Secretary of the Navy Richard Danzig announced to the press on 6 January 2000 that the Navy's newest surface combatant, DD-21, will be built with an integrated power system. Here's why:
Today's warships, just as those first wired for electricity almost 100 years ago, are constructed with segregated propulsion and electric power systems. These systems generally are reliable and work well under normal circumstances; however, they suffer from serious drawbacks, the most notable of which is that existing electrical-distribution systems are unable to maintain power continuity when and immediately after the ship suffers a hit. This weakness has been identified in published reports describing recent battle-damage incidents, including the Exocet hits on the Stark (FFG-31) and the Samuel B. Roberts (FFG-58) and Princeton (CG-59) mine strikes.
Many short-term, Band-Aid approaches to correcting the shortcomings of current AC electrical-distribution systems have been implemented. Solid-state automatic bus transfer switches (ABTs), for example, reduce dramatically the transfer time from normal to alternate power sources. Unfortunately, they remain far too slow for the increasingly sensitive computers and electronics equipment being installed in fleet units—both surface and submersible. Solid-state ABTs also are relatively expensive and require embedded control computers to function properly. In part because of the need for continuous power supply, many combat-system components now are being fitted with uninterruptible power supplies (UPSs) to maintain a ride-through capability during power system transients. The many batteries comprising such UPSs, however, are located in various spaces throughout the ship, creating both a maintenance problem and a hazardous waste issue.
In addition, combat ships—including today's cruisers, destroyers, and carriers—do not have enough installed electric power generation capacity to handle mid-life upgrades to combat-system equipment. Maintaining a clean electric power supply to the sensitive electronics in modern combat- system equipment is a continual problem in today's fleet and is likely to get worse in the 21st century.
Since the introduction of electricity on board, warships have experienced a steady and increasingly significant growth in the demand for electric power (see Figure 1). Each new generation of combat-system equipment requires more electric power than its predecessors. In the C4I arena, today's focus on network-centric warfare and the migration to commercial off-the-shelf (COTS) computers require the production of both larger amounts and higher quality electricity from the ship's power system. To achieve these goals cost-effectively, the Navy is committed to developing an integrated power system capable of meeting the needs of a warship—propulsion, lighting, communications, combat systems, and so on. Key questions to be resolved include:
- How can we best achieve and demonstrate the advantages of the integrated power system (i.e., which technologies represent the best trade-off between performance, cost, and risk)?
- How can COTS systems and developments be leveraged to stretch design, development, and procurement budgets without sacrificing mission-critical requirements?
Looking ahead, the electric power needs of a warship will increase significantly as electromagnetic weapons become a reality? Although directed-energy weapons still are many years from fleet introduction, other weapons may be introduced in the near term that demand orders of magnitude more electric power than is required today. Electromagnetic rail guns, which fire hypersonic projectiles, are in development, and prototypes of the electro-thermal gun have been demonstrated in the laboratory. Also, the focus on littoral warfare has revitalized interest in active sonar for both antisubmarine and anti-mine warfare. Theater ballistic missile defense efforts may result in radar power consumption four to five times greater than present levels. The Navy also is pursuing electromagnetic launch and recovery of aircraft for future carriers. All of these possibilities will require much more installed electric generation capacity than exists in today's ships and will make a widely applicable, cost-effective solution an imperative in future surface ship design, development, and construction. Hence, the interest in an integrated power system ship.
Integrated Power Systems
Integrated power systems will be more efficient and will reduce noise signatures and improve the performance of fleet units. An integrated electric propulsion system will solve the generating capacity problem for any weapon system now envisioned. For example, an Arleigh Burke (DDG-51)-class destroyer has about 79 megawatts (MW) installed power dedicated to propulsion and 7.5 MW installed power dedicated to generating electricity. An integrated electric ship, built to meet the same mission requirements, would require only 66 MW installed capacity for all purposes and have a higher top speed (see Figure 2). Many other advantages to an integrated electric approach have been documented in various studies over the years and are summarized in Table 1.
Some of the benefits listed in Table 1 already are being enjoyed by ships in the commercial sector, where integrated electric propulsion systems have become commonplace, completely taking over some market segments. The latest trend in this arena is toward "podded propulsors" that integrate electric motors and propellers in a faired pod beneath the ship. Azimuthing versions, conceptually similar to the auxiliary power units installed on the FFG-7 class, eliminate the need for rudders and steering gear and improve a ship's stopping distance and turning radius dramatically (Figure 3). Such systems are becoming common in the cruise ship industry, and at least three companies are marketing them commercially in ratings up to 20 MW.
The Navy has been researching electric propulsion and integrated electric power systems for more than 30 years. The Integrated Power System Program Office (PMS-510) is charged with developing integrated electric propulsion for the Navy.
Technology finally has progressed to the point where the Navy will introduce an integrated electric warship to the fleet before the end of this decade. Unlike previous developments, which had concentrated on high-performance, full-military-specification components, the current post-Cold War effort is focused on affordability, using commercially derived technology and modifying designs to the minimum extent necessary. The commercial market also demands extremely rugged, highly reliable propulsion and generation equipment with 30-50-year lifetimes, so such a modified-COTS approach is a reasonable way to acquire power systems without incurring the huge cost of maintaining a unique military infrastructure. In addition, the mission and equipment performance requirements for various fleet units can be very different, and common development may not be cost-effective for every type of ship or ship system. Thus, the modified COTS acquisition strategy is attractive and flexible as well as being consistent with DoD Directive 5000.1 and DoD's various acquisition reform initiatives.
The Navy's integrated power system program recently completed a full-scale, shore-based proof-of-concept demonstration at Naval Surface Weapons Center in Philadelphia. The test site, depicted in Figure 4, consists of an LM2500 generator set, an Allison 501 generator set, a set of commercial 4 160- volt switchgear, a 25,000-horsepower induction motor and Pulse Width Modulated (PWM) motor drive, and 1-1/2 zones of ship service power distribution equipment. All critical components were designed and built to military requirements for shock, vibration, and electromagnetic compatibility.
Commercial equipment was adapted to the military environment by making limited design changes and by using shock mounts rather than hard mounting. As a cost-saving measure, however, some low-risk components were not constructed to their shock designs in the development program. Vibration requirements also affected the commercial designs. Generally, military requirements were met at a cost premium of about 10% over the commercial price—significantly less than the 10-20 times cost penalty typical for full-military-specifications components. Contrary to what some detractors have claimed, test results indicate that this type of propulsion equipment can be built to meet the stringent military requirements being placed on our new surface warship designs.
Many organizations, within both the Navy and industry, have claimed that a common propulsion motor development for submarines and surface ships would be more cost-effective than separate developments. At first glance, this seems logical, but when total fleet life-cycle costs are considered, a more than $1 billion savings might be possible by allowing the surface fleet to use the modified-COTS motors rather than the military-unique designs required by submarines. That is not to say that other technologies or components should not be developed in concert between the two communities. Solid-state power converters and motor drives may exhibit significant commonality between these applications. But it is not cost-effective for the surface ship community to be required to use propulsion motors designed to meet submarine requirements.
Integrated Fight-Through Power
To solve the fleet's power continuity problem, the Navy's integrated power systems program has been developing a concept known as integrated fight-through power. The core of this concept is a DC zonal distribution system to which all vital sensitive loads would be connected. This is a radical departure from the conventional AC distribution system used on today's ships (see Figures 5 and 6). By distributing DC power, you can eliminate automatic bus transfer switches and circuit breakers. Transfer from normal to alternate sources is accomplished by "auctioneering" diodes in a DC system. Diodes are simple, cheap, and require no computerized control. They function as electrical "check valves," allowing current flow from whichever source offers the highest voltage. The conceptual design illustrated in Figure 6 also features solid-state power converters that electrically isolate each zone, preventing the propagation of electrical faults throughout the ship.
To fully appreciate this design, let's look at what happens today when a ship takes a hit. When an AC distribution system is battle damaged (Figure 5), numerous arcing ground faults are created from the weapon's effects on live power cables. Because today's protection systems are designed for a single bolted fault, not multiple arcing faults, up to 0.3 seconds passes before the circuit breakers trip. During this time the voltage sags and high-frequency harmonics are transmitted throughout the ship through the copper cables and bussbars that connect to the loads. Sometimes current division from multiple faults causes a complete failure of the circuit protection, resulting in class C fires and power disruption to all loads. These power disturbances trip sensitive electronics equipment such as command and decision systems and fire-control computers, taking them off line. The time required to reboot and reload software effectively puts the ship out of the battle until her combat systems can be brought back on line.
With the DC system depicted in Figure 6, this cannot happen. Because it uses solid-state power converters (shown as PCMs) to electrically isolate each zone, a fault (bolted or arcing) in any zone will cause the power converter feeding that zone to shut down. Sensitive loads in other zones will never see any disturbances of their input power. Equipment within the damaged zone might lose input power, depending on the extent of damage, but as long as a single generator remains on line, equipment in all other electrical zones will be unaffected. Sufficient energy storage capability can be included in the ship's design to support limited operations without any generators until one can be brought back on line.
The highly reliable solid-state electronics used to manage power flow are easily controlled by the ship's mission-control computers, enabling automatic system reconfiguration and isolation of individual areas after battle damage or even before damage occurs, based on the threat being presented to the ship.
Combat Systems Loads
Another feature of the DC distribution system is the ability to tailor the power interfaces to combat systems power usage. Today, all combat systems are powered by either 60 or 400 Hz AC because that has been the standard interface. The first thing many users do with that power, however, is rectify it to DC for use within their systems. Radar and sonar are typical examples of loads that use large amounts of DC power. The developers of these systems spend significant amounts of money placing EMI filters on their input rectifiers (which add weight and volume) so they do not pollute with harmonics the AC bus to which they are connected.
With a DC distribution system, the power interface to combat systems can be tailored to whatever is optimum for that particular system. This is easily accomplished by changing the output voltage set-point in the control software for the solid-state power converters. The combat system developer could eliminate the input rectifier, EMI filter, and uninterruptible power supply batteries—creating measurable savings and performance improvements for the combat system. The DC distribution system would have less AC power to create, making the power system less expensive, smaller, and lighter. Ultimately, the ship becomes less complex, more reliable, easier to maintain, and less dependent on a large crew to operate.
The main argument against a DC distribution system is the size and cost of the many power converters that replace the traditional transformers, ABTs, and circuit breakers of an AC system. When only the electrical systems are considered, DC systems appear to be more costly. If one considers the affect on the combat system, auxiliary systems, and the ship as a whole, however, this higher cost could be more than offset.
Toward an All-Electric Fleet
Integrated power systems provide the flexibility to direct electric power to propulsion or ship's service use as required. This will allow the commanding officer—not a design engineer—to decide where the power is needed in real time when faced with a need. Because the installed generating capacity will be large enough to handle the propulsion load, any future combat systems upgrades requiring additional electric power can be accommodated without major modifications to the electric plant of an IPS ship.
The integrated fight-through power concept will prevent ship-wide power outages, which are common today when battle damage occurs, and will enable our ships to stay in the fight. DC zonal distribution will allow ships to tailor power interfaces to usage loads, with potentially tremendous cost savings.
The modified commercial approach to procurement makes maximum use of scarce Navy financial resources, investing in military-unique development only when COTS products are insufficient. This will allow the Navy to take advantage of technology developments in the rapidly advancing commercial marine industry without incurring the overhead of a separate military industrial base. In those areas where commercial technology cannot meet the Navy's needs, such as solid-state power converters and motor drives, there is no choice but to develop new technology. Even those costs can be minimized, however, if development is undertaken with an eye toward commercialization.
The Navy's commitments are not diminishing, but its budgets are. The acquisition community must find more cost-effective ways to equip, maintain, operate, and crew the 21st-century fleet. The integrated power system program is one way to meet these challenges while improving military effectiveness. The choice of how to power the next generation of Navy ships is obvious.
Commander (Select) McCoy, an engineering duty officer, is Deputy Program Manager and Deputy Technical Director for the IPS program office. He has nine years experience in the electric propulsion field and holds a doctorate in design of electric machines, as well as degrees in mechanical and electrical engineering and naval architecture.