Engineer's Notes: connecting generation
Every connection is a study
A generator, PV plant or battery changes fault levels, voltages, protection and power quality on the network. Network operators ask for connection studies before approval, and the requirements grow with the size of the plant.
Synchronising is a skill
Closing a generator breaker out of phase can push several times rated current through the machine and damage the shaft and coupling. Check-synchronising relays and auto-synchronisers prevent it, but operators still need to understand what they are watching.
Voltage rise limits solar
On long or weak feeders, voltage rise usually limits how much generation can connect before thermal ratings do. Volt-var control, curtailment and stronger connections increase hosting capacity.
Never feed a dead network
Loss-of-mains protection, inverter anti-islanding, intertrips and interlocked changeovers all exist for one reason: generation must not keep a network live that people believe is dead.
Frequently asked questions
What conditions must be met before closing a generator onto the grid?
Voltage, frequency and phase angle must match closely, and the phase sequence must be correct. A typical check-synchronising window is within 5 % voltage, 0.1 Hz and 10 degrees, with the generator running slightly fast so it picks up load when the breaker closes.
Why does distributed generation cause voltage rise?
Exported power flows back along the feeder towards the substation, and the voltage drop across the feeder resistance and reactance reverses into a rise, roughly (P x R + Q x X) / V squared. Absorbing reactive power or curtailing output reduces it.
What is islanding and why is it dangerous?
Islanding is when generation keeps supplying part of the network after the utility supply has been disconnected. Line crews may find a supposedly dead circuit live, and reclosing onto an out-of-phase island can damage equipment. Loss-of-mains protection and inverter anti-islanding must disconnect the generation, usually within 2 seconds.
What is a volt-var curve?
An inverter control setting that makes the inverter absorb reactive power when its voltage is high and supply reactive power when it is low. AS/NZS 4777.2 and IEEE 1547 define default curves that help keep feeder voltages within limits as more solar connects.
How do I check a capacitor bank for harmonic resonance?
Estimate the parallel resonance order as h = sqrt(Ssc / Qc), where Ssc is the fault level and Qc the bank size. If h is close to 5, 7, 11 or 13, harmonic currents can be amplified, so use a detuned bank with a series reactor.
Topics: synchronising · synchroscope · check-sync relay · capability chart · excitation · voltage rise · hosting capacity · volt-var · volt-watt · short-circuit ratio · loss of mains · RoCoF · anti-islanding · intertrip · NVD · BESS · PV inverter · capacitor bank · resonance · DRRG · AS/NZS 4777 · Part 6 · IEEE 1547
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