SOLA2540 Chap.3 Storage, Inverters and Protection
Storage, Inverters and Protection
Component selection is a matrix of energy, power, duration, temperature and connection constraints. Battery sizing begins with DC-bus daily energy, autonomy, nominal voltage and maximum depth of discharge, but the selected bank must also meet maximum and surge current at the relevant discharge rate and temperature.
Inverter selection begins with credible simultaneous apparent power, not daily watt-hours, and attaches a duration to continuous, short-term and surge ratings. Controllers must match array voltage and current while providing the correct charging regime.
Cables must satisfy current capacity and voltage drop, and overcurrent protection must interrupt the available fault current near its source while isolating the affected section. This chapter practises the closing checks that turn calculated capacities into a coherent and protectable system.
What this chapter covers
- 01
Battery energy capacity depends on daily DC-bus energy, autonomy, bus voltage and maximum depth of discharge
- 02
Battery power capability and surge duty are checked separately from stored energy
- 03
The selected discharge-rate capacity and operating temperature must match the application
- 04
Inverter running demand is expressed in apparent power when load power factor is below one
- 05
Continuous, duration-limited and surge ratings cannot be substituted for one another
- 06
Controller checks include array voltage, array current, battery charging regime and temperature compensation
- 07
Cable sizing satisfies both current-carrying capacity and voltage-drop limits
- 08
Protection placement, interruption rating, labelling and selectivity follow the actual fault-energy path
Worked example · free
Size battery energy and report the unfinished checks
- +1Uncorrected capacity is 5,200 × 3 /(48 × 0.50) = 650 Ah.
- +1Apply the temperature factor: 650 × 1.08 = 702 Ah required at the specified discharge-rate basis.
- +1For a 720 Ah selection, daily depth of discharge is 5,200/(48 × 720) = 0.1505, or 15.1% per day.
- +1The selection remains provisional until discharge-rate, maximum-current, surge, charging-current and configuration checks pass.
Key terms
- Battery autonomy
- The agreed period for which storage supplies the design load without renewable input, considering service criticality, resource intermittency, backup and acceptable loss of load.
- Depth of discharge
- The fraction of rated battery capacity removed. The design maximum limits usable energy and is selected with battery characteristics and cycle-life objectives in mind.
- Discharge rate
- The time basis over which battery capacity is specified. Available ampere-hours can depend on how quickly energy is withdrawn, so the rating basis must match the load duty.
- Apparent power
- Voltage-current loading measured in volt-amperes. It can exceed real power when power factor is below one and is central to inverter and conductor checks.
- Voltage drop
- The reduction in voltage along a cable caused by current and impedance. A cable may carry current safely yet deliver unacceptable voltage or waste excessive energy.
- Protection selectivity
- Coordination that allows the protective device nearest a fault to operate without unnecessarily disconnecting healthy sections of the installation.
Storage, Inverters and Protection FAQ
Is the battery ampere-hour result the final selection?
No. The energy calculation gives a minimum capacity for the stated daily demand, autonomy, bus voltage and usable depth of discharge. Apply temperature and use the correct discharge-rate basis, then compare commercial configurations. Separately verify maximum and surge current, charging current, cycle duty, maintenance, transport and safe installation. Recompute achieved autonomy and daily depth after choosing the actual bank.
Why does inverter sizing use volt-amperes?
An inverter carries voltage and current, so loads with power factor below one demand more apparent power than their real-watt consumption suggests. Convert each relevant AC load using its running power factor, build a credible coincidence case, and attach durations to continuous and short-term ratings. Then test motor or compressor start events against the surge rating and allowed surge duration.
What makes a cable selection complete?
Check current-carrying capacity, voltage drop, insulation and environmental suitability for the circuit. Low-voltage battery and inverter paths can carry high current, so current capacity may control even on short runs; other extra-low-voltage paths may be dominated by voltage drop. The cable length and cross-section in the worksheet must match the single-line diagram and the protection device selected for that conductor.
Where should battery protection be placed?
Battery conductors contain a high-energy fault source, so overcurrent protection should be mounted as close as practicable to the battery terminals while avoiding ignition hazards and providing suitable DC interruption. The required protected conductors depend on the earthing arrangement. Each string in a multiple-string bank needs appropriate consideration, equalised cable paths and a clear isolation method.
Assessment move
Use a constraint matrix rather than memorising one rating per component. For the battery, write energy capacity, discharge-rate basis, temperature, maximum demand, surge and charging current in separate rows. For the inverter, convert running watts to volt-amperes, list credible simultaneous loads, then add start events with their durations.
For the controller, calculate hot and cold array voltages, maximum array current and the required battery charging regime. For every cable, pair current capacity with voltage drop and check that the protection device is suitable for the circuit type and available fault energy. Practise a two-pass solution: the first pass calculates a minimum, and the second pass tests the selected commercial equipment.
In written responses, resist the phrase “therefore select” until the second pass is complete. Draw a single-line diagram and annotate each branch with normal current, surge or fault duty; this reveals when the same device has been asked to satisfy incompatible roles. Finish by checking selective isolation and service continuity: a faulted branch should be made safe without unnecessarily removing critical healthy loads.
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