Lab 08 · Playable · Sandbox mode

The Voltage Game.

A 20-kilometre 11 kV rural feeder. Four load buses. Solar panels on every roof. Play with the settings and watch what happens to the one voltage the whole feeder has to share.

Turn the sun up. Push the load down. Try the tap changer. Switch on the inverter Q(V) droop. The grid pushes back — and you'll see where.

The feeder.
within band near limit tripped reverse flow
SUBSTATION 11 / 0.4 kV
Controls
Feeder load 100%
All four buses scale together. Peak per bus = 300 kW / 145 kvar.
Solar PV output 0%
Percentage of installed PV capacity being generated right now.
PV installed capacity 400 kW / bus
Per-bus rooftop PV, uniformly distributed.
Substation tap position 0
Each step = ±2.5% at the substation bus.
Q(V) inverter control Inverters absorb reactive power when local V rises.
Quick scenarios
Voltage profile · substation to feeder end —
TRIP 1.10 STAT +5% NOMINAL 1.00 STAT −5% TRIP 0.90
Head-end power
— kW
Feeder losses
— kW
Worst bus voltage
— pu
Buses out of band
— / 4
Move a slider to begin. The feeder responds instantly.
The physics · a short primer

Why solar makes voltage go the wrong way.

In a traditional distribution feeder, power flows one way — from the substation out to the loads. Along that path, current running through the line's resistance and reactance drops the voltage. The further from the substation, the lower the voltage. Engineers size taps and reactive compensation to keep the end-of-feeder voltage inside statutory limits (in India, typically ±5% at 11 kV).

Rooftop solar breaks that assumption. When a household exports power to the grid, current at that bus reverses direction. If enough houses export at the same time — noon, blue sky, low load — the entire feeder can flip into reverse power flow. And the same physics that used to drop the voltage now raises it.

The linearised voltage drop across a segment is roughly:

ΔV ≈ (R · P + X · Q) / Vnom

On rural feeders R and X are similar magnitudes (unlike transmission, where X ≫ R). So active power P — the thing solar exports — has a strong voltage effect. Reverse the sign of P and you flip the sign of ΔV.

Tap changer — the blunt instrument.

The substation transformer can raise or lower voltage in steps. Effective at the head, weaker at the tail. It's slow — usually not designed to chase cloud transients.

Q(V) inverter droop — the smart trick.

Modern PV inverters can absorb or inject reactive power based on local voltage. When V rises, they absorb Q. It works locally, autonomously, and doesn't curtail active power. India's CEA now mandates this capability.

Curtailment — the last resort.

If nothing else holds voltage in band, inverters trip on overvoltage or throttle back active power. Households lose export revenue. Everyone loses trust in the connection.