Control and Renewable Energy Systems
This domain joins control engineering to generation: contactor and relay logic, motor starting, PLC programming, and the design and safe grid connection of PV, heat pump, wind and battery systems.
Motor control
A direct-on-line starter uses a contactor with a seal-in contact, a stop button and a thermal overload relay. Star-delta connection reduces starting current and starting torque to one third, with a changeover delay and mechanical interlocking. A variable frequency drive rectifies, smooths and inverts (PWM) to control speed and torque, and produces harmonic and earth leakage currents that affect RCD selection - a type B device is generally needed for three-phase drives without filtering. Emergency stop and guard interlocks must remove power through the contactor, not through the PLC alone.
PV design
Standard test conditions are 1000 W/m² and 25 °C cell temperature. Open-circuit voltage rises as temperature falls, typically around −0.3%/°C, so string voltage must always be checked on the coldest expected morning.
Worked example
Ten modules with Voc 41.2 V and a coefficient of −0.30%/°C on a site with a minimum of −10 °C. ΔT = 35 K, so Voc rises by 41.2 × 0.003 × 35 = 4.3 V to 45.5 V per module, giving a 455 V string. If the inverter maximum DC input is 500 V and the system limit is 1000 V, the design is acceptable - but check that the hot-day MPP voltage still exceeds the inverter start voltage.
Traps
Sizing strings at 25 °C only; using AC-rated isolators on PV DC; omitting anti-islanding protection and the ESB Networks connection application; confusing kWp with annual kWh; and installing battery storage without reviewing changeover, earthing and export limitation.