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IEEE PES Innovative Smart Grid Technologies Europe

Quantum-Compatible Unit Commitment Modeling through Logarithmic Discretization

Tyler Christeson, Amin Khodaei, Rozhin Eskandarpour

Published Oct 14, 2024• 2 citations
Publisher

Abstract

The unit commitment (UC) problem is essential to operating the power system in an efficient and reliable manner. However, modern power systems are growing increasingly complex with the growing penetration of distributed energy resources, making classical approaches to the UC problem computationally inefficient. Quantum computing approaches have been investigated for the UC problem due to the potential to outperform classical solutions. In many quantum models, including quadratic unconstrained binary optimization, or QUBO, the problem must be reformulated by discretizing continuous variables into "bins" or "segments" to ensure compatibility with the quantum processing unit (QPU). This would increase the number of variables due to discretization and lead to sub-optimal solutions. In this paper, we propose a new reformulation strategy for converting optimization problems into QPU-compatible binary quadratic models while creating significantly fewer binary variables, resulting in faster computation, increased efficiency, and further scalability. The performance of this approach is examined in terms of computation time and solution optimality to demonstrate its advantages and to illustrate the potential capability to solve the UC problem more efficiently than classical solutions.

DOI
10.1109/isgteurope62998.2024.10863738

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IEEE Power Engineering Society General Meeting•Jul 27, 2025•1 citation
The unit commitment (UC) problem is essential to operating the power system in an efficient and reliable manner. However, modern power systems are growing increasingly complex with the growing penetration of distributed energy resources, making classical approaches to the UC problem computationally inefficient. Quantum computing approaches have been investigated for the UC problem due to the potential to outperform classical solutions. In many quantum models, including quadratic unconstrained binary optimization, or QUBO, the problem must be reformulated by discretizing continuous variables into "bins" or "segments" to ensure compatibility with the quantum processing unit (QPU). This would increase the number of variables due to discretization and lead to sub-optimal solutions. In this paper, we propose a new reformulation strategy for converting optimization problems into QPU-compatible binary quadratic models while creating significantly fewer binary variables, resulting in faster computation, increased efficiency, and further scalability. The performance of this approach is examined in terms of computation time and solution optimality to demonstrate its advantages and to illustrate the potential capability to solve the UC problem more efficiently than classical solutions.
The unit commitment (UC) problem stands as a critical optimization challenge in the electrical power industry. It is classified as NP-hard, placing it among the most intractable problems to solve. This paper introduces a novel hybrid quantum-classical algorithm designed to efficiently (approximately) solve the UC problem in polynomial time. In this approach, the UC problem is decomposed into two subproblems: a QUBO (Quadratic Unconstrained Binary Optimization) problem and a quadratic optimization problem. The algorithm employs the Quantum Approximate Optimization Algorithm (QAOA) to identify the optimal unit combination and classical methods to determine individual unit powers. The proposed hybrid algorithm is the first to include both the spinning reserve constraint (thus improving its applicability to real-world scenarios) and to explore QAOA warm-start optimization in this context. The effectiveness of this optimization was illustrated for specific instances of the UC problem, not only in terms of solution accuracy but also by reducing the number of iterations required for QAOA convergence. Hybrid solutions achieved using a single-layer warm-start QAOA (p=1) are within a 5.1 % margin of the reference (approximate) classical solution, while guaranteeing polynomial time complexity on the number of power generation units and time intervals.
The penetration of distributed energy resources (DERs) within the microgrids (MGs) increases the complexity of Unit Commitment (UC) problem. In this paper, the idea of quantum computing (QC) is introduced in power system distributed optimization to solve the combinatorially complex UC problem. To take advantage of QC, the quantum-amenable model of UC problem is innovatively developed. Then, by using the hybrid nature of quantum-inspired alternate direction method of multipliers (ADMM) and by exploiting the superposition and entanglement of quantum bits (qubits), the classical ADMM coordinates the subproblems to obtain feasible solutions. Additionally, this paper uses a decomposition and coordination-supported framework for quantum optimization, which allows to solve problems larger that currently available quantum computers are capable of solving. The quantum distributed UC results are compared with those from its classical counterpart, which validate the efficacy of quantum computing,
This paper proposes a hybrid quantum-classical algorithm to solve a fundamental power system problem called unit commitment (UC). The UC problem is decomposed into a quadratic subproblem, a quadratic unconstrained binary optimization (QUBO) subproblem, and an unconstrained quadratic subproblem. A classical optimization solver solves the first and third subproblems, while the QUBO subproblem is solved by a quantum algorithm called quantum approximate optimization algorithm (QAOA). The three subproblems are then coordinated iteratively using a three-block alternating direction method of multipliers algorithm. Using Qiskit on the IBM Q system as the simulation environment, simulation results demonstrate the validity of the proposed algorithm to solve the UC problem.
Unit commitment (UC) problems faced by Independent System Operators on a daily basis are becoming increasingly complex due to the recent push for renewables and the consideration of sub-hourly UC to accommodate the increasing variability in the net load. A disruptive solution methodology to address the growing complexity is therefore required. Quantum computing offers a promise to overcome the combinatorial complexity through the use of the so-called “qubits.” To make the best use of near-term quantum computers to solve UC problems with a much larger number of binary variables than the number of qubits available, this paper devises a novel solution methodology based on a synergistic combination of quantum computing and Surrogate Lagrangian Relaxation (SLR) to solve UC problems. Our new contributions include: 1) A Quantum-SLR (QSLR) algorithm incorporating quantum approximate optimization algorithm (QAOA) into the SLR method, which overcomes the fundamental difficulties of previous LR-based quantum methods such as zigzagging of multipliers and the need to know or estimate the optimal dual value for convergence; 2) A Distributed QSLR framework (D-QSLR) capable of coordinating local quantum/classical computing resources with those within neighborhoods and, in the meantime, protecting data privacy; 3) A Quantized UC model to obtain accurate commitment unit subproblems decision by using a quantum machine; and 4) A time-unit-decomposed quantum UC approach to overcoming the quantum resources’ limitations. Promising quantum test results validate the effectiveness of QSLR and the scalability of the UC-oriented D-QSLR algorithm, which demonstrate QSLR’s enormous potential in UC optimization.
Stochastic Unit Commitment (SUC) has been proposed to manage the uncertainties driven by renewable integration, but it leads to significant computational complexity. When accelerated by Benders Decomposition (BD), the master problem becomes binary integer programming, which is still NP-hard and computationally demanding for classical methods. Quantum Annealing (QA), known for efficiently solving Quadratic Unconstrained Binary Optimization (QUBO) problems, presents a potential solution. However, existing quantum algorithms rely on slack variables to handle linear binary inequality constraints, leading to increased qubit consumption and reduced computational efficiency. To solve the problem, this paper introduces the Powell-Hestenes-Rockafellar Augmented Lagrangian Multiplier (PHR-ALM) method to eliminate the need for slack variables, making qubit consumption independent of the increasing number of Benders cuts. To further reduce the qubit overhead, quantum ADMM is applied to break large-scale SUC into smaller blocks for sequential solutions, which does not scale with the number of generators. Finally, the simulation results on both 4-generator and the IEEE bus-118 systems demonstrate the feasibility and scalability of the proposed algorithm, indicating its superior qubit and runtime efficiency over classical and baseline quantum approaches on the D-Wave QPU platform.
Mixed-integer linear programs (MILPs) are fundamental to power system operations but are challenging to solve due to their NP-hard nature. Quantum annealing, which seeks to find the ground state of an Ising model–a minimization of a quadratic binary function–offers potential speedups for solving combinatorial optimization problems. However, converting MILPs into quadratic unconstrained binary optimization (QUBO) problems often requires lots of slack variables, leading to significant qubit overhead and limiting the capability of quantum annealers. In this work, we propose an efficient formulation approach that generates a QUBO representation of a targeted MILP without introducing auxiliary binary variables. Geometrically, our method approximates the value function of the MILP with a quadratic binary function leveraging a simple convex optimization subroutine. We prove the existence of the QUBO representation, which preserves the MILP’s optimal solution and objective value with the same number of binary variables. The proposed approach is validated on several instances of the security-constrained unit commitment (SCUC) problem, with test cases featuring up to 288 binary variables, demonstrating its efficacy in reducing qubit overhead for quantum-classical hybrid Benders decomposition frameworks.