Research on energy flexibility demand analysis and evaluation method of power systems with a high proportion of new energy

With the increase of the scale of highly uncertain power supply in the system, the system needs more and more energy flexibility. Existing studies focus more on short-term energy demand volatility changes and lack quantitative means to analyze the system’s demand for energy flexibility at different time scales. Aiming at the above problems, this paper proposes an energy flexibility demand analysis and evaluation method for power systems with a high proportion of new energy. It constructs an energy flexibility demand evaluation index system at different time scales. Combined with the analysis of domestic provincial cases, the results verify the effectiveness of the proposed method.


Introduction
Considering the volatility and randomness, with the increase of the proportion of new energy, such as wind and solar photovoltaic, in the energy structure, the range and rate of change of the system's residual load will increase significantly, and the demand for improved flexibility is also increasing [1,2].Flexibility measures will play an increasingly important role in the power system in the future.
The power balance between generation and load is an important basis for the safe operation of the electric power system.However, due to the development of power supply technology and the transformation of the power supply structure, in the process of electric power system scheduling, the power balance may be tracked by the power generation regulation capacity, which leads to the failure to meet the power balance and must generate power abandonment or cut load [3,4].
In analyzing energy flexibility demand, some research has been carried out at home and abroad.In [5], it studies the basic principles and key challenges of flexible supply and demand balance.It puts forward the evaluation method of flexible supply and demand balance, the simulation calculation method of flexible supply and demand balance, and the simulation scenario construction method considering different weather processes.In [6], it analyzes the influence of various regulatory resources on the flexibility of the power system in the case of wind power grid-connection and proposes a holisticlocal flexibility evaluation method.In [7], flexibility improvement strategies such as thermal power unit performance transformation, demand-side response based on the virtual power plant, and demand-side response based on flexible load are proposed.A full-timing system operation model with flexible resource constraints and multi-day coordinated operation is constructed.In [8], it analyzes the evolution trend of flexibility demand generated by a high proportion of new energy, forecasts the operation characteristics of various flexible resources in the source network, and proposes some policy suggestions.

Energy flexibility demand analysis evaluation index system
From the direction perspective, energy flexibility demand can be divided into up-regulated energy flexibility demand and down-regulated energy flexibility demand.From the time scale perspective, energy flexibility demand can be divided into daily energy flexibility demand, monthly energy flexibility demand, and annual energy flexibility demand.The definition and connotation of different indicators are introduced below.

Up-regulated energy flexibility requirements
The up-regulated energy flexibility demand of a certain period refers to the additional power level provided by flexible resources other than new energy units, such as wind turbines, to meet the net electricity demand of the system during this period.The calculation formula is as follows.

𝐸
* , 0 where  is the up-regulated energy flexibility demand at time period t+1;  is the load demand of the system in time period t+1;  is the output level of new energy such as scenery on time period t+1 of the system;  is the total output level of other resources except the scenery and other new energy units in time period t of the system;  is the length of a period of time.
According to the formula, when the load demand at the time period t+1 is lower than the sum of the new energy output at the time period and the output of other power sources at the previous time period, the up-regulated energy flexibility demand at the time period t+1 is 0.

Down-regulated energy flexibility requirements
The down-regulated energy flexibility demand of a certain period of time refers to the low net electricity demand of the system at a certain period of time.To meet the power balance and improve the power generation of volatile new energy such as scenery as much as possible, flexibility resources other than scenery and other new energy units are required to reduce the additional power level.The calculation formula is as follows.
where  is the down-regulated energy flexibility demand at time period t+1;  is the load demand of the system in time period t+1;  is the output level of new energy such as scenery on time period t+1 of the system;  is the total output level of other resources except the scenery and other new energy units in time period t of the system.
According to the formula, when the load demand at the time period t+1 is higher than the sum of the new energy output at the time period and the output of other power sources at the previous time period, the down-regulated energy flexibility demand at the time period t+1 is 0.

Intraday energy flexibility requirements
The intra-day energy flexibility demand refers to the algebraic value that continuously increases the energy flexibility demand, and the algebraic value that continuously decreases the energy flexibility demand, considering the load changes and new energy output changes of a day.The calculation formula is as follows.
where  , is the sum of up-regulated energy flexibility requirements at time periods t 1 to t 2 .According to the formula, in the period t 1 to t 2 , when the up-regulated energy flexibility demand in a certain period is 0, the total up-regulated energy flexibility demand in the period is 0.
where  , is the sum of down-regulated energy flexibility demands from time periods t 1 to t 2 .According to the formula, from t 1 to t 2 , when the down-regulated energy flexibility demand in a certain period is 0, the sum of the down-regulated energy flexibility demand in this period is 0.
Based on the obtained sum of up-regulated energy flexibility demands in continuous time periods and down-regulated energy flexibility demands in continuous time periods, intra-day energy flexibility demands can be obtained as follows.

𝐸
, ,   , where  is intraday energy flexibility demand;   , is the days of continuous time flexibility increase energy demand on the maximum sum;   , is consecutive periods for days on the maximum flexibility to cut energy demand combined.
According to the formula, the intra-day energy flexibility demand is the greater of the maximum sum of the up-day energy flexibility demand and the maximum sum of the down-day energy flexibility demand in the intra-day continuous period.

Monthly energy flexibility requirements
The monthly energy flexibility demand refers to the maximum algebraic value of increasing energy flexibility demand in consecutive days and decreasing energy flexibility demand in consecutive days, considering the daily load changes and new energy output changes in a month.The calculation formula of intra-day upward energy flexibility demand is as follows.

𝐸
, 0 where  is the energy flexibility requirement increased on day+1;  is the total power consumption of the system on day+1;  is the total power generation of fluctuating new energy such as system scenery on day+1;  refers to the total power generation of other resources in the system on the first day except new energy units such as scenery.
The algebraic and computational formula for the intra-day upward adjustment energy flexibility requirement on consecutive days follows.
where  , refers to the sum of energy flexibility requirements up-regulated in the days from day 1 to day 2 .According to the formula, from day 1 to day 2 , when the intra-day up-regulated energy flexibility demand on a certain day is 0, the sum of intra-day up-regulated energy flexibility demand within the period is 0.
Similarly, the formula for intra-day reduction of energy flexibility demand is as follows.
where  refers to the intra-day reduction of energy flexibility demand on day+1;  is the total power consumption of the system on day+1;  is the total power generation of fluctuating new energy such as system scenery on day+1;  refers to the total power generation of other resources in the system on the first day except new energy units such as scenery.
The algebraic and computational formula for intra-day reduction of energy flexibility requirements on consecutive days follows.
where  , refers to the sum of energy flexibility requirements reduced during the day from day 1 to day 2 .According to the formula, from day 1 to day 2 , when the intra-day reduced energy flexibility demand on a certain day is 0, the sum of intra-day reduced energy flexibility demand within the period is 0.
Monthly energy flexibility demand can be obtained based on the sum of intra-day up-regulated energy flexibility demands in a continuous period and the sum of intra-day down-regulated energy flexibility demands in a continuous period as follows.
where  is the monthly energy flexibility demand;   , is for monthly on consecutive days in the days of a maximum of flexibility increase energy demand combined;   , is for monthly on consecutive days in the days after the maximum flexibility to cut energy demand combined.
According to the formula, the monthly energy flexibility demand is the greater sum of the upregulated energy flexibility demands on consecutive days and the maximum sum of the down-regulated energy flexibility demands on consecutive days in a month.

Annual energy flexibility requirements
Annual energy flexibility demand refers to the number of algebras with continuous monthly upward energy flexibility demand and the number of algebras with continuous monthly downward energy flexibility demand, considering the monthly load changes and new energy output changes within the year.
The formula for calculating the monthly upward energy flexibility demand is as follows.
where  is the monthly upward energy flexibility demand in the first month+ 1;  indicates the total power consumption of the system in the first month+ 1.  is the total power generation of fluctuating new energy such as system scenery in the first month+ 1;  refers to the total power generation of other resources in the system except for new energy units such as scenery in the first month.
The algebraic and computational formula for the monthly upward revision of energy flexibility requirements on consecutive months is as follows.

𝐸
where  , is the sum of monthly increased energy flexibility requirements from month 1 to month 2 .According to the formula, when the monthly increased energy flexibility requirement in month 1 to month 2 is 0, the total monthly increased energy flexibility requirement in the period is 0.
Similarly, the formula for the monthly reduction of energy flexibility demand is as follows.
where  refers to the monthly reduction of energy flexibility demand in month+ 1;  indicates the total power consumption of the system in the first month+ 1.  is the total power generation of fluctuating new energy such as system scenery in the first month+1;  refers to the total power generation of other resources in the system except for new energy units such as scenery in the first month.
The algebraic and computational formula for the monthly reduction of energy flexibility requirements on consecutive months is as follows.
where  , is the total of monthly reduced energy flexibility requirements from month 1 to month 2 .According to the formula, when the monthly reduced energy flexibility requirement in month 1 to month 2 is 0, the total number of monthly reduced energy flexibility requirements in this period is 0.
Based on the obtained sum of monthly up-regulated energy flexibility demands in continuous time periods and the sum of monthly down-regulated energy flexibility demands in continuous time periods, the annual energy flexibility demands can be obtained as follows.
where  is the annual energy flexibility requirement;   , ^ up) is for the monthly increase energy demand flexibility on consecutive month in the sum of the maximum;   , is for annual on consecutive month in the monthly maximum flexibility to cut energy demand combined.
According to the formula, the annual energy flexibility demand is the greater of the maximum sum of monthly upward energy flexibility demands and the maximum sum of monthly downward energy flexibility demands in consecutive months of a year.

Case introduction
We take the province with the largest installed new energy capacity as an example.In 2025, this province's total social electricity consumption will be 178.1 billion kWh, with the installed capacity of thermal power 413, 98 MW, hydropower 10, 030 MW, wind power 38, 530 MW and photovoltaic power 41, 690 MW.

Results analysis
Using the time series production simulation method, the annual generation plan of the unit can be obtained.According to the generation plans of various power sources obtained and combined with the energy flexibility analysis and evaluation indexes proposed in this paper, the daily energy flexibility demand, monthly energy flexibility demand, and annual energy flexibility demand can be obtained, respectively.
We take one day as an example.The intra-day energy flexibility demand is 5576.6MWh on that day, and the intra-day energy flexibility demand distribution is shown in Figure 1.It can be seen that the energy flexibility demand is down-regulated in three consecutive hours of the day.Still, it is upregulated in the next three hours.The energy flexibility adjustment capacity of the day needs to meet the continuous adjustment and direction change of 3 hours.

Figure 1 Intraday energy flexibility demand distribution
Taking a certain month as an example, the monthly energy flexibility demand of that month is 193291.49MWh, and the distribution of monthly energy flexibility demand is shown in Figure 2. As can be seen, the energy flexibility demand will be up-regulated in three consecutive days.Still, it will be down-regulated in the following two days.The energy flexibility adjustment ability must meet the continuous adjustment and direction change for several days a month.

Conclusion
Based on the results and discussions presented above, the conclusions are obtained as below: (1) From the perspective of intra-day energy flexibility demand, the energy flexibility of the system needs to meet the continuous adjustment and direction change for several hours.
(2) From the perspective of monthly energy flexibility demand, the energy flexibility of the system needs to meet the continuous adjustment and direction change for several days.
(3) With the continuous increase of the proportion of new energy, from the perspective of annual energy flexibility demand, the energy flexibility of the system needs to meet the continuous adjustment and direction change for several months to better promote the consumption of new energy.

Figure 2
Figure 2 Monthly energy flexibility demand distribution From the perspective of the whole year, the annual energy flexibility demand is 11102263.4MWh, and the annual energy flexibility demand distribution is shown in Figure 3.The energy flexibility adjustment capacity of the system needs to meet the continuous adjustment and direction change for several months within a year.

Figure 3
Figure 3 Annual energy flexibility demand distribution