Christian Conficoni

MS3: A Mediterranean-stile job scheduler for supercomputers - do less when it's too hot!

Supercomputers machines, HPC systems in general, embed sophisticated and advanced cooling circuits to remove heat and ensuring the required peak performance. Unfortunately removing heat, by means of cold water or air, costs additional power which decreases the overall supercomputer energy efficiency. Free-cooling uses ambient air instead than chiller to cool down warm air or liquid temperature. The amount of heat which can be removed for-free depends on ambient conditions such as temperature and humidity. Power capping can be used to reduce the supercomputer power dissipation to maximize the cooling efficiency. In this paper we present a power capping approach based on Constraint Programming which enables to estimate at every scheduling interval the power consumption of a given job schedule and to select among all possible job schedules the one which maximizes the supercomputer efficiency.

Energy-aware cooling for hot-water cooled supercomputers

Hot-water liquid cooling is a key technology in future green supercomputers as it maximizes the cooling efficiency and energy reuse. However the cooling system still is responsible for a significant percentage of modern HPC power consumption. Standard design of liquid-cooling control relies on rules based on worst-case scenarios, or on CFD simulation of portion of the entire system, which cannot account for all the real supercomputer working conditions (workload and ambient temperature). In this work we first introduce an analytical model, based on lumped parameters, which can effectively describe the cooling components and dynamics, and can be used for analysis and control purposes. We then use it to design an energy-optimal control strategy which is capable to minimize the pump and chiller power consumption while, meeting the supercomputer cooling requirements. We validate the method with simulation tests, taking data from a real HPC cooling mechanism, and comparing the results with state-of-the-art commercial cooling system control strategies.

A synchronous coordinates approach in position and speed estimation for Permanent Magnet Synchronous Machines

A novel and simple observer scheme is proposed for rotor speed and position reconstruction for Permanent Magnet Synchronous Machines. The reference frame adopted in the observer is pushed toward the synchronous one by forcing it to be intrinsically aligned with the estimated back-emf vector and by designing suitable adaptations law for its speed and angle along with the back-emf amplitude. Stator flux dynamics are not used in this approach, leading to an improved robustness with respect to voltage and current measurement uncertainties. Stability analysis is carried out by using singular perturbation approach. Effective tuning rules are drawn exploiting insightful linearization of the proposed nonlinear adaptive observer. Simulations show the properties of the presented method.

Adaptive Observer for Three Phase Source Voltage Under Unbalanced Conditions

Abstract In this paper the problem of estimating amplitude, phase and frequency of the main component of three-phase line voltage under unbalanced conditions is considered. Unbalancing is a common condition and generates a so-called negative sequence in the line voltage, which may affect relevantly estimation of main components dynamics. Different adaptive schemes are proposed and analyzed. First of all, a straightforward solution derived by LTI observer with frequency adaptation is analyzed and its high sensitivity to negative sequence component is underlined. Hence, an adaptive nonlinear observer is proposed, exploiting the properties of a synchronous reference frame to represent the line voltage. Its larger robustness to negative sequence and voltage harmonics is verified by analysis and simulations. Finally, the latter solution is enhanced by adding the negative sequence model to the basic observer structure. This allows for perfect compensation if the unbalancing effects. Adaptation law for this solution is fairly different form the previous one and a completely different stability analysis is required. Simulation results are also provided to demonstrate the effectiveness of the proposed scheme even under large unbalancing.

Globally asymptotically stable reconstruction of flux, rotor position and speed for permanent magnets machines

An original full-order observer is presented for stator fluxes and speed reconstruction for Permanent Magnet Synchronous Machines. The estimation is carried out in a fixed reference frame exploiting the stator currents and voltages as the only known variables. Global asymptotic convergence properties of the proposed scheme are drawn, casting the estimation problem into adaptive systems theoretical framework. Insightful modification to the flux estimation dynamics is proposed in order to improve the observer performance in the low speed region. Simulation tests asses the features of the presented method.

Induction motor sensorless observer aligned with rotor flux derivative

A novel observer is proposed to reconstruct rotor speed and rotor flux vector in induction motors. The presented solution is characterized by a simple structure, based on a rotating reference frame which is pushed toward perfect alignment with the time-derivative of the rotor flux vector with respect to the stator windings reference frame. This approach allows to derive rotor speed and flux estimations without relying on pure integration of the stator flux dynamics and without using typical adaptive schemes which exploit the product between flux components and speed in the back-EMF of the stator current dynamics. Therefore, very good robustness is achieved with respect to voltage and current measurement uncertainties. Stability analysis is carried out by singular perturbation arguments, guaranteeing semiglobal practical asymptotic convergence. Effective tuning rules are derived considering a meaningful linearization of the proposed nonlinear observer. Simulations are presented to show the effectiveness of the solution.

Control of Shunt Active Filters With Actuation and Current Limits

In this brief, a plug-in unit is presented to manage control voltage saturation and maximum current limit in shunt active filters (SAFs), where unconstrained control algorithms are already defined. The proposed unit extends the operating region of such devices, i.e., under large transients and overload conditions, with performance guarantees. Therefore, improved robustness, availability, and composability of SAFs are obtained. The solution is composed of two parts. An antiwindup (AW) unit is defined to deal with control input saturation by modifying the current references through a suitably designed additional dynamics. In addition, a current saturation strategy is formulated. Again the current reference is modified, accounting for the limitations of the system, augmented with the AW scheme. The approach is valid for any kind of unconstrained controller adopted to steer SAFs. Here, results are presented considering an internal-model-based current controller. Simulation and experimental tests confirm the effectiveness of the method.

Modeling and control design for power systems driven by battery/supercapacitor hybrid energy storage devices

This paper addresses several important problems in a typical power system driven by battery/supercapacitor hybrid energy storage devices. The currents in the battery and the supercapacitor are actively controlled by two bidirectional buck-boost converters. Detailed investigation is conducted on deriving two state-space averaged models for the whole interconnected system. The control objective is to track the references of two variables. The control design problem is converted into a numerically efficient optimization problem with linear matrix inequality (LMI) constraints. When the optimization algorithm is applied to an experimental system, the resulting optimal control law is very close to a simple integrator control (with two inputs and two outputs). The simple integrator control is applied to the system in both simulation and experiment. The results confirm the effectiveness of the modeling and control design methods.

Anti-windup scheme for current control of Shunt Active Filters

An anti-windup technique for current control of Shunt Active Filters is presented. The main purpose of the proposed solution is to preserve the unconstrained tracking error dynamics, even under saturated conditions. A suitable modification of current references is imposed through the design of an additional dynamics. The proposed approach is valid for any kind of controller adopted to steer SAF current; in this work it has been applied on an internal model based controller. A variant of the basic approach is proposed to improve performances. Simulation tests confirm the effectiveness of the presented solution.

Integrated Energy-Aware Management of Supercomputer Hybrid Cooling Systems

Advanced cooling systems and optimization strategies are critical to operate modern supercomputers and high-performance computing systems in an energy-efficient fashion. Hybrid architectures combining emerging liquid cooling with traditional air cooling are a promising solution. Standard management techniques maintain these systems at fixed operating points, typically without coordination between the diverse cooling knobs. In this paper, we propose an energy-aware optimization strategy exploiting heterogeneous cooling systems in a holistic fashion with the goal of minimizing the overall cooling system power consumption, while at the same time meeting the system thermal constraints. To this purpose, we developed a modeling approach to build a low-order analytical model, which captures the overall thermal behavior of the system. Then, this compact and computationally manageable model is exploited to set and solve a treatable optimization problem, leading to definition of an energy-optimal cooling strategy. The proposed method is presented taking Galileo as real-life case study. Galileo is a high-performance computing system with hybrid cooling architecture recently installed at CINECA (a supercomputing facility located in Italy). The cooling strategy resulting from the proposed approach is compared with common strategies in order to assess the efficiency advantages.