The provincial power grid in China usually contains hundreds of substations and thousands of buses. Considering the physical model of whole power network is too complex, while the actual power grid is often radial, and the new energy station mainly connected to the terminal grid, there will not be many lines overload after the optimization of power network structure, so the aggregation grid model can be used to meet the requirements of the time series simulation. The aggregation model of power grid is shown in Figure 1.

Region A, B and C are renewable energy blocked areas which restricted by the cross section, the aggregation model is established based on the principles: a) aggregation model does not consider the detailed topological structure of the power grid, the power source and load are not affected by their physical location; b) the power consumption balance of the power network is not affected by the power network aggregation model.

Thermal power plant is an important member of the power system in China. It is very important for the accommodation of renewable energy. Thermal power units can be divided into condensing steam turbine, backpressure turbine and extraction turbine. Condensing steam turbine is exclusively for power generation, while back pressure turbine and pumping turbine can also heat, heating unit has greater influence on the accommodation of renewable energy.

1) Back pressure turbine

Back pressure turbine is a kind of heating units, its working characteristics are shown as Equation (1) and Figure 2(a). is the ratio of the power output and the thermal output of the unit, is the thermal output of the unit, is the power output of the unit.

2) Pumping turbine

The working characteristics of pumping turbine are shown as Equation (2) and Figure 2(b). When the heat output is fixed, power output can be in a certain range which is decided by and. and is the minimum and maximum power output.

Hydroelectric power plant can be divided into two categories, one is the run-off hydropower plant without capacity of regulation, and one is adjustable hydropower plant with adjustable reservoir. The operation of hydropower plant is closely related to water resources. Power generation of the run-off hydropower plant is basically determined by the flow of the river. Adjustable hydropower plant does not have minimum output limit, but reservoir inflow and reservoir capacity is the limit factors of the power output as shown in Equation (3).

is the initial power generation capacity of the reservoir, is the power generation capacity of the inflow water, is the actual power output, is the initial power generation capacity of the reservoir in next cycle, and is minimum and maximum power generation capacity of the reservoir.

The output of renewable energy can be viewed as time sequence in time series production simulation. The change characteristics of the sequence should be consistent with the actual renewable energy resources.

1) Optimization Objective

REPS regards the maximum accommodation of renewable energy power as the optimization goal, considering renewable energy output is different in different locations, so renewable energy is calculated separately for different power grid, As shown in Equation (4):

N is the total number of aggregation networks in the system, n is expressed as an aggregation power network, T is the total simulation time, t is the step, and is the wind power and PV power output of network n at time t.

3) Constraint Equations

Constraint Equations include reserve capacity constraint in Equation (5), load balance constraint in Equation (6), conventional unit output constraint in Equation (7)-(10), number of running units constraint in Equation (11), conventional unit power constraint in Equation (12) and regional transmission capacity constraint in Equation (13).

and is the positive and negative reserve capacity., and is the total number, maximum and minimum output of unit j in network n at time t. and is the op-

timized power and running number of unit j in network n at time t. is the system load of network n at time t. and is the up and down ramp rate. is the transmission power of line i in network n at time t. and is maximum and minimum transmission capacity of line i.

The boundary conditions of power grid operation is an important basis for the calculation of the renewable energy accommodation, the boundary conditions mainly include reserve capacity, conventional power supply mode, maximum and minimum technical output of the units, load data, renewable energy output sequence and transmission line principles. Boundary conditions of renewable energy accommodation calculation in the model province are shown in Table 1.

In the annual assessment calculation of the next year, assuming the stability limit of the renewable energy channel is same with the previous year, and large-scale hydropower station will be put into operation in neighboring province, the installed renewable energy capacity of the model province is expected to increase 24 percent year- on-year, and the power network structure has little change compared to last year, so the new energy transmission channel has no change. Prediction of renewable power accommodation of the model province is shown in Table 2.

The monthly consumptive capacity assessment of the model province is shown in Figure 2.

From the results of the simulation, we can find that the channel stability limits is the main limiting factor for renewable energy accommodation in the model province, sending ability is limited by the grid structure constraints, due to large-scale hydropower station will be put into operation in neighboring province, the neighboring province power no longer needs power from the model province in summer, so the situation of renewable energy curtailment will continue.