# Abstract

The energy transition entails a rapid uptake of renewable energy sources. Besides physical changes within the grid infrastructure, energy storage devices and their smart operation are key measures to master the resulting challenges like, e. g., a highly fluctuating power generation. For the latter, optimization based control has demonstrated its potential on a microgrid level. However, if a network of coupled microgrids is considered, iterative optimization schemes including several communication rounds are typically used. Here, we propose to replace the optimization on the microgrid level by using surrogate models either derived from radial basis functions or neural networks to avoid this iterative procedure. We prove well-posedness of our approach and demonstrate its efficiency by numerical simulations based on real data provided by an Australian grid operator.

# Zusammenfassung

Die Energiewende bringt einen raschen Zuwachs eneuerbarer Energiequellen mit sich. Neben den physikalischen Veränderungen der Netzinfrastruktur spielen Energiespeichereinheiten und deren intelligente Nutzung eine entscheidende Rolle, um die sich ergebenden Probleme wie z. B. die stark schwankende Energieerzeugung zu bewältigen. In Bezug auf Letztere haben optimierungsbasierte Steuerungstechniken ihr Potential auf Microgrid-Ebene unter Beweis gestellt. Betrachtet man jedoch ein Netzwerk gekoppelter Microgrids, werden üblicherweise iterative Optimierungsansätze gewählt, welche mit mehreren Kommunikationsrunden einhergehen. Um derartigen Kommunikationsschleifen vorzubeugen, schlagen wir vor, den Optimierungsschritt auf Microgrid-Ebene durch den Einsatz geeigneter Ersatzmodelle zu vermeiden. Den hier verwendeten Ersatzmodellen liegen zum einen radiale Basisfunktionen und zum anderen neuronale Netze zugrunde. Wir zeigen, dass unser Ansatz wohlgestellt ist und demonstrieren die Effizienz anhand numerischer Simulationen basierend auf realen Daten eines australischen Verteilnetzbetreibers.

## 1 Introduction

The share of renewable energy sources rapidly increases; also due to more and more installed devices like e. g., solar panels at household-level. Hence, households become *prosumers*, i. e., power is not only consumed but also produced. Therefore, energy generation and distribution takes place in a distributed way. In particular, energy can be transmitted bidirectionally between the grid and the prosumers, which results in a paradigm shift in the grid organization. In addition, prosumers may also possess some kind of energy storage device in order to manipulate their power demand profiles by either charging or discharging. From the grid operator’s perspective it might be beneficial that charging decisions are not made based on local information only. Instead taking into account information on the entire grid may improve the system-wide operation, e. g., to flatten the overall power demand within the grid in order to facilitate the power supply. [1]. In order to achieve this goal, communication is needed. In the future, each household shall be equipped with a smart meter which yields so-called *smart homes*. Smart meters collect data and communicate with the grid operator automatically.

A straight-forward way to optimally operate the overall system is to formulate one large-scale optimization problem and to solve it in a centralized way, see, e. g., [2]. This approach, however, is hard to realize in practice. One of the disadvantages is that some central node needs the complete information about the grid, which is, e. g., due to data privacy, not desirable. Alternatives are decentralized or distributed optimization algorithms. In [3] the authors propose a decentralized approach to steer energy storage systems in order to avoid over-capacity of pole transformers while maintaining a high charging amount of energy storage systems in low-voltage distribution systems. The other option mentioned above are distributed optimization methods such as distributed dual ascent [4], Alternating Direction Method of Multipliers (ADMM) [5] or Augmented Lagrangian based Alternating Direction Inexact Newton (ALADIN) [6]. These algorithms use a star-shaped communication topology, i. e., each smart home communicates only with the grid operator and does not share any information with its neighbours. Nevertheless, in every iteration each household has to transmit specific (personal) data to the grid operator, see also [7] and [8] for an application of ADMM and ALADIN to electrical networked systems, respectively. In order to exploit the potential of these algorithms they are typically embedded within a Model Predictive Control (MPC) framework. MPC is a state-of-the-art technique to tackle optimal control problems by solving finite-dimensional optimization problems successively, see e. g., [9] for an introduction to MPC and [10], [11] for MPC approaches in electrical networks.

An alternate option to steer the power demand of local agents besides battery control is to schedule so-called controllable loads. Controllable loads can be shifted in time to avoid bottlenecks in the energy supply, see e. g., [12], [13]. There is also a large potential in the context of stochastic optimization of smart grids. For weather forecasting methods we refer to [14]. How to integrate electrical vehicles into the electricity network under uncertainties is described in [15].

Considering the power networks described so far, it is assumed that exchange of energy within the grid is possible at any time and does not cause any losses or additional costs, which might (approximately) hold for domestic nets, e. g., a town. In this paper, we refer to these grids as microgrids (MGs). In [16], [17], the concept of coupled MGs is used to tackle large-scale problems incorporating several MGs. In the latter, the authors show that even if each single MG is optimally operated, there is still room for improvement if energy can be exchanged among MGs. Therefore, a second optimization problem is solved on a higher grid level in order to optimally exchange energy resulting in a bilevel optimization problem [18].

In [19], the authors propose to replace the distributed optimization routine on the lower grid level by a surrogate model in order to speed-up the calculation and further reduce communication effort. Here, Radial Basis Functions (RBFs) [20] are used to approximate the input-output behaviour of ADMM within the framework of coupled MGs established in [17]. Besides RBFs there are various methods to learn the behaviour of a complex function. Artificial Neural Networks (NNs) are one of the most popular representatives of modern artificial intelligence techniques and are often used in practice due to their success in various application fields, see e. g., the survey article [21]. In [22] the authors forecast loads in a power grid using NNs, whereas in [23] NNs are used in an optimal power flow framework. The main advantage of using surrogates is that communication effort can be reduced.

In this paper, we extend the idea of coupled microgrids established in [17] by proposing an iterative *bi-directional* optimization routine in order to improve the overall performance. Due to its iterative structure, however, our method comes along with a strong need for communication between smart homes and grid operator. As a remedy we present two approaches to reduce the communication effort by substituting the optimization on microgrid level via surrogate models. A main difference compared to [19] lies in the different input-output map that is replaced by the surrogate models, for which we can show that each input uniquely determines an (optimal) output. Furthermore, we also take NNs as potential surrogate models into account and study the performance of the resulting approximations numerically in an MPC framework. Our simulations show that the proposed method approximately recovers the performance based on using ADMM but significantly reduces the communication burden. The effect of applying surrogate models within MPC extends our previous work [19] where a surrogate model based on RBFs was only applied in a static optimization problem.

The paper is structured as follows: In Section 2 we formulate a mathematical model for coupled microgrids that consists of two hierarchy levels, and introduce optimization problems corresponding to each of them. In the consecutive section, we propose an iterative scheme that requires the solution of a distributed optimization problem on the lower level which is solved using ADMM. In Section 4, we investigate the impact of disturbances w. r. t. the lower-level solution on the performance measured in terms of the upper-level objective function. Based on the results, we propose to replace ADMM by surrogates in order to reduce communication effort and computation time. The performance of the optimization scheme incorporating surrogates is analysed in an MPC framework in Section 5.

## 2 A model for coupled microgrids

We consider a system of coupled microgrids (MGs) and call it a *smart grid*. Each MG consists of several residential energy systems (agents) coupled through the grid operator, which can be seen as Central Entity (CE). The coupling of the microgrids is done through a network, where some MGs are connected by a transmission line and others are not connected, cf. Figure 1.

### Figure 1

### 2.1 Upper-level model: Energy exchange

We assume that we have *κ*, for *κ* has an average power demand *n*. Given this, we can compute the total power demand *κ*, but this is not necessary for the rest of the discussion.

Let *ν* to MG *κ*. We enforce *κ* is given by its own total power demand *k* for *N* timesteps of each MG in a least-squares sense. The objective function is thus given by

Here, the vector

We are interested in minimizing (1) under the following constraints: All exchange rates *δ*,

*ν*to MG

*κ*at time instance

*n*. Constraints (2b) and (2c) ensure that the whole energy of each MG is

*scheduled*and that exchanges via transmission lines can only occur in one direction during one time step. We denote the feasible set of (2) by

The efficiency of a transmission line does not depend on the direction of the transfer, i. e., the matrix *η* is symmetric. Furthermore, we assume no loss without transport, i. e.,

### 2.2 Lower-level model: Single microgrid

As we have seen in the previous section, we consider an average power demand at each MG as well as some desired quantity *i*-th system, *κ*, can be described by the discrete time system dynamics,

### Figure 2

The system can be controlled by charging

*N*,

For a concise notation we introduce the set

of feasible outputs over the next *N* time steps, *κ* we use the Cartesian product, e. g.,

The output quantity in (3b) is the power demand *κ*. The average power demand

where

Let us for the moment ignore the coupling described in Subsection 2.1. Then,

Therefore, the overall objective

per MG with local objective function

### 2.3 Fully coupled optimization problem

We are interested in optimizing the function (1). This function, in general, depends on *δ* as well as on *u*, which we have to find in such a way that

Note that due to constraint (2c) the optimization of *δ* is non-convex. Furthermore, the large scale of the optimization w. r. t.

## 3 Bidirectional optimization

We propose to tackle the optimization problem (7) in a *bidirectional* way, i. e., we first find an optimal *δ* being the identity and then optimize (7) w. r. t. *δ* for fixed

### 3.1 Bidirectional optimization scheme

Assume that each MG *κ*,

and hence, the difference

is minimized further. One could think about fixing *δ* and finding an optimal *δ* but also the *δ*’s from the previous optimization step. Intuitively, the difference *κ*. This yields the modified lower-level optimization problem

where *δ* and a given previous

Based on the updated reference value we solve (8) and (2) to improve the battery usage and the energy exchange and repeat the optimization until some terminal condition is satisfied, e. g., performance improvement less than a pre-defined tolerance or maximal number of iterations exceeded. This procedure is summarized in Algorithm 1. Note that we only update the reference

### Algorithm 1

Neither convergence nor the interpretation of a potential limit of Algorithm 1 is clear a priori. Figure 3, however, experimentally shows convergence of the proposed scheme and a continuous improvement of the upper-level performance index. Here, we ran 10 iterations of the optimization scheme and plotted both the objective function values before and after the energy exchange within each iteration. The values stagnate after four iterations indicating that additional iterations do not further improve the overall performance. The next subsection elaborates on how to solve (8) in a fully distributed way using ADMM.

### Figure 3

### 3.2 Distributed optimization via ADMM

In this section we briefly discuss how to solve the lower-level optimization problem (6) or (8) using an Alternating Direction Method of Multipliers (ADMM) approach. We consider a single MG and therefore omit the index *κ*. Since the averaged output quantity appears in the objective function (6) or (8), we need to introduce an auxiliary variable *a* in order to decouple the lower-level optimization in the following way,

*z*. ADMM is an optimization scheme to solve (9) based on the augmented Lagrangian

in a distributed way, cf. [5]. Following [7], the ADMM algorithm for (9) yields the three-step iteration

*i*,

According to Theorem 3.1 in [7] the optimization scheme (10) converges in the following sense.

### Theorem 1.

*Consider Problem* (9) *with g being strictly convex, closed and proper and let the iterates**be computed according to* (10)*. Then the following following statements hold true:*

According to [7] and the references [5, Section 3] and [4, Appendix C] therein, problem (6) fulfils the assumptions of Theorem 1.

## 4 Surrogate models for ADMM

This section is dedicated to surrogate models for the optimization routine (10) within a single MG. For simplicity of notation we omit the index *κ*.

Due to the distributive structure of ADMM, the residential energy systems do not need to share information with their neighbours but only with the CE, see also the star-structure in Figure 2. In each iteration *ℓ* of ADMM, subsystem *i* has to transmit its solution

for all feasible

Figure 4 (top) shows that if the approximation (11) is sufficiently accurate, the impact on the performance of the optimization scheme is negligible. Here, the costs

### Figure 4

Note that (11) might yield approximations to the solution

### 4.1 Well-posedness

The following proposition states that for equality in (11), a proper mapping is defined. For a concise notation we replace the index *i* here.

### Proposition 1.

*Consider φ given by* (11)*, where**describes the optimal solution of* (6) *computed via ADMM, i. e.,**. We assume all hyper-parameter to be fixed meaning that**in* (3)*–*(4) *are constant over time for all**. Then φ is a mapping, i. e., for all**, there exists a uniquely determined**such that**is the solution to the optimization problem* (6)*.*

### Proof.

First note that ADMM yields the unique solution of (6), see e. g., [5]. Furthermore, there are no constraints on

where

### 4.2 Radial basis functions approximation

Radial Basis Functions (RBFs) are used to interpolate functions based on a set of sampling data. We briefly recap some basics on RBFs. For a detailed introduction to theory and application see e. g., [20], for a similar approach where RBFs are used to replace ADMM we refer to [19].

Let

where *ψ* yields support close to the sampling data *B* are determined by interpolation conditions, cf. [19], [20].

In Figure 5, a possible fit via RBFs is visualized. Here, we interpolated given data from two-weeks of optimization (4540 data points) based on sampling data picking each 25-th data point to train (12). Then, we tested `Matlab` toolbox `DACE` [27]. Note that the evaluation time of the RBF approximation grows with the number of data points used. Already with 180 data points to train (12) with

### Figure 5

### 4.3 Neural networks approximation

Neural Networks (NNs) are a state-of-the-art method in artificial intelligence frameworks. Based on huge amounts of data *l*-layers as an approximation to the mapping (11), i. e.,

where *σ* denotes the sigmoid function, and the weights *l* determine the number of rows and columns of `Matlab`’s built-in toolbox `nftool`.

The overall goal of the approximation (13) is to be sufficient in the sense of the MPC performance shown in Figure 6. Our experiments in Figures 4 (bottom) and Figure 5 show that with one hidden layer of ten neurons only, a satisfying approximation on a 24-hours time window can be achieved if the training data is large enough. Note that NNs benefit from big data. In our case study, we trained the NN only on data corresponding to two weeks.

## 5 Numerical proof-of-concept

Model Predictive Control (MPC) is a method to tackle optimal control problems on an infinite time horizon by solving a series of finite dimensional optimization problems instead, see e. g., [9] for an introduction to non-linear MPC.

### 5.1 Model predictive control (MPC)

Consider the optimal control problem (6). In order to provide an optimal control sequence over an arbitrary long time horizon we use MPC. To this end, at current time instance *δ*. Then, only the first instances

### Algorithm 2

Note that Problems (6) or (8) and (2) have to be solved in order to determine

are realized at each time step

### 5.2 Usage of surrogate models in MPC

We compare the performances using ADMM, RBFs, and NNs on the lower-level, i. e., in Step 4(b) of Algorithnm 1. In all numerical simulations we set ^{[1]} The battery parameters were randomly chosen with mean values

For simplicity of the numerical computation, we only replaced the lower-level optimization routine for MG 1 and, thus, avoid training a separate surrogate model for each MG. We used `Matlab` for implementation.

Results on the MPC closed loop can be found in Figure 6 and Table 1. In Figure 6 the closed-loop performances of ADMM (black line) compared to perturbed ADMM, and ADMM (black line) compared to the two surrogate models are visualized. Similar to the open-loop case, small disturbances in ADMM have little impact and RBFs outperform the NN. The first column of Table 1 compares the sum of all MPC closed-loop performances using ADMM, RBFs and a NN while in column 2 the average runtimes of these approaches are reported. Note that when using a surrogate, we call ADMM once per MPC iteration. As elaborated in [7] in each ADMM iteration an *N*-dimensional vector has to be transmitted twice. Hence, both surrogates reduce the need for communication. Two great advantages of ADMM are that the local optimization (10a) can be parallelized and the global optimization is independent of the size of the MG. However, a single function evaluation such as (12) or (13) is faster than running the entire ADMM optimization routine.

### Figure 6

### Table 1

closed-loop cost | runtime [ms] | |

no control | 12,228 | — |

ADMM | 4,416 | 2.5 |

RBFs | 4,529 | 1.2 |

NNs | 5,598 | 0.05 |

Note that in column 2 of Table 1 we ignored the communication between smart homes and CE which is needed to apply ADMM in practice. However, the runtime of ADMM impairs when executed in an actual smart grid while surrogates do not require additional communication.

In order to improve the performance of the NN, more sampling data has to be generated to increase the training set significantly. To avoid large offline computation times, we chose

## 6 Conclusions

In this paper we recalled an optimization problem arising in large-scale electrical networks. We proposed an iterative *bidirectional* optimization scheme to tackle this problem in a distributed way, and showed numerically that a small error on the lower level does not have noticeable impact on the performance. Based on this observation, we replaced the lower-level optimization by surrogate models using radial basis functions and artificial neural networks. The numerical results show the potential of using these surrogates to reduce communication effort and computational time in MPC while preserving the overall performance.

**Funding source: **Bundesministerium für Bildung und Forschung

**Award Identifier / Grant number: **05M18EVA

**Award Identifier / Grant number: **05M18SIA

**Funding source: **Deutsche Forschungsgemeinschaft

**Award Identifier / Grant number: **WO 2056/6-1

**Funding statement: **The authors gratefully acknowledge funding by the Federal Ministry for Education and Research (BMBF; grants 05M18EVA and 05M18SIA). Karl Worthmann is indebted to the German Research Foundation (DFG; grant WO 2056/6-1).

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**Received:**2019-07-05

**Accepted:**2019-09-06

**Published Online:**2019-11-30

**Published in Print:**2019-11-18

© 2019 Baumann et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.