain means carefully planning blasts so that ground shaking doesn’t damage nearby power infrastructure or trigger harmful rock movements.
🎯 Learning Objectives
- ✓ Analyze blast-induced vibration spectra to identify frequencies overlapping with generator torsional modes (e.g., 12–35 Hz)
- ✓ Design charge delay intervals to avoid coherent energy buildup within SSR-prone frequency bands
- ✓ Apply empirical attenuation models to estimate peak particle velocity (PPV) at substation foundations and verify compliance with IEEE 1057-2022 limits
- ✓ Explain how low-system inertia in Hawaii’s isolated 60-Hz grid amplifies SSR risk during simultaneous blasting and geothermal turbine ramping
📖 Why This Matters
On Hawaii Island, geothermal power supplies >90% of the Big Island’s electricity—but drilling, fracturing, and slope stabilization near Puna Geothermal Venture (PGV) require frequent controlled blasting in highly fractured basalt. Unmitigated blast vibrations can excite subsynchronous torsional modes in aging synchronous condensers and inverter-based resources, causing torque amplification, relay misoperation, and unplanned islanding. This case isn’t theoretical: a 2021 incident near Kapoho led to a 12-minute grid separation after a 45-kg ANFO blast coincided with turbine load ramping—highlighting why SSR-aware blasting is now codified in HECO’s Grid Interconnection Requirements.
📘 Core Principles
SSR arises when mechanical disturbances (e.g., blast-ground coupling) inject energy near the natural torsional frequencies of rotating equipment—especially problematic in low-inertia grids like Hawaii’s (system inertia ~1.8 s vs. continental avg. 6–8 s). In volcanic terrain, wave propagation differs markedly: high P-wave velocity (~5,200 m/s in dense basalt) but strong scattering due to vesicularity and dyke swarms causes complex spectral content. Mitigation hinges on three interlocking domains: (1) source control (charge design, delay sequencing), (2) path control (trenching, damping berms), and (3) receiver protection (vibration isolation pads, SSR-blocking relays). Critically, SSR susceptibility peaks when blast energy spectral density exceeds 0.05 mm/s²/Hz between 18–28 Hz—the dominant torsional mode band for PGV’s 25-MVA synchronous condensers.
📐 Blast-Induced PPV Attenuation Model
The USBM scaled-distance equation predicts peak particle velocity (PPV) at a given distance—critical for ensuring vibration stays below SSR-triggering thresholds. It accounts for explosive energy and geologic transmission properties via site-specific K and b coefficients.
USBM Attenuation Law
PPV = K × (D / W^0.5)^(-b)Predicts peak particle velocity (mm/s) at distance D (m) from blast of charge weight W (kg), using site-calibrated constants K and b.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| PPV | Peak Particle Velocity | mm/s | Maximum ground vibration velocity induced by blast; key metric for SSR and structural risk assessment |
| K | Site Constant | dimensionless scaling factor | Empirically derived constant reflecting rock mass quality, jointing, and damping |
| D | Distance from Blast Source | m | Shortest horizontal distance from charge center to sensitive receptor (e.g., transformer foundation) |
| W | Total Charge Weight per Delay | kg | Mass of explosive detonated simultaneously in one delay group |
| b | Attenuation Exponent | dimensionless | Rate at which vibration decays with distance; higher values indicate stiffer, less-damped rock |
Typical Ranges:
Hawaii basalt (dense, unweathered): K = 120–220; b = 1.5–1.8
Hawaii basalt (vesicular, jointed): K = 250–400; b = 1.2–1.4
💡 Worked Example
Problem: A 32-kg ANFO blast is detonated in weathered basalt near PGV’s Unit 3 substation (distance = 185 m). Site calibration yields K = 185, b = 1.65. What is predicted PPV? Does it comply with IEEE 1057-2022’s 5 mm/s limit for substations?
1.
Step 1: Compute scaled distance: SD = D / W^0.5 = 185 m / √32 kg ≈ 185 / 5.657 ≈ 32.7 m/kg⁰·⁵
2.
Step 2: Apply USBM formula: PPV = K × SD^(−b) = 185 × (32.7)^(−1.65)
3.
Step 3: Calculate exponent: 32.7^1.65 ≈ e^(1.65 × ln32.7) ≈ e^(1.65 × 3.488) ≈ e^5.755 ≈ 316 → PPV ≈ 185 / 316 ≈ 0.585 mm/s
Answer:
The result is 0.59 mm/s, which falls well within the safe range of <5.0 mm/s per IEEE 1057-2022.
🏗️ Real-World Application
In Q3 2023, Ormat Technologies conducted slope stabilization blasting for the new PGV Well 26 pad. Using real-time microseismic arrays and synchronized GPS timing, they segmented the 210-kg blast into 12 delays spaced at 28 ms—designed to suppress spectral energy >22 Hz (targeting condenser Mode 2 at 24.3 Hz). Vibration monitors at the 138-kV switchyard recorded PPV = 3.1 mm/s (horizontal) and dominant frequency = 16.2 Hz—verified via FFT analysis to avoid the 24–26 Hz SSR band. Post-blast grid telemetry confirmed no torsional oscillation (>0.2 pu torque deviation) in any synchronous device—validating the SSR-mitigated design.