1、 The balance problem caused by uncertainty

New energy has strong volatility, with fluctuating output; At the same time, new energy naturally has uncertainty, and there may not be wind or light when you use electricity. In the current power grid, the proportion of new energy is not yet high, and thermal power units are fully responsible for bearing this fluctuation and uncertainty: less wind and solar power generation, more thermal power generation. There is more wind and solar power generation, but less thermal power generation.

But what about the future? If new energy becomes our primary power source, how can we solve the problem?

2、 Defects in regulation ability interfere with the stability of the power grid

To ensure the normal operation of the power system, two important stability concepts are involved: frequency stability and voltage stability. The traditional synchronous generator is the biggest pillar to maintain these two stability. However, new energy sources such as wind and solar power do not have the ability of traditional generators to maintain voltage and frequency, and even consume a large amount of reactive power when connected. They may also disrupt the stability of the power system and pose risks to its safety.

Frequency stability: refers to the ability of the power system to maintain a stable frequency even when there is a significant imbalance between power generation and load demand due to severe disturbances, mainly handled by the frequency modulation device in synchronous generators.

The frequency is closely related to the speed of the generator. Generally, a motor will maintain a speed of 3000 revolutions per minute (1 pair of poles, 50 revolutions per second) or 1500 revolutions per minute (2 pairs of poles, 25 revolutions per second), corresponding to 50Hz AC power supplied by the power grid. When the power supply is lower than the electricity consumption, the speed of the generator will decrease, converting a portion of the kinetic energy stored in the huge rotor of the generator (as shown in the figure below) into electrical energy and supplying it to the grid. The decrease in speed brings about a decrease in voltage frequency, for example, to 49.8Hz. At this point, the frequency regulation mechanism will start, increasing the steam flow for thermal power plants and the water flow for hydropower plants to output more power, accelerating the generator and maintaining a frequency of 50Hz.

However, wind power and photovoltaics often rely on the weather to make a living. When the frequency drops, there is no additional way for them to output more power, and their frequency regulation ability is far lower than that of traditional thermal power units.

Voltage stability: refers to the ability of a power system at a given operating point to maintain all node voltages at an acceptable value after being subjected to disturbances. It relies on the system's ability to maintain or restore a balance between load demand and load supply.

When a generator set generates electricity, in addition to outputting active power, it also outputs reactive power, which is the core of maintaining the voltage of the power system. However, wind and solar power basically do not have the ability to provide reactive power to the grid, or can only provide very little reactive power. In many cases, they may still need to absorb reactive power from the grid to maintain operation, which reduces the stability of grid voltage. At the same time, the output power of wind and solar power generation fluctuates greatly (due to the instability of wind and solar resources), and changes in power will also bring voltage fluctuations, which will make the grid voltage more unstable.

Reactive power: In the power grid, alternating magnetic fields need to be constructed to build energy bridges. Some of them absorb energy from the power source, while others release energy during time. The average power of this process is 0, but the power required to establish these magnetic fields is called reactive power.