Impacts of pumped storage power station on large-scale wind power integration into grid

Shuguang SONG, Xueshan HAN, Dayang YU School of Electrical Engineering

Shandong University Jinan, China

songshuguang@mail.sdu.edu.cn

Abstract—In this paper, a method to calculate online units’ minimum capacity correspond to a certain capacity of wind power is proposed firstly. Then as a comparison, PSPS is added into the model and corresponding results can be achieved. By comparing the results, effects of PSPS on wind power integration into grid can be obtained. In this paper, we take Shandong power grid as example and analyze its situations. It should be noticed that when capacity of wind power is very low, utilization rate of wind power is 100% without PSPS, which means because of existence of peak load regulation ability of traditional units, grid can accommodate a certain capacity of wind power without PSPS. It indicates that PSPS is not necessary for power grid to accommodating wind power at low level of wind power.

Keywords-wind power; pumped storage power station (PSPS); peak load regulating

I. INTRODUCTION Since Renewable Energy Law has been implemented in

2005, wind power installed capacity has grown for five consecutive years with over 100 percent rate in China [1]. By 2020, China is expected to have 150 million kilowatts wind power installed. With the rapid growth of wind power capacity, however, ability of wind power accommodation is relatively insufficient, so large amount of wind power cannot be accepted by grid and was forced to abandon [2]. Research has found that wind power fluctuation profile is opposite with load in the same area [3, 4]. This increases the peak-valley ratio, makes the grid add load reserve capacity as wind power integrates into grid. Nowadays, China's electric power demand is supplied mainly by thermal power units. Deeply regulating peak load with thermal units not only reduce unit’s lives, but also increase coal consumption and greenhouse gas emissions, weaken the wind power advantage in environmental protection [5].Therefore, it’s an efficient way to change the power source structure and increase the peak load regulation capacity for improvement of the ability of wind power accommodation[6].

The increase in renewable energy has led many to believe that large-scale electricity storage could solve its associated problems. Pumped storage power station has 200% regulation

coefficient and response fast [7], can effectively cut peak load and fill valley load. Much of the work in the literature on storage has examined hydro-storage with wind power. These works generally examine the influence of using storage to provide a more predictable power source [8] [9]. However, it’s not always suitable for construction of pumped-storage power station in the wind farm nearby. A more feasible measure is to increase peak load regulation capacity by operating PSPS in a whole grid. In this paper, a method to evaluate ability of wind power accommodation is proposed firstly; then results of Shandong power grid is obtained as an example. At last, we analyze the result and impact of PSPS on wind power integration into grid.

Evaluation targets of wind power integration into grid include utilization rate, proportion in the energy consumption, online units commitment capacity, etc. Wind power has to be curtailed when it is larger than peak load regulation capacity. But curtailment is necessary for improve wind turbines capacity and proportion of wind power in the total energy demand. Online units commitment capacity would decrease because wind power has a capacity credit.

II. THE OPTIMAL MODEL The model used to carry out analysis is based on the model

used in CEPRI report [10]. In this analysis, deterministic values of wind and load are used, where the wind data used are statistic value. This means that the uncertainty of wind isn’t accounted for; however, the variability is still seen.

A. Original Calculation Method of Wind Power Accommodation Figure 1 gives a diagram for calculating peak load

regulation capacity of a power grid. The difference between real grid-wide generation output .G rea lP and minimal output of generation unit commitment capacity of the power grid is just the peak load regulation capacity that can be used for offsetting injected wind power.

This work is supported by Natural Science Foundation of Shandong Province (No. ZR2010EQ005).

978-1-4577-0365-2/11/$26.00 ©2011 IEEE 1081

Figure 1. Diagram for calculating capacity of peak load regulation

Capacity of peak load regulation that can be used for offsetting injected wind power can be calculated with following formulate:

Grid-wide maximum generation load:

. .max .max( ) /[(1 )(1 )]G real Load Trans Loss GenP P P K K= + − − (1)

Online units commitment capacity of the power grid:

. .max ReGN G real serveP P P= + (2)

Real grid-wide generation output:

. ( ) / [(1 )(1 )]G real Load Trans Loss GenP P P K K= + − − (3)

Minimal output of grid-wide generation commitment:

.min .(1 )GN GN G adjustP P K= × − (4)

Grid-wide reserve capacity:

. . max .max Reserve Spin Contingency

G real Load G

P P P

P K P

= +

= × + (5)

Total peak load regulation capacity of the power grid:

. .minbalance G real GNP P P= − (6)

However, the results obtained by formulates above are conservative because wind power has capacity credit. According to achievement obtained already, wind power forecasts will be more and more accurate. Operator can schedule generation unit commitment capacity with accurate wind power forecast.

B. Operation Rules of PSPS PSPS is a special power station that it can not only generate

as a power source but also pump water. However, PSPS’s output depends on its reservoir condition. It can’t generate while its reservoir is close to lower limit, it also can’t pump water while its reservoir is near to upper limit. The volume of water stored in the reservoir at time t+1 1tW + is denoted by the following formulation:

,1 ,

H tt t P t p

h

PW W P t tη

η+ = + ⋅ ⋅ Δ − ⋅ Δ (7)

Here, we simplify the expression of the water volume in the reservoir and express as energy stored instead [11]. In this formulation, ,P tP denotes power of pumps at time t ,

,H tP denotes power of hydro turbines at time t , pη and hη

denote efficiency of pump and hydro turbine respectively, tΔ denotes time interval during every moment. The energy stored in the reservoir is restricted by the following bound as mentioned above:

min t maxW W W≤ ≤ (8)

Output of hydro turbines and pumps is limited to PSPS stalled capacity:

, , ,H min H t H maxP P P≤ ≤ (9)

, , ,P min P t P maxP P P≤ ≤ (10)

The formulations above consist of the PSPS operation regulation. For this analysis, a pumped storage power station is added. This was a plant with a maximum generating and pumping capacity of 1GW, and round trip efficiency of 75%. The reservoir is assumed to be large enough to store energy to generate at maximum for 5 hours, i.e. 5GWh.

C. Changes Made for Wind Power Although wind power output is intermittent and stochastic,

hourly wind power can be predicted to achieve a certain precision because of the regional correlations of hourly wind power fluctuations of large wind farm [12] [13]. The diurnal wind fluctuation pattern is important to the grid operation because the grid operators have to schedule the hourly production of the units to match the generation to the demand. There are strong correlations among wind zones connected to the North China grid on the hourly level. The seconds-level and minutes-level random fluctuations that can be offset by dispersed wind farm have little influence on the hourly fluctuation pattern. In this paper, we use capacity factor to explain how wind power output fluctuation. Capacity factor

,capa tW means the utilization rate of wind turbines at time t . Specific wind power output can be obtained by multiplying wind capacity factor at every moment by wind power installed capacity ,W capaP .

PG.

KGen×PG..real PW

Power Grid

PG×(1-KLOSS) PTrans

PLoad

PG

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D. Improved Model for Calculating The Ability to Accommodate Wind Power Based on the contents above, an improved model used to

analyze the impact of PSPS on wind power integration into grid is proposed. The model is described as follows by formulations (1) ~ (11). The objective function is maximizing the amount that wind power is used by grid at a certain wind power installed level.

,1

T

W tt

Max P=∑ (11)

Constraint conditions are made up of load equalization constraint; bound of wind power used by grid; reserve capacity constraint and constraints of conventional units output, etc.

, , , , ,. . G t W t H t L t P ts t P P P P P+ + = + (12)

, , ,0 W t capa t W capaP W P≤ ≤ ⋅ (13)

, ,( ) (1 ) (1 )G t G capa Reserve Gen LossP P P K K≤ − ⋅ − ⋅ − (14)

( ), , 1 (1 ) (1 )G t G capa adjust Gen LossP P K K K≥ ⋅ − ⋅ − ⋅ − (15)

, ,maxG capa L ReserveP P P= + (16)

,maxReserve L Load ContigencyP P K P= ⋅ + (17)

In the formulation (12), ,H tP and ,P tP can’t exist at same

time as mentioned above; ,W tP denotes wind power can be

accepted by grid at time t actually, and its upper bound is the power could be generated by wind farm at time t , which is denoted as , ,capa t W capaW P⋅ . Specific meanings are explained in the text above.

III. EXAMPLE PROBLEM AND RESULT ANALYSIS Here we take a week situation of Shandong grid in 2009 as

an example and use the model built above to analyze it. We compile the program that calls CPLEX 12.1[14] with C++ to calculate the problem. As a comparison, the situation without PSPS is also calculated. Wind power was increased in steps of 10GW, from 10GW installed to 40GW installed. Storage with a generating and pumping capacity of 1GW was added, which had a storage size enough for 5 hours of operation, i.e. 5 GWh. The load curve and wind power capacity factor is shown in Figure 2.

0

10

20

30

40

1 25 49 73 97 121 145Hour

Pow

er(G

W)

Load DemandMaximum operating coal-fired capacityMinimum coal-fired output limit

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1 25 49 73 97 121 145Hour

Capa

city

fact

or

(a) Load curve of Shandong Power Grid (b) wind power capacity factor curve

Figure 2. Load and wind power capacity in a week of Shandong Grid

Figure 3 and Figure 4 show, existence of PSPS improve the ability to accommodate wind power for grid. Figure 3 shows the utilization of wind power in the system for increasing amounts of installed wind with and without the storage. Here, wind will be curtailed if there is too much of it being produced. As can be seen, very little wind is curtailed at 10GW of wind installed. However, there is a sharp decrease in utilization rate of wind power at higher levels of installed wind. A significant increase in utilization can be seen when storage is added. The number of utilization with storage is about 3 percentages larger than that without storage.

Figure 4 shows the change in the percentage of total energy provided by wind with and without storage. As the amount of wind stalled increases above 13GW, storage allows the wind installed to meet significantly more of the energy requirements. For example, to meet 30% of the energy from wind, without

storage, 30.88GW of wind would need to be installed. Only 28.8GW of wind needs to be installed with 1GW PSPS.

Figure 5 shows online units commitment capacity with and without PSPS. It can be seen that the capacity decrease as increasing wind installed no matter with or without PSPS. It should also be noted that online units commitment capacity with 1GW PSPS is 1GW smaller than that without PSPS. This implies that 1GW PSPS is full used when wind installed capacity is between 10GW and 40GW.

1083

60

70

80

90

100

10 20 30 40 wind installed (GW)

% o

f win

d po

wer

use

d

No storage With 1GW storage

Figure 3. Wind power used over different installed wind levels, with and without storage

10

20

30

40

10 20 30 40 wind installed (GW)

Ener

gy p

rovi

ded

from

win

d

No storage With 1GW storage

Figure 4. Energy provided by wind for increasing wind levels, with and without storage

28

29

30

10 20 30 40Wind installed（GW）

Onl

ine

units

com

mitm

ent.

capa

city

(GW

)

No storage With 1GW storage

Figure 5. Online units commitment capacity as wind added to system

Table Ι shows energy provided by conventional units as increasing wind installed with and without PSPS. Energy provided by conventional units is almost the same with and without PSPS when wind installed capacity is below 10GW.

This is because PSPS can’t always generate or pump. PSPS pumps with energy supplied by grid and generate to grid with the energy stored in the reservoir. Its operation doesn’t decrease total generation of system, but increases it. It can be seen that PSPS doesn’t display obvious effect until a considerable wind installed capacity reaches. While wind power installed capacity is above 10GW, some wind power would have to be curtailed. PSPS just utilize this power to pump water and then generate at a proper time.

TABLE I. ENERGY PROVIDED BY CONVENTIONAL UNITS WITH AND WITHOUT PSPS

Wind installed（

GW）

Energy provided by conventional units without PSPS（

*103GWh）

Energy provided by conventional units with 1 GW PSPS（*103GWh

） 0 4.4212 4.4212

10 3.8823 3.8820 20 3.4169 3.3926 30 3.1097 3.0655 40 2.9439 2.8812

IV. CONCLUSIONS This paper examined the influence of PSPS at increasing

wind power installed. It shows that PSPS increases utilization of wind power and increases the percentage of energy provided by wind, decrease online units commitment capacity at a considerable wind installed level. However, PSPS doesn’t show obvious impact at lower wind level. Considering the capacity level is already very high, it will be more infeasible for PSPS to improve the ability of wind power accommodation.

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