How Much Autonomy? IPS Control System - A Case Study

Copyright «::. If AC Control Applications in Marine Systems, Glasgow, Scotland, UK , 2001

IFAC

C:OC> Publications www.elsevier.comllocate/ifac

HOW MUCH AUTONOMY? IPS CONTROL SYSTEM - A CASE STUDY Henry J. Hegner, Bipin B. Desai, Kenneth A. Lively

Henry Hegner: Naval Sea Systems Command, Washington. D.e. Bipin B. Desai: Gibbs and Cox. Ne . . . ' York. NY Kenneth A. Live~l ': Anteon Corporation. Annapolis. MD Abstract: The Integrated Power System (lPS) Program of the U. S. Navy has been conducting technology demonstrations of electric drive and dc zonal ship service power equipment. Integrated power system provides electric power to all electric loads; propulsion and ship service. Six fundamental functions were identified from the examination of the required functions of such an electric power system. These six functions comprise the basis of the IPS modules: power generation, power distribution, power conversion, energy storage, power loads (of which propulsion motor modules are a subset), and power management. Power management module is the core of the IPS control system and is entirely embedded in software. Of the many benefits sought by the IPS , reduced manning is a major one. In addition, all concepts of the IPS control system will have to accommodate the trend toward all electric re-configurable ship. This paper outlines the philosophy to fully integrate the IPS control system hardware and the power management software into the ship wide control network. The topic of the paper addresses how autonomy is investigated to achieve the top-level goals of reduced manning, all electric re-configurable ship and the fundamental performance of the IPS. The paper ends with some provocative issues along with the most current approach for power management under consideration. Copyright © 2001 IFAC Keywords: Automation, Autonomous Control, Control System Design, Intelligent Control, Power System Control, Ship Control I . INTRODUCTION

functionally specified and programmed without being hindered by the network specifics. Accordingly, the goal is to make the IPS modules capable of interacting with their environment and able to communicate with one another over a network. As an intelligent node , each module will be capable of handling its share of the overall control task for the IPS, thereby distributing the control functions throughout the ship. The power flow will be controlled and monitored at every module . Each module will detect and identify malfunctioning module( s) and reconfigure the system to continue to support as many loads as practical. These intelligent and sophisticated modules will offer the ship system designer easy implementation of distributed sense and control networks . They will provide flexible reconfiguration capability after both intended (equipment upgrades) and unintended (casualty) system changes. and will degrade gracefully upon failure of control system or controlled components or network paths. The IPS control system architecture with computing resources in each module is described in Desai. et at. ( 1999).

The U.S. Navy Integrated Power System (lPS) program is developing integrated electric power technology for US Navy ships. The system is based on an open. modular architecture using modules from multiple vendors. Although having multiple vendors will promote competition, it mandates a strong emphasis on interoperability between modules during design and testing phases of the development. Because of this modular nature and the control requirements for the electronic switching power supplies used in the IPS. computing resources are available throughout a ship within the IPS modules. Consideration is therefore given to process inputs from sensors and to coordinate outputs to actuators intelligently at the module level. Information is communicated across the ship ' s network only when required and serves to facilitate interoperability and human intervention. It is also contemplated to view the control and monitoring functions as networked control applications that are independent of the specific communication hardware and network implementations. This requires that each module/component not only act as an intelligent node in the networked architecture, but also be

The basic IPS architecture is shown in Figure I. The architecture allows multiple distribution levels. with

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power sources and loads possible at each level. One configuration being studied at present has power sources only at the top distribution level, which provides power at 4160 VAC, 3 phase, 60 Hz. Propulsion motors are supplied power from this level. Power conversion modules (PCMs) convert the 4160 V AC to 1000 VDC for distribution fore and aft on the ship. Two of these 1000 VDC "buses" are employed, one on the port side of the ship and one to starboard, providing redundant sources of power to each of several zones on the ship. At each zone, the 1000 VDC power is converted again to 800 VDC, which supplies large loads directly, or provides input power to the third and final power conversion module: 800 VDC to 450 VAC. Most existing ship service loads are supplied from the 450 VAC, although next generation ships are likely to have more DC loads, thus eliminating the individual AC to DC power supplies inherent in most electronic equipment. As technology advances, power sources will migrate downward in the architecture, and energy storage modules will be provided to accommodate temporary power needs.

insertion of emerging technologies. The critical issues for selection of the architecture are allocation of functions to satisfy the above, and capturing the commercial networking infrastructure. At the top of the list for functional allocation is control. Thus, the fundamental control philosophy is based on an autonomic concept wherein routine evolutions occur without either a supervisory control or operator intervention. This philosophy also supports reduced manning. However, in an emergency, rapid responsive actions, either from a supervisory station or from any other station, are possible. Operator interaction with the control system must be kept to a high level, with the embedded controllers relied on to carry out individual control actions. In order to achieve this level of autonomy, all control actions based on internal intelligence are embedded in the local module controllers. Computer-based technology provides facilities that will support more than mere integration of protective and speed oriented functional processes into automation. Many shipboard functions, including power management and control, which were traditionally performed manually can now be accomplished by the automation system using computer hardware and software. The control interfaces between the modules are based on the concept where the performance and behavior of the modules do not directly rely on the performance and behavior of other modules except for the input power

2. ARCHITECTURE The IPS control system architecture has to be suitable for its performance needs, ship's doctrinal needs, reduced manning requirements, and must facilitate 468

requirements. The control system architecture will permit the modules to be self-starting upon application of the input power. The control system will permit the system to be capable of autonomous operation without support from external supervisory function(s). At the same time, remote operator control and monitoring will be supported. All modules will interface via the ship's communications network. The data exchanged between modules will be based on messages passed between modules using the ship's communications network. Each module will have a controller which will consist of a control and monitoring function for that module (local controller), a Power Flow Manager (PFM) function, external interface functions, a data port capable of connecting to a portable computer, and, if necessary, a Human Machine Interface (HMI) function. During steady state operation for a given ship's operating mode, the system will operate autonomously, receiving power allocations from a Supervisory Control Function. In the event of a loss of input to the module, the module will be capable of sustaining the local control and communication via the ship's communications network for a sufficient length of time to permit fault detection, isolation, and system reconfiguration. The algorithms used to perform fault detection, isolation and reconfiguration will be simple and efficient to leave all but the faulted section energized. A system or main bus fault in an electrical zone will be detected, isolated, and, if appropriate, the distribution system reconfigured to restore power to the unaffected portion of the bus.

in each module performs these tasks. It receives inputs from the module's sensors to determine the module's health and present status. It also receives commands from its own Human Machine Interface (HMI), the Power Flow Manager (PFM) function and messages from a ship's control system via the external interface function. The internal control function acts on various inputs to maintain the module in the commanded state unless a state change is commanded or conditions within the module warrant a state change (i.e. internal failure) . The parameters for monitoring and the type of monitoring, such as analog/discrete, are selected to permit automatic reconfiguration, system health assessment, and on-line real time condition assessment. In order to implement condition based maintenance, an on-line assessment capability that can predict time and mode of failure is yet to be investigated. The primary functions related to the power system are: • • • •

On/Off and Open/Close controls of devices Power Flow Management System Stability Fault Detection, Isolation, and Reconfiguration

3.2 On/Off or Open/Close controls of devices On/Off or Open/Close controls will be accomplished within each module. Any occurrence that requires an action to protect the module is captured by the processor in the module which in turn generates appropriate commands to isolate that occurrence. If the operator wants to control a device, it can be done at the module's Human to Machine Interface (HMI) or via a message sent from a remote station. Sending or receiving messages over the network supports any On/Off or Open/Close operation required for reconfiguration.

3. APPROACH TO AUTONOMY One of the approaches is to embed all intra-related functions of a system/subsystem within the system/subsystem so that it instills as much autonomic response as possible. The individual system/subsystem dynamics are, therefore, reduced to the local and functional level as opposed to a ship wide or system level; and thus are controlled by the designers of the individual system/subsystem (e.g. power conversion) instead of by the designers of the ship wide networked control system. This approach allocates and separates information functionality to the ship's control system and power management functionality to the power system. For the IPS ship, the power system includes propulsion functionality in addition to power generation and distribution. Essentially, each piece of equipment/module in the power system contains a software object in order to leverage the computational capability. These software objects operate, control and maintain that equipment using system data and algorithms. Instead of relying on a master/slave paradigm, each software object will be designed to accomplish a peer-to-peer functionality. Of course, the power system will use and interface with the ship's networked control architecture but only to the extent of message transfer needs and to support integrated services philosophy.

3.3 Power Flow Management Operational Scenario Description (OSD)s will correlate the expected IPS load, the priorities of these loads, and the desired electrical plant configuration, with ship missions. Operational Scenarios combine expected notions of operational readiness, material readiness, and various ship special operating conditions into a single set of contexts under which the IPS is configured and controlled. Each Operational Scenario determines the nominal power requirement for the ship, including propulsion and ship service loads, establishes the relative priorities of all shipboard loads, and specifies an appropriate IPS configuration. Operational Scenario information can be entered at a Supervisory Control workstation and communicated to other IPS controllers as required via the shipboard communication network. Default information stored at each IPS local controller permits the controller to start up and operate even when communications are not present.

3.1 Primary Functions Each module is designed to be self-regulating and self-protecting. The control and monitoring function 469

controlling the distribution or flow of power from sources to loads. A detailed discussion of PFM logic was presented by Lively, et al. (2000), in which power was allocated and monitored at each level of the power distribution hierarchy. A simplified version ofPFM logic is shown in Figure 2, wherein power allocations are only made at the first level of the power flow hierarchy. A specific operational scenario identifies the maximum expected power requirement for both propUlsion and ship service loads. The maximum desired ship speed equates to a power allocation that must be reserved for propulsion (ALP(Prop)). Whenever a new operational scenario is placed in effect, a Supervisory Control station will determine the power required for the expected

The IPS control philosophy also considers power to be a resource that is generated by power sources and consumed by various loads. Resource Management is performed within the context of the ship's current Operational Scenario, and within any constraints presented by system faults, configuration, or equipment malfunction. This function is performed by the module's Power Flow Manager (PFM), and is essentially the task of determining how much power must be provided in various scenarios to meet the power demands of the propUlsion and ship service loads, and how to respond to varying conditions of loading. PFM is a distributed function, in that all IPS modules having controllers are responsible for

Calculate desired AVPD = ALP(Prop) + L(On-Line PCM-4 Ratings)

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Figure 2 - PFM Flow Chart 470

propulsion and ship service loads. This power level is referred to as the Desired Available Power (A VPD). A Supervisory Control station will then determine the combination of generating capacity and plant configuration that meets or exceeds that power requirement, and then take action to bring on-line the required generators and achieve the appropriate plant configuration. On-line generating capacity is referred to as the ship's Available Power (AVP(Ship)). Problems with generator availability, distribution system faults, equipment malfunction, or other considerations may make it impossible to bring online sufficient power to meet the Desired Available Power level associated with an Operational Scenario. In this case, disconnectable ship service loads or increments of propulsion power are shed as shown in Figure 2.

from impacting power quality in adjacent zones. The fault will be removed either through shutdown of a module or the opening of a circuit disconnect. Load shedding may occur during this phase and IPS modules may be placed in allowable overload status. Localization Phase: Module controllers evaluate event parameter values to determine the location of the fault. Isolation Phase: Module controllers open disconnects to isolate the faulted portion of the distribution system or the faulted module. Reconfiguration Phase: Module controllers reconfigure the plant to reroute and restore power to unaffected portions of the system and regain power to loads. Reconfiguration will regain system redundancy where possible, remove overload conditions, and restore power availability to loads shed due to the fault.

3.4 System Stability The IPS architecture leverages the advances made in power electronics technology. The modules containing such power electronics can present differential negative impedance which could result in power system instability if proper care is not taken in the design of the interfaces. The philosophy that is used to ensure stability does not rely on the external control system. An effort is being made to define boundaries on the input and output impedances of the interfacing modules which will prevent any instability.

Module controllers will communicate with higher level controllers when required for system level coordination, and for diagnosis of the source of the fault. In order to detect high-impedance faults, module controllers will report input and output current values to a higher level controller on a periodic basis. The higher level controller will compare input and output current values to determine the presence of a fault. If a fault is detected, the higher level controller will take the appropriate action to isolate the fault. Individual module controllers will continue to provide self-protection from faults in the absence of communications.

3.5 Fault Detection, Isolation, and Reconjiguration The IPS electrical fault protection control functionality is distributed among module controllers and automatically detects and isolates faults. The system will provide automatic reconfiguration of the plant after a fault is isolated to regain redundant power feeds to each zone if possible. Electrical faults are considered to be of one of the following categories: high impedance, low impedance (bolted), and internal module faults . High impedance faults present some difficulties, however, in that high impedance faults in DC distribution circuits may only be momentary, quickly becoming low impedance faults, whereas high impedance faults in AC distribution circuits may be difficult to differentiate from normal load impedances.

A very high degree of autonomy is achievable. The fault detection, isolation and re-configuration process uses the external control system for its operation. However, an effort is on-going to develop an algorithm that may not need support from an external control system.

The occurrence of a fault will trigger a five step process as follows :

Lively, K., McCoy, T., Thompson, T., Zivi, E. (2000). Advanced Control Concepts for an Integrated Power System (IPS) Warship, Fifth International Naval Engineering Conference.

4. CONCLUSION

REFERENCES

Event Phase: A fault occurs somewhere in the distribution system; this could be a low/high impedance fault in an IPS module, a fault in the distribution network or a module internal fault

Desai, B., Hegner, H. , McCoy, T., Robey, H. (1999). Distributed Intelligent Sense and Control System for Integrated Power System, Twelfth Ship Control System Symposium.

Reaction Phase: A number of autonomous or reflexive actions take place immediately to maintain continuity of power and to prevent faults in one zone

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