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This paper comes up with a concept of synergetic advanced dispatch in order to deal with the ever-increasing uncertainty in power grid: Decision is made with respecting to AGC units and active load on the basis of synergetic unit combination such that active load’s advantages in regulation speed is put to full use in achieving efficient cooperation with renewable energy power. Meanwhile, factoring in allowable frequency variation range during decision-making may help to reduce AGC units’ regulation load and improve power grid's capacity of accommodating renewable energy power. Calculation example analysis suggested that the model and technique presented in this paper is capable of efficient coordination between active loads and renewable energy power, delivering friendly transition with day-ahead dispatch and AVC control.

Advanced dispatch is a link that relates day-ahead dispatch to automatic generation control (AGC). Research on traditional power grid advanced dispatch is approaching to its mature stage at present [

New technologies emerge in recent years, such as electric automobiles, energy storage, controllable load, and interruptible load, allowing changing their own generation or consumption power to a certain extent for what called active loads. Some researchers studied the control methods for active loads participating in primary [

This paper studies synergetic advanced dispatch with integration of renewable energy. It makes decision for AGC unit and active load response on the basis of the result of synergetic unit combination such that active loads’ advantage in regulation speed may be put to good use in coping with uncertainty associated with renewable energy power generation and conventional loads in order to realize an efficient cooperation. Additionally, the model has taken into account the fact that frequency variation is allowable within a certain limit, which has the effect of expanding the feasible region [

Synergetic dispatch of power grid requires intimate coordination of decision- making at various time scale such as day-ahead level (unit combination), advanced level, and real time level, among which, synergetic advanced dispatch is the link that relates day-ahead dispatch to AGC control.

With non-AGC units, in synergetic dispatch, it is AGC units and active loads that take up reserve and the capability is therefore greater in coping with uncertainties; because of this, synergetic advanced dispatch decision-making no longer considers the re-decision-making of non-AGC unit output power, so that non-AGC units are operated according to the power generation schedule specified in day-ahead dispatch.

With AGC units, synergetic unit combination configuration specifies reserve capacity whereby AGC units may cope with uncertainties in advanced time level. In synergetic advanced dispatch, the reserve response quantity of AGC units shall be determined according to super-short period prediction outcomes of renewable energy power generation and conventional loads.

With active loads, synergetic unit combination is a process that allocates this finite dispatch resource and develops a power storage schedule, depending on daily periodic pattern of renewable energy power generation and conventional loads. The conflict shall be avoided that decision-making in synergetic advanced dispatch is contradictory with synergetic unit combination, otherwise the power demand by active loads might not be satisfied.

Synergetic advanced dispatch provides a base point which may be used by AGC units and by active loads while participating in AGC control, and dispatch errors as well as power fluctuations occurring in the dispatch period need to be balanced by AGC control.

According to [_{t} in period t during synergetic advanced dispatch, then the adjustment quantity ΔP_{A,t} of active loads can be written as:

where,

Then, the adjustment quantity ΔP_{AGC,t} of AGC units can be expressed as below:

where,

After regulation of active loads and AGC unit, if there is:

Suppose a system frequency regulation coefficient of β_{t} at time period t, when Equation (3) is valid the power grid frequency deviation Δf_{t} is:

Rejection of renewable energy power or switching off loads becomes necessary only when ∆f_{t} is out of the allowable frequency scope.

Assuming Ω_{R} and Ω_{D} are uncertainty sets corresponding to renewable energy power generation and conventional loads, then the power p_{R} of renewable energy power generation and the power p_{D} of conventional loads shall satisfy:

Advanced dispatch decision-making outcomes shall satisfy the uncertainty set of renewable energy power generation and conventional loads so that it is modeled as below:

where, in (6) and (7), f is the cost function; u is the control quantity needing advanced decision; y is a buffer quantity designed to cope with potential fluctuations; h is the equality constraints, g is the inequality constraints,

With minimization of AGC units’ operation cost and rejected renewable energy power as the objective, the objective function of this model may be written as:

where, T is the number of time periods, N is the number of AGC units, P_{G}_{,i,t} is the base point of AGC unit i at time period t, a_{i}, b_{i} and c_{i} are characteristic coefficients of AGC unit i.

Suppose that the predicted interval of renewable energy power generation for time period t is_{t} being the penalty coefficient.

Constraints mainly include:

1) Base point power equilibrium constraint

where, P^{*}_{N}_{AGC,t} is the sum of output power of non-AGC units in time period t; P_{R}_{,t} is scheduled renewable energy power generation value in time period t and is a decision quantity; D_{t} is the super short period power prediction expected value of conventional load in time period t; P_{A,t} is the charge/discharge power of active loads in time period t and is also a decision quantity, with P_{A,t} > 0 indicating the active loads are being charged.

2) Operational constraints of active loads

where, _{t} is the energy stored by active loads in time period t;

3) Coping with uncertainty constraints

Supposing conventional load distribution interval is

where,

An illustration is provided below using the upper boundary

In the case of active loads, the actual adjustment quantity

with the rest of adjustment quantity demand being

where,

Similarly, when the net load drops to the lower boundary

4) Upper and lower limit constraints of AGC unit output power:

where,

5) Constraint of AGC unit creeping speed:

where, r_{G,i} is the maximum adjustment speed of unit i, ∆t being the duration of time.

6) Constraint of frequency deviation:

where,

7) Constraint of renewable energy power generation operation interval:

8) Constraint of AGC unit participation factors:

In the model above, Equation (8) is in quadratic form, Equations (15) and (17) are logical expressions, and Equations (16) and (18) contain nonlinear expression of the product of participation factors. To deal with that, the following measures are taken:

Firstly, the quadratic objective function is piecewise linearized.

Secondly, logical expressions (15) and (17) are converted into a general form by introducing 0 - 1 auxiliary variables. For (15), 0 - 1 quantity

where, M is a constant with a high value and

Analysis discovers that (15) is equivalent to (28)-(30). Similarly, logical expression (17) can be converted to a general form consisted of (31)-(33).

Finally, AGC unit participation factors are taken as constant using the following equation in this paper.

The model described above was simulated using a 10-unit system in which unit 1 - 6 are supposed to be AGC units and units parameters, load curve and day-ahead dispatch results are given by [

Suppose that day-ahead dispatch sets active load power at 25 MW and traditional advanced dispatch does not consider the adjustment for active load so that the charging power is treated as a fixed constant in decision-making. With traditional dispatch, the following figure compares the wind power operation interval as predicted and the operation interval as actually established in decision-making.

As is seen from

The model and computation method proposed in this paper, in contrast, have

factored in active load reserve response and frequency regulation effect, so it is no longer needed to reject wind power in this example, hence completely satisfying the predicted wind power operation interval.

To validate dispatch outcomes, Monte Carlo simulation was performed during which wind power output power values were generated randomly to simulate actual charge/discharge power of active loads and variation in power grid frequency.

As is evident from

In the time period when active loads are unable to satisfy power uncertainty, active power equilibrium may be achieved by readjusting the AGC unit output power or by frequency regulation effect. If in AGC control, the objective is to reduce AGC unit regulation to a minimum and let power fluctuation be preferably taken up by frequency regulation effect, then the simulated frequency is as shown in

deviation is larger, but remains within the allowable deviation range.

This study investigated synergetic advanced dispatch, coming to the following major conclusions: 1) Coordination with reserve configuration in synergetic unit combination and factoring in adjusting effect of active loads are able to make full use of active load’s advantages in regulation speed and achieve more cooperation with renewable energy power generation; 2) Consideration of active loads and frequency regulation effect in advanced dispatch model is conducive to alleviate AGC units’ regulation pressure in the control process and avoid conservative decision; 3) Synergetic advanced dispatch realizes friendly transition from day- ahead dispatch to AGC control and improves power grid’s adaptation to renewable energy power.

Yang, S., Jiang, Z.F., Li, S., Li, W.B., Liu, G.J., Cao, X.Y., Wang, N. and Zhang, L.N. (2017) Synergetic Dispatch of Power System with Integration of Large-Scale Renewable Energy. Energy and Power Engineering, 9, 589-597. https://doi.org/10.4236/epe.2017.94B065