DE102012201969A1 - Method for determining maximum electric energy of electric energy storage device, involves defining reproduced preset function and predefined function to satisfy preset formula for calculating absorbed electric maximum - Google Patents

Abstract

The method involves providing specific function for current network load distribution. The value representing entry capacity of electricity network is calculated. The maximum power consumption capacity of power system is determined to reproduce predefined function by adding value to function values of specific function. The actual electric energy supply of renewable energy sources is provided in power system to reproduce preset function. The reproduced preset function and predefined function are defined to satisfy preset formula for calculating absorbed electric maximum.

Description

The invention relates to a method for determining a maximum electrical work W _{max to be received by} an electrical energy store for a power grid fed with conventional and renewable energy sources.

Renewable energy sources, such as wind and sun, are increasingly being used to generate electricity. In contrast to conventional energy production through the combustion of fossil fuels, the energy supply produced by renewable energy sources can not be easily adapted to current energy requirements. The energy supply generated by renewable energy sources can be considerably larger or considerably smaller than the current energy requirement.

Network operators are obliged to accept the electrical energy generated by renewable energy sources. This can lead to an overload of conventional power grids. To counteract such an overload, the respective power grid can be expanded. However, the expansion of a power grid is extremely time-consuming and costly because of the creation of new tracks required. Alternatively, the overload of a power grid can be counteracted by the involvement of electrical energy storage.

Suitable energy stores are for example pumped storage, chemical storage, fuel cells, batteries or the like.

With regard to the use of an energy storage, it is necessary to provide information about its design, in particular to determine a maximum aufzunehmende electrical work of the electrical energy storage. The determination of a maximum electrical energy to be included in a given power network energy storage is extremely complex because of the large number of variables to be considered.

The object of the present invention is to eliminate the disadvantages of the prior art. In particular, it is intended to specify a method which is as simple and efficient as possible for determining a maximum electrical work of an electrical energy store to be accommodated.

This object is solved by the features of claim 1. Advantageous embodiments of the invention will become apparent from the features of claims 2 to 5.

, wobei Θ(t) die Heaviside-Funktion ist.According to the invention, a method is proposed with the following steps for determining the maximum electrical work W _{max of} an electrical energy store for a power grid fed with conventional and renewable energy sources:
Providing a first function | P _load | (t) representing a power distribution specific load distribution over time;
Calculating a value | P _{E, O} | representing a feed-in capacity of the power grid, the power grid being unloaded;
Determining a second function | P _{E, max} | (t) representing the maximum power capacity of the power network by adding the value | P _{E, O} | to the function values of the first function;
Superimposing the second function | P _{E, max} | (t) with a third function | P _{unit representing} the actual electrical energy input of the renewable energy sources into the power grid _. | (T); and
Calculation of the maximum electrical work W _{max to be taken} according to the following formula:

where Θ (t) is the Heaviside function.

For the purposes of the present invention, a "power grid" is understood to be a galvanically isolated power grid of a voltage level. The power grid may be connected to one or more feed points to another power grid of a superimposed voltage level.

To carry out the method according to the invention, first a first function | P _load | (t) is provided which represents a power distribution specific to the power network over time. The first function represents a load that is summed in total over time at the feed points from the superimposed voltage level.

In a second step, a value | P _{E, O} | calculated, which represents a supply capacity of the power grid, the power grid is without load. The calculation of the value | P _{E, O} | is carried out by at least at any time t by means of a conventional network calculation software for stored loads and taking into account the feed-in power supplied by renewable energies, the limit of the maximum free capacity of the power grid is calculated. At a time t, | P _{E, O} | from the distance between | P _load | and | P _{E, max} |.

In a further step, the determination of a second function | P _{E, max} | (t) which returns the maximum power capacity of the power grid is determined by adding the value | P _{E, O} | to the function values of the first function | P _load | (t). Thus, the second function | P _{E, max} | (t) can be determined quickly and easily.

In a further step, a third function | P _{injection representing} the actual electrical energy input of the renewable energy sources into the power grid is _generated. | (t) provided. The third function | P _Einsp. | (t) represents a "actual feed-in" expected by renewable energy sources during the period under consideration. The third function | P _Einsp. | (t) results z. From values measured in the past.

, wobei Θ (t) die Heaviside-Funktion ist.Finally, the maximum electrical work W _{max to} be recorded is calculated according to the following formula:

where Θ (t) is the Heaviside function.

The above formula is given by the following formula which describes the degree of filling D _n (t) of the memory at time n by recursive calculation, where D _n (t) = 100% is set:

In the above formula, the difference of the powers P _Einsp. and P _{E, max are integrated} over time t and set in proportion to W _max . This ratio is calculated with the Heaviside function of the integral of the difference of the powers P _Insp. and P _{E, max} multiplied. This ensures that the degree of filling D _n (t) of the memory can not be <0.

The method according to the invention is simple and efficient. Using the first function | P _Last | (t), namely the load curve, which is usually present for the power grid, and the third function | P _Inp. | (t) can be calculated so quickly and easily the maximum electrical work W _{max of} an electrical energy storage device for the relevant power grid.

It has surprisingly been found that the value | P _{E, O} | is substantially constant, ie | P _{E, O} | (t) can be considered as parallel to the x-axis. It is advantageously sufficient to determine the value | P _{E, O} | only once. This results in a significant simplification in the determination of a maximum aufzunehmenden electrical W _max work of an electrical energy storage.

The proposed determination of maximum electrical work W _{max of} the electrical energy store can be further refined. To determine the maximum electrical work W _{max of} the electrical energy store that is to be accommodated, it is possible to determine, at time t _n, at P _Einsp. > P _{E, max} available power output P _output from the power grid, a maximum recording power of the energy store P _max.input and a recording _efficiency η _{input of} the energy storage are taken into account. Furthermore, to determine the maximum electrical work W _{max of} the electrical energy store to be recorded at time t _n at P _Einsp. > P _{E, max} available recording power P _Recording in the mains, one maximum output power of the energy storage P _max.output and a discharge efficiency η _{output of} the energy storage are taken into account.

According to a further advantageous embodiment, a maximum power to be _{absorbed by} the energy store can be calculated from a maximum difference _value between the third | P _Einsp. | (t) and the second function | P _{E, max} | (t) over time t.

The method according to the invention will be explained in greater detail below by way of example. Show it:

1 the first, second, third and fourth functions over time,

2 the result of a computational determination of | P _{E, O} | (t) and

3 a schematic overview of the boundary conditions for the design of an electrical energy storage.

1 shows the first function | P _load | (t) as well as the value | P _{E, O} | assumed here to be constant, which represents a feed-in capacity of the power grid, the power grid having no load st. By adding the first function | P _load | (t) with the value | P _{E, O} | The result is the second function | P _{E, max} | (t), which represents the maximum power capacity of the power grid. The function values of the third function | P _Einsp. | (t) result from measured values.

If | P _Insp. | (t)> | P _{E, max} | (t), describes an area between two adjacent intersections, eg. B. t ₁ , t ₂ and t ₃ t _{4 of} the second | P _{E, max} | (t) and third function | P _Einsp. | (t) a work which the electric energy store must absorb. In this case, the electrical power generated by renewable energy sources is greater than the power capacity of the grid.

If | P _{E, max} | (t)> | P _Insp. | (t), then the between two adjacent intersections, z. B. t ₂ , t ₃ and t ₄ , t _{5 of} the second | P _{E, max} | (t) and third function | P _Einsp. Surface (t) the work again, which can give off the electrical energy storage. In this case, the electrical power generated by renewable energy sources is less than the maximum power capacity of the grid.

The in the lower count of the 1 reproduced fourth function indicates the degree of filling of the memory. By the time t ₂ , the degree of filling increases from 0% to about 60%. In the interval between the times t ₂ to t ₃ , the degree of filling then drops to about 45%. In the time interval between the times t ₃ and t ₄ , the degree of filling increases to about 75%. Finally, the degree of filling drops again to 0% until time t ₅ .

After the in 1 explained method steps is | P _{E, O} | assumed to be constant. This assumption substantially simplifies the proposed method.

2 shows the result of a computational verification of the above assumption. For this purpose, | P _{E, O} | (t) is determined by means of a conventional network calculation software (Power Factory). The loads are stored. Subsequently, at any point in time, the feed-in power in the power grid is gradually increased until a limit value violation occurs. If one repeats this process for all times, the results in 2 shown calculated second function | P _{E, max} | (t). By subtracting the first function | P _load | (t) known for the relevant power grid, the function of the feed-in capacity over the time of the power grid | P _{E, O} | (t) results.

How out 2 can be seen, the values P _{E, O} fluctuate over time only in a relatively small range by a constant value. In the calculations, with respect to an average value, a maximum deviation of at most 8% of the electric work W _{max of} an electrical load has to be obtained. This deviation can be accepted in the determination of the maximum energy storage, in particular with regard to the inaccuracy of a prognosis underlying the third function. As a result, it is advantageously sufficient to calculate the value | P _{E, O} | only once. This results in a considerable simplification for the inventive method.

3 shows in an overview of other parameters that are important for the design of an electrical energy storage. These parameters can also be determined quickly and easily based on the present invention.

The power which an energy storage device has to absorb (charge storage) or can give off (emptying of storage) results from the in 2 shown distances between the second | P _{E, max} | (t) and third function | P _Einsp. | (T).

The maximum over the times 1... X gives the maximum power which the energy store must absorb.

A maximum duration of use of the energy storage is the maximum time in which the energy storage continuously absorbs power and can be calculated from C _n as follows: J _n = If (C _n >0; J _n-1 + 1; 0)

The maximum of the values J ₁ ... J _x multiplied by the time interval Δt gives the maximum duration of use R: R = Max (J _n ) · Δt

With the out 3 apparent boundary conditions, the work W _{max is} calculated. Storage capacity or the maximum to be taken electrical

Whether there is a storage charge or discharge results from the sign of C _n .

In the next step, the efficiency and the performance limits in the conversion processes from the power grid to the energy storage and from the energy storage to the power grid are taken into account. These constraints change C _n to give the performance that will actually contribute to changing the degree of memory filling in the next steps. The value of C _n is changed in each case by one of the two efficiencies and limited by one of the power limits.

This can be taken into account by the following calculation steps:

Influence of η _output : ΔP _out = When (C _n <0; C _n / η _output ; C _n )

Discharge of the energy storage: The values for C _n > 0 remain the same, the others are increased because less power actually arrives at the power supply due to the conversion loss.

Influence of the performance limits: ΔP _limited = If (ΔP _out <0; If (ΔP _out <P _max.output ; P _max.output ; ΔP _out ); If (ΔP _out > P _max.input ; P _max.input ; Δ P _out )

The power limits are taken into account for charging and discharging the energy store. During the discharge of the energy store, it is already taken into account in the preceding step that more energy can be drawn from the energy store as a result of the loss than is absorbed by the power grid.

Influence of η _input : ΔP _in = when (ΔP _limited <0, ΔP _limited , ΔP _limited · η _input )

Charging of the energy storage, values for C _n <0 remain the same, the others are lower, since the conversion loss less power at the energy storage arrives.

In the next step, the respective power is multiplied by the time interval, the delta of the work is calculated: ΔW = ΔP _in · Δt (eg from MW become MWh)

Now a value for W _{max has to be} assumed, which has to be adjusted after the next step. Thus, from delta of work and assumed W _max, the percentage delta of a charge level of the energy store can be calculated: Δ% = ΔW / W _max

The degree of filling D _{n of} the energy store is calculated here as follows: D _n = when (D _n-1 + Δ% <0;0; D _n-1 + Δ%) Max (D _n ) <100%

The maximum filling level must not exceed 100%, therefore the value W must be adjusted.

With these results, the storage duration and the number of cycles of the energy storage are calculated. First, the auxiliary value "memory status G _n " is introduced. It indicates at any time whether the energy store is currently empty or not and is calculated as follows: G _n = If (D _n >0;1; 0)

The auxiliary quantity H _n summed up the immediately successive times at which G _{n is} equal to 1. H _n = If (G _n = 1; H _n-1 + 1; 0)

The maximum of H _n multiplied by the time interval gives the maximum storage duration S: S = Max (H _n ) · Δt

The number of status changes I _n in the energy store is used to calculate the number of cycles: I _n = when (G _n = G _n-1 ; I _n-1 ; I _n-1 + 1)

Half of the status changes calculate the number of cycles I: T = I _n / 2

Used formula symbols

• P _{max. Input}

  maximum performance in the conversion process from network to storage;

  P _max.output

  maximum performance in the forestation process from storage to network;

  W _max

  maximum work of the memory (storage capacity);

  η _input

  Efficiency in the conversion process from network to storage;

  η _output

  Efficiency in the conversion process from storage to network;

  R

  Period during which the storage tank must absorb the energy (duration of use);

  S

  Period in which the memory must store the energy (storage duration);

  T

  Number of storage cycles;

  P

  Selected time interval;

  P _{unit, n}

  Actual supply to a power grid;

  P _{E, max, n}

  maximum capacity of a power grid for feeding;

  C _n

  Power which has to take a memory from the network (memory charge) or which the memory can deliver to the network (memory discharge);

  D _n

  percentage filling level of the store;

  ΔP _{out, n}

  Power reduced by efficiency during storage discharge;

  ΔP _{limited, n}

  Performance after consideration of performance limitations;

  ΔP _{in, n}

  Performance reduced by efficiency during storage charging;

  ΔW _n

  Delta of the work from the memory or into the memory;

  Δ% _n

  percentage delta of the memory fill level (delta work to total memory capacity);

  G _n

  Memory status (status 0 means memory empty, status 1 means energy in memory);

  H _n

  consecutive periods of memory status 1;

  I _n

  Sum of status changes;

  Y _n

  successive periods of storage charging.

For the preceding characters, n ∈ N

Claims (5)

Method for determining a maximum electrical work W _{max of} an electrical energy store for a power grid fed with conventional and renewable energy sources, comprising the following steps: providing a first function | P _load | (t) representing a power distribution specific to the power grid over time ; Calculating a value | P _{E, O} | representing a feed-in capacity of the power grid, the power grid being unloaded; Determining a second function | P _{E, max} | (t) representing the maximum power capacity of the power network by adding the value | P _{E, O} | to the function values of the first function; Providing a third function | P _{injection representing} the actual electrical energy input of the renewable energy sources into the power grid _. | (T); and calculating the maximum electrical work W _{max to be taken} according to the following formula:

where Θ (t) is the Heaviside function. Method according to claim 1, wherein only one value | P _{E, O} | is calculated. Method according to one of the preceding claims, wherein for determining the maximum electrical work W _{max of} the electrical energy store to be recorded at time t _n at P _Insp. > P _{E, max} available power output P _output from the power grid, a maximum recording power of the energy store P _max.input and a recording _efficiency η _{input of} the energy storage are taken into account. Method according to one of the preceding claims, wherein for determining the maximum electrical work W _{max of} the electrical energy store to be recorded at time t _n at P _Insp. <P _{E, max} | existing recording power P _recording memory P _max.output and a discharge efficiency η _{output of} the energy of the power grid, a maximum output power of the energy storage energy are considered. Method according to one of the preceding claims, wherein a maximum energy to be _{absorbed of} the energy store from a maximum difference _value between the third | P _Einsp. | (t) and the second function | P _{E, max} | (t) over the time t is determined.

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