PW-C4 · Transmission and per-unit

Voltage regulation: hold the voltage by steering reactive power

When the load changes, the receiving voltage swings because of the I·Z drop — sagging under heavy load, rising under light load. Yet the voltage must stay within an allowed band. Drag a capacitor that fills in reactive to cut the drop, and learn how a sagging voltage is pulled back inside the band.

Grow the capacitor, bring the voltage into the band

Under heavy load the receiving voltage sags below the lower limit (red). Drag the handle to grow the capacitor reactive Q_C. The lagging reactive in the line falls, the X drop shrinks, and the voltage marker rises into the green band (0.95 to 1.05). Grow it too far and it overshoots into over-voltage.

Drag the handle left and right to set the capacitor size.

Receiving voltage (allowed band 0.95 to 1.05)

V_r ≈ 0.880 pu

Under-voltage · below band

Q_C ≈ 0.00   Q_L = 0.30

Voltage swings with the load

The receiving voltage is below the sending voltage by the I·Z drop the load current makes across the line impedance. So at a heavy midday load the voltage falls sharply, while at a light midnight load the drop is small and the voltage barely moves — or on a long line the charging current can even push it above the sending voltage (the Ferranti effect). Equipment works properly only near its rated voltage, so however the load changes the voltage must be kept within an allowed band.

Reactive plant pushes the voltage

In the drop e ≈ I(R cosφ + X sinφ), an HV line has a large X, so the X sinφ term — the reactive current — dominates the drop. Putting a capacitor by the load to fill in the lagging reactive lowers the reactive current in the line, shrinks the drop and raises the receiving voltage. Conversely, under light-load over-voltage a shunt reactor absorbs reactive to pull the voltage down. These devices that push the voltage by supplying or absorbing reactive are the reactive-compensation plant.

ObserveVR(%) = (V_nl − V_fl) / V_fl × 100

Regulation is the ratio of the no-load to full-load voltage difference.

Choosee ≈ I(R cosφ + X ?)

On HV lines the X-times-sinφ term dominates the drop.

Fill inQ_C ↑ → V_r ?

Filling reactive with a capacitor raises the voltage.

On your own과전압 → 리액터로 V_r ?

Light-load over-voltage is lowered with a reactor.

Tap changing · directly by ratio

Another handle is the transformer tap. Changing the winding tap to adjust the turns ratio raises or lowers the secondary voltage directly. In heavy-load hours the ratio is lowered to raise the voltage, and the reverse in light-load hours. An on-load tap changer (OLTC), which changes taps without interrupting the load, is common. Operating reactive plant and tap changing together holds the receiving voltage within its band even as the load swings all day.

Back to the first screen

Growing the capacitor lifted the red, sagging voltage marker into the green band, and growing it further spilled over into over-voltage. The capacitor filled the line’s lagging reactive and cut the X sinφ drop. Voltage regulation is exactly this — supplying or absorbing reactive, and when needed changing the turns ratio with a transformer tap, to tie the voltage inside the narrow strip of its allowed band however the load swings.

The voltage regulation — as the load changes, the I·Z drop sags the receiving voltage (heavy load) or raises it (light load, Ferranti). Regulation VR = (V_no-load − V_full-load)/V_full-load. Since the X sinφ term dominates the drop, reactive plant (capacitor = supply reactive → voltage up, reactor = absorb reactive → voltage down) and transformer tap changing (adjusting the turns ratio) hold the voltage within an allowed band (such as ±5%) however the load swings.

The next step

That was the transmission side, sending power far. The next unit (PW-C5) looks at the distribution systems that divide that power down to customers: connections such as single-phase two-wire, three-phase three-wire and three-phase four-wire, and layouts from a radial feeder reaching one way, to a loop or a multi-path network — how a distribution grid is built by weighing reliability against cost closes section C.