Power grid reform: What is grid-forming inverter control?

How electric power system works? 

An electric power system generates electricity on demand. That is to say, the power output equals the electrical load of an electric power system.  All generators, power distributors, and end users in the same grid must work with AC currents at the same power grid operating frequency. For example, Taiwan’s operating frequency is 60 Hz, at which power of all rotators of fossil fuel, nuclear, or hydro power stations produced. When electrical load is lower than power output, frequency will rise, and vice versa. Abnormal frequency, no matter too high or too low, damages equipment of the system. Therefore, to sustain grid operating frequency, power companies adjust power output of generators to levels that equals electrical load.

Generally, power companies dispatch generator sets and adjust electricity purchase volume after estimating the power consumption of each region based on time, temperature changes, historical data, and electrical load forecast, and with consideration of characteristics of different generator sets.

For example, Taiwan classifies energy sources into three categories. The first category includes coal and nuclear, which can serve as baseload power sources, providing stable and continuous power supply. The second category includes gas, pumped-storage hydroelectricity, and lithium-ion battery storage system, which responds to changes of electrical load immediately. These sources are mostly used as intermediate and peak load power sources, due to higher electricity supply costs. The third category includes the unpredictable and non-dispatchable variable renewable energy (VRE), such as wind and solar. The VRE must be integrated with ESS to function as a stable and reliable source of power.

When power consumption is higher than the expectation of power companies, and generator sets fail to respond due to inspection or malfunction, frequency of the gird will drop. When the frequency drops to a certain level, the electric power system will have to cut itself off the grid, lest the entire system breaks down. This condition is commonly known as “tripping.
 

What is inertia?

Conventionally, rotators of electromagnetic rotating machines produce the frequency of AC-grid systems. A traditional generator set weighs tonnes, and the kinetic energy stored within is called inertia, which gives generators the tendency to keep rotating and prevent changes in the frequency of the AC-grid system. The inertia buys time for the system to rebalance supply and demand. However, renewable energies, such as wind and solar powers, are connected to the grid through power electronic equipment, with no turbines absorbing the imbalance of energy. Therefore, frequency of the system is more likely to drop when disruptions occur. With shares of renewables increasing as more and more wind and solar powers are connected to the grid, electricity power system will have to transform accordingly.

The grid-connection of VRE is categorized into six phases:

• Phase 1 and 2: VRE shares are relatively low. Equipment of existing grids can handle or make slight adjustment.
• Phase 3 and 4: The system experience significant changes in the mode of power distribution. Upgrades are necessary for the system to process large shares of VRE.
• Phase 5 and 6: Long-lasting VRE surplus and power shortage would occur. Long-duration ESS are required for the system to distribute different energies throughout the year, such as hydrogen storage or other energy storage technology.

Rising share of VRE, six phases of grid transformation

Source： IEA
 

What is grid-following and grid-forming inverters?

During the transformation of electric grids, more and more ESS replace generator sets of traditional energy sources to provide short-term power supply services. The conventional way of grid connection uses the grid-following inverter control. In the grid-following mode, the inverter controls power output by tracking the voltage angle and frequency of the grid. Inverters can respond to frequency of the grid immediately but cannot function once signals of frequency and voltage are lost. As more ESS are connected to grids, frequency fluctuations may result in chain reactions, causing the entire grid system to breakdown. In the grid-following mode, frequency control still relies  on generation sets.

In the grid-forming mode, inverters work as traditional synchro generators, controlling frequency actively, keeping frequency and power output sufficient comparing to demand load any time, ensuring stable operation of the electric power system.