PW-E1 · Renewables and the grid

Connecting solar and wind: an inverter has no inertia

The grid so far has assumed huge synchronous generators turning. Solar is DC and wind is variable frequency, so they connect through an inverter, which carries almost none of the inertia that a synchronous machine’s spinning mass provides. Drag the renewable share and see that, as inertia falls, the same disturbance makes frequency drop faster and deeper.

Drag the renewable share and watch the frequency

The curve is how grid frequency moves just after a generation-loss event. Drag the handle to raise the renewable share. As synchronous machines thin out and inertia falls, the same-sized event makes frequency drop more steeply and the nadir go deeper. Cross below the green line and under-frequency load shedding fires.

Drag the handle left and right to set the renewable share.

Frequency after the event (60 Hz base)

nadir ≈ 59.00 Hz   RoCoF ≈ 0.78 Hz/s

Load shedding fires (UFLS)

r ≈ 20%   H ≈ 4.6 s

The inverter stands in between

A synchronous generator connects directly to the grid, its rotor turning as one with grid frequency. But solar gives DC, and wind gives AC whose frequency drifts with the wind. Neither can join a 60 Hz grid as is, so a power-electronic device — the inverter — sits in between and synthesizes that output to match the grid’s frequency, voltage and phase exactly. An inverter is a smart device that measures the grid and actively synchronizes, but that very fact leads to the absence of inertia.

Inertia holds the frequency up

Grid frequency is a needle reflecting the balance of generation and load. When generation suddenly falls short the frequency drops, and the rate of that drop, RoCoF = ΔP/2H, is inversely proportional to the system inertia H. A synchronous generator’s heavy spinning rotor holds ½Jω² of kinetic energy and, at the instant of the event, releases it automatically to hold the frequency falling slowly. An inverter electronically decouples its energy source — sun, wind or battery — from grid frequency, so without extra control it inherently provides none of this inertia.

ObserveH_inv ≈ 0

An inverter inherently has almost no inertia.

ChooseRoCoF = ΔP / ?

The rate of frequency change is inverse to inertia.

Fill inr ↑ → H ↓ → RoCoF ?

More renewables means less inertia and a larger RoCoF.

On your ownH ↓ → nadir ?

Lower inertia means a deeper frequency nadir.

What low inertia costs

As the renewable share rises and pushes out synchronous machines, system inertia falls, and the same generation loss makes frequency drop faster (higher RoCoF) and deeper (lower nadir). Too deep, and under-frequency load shedding fires to cut load on purpose and prevent collapse; further still leads to a wide-area blackout. The remedies are to give inverters synthetic-inertia (virtual-inertia) control so they mimic inertia, add the fast frequency response of batteries, and keep enough synchronous condensers. The lack of inertia is exactly the core engineering challenge of the renewable transition.

Back to the first screen

The size of the event stayed the same, yet the higher the renewable share the more steeply the frequency curve fell and the deeper the nadir. What changed was not the event but the amount of rotating inertia there to cushion the shock. An inverter synchronizes to the grid cleverly, but having no heavy rotor like a synchronous machine, it gives no inertia to hold the frequency up. The renewable transition wins clean energy but leaves the homework of how to restore this inertia — homework that leads into the stability story of the next unit.

The connection of solar and wind — renewables connect through an inverter (power electronics), not a synchronous generator, actively synchronizing their output to grid frequency and phase. The key difference is inertia: a synchronous machine’s spinning mass slows frequency change with kinetic energy (RoCoF = ΔP/2H), while an inverter has inertia ≈0. The higher the renewable share, the lower the inertia, so the same event gives higher RoCoF and a lower frequency nadir. Synthetic inertia and fast frequency response are the remedies.

The next step

Having begun the story of inertia and frequency, the last unit (PW-E2) brings together grid stability and frequency. Supply-demand balance sets the frequency, and inertia (primary), the governor (secondary) and automatic generation control (tertiary) restore it to 60 Hz in turn. And on the power-angle curve from D1, when the angle opened by a fault exceeds the equal-area condition the generator loses synchronism — transient stability — so every piece so far fits into the single picture of stability.