🎓 Lesson 11 D5

Solid-State Breaker Sizing and Coordination with AC Protection Devices

A solid-state breaker is an electronic switch that quickly stops DC current in microgrids—like a high-speed, no-moving-parts circuit breaker for batteries and solar inverters.

🎯 Learning Objectives

✓ Calculate required voltage and current ratings for solid-state breakers based on system DC voltage, maximum fault current, and clearing time constraints
✓ Design coordination logic between solid-state breakers and upstream AC-side protection (e.g., molded-case circuit breakers or relays) to ensure selective tripping under hybrid fault scenarios
✓ Analyze thermal and switching energy limits of SiC-based SSBs using manufacturer datasheets and IEEE 1547-2018 fault ride-through requirements
✓ Explain the impact of cable inductance and inverter short-circuit capability on SSB sizing and coordination margins
✓ Apply IEC 62304 and UL 1741 SB Annex D criteria to verify SSB compliance with microgrid safety and interoperability standards

📖 Why This Matters

In modern DC and hybrid microgrids—especially those integrating battery energy storage systems (BESS), photovoltaic inverters, and fuel cells—traditional AC breakers fail catastrophically during DC faults due to sustained arcing and inability to interrupt rising current. Solid-state breakers are now essential for safe, fast, and selective protection—but mis-sizing or poor coordination with AC-side devices leads to nuisance trips, equipment damage, or uncontrolled cascading faults. This lesson bridges theory and practice: you’ll learn how to size SSBs not just for voltage/current, but for *system-level coordination*, ensuring reliability when microseconds matter.

📘 Core Principles

Solid-state breaker sizing hinges on three interdependent domains: (1) Electrical stress limits—peak voltage (including transient overvoltages from inverter switching or lightning), continuous current (including harmonic derating), and fault current magnitude/di/dt; (2) Thermal and switching energy management—junction temperature rise during conduction and energy dissipation during turn-off (E = ∫v(t)·i(t) dt); and (3) Coordination physics—ensuring the SSB clears before upstream AC breakers (e.g., LV MCCBs) reach their let-through energy (I²t) threshold, while respecting inverter fault-current contribution duration per IEEE 1547-2018 Section 5.3. Coordination requires time-current characteristic (TCC) overlap analysis across both AC and DC domains, accounting for propagation delays, sensor bandwidth, and control-loop latency—unlike AC-only systems where TCC curves align naturally at 50/60 Hz.

📐 Critical Clearing Energy and Coordination Margin

The key sizing criterion is ensuring the SSB’s interrupting energy rating exceeds the fault energy it must absorb *before* upstream AC protection operates. This is quantified using the let-through I²t of the nearest AC breaker and the SSB’s clearing time. The coordination margin ensures selectivity.

Coordination Energy Margin

E_{SSB} ≥ I_{fault}^2 × t_{clear}

Minimum interrupting energy rating required for solid-state breaker to safely absorb fault energy during its clearing interval.

Variables:

Symbol	Name	Unit	Description
E_{SSB}	SSB interrupting energy rating	A²·s	Maximum energy the SSB can safely interrupt without failure
I_{fault}	Available DC fault current	A	Peak fault current contributed by inverters, batteries, or PV during clearing window
t_{clear}	SSB clearing time	s	Time from fault detection to current zero (including sensing, logic, and semiconductor turn-off delay)

Typical Ranges:

Commercial BESS (1.5 kVdc): 1000 – 5000 A²·s

Utility-scale solar + storage hybrid: 3000 – 15,000 A²·s

💡 Worked Example

Problem: A 1.5 kV DC BESS string feeds into a 400 V AC/DC bi-directional inverter. Upstream AC protection is a 63 A Type B MCCB (Schneider iC60N) with I²t let-through of 1200 A²·s at 10 kA prospective fault. Inverter contributes 2.5 kA DC fault current for 20 ms before current-limiting engages. SSB clearing time is 250 µs. Calculate minimum required SSB energy rating and verify coordination margin.

1. Step 1: Compute worst-case fault energy seen by SSB = (inverter fault current)² × (time until current limiting) = (2500 A)² × 0.02 s = 125,000 A²·s.

2. Step 2: Compare to AC breaker’s let-through: 1200 A²·s — this is the energy the AC breaker *allows through* before clearing; since SSB clears in 250 µs, it must absorb only its own share: (2500 A)² × 0.00025 s = 1562.5 A²·s.

3. Step 3: Verify coordination: SSB clears in 250 µs << AC breaker’s typical clearing time (10–50 ms), so energy absorbed by SSB (1562.5 A²·s) must be ≤ SSB’s rated interrupting energy (e.g., ≥ 2000 A²·s per datasheet), providing a 28% margin.