[Paper Review] A Fully Controllable Power System - Concept for FACTS and HVDC Placement

Spyros Chatzivasileiadis, Göran Andersson|arXiv (Cornell University)|Jun 27, 2015

Power System Optimization and Stability7 references3 citations

TL;DR

This paper proposes a fully controllable power system using FACTS and HVDC controllers to decouple market dispatch from security constraints, eliminating redispatching costs. By introducing the controllability vector and two placement algorithms, it determines the minimum number of controllers needed—nearly nL−nB+1 for series placement—and shows that optimal placement enables full system control with zero 'Cost of Security' in ideal conditions.

ABSTRACT

This paper puts forward the vision of fully decoupling market operations from security considerations through controllable power flows. In "A Fully Controllable Power System", power system security is no longer dependent on the location of the power injection points. In the ideal case, this leads to the elimination of redispatching costs, which amount to several million dollars per year in large systems. This paper determines the upper and lower bounds for the number of controllable lines and number of controllers to achieve this decoupling in any system. It further introduces the notion of the controllability vector CV, which expresses the effect of any controller on the AC line flows. Based on two alternative definitions for controllability, two controller placement algorithms to maximize controllability are presented and their results are compared.

Motivation & Objective

• To decouple market operations from security constraints in power systems by enabling full control over AC line flows.
• To determine the theoretical minimum number of FACTS and HVDC controllers required to achieve full system controllability.
• To develop and compare two controller placement algorithms that maximize system controllability under limited controller availability.
• To introduce the controllability vector (CV) as a metric for quantifying controller impact on line flows.
• To extend the framework to TCSCs and PSTs, showing applicability beyond HVDC lines.

Proposed method

• The paper uses the DC power flow approximation and Power Transfer Distribution Factors (PTDFs) to model line flows as linear functions of bus injections and controller setpoints.
• It defines the controllability vector (CV) as the partial derivative of AC line flows with respect to a controller’s active power injection, quantifying its impact on all lines.
• Two controller placement algorithms are proposed: one based on solving a linear program (LP) to maximize total flow change, and another using a vector-based heuristic relying on CV magnitude and power angle (cosφ) constraints.
• The LP-based method optimizes for maximum absolute change in line flows under a given control action limit (ΔPc ≤ p), while the vector-based method uses a greedy selection of high-impact locations.
• The framework is extended to TCSCs and PSTs by modeling their impact as derivatives of line flows with respect to reactance (xij) and phase shift (δ), respectively.
• Theoretical bounds are derived: the maximum number of fully controllable lines is nL−nB+1 for series placement and nB−1 for parallel placement.

Experimental results

Research questions

• RQ1 What is the minimum number of FACTS and HVDC controllers required to make any power system fully controllable?
• RQ2 How can controller placement be optimized to maximize system-wide controllability under a limited number of controllers?
• RQ3 What is the impact of different controllability definitions—based on absolute flow change vs. vector magnitude—on controller placement outcomes?
• RQ4 How do loading patterns and generator costs affect the feasibility and cost-effectiveness of achieving a fully controllable system?
• RQ5 Can the proposed framework be extended to other flexible AC transmission system (FACTS) devices such as TCSCs and PSTs?

Key findings

• The theoretical upper bound on the number of fully controllable AC lines is nL−nB+1 when controllers are placed in series, and nB−1 when placed in parallel.
• The minimum number of controllers required to achieve full controllability is also nL−nB+1 for series placement and nB−1 for parallel placement.
• The vector-based controller placement algorithm achieves comparable results to the LP-based method with significantly reduced computation time, though it requires careful calibration of heuristics like the cosφ limit.
• Placing two HVDC lines in parallel results in no additional control capability, as it only adds one degree of freedom, leading to infeasible solutions.
• The controllability vector (CV) effectively quantifies the impact of each controller on all AC line flows, enabling systematic placement optimization.
• For TCSCs and PSTs, the CV framework can be adapted, but the sensitivity vectors depend on the system operating point, requiring multi-point evaluation for robust placement.