Due to the random fluctuation and non-storage of wind and light resources, the new energy generation output has random fluctuation, intermittency and low schedulability. After a large number of power systems are connected, the balance mechanism of the traditional power system based on deterministic generation tracking load is no longer applicable, and the power and power balance faces great challenges. It is necessary to coordinate the operation of flexible resources such as source, network, load and storage to maintain the power balance of the system, such as insufficient flexibility resources will lead to difficulties in local consumption. Due to the low guaranteed output of new energy, the system faces the risk of insufficient power supply reliability in extreme cases.
In 2019, the total installed capacity of wind power and photovoltaic power generation (including rooftop photovoltaic) in California, the United States, was 3.34× 107kW, accounting for more than 34% (close to the predicted value of China's installed capacity of new energy in 2030), and the high proportion and volatility of new energy access has brought challenges to the dispatch operation and reliable power supply of the local power grid. In mid-August 2020, continued high temperatures caused a surge in electricity demand in California; On the evening of August 14, due to the zero photovoltaic power generation output, wind power output decreased significantly due to the weather, new energy output was only 3.257×106 kW (less than 10% of the installed capacity of new energy), resulting in a power shortage, more than 400,000 users were cut off, lasting for about 1 hour; On August 15, more than 200,000 customers were subject to power cuts in turn.
In China, taking a provincial power grid in Northwest China with a high proportion of new energy as the object, the 2030 power balance simulation analysis was carried out. In order to achieve the new energy power target, the installed capacity of new energy in the provincial grid in 2030 will account for more than 50%, and the penetration rate of new energy (the ratio of installed capacity to the maximum annual load) will exceed 200%. In intra-day operation, some periods of new energy, about 1000 hours a year all powered by new energy, about 1800 hours of new energy power surplus; The annual new energy power limiting rate exceeded 10%, of which about 60 hours of new energy power limiting exceeded 30% of its installed capacity. In some periods, the new energy output is low, and the period when the output is less than 10% of the installed capacity exceeds 4200 hours, and the superimposed load summit leads to the tight balance of the system and even the power shortage.
It is necessary to flexibly adjust conventional power supply, mutual benefit between provinces of Northwest power grid and large capacity energy storage to maintain power balance. It can be seen from this analysis that under the condition that the installed capacity of new energy reaches 50%, the power balance of the system faces great challenges, and there will be a coexistence of energy and power oversupply (abandoned power) and short supply (limited power). In addition, it can face multiple transitions of power abandonment and power rationing states within a day, and the corresponding conversion period is sharply shortened.
In the future, with the continuous improvement of the proportion of new energy access, the mismatch between supply and demand in the power system will be more significant, and the problem of coexistence of efficient consumption and energy security will be faced.
- Safe and stable operation
The power system is characterized by a high proportion of new energy generation, a high degree of electricity electronization, and a high degree of AC/DC coupling. The operation characteristics of the power grid tend to be complex, and the difficulty of regulation is increasing, which has a direct impact on the safe operation of the system.
The equipment using the power electronic converter interface lacks the "inertia" in the traditional sense. After large-scale access, the new energy will occupy the boot space of the conventional unit, resulting in the reduction of the system's moment of inertia and frequency modulation capacity, which also leads to the prominent problem of system frequency stability and the increase of the risk of exceeding the limit. The frequency tolerance of new energy connected to the grid is low, and it is more likely to be large-scale off-grid due to frequency anomalies after encountering large disturbances, resulting in chain accidents.
The voltage-reactive response characteristics of new energy power generation and converters in steady and transient processes determine the influence characteristics on voltage stability. The dynamic reactive power supporting capacity of new energy equipment is weaker than that of conventional generator sets, and the contribution to short circuit current is insufficient. New energy equipment is usually connected to the end of the power grid, the short circuit of the power grid is relatively low, and the voltage stability problem is easy to occur. With the rapid increase of the proportion of new energy, the dynamic reactive power reserve and supporting capacity of the system decrease sharply, the system strength decreases, and the problems of voltage stability and transient overvoltage will become more prominent.
The permeability of new energy power generation, unit type, access location, control parameters, operating conditions, etc. will affect the power Angle stability of the system. The large-scale access of new energy will make the power Angle stability characteristics complex and increase the uncertainty, and the previous "pre-plan" security control strategy is difficult to configure, and the mismatch risk will increase, affecting the power grid security. The control characteristics of the converter make it have different power Angle synchronization stability from the synchronous generator, which will introduce new stability connotation and bring the synchronous instability risk between the new energy generation and the system.
Some wide-band oscillations appear one after another. The characteristics of power electronic equipment, such as control dominance, broadband dynamic, and complex interaction, make the stability of the new power system no longer limited to the traditional power frequency and electromechanical time scale, but bring about new problems of power electronic equipment network stability on the electromagnetic time scale, and the corresponding frequency range is extended to 0.1~1000 Hz. In recent years, wind power gathering areas in Hebei, Xinjiang and other provinces have been oscillating, endangering the safety of equipment and power grid operation.
- Market mechanism
At present, the new energy plan still accounts for a high proportion of electricity. Considering the matching of pre-emptive power purchase, after a high proportion of new energy is connected to the power system in the future, the existing security purchase policy will not be able to fully connect with market-oriented transactions. The installed capacity of wind power and photovoltaic power generation continues to expand, and the randomness and volatility of output will lead to priority power generation (guaranteed quantity and price) and priority power purchase in terms of electricity, curves, and prices can not match, forming a large number of unbalanced funds, affecting the operation of the power market and the consumption of new energy.
In order to adapt to the intermittent characteristics of new energy, on the one hand, the development of the power market needs to shift to a more refined time dimension and a more accurate spatial granularity. The current power market design is suitable for the power system based on traditional energy, and after the proportion of new energy is greatly increased, the system's demand for flexible resources will be greatly increased, and the supporting market mechanism that can tap the value of flexible resources needs to be formed. On the other hand, in the future, the high proportion of new energy power market will present the characteristics of low marginal cost and high system cost, the responsibilities and rights of new energy in the market have not been clearly defined, and the market-oriented mechanism of system cost has not been established, increasing the difficulty of market design.
The existing power market does not fully reflect the green attribute of new energy, resulting in the optimal allocation of market resources can only consider the attribute of electricity price and cannot reasonably reflect the comprehensive value of a high proportion of new energy. At present, although there are consumption trading markets, voluntary green certificate trading markets and carbon markets, the coordination and connection between various markets and power markets are not perfect, and there are cross-subsidies and other problems. The quantitative calculation of the comprehensive impact of various markets on the development of energy and power has not yet been carried out, and the actual effect of various mechanisms is difficult to predict.
