Medium Voltage Alternating Current System Grounding Circuit

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Directed Energy (DE) OBJECTIVE: Develop a compact system grounding circuit for Medium Voltage Alternating Current (MVAC) applications between 1 and 15 kV that can operate continuously with a ground fault, is air cooled, works with a ground fault having a direct current (dc) component, dissipates less than 3 kW of heat during a ground fault, and prevents high transient voltage spikes due to intermittent ground faults. DESCRIPTION: To enable continued operation in the case of a single ground fault, shipboard MVAC power systems typically are high resistance grounded using a grounding transformer and a High Resistance Grounding (HRG) resistor. The HRG circuit ensures line to ground transient voltages during a ground fault (particularly during intermittent ground faults) are not excessive and that under normal operation, the neutral to ground voltage is nearly zero. During a ground fault, the current through the HRG resistor is equal or marginally higher than the capacitive charging current of the system. This current can be on the order of 10 amps and the voltage would be the line to neutral voltage (The line-to-line voltage divided by the square root of three). In a 13.8 kV system, this can result in the HRG resistor dissipating roughly 80 kW. This amount of heat requires special accommodations for heat removal (water cooling for example) that complicate ship integration and drive costs. Alternately, the ground fault must be cleared within a few minutes before the space overheats, possibly impacting the safety of the ship. Details on HRG systems may be found in IEEE Std 45.1 (Recommended Practice for Electrical Installations or Shipboard Design) and IEEE Std 3003.1 (Recommended Practice for System Grounding of Industrial and Commercial Power Systems). Continuous operation with a ground fault is required to ensure vital electrical loads such as propulsion and combat systems do not lose power during critical time periods. The goal is to allow for locating and clearing the fault(s) in a non-critical time period. Since the length of time the ship will be in a critical time period is unknown, the HRG system should have the ability to operate continuously. Another issue is that if a rectifier load is directly attached to the MVAC system, a ground fault on the Direct Current (DC) side of the rectifier can result in a DC current in the grounding transformer; this DC current can result in saturation of the grounding transformer leading to the grounding transformer and resistor overheating and potential catastrophic failure. Research and Development along with Innovation are required to fulfill the functionality of an HRG circuit while dissipating less than 3 kW, enabling an air-cooled solution with a heat load that can be accommodated by the ship's ventilation system. In addition to the requirement to limit heat dissipation to 3 kW, the proposed solution should not require more than 2 square meters deck space (threshold) or 1 square meters deck space (objective). The proposed solution should be capable of accommodating 10 Amps (A) of ground current (threshold) or 20 A of ground current (objective). The power quality of the MVAC system should be assumed to be in accordance with MIL-STD-1399-300.2. The shipboard equipment should be shock qualified to Grade A (MIL-DTL-901) and constructed to operate in the shipboard environment (MIL-DTL-917). The solution may include power electronic devices with advanced controls to achieve the desired properties. Repair parts should be capable of passing through Navy standard hatches 26 x 66 and doors 30 x 60 . This MVAC System Grounding Circuit should be applicable to all shipboard MVAC systems up to 15 kV (both commercial and naval) and terrestrial industrial power systems where high reliability power is required (such as for process controls). PHASE I: Provide a concept design of an MVAC System Grounding Circuit fulfilling the desired functionality listed in the Description. The feasibility of the MVAC System Grounding Circuit shall be demonstrated through Computer Aided Design (CAD) models and circuit simulation. The Phase I Option, if exercised, should include the identification of key knowledge gaps and risks, initial design specifications, capabilities description, test plan, and test procedures for a prototype solution in Phase II. The prototype should address the key knowledge gaps and risks. A draft procurement specification for the production system should also be delivered. PHASE II: Update the initial design specifications (if necessary) developed during the Phase I Option and produce a prototype system for testing in accordance with the test plan and test procedures. Include, if required, component testing to close knowledge gaps prior to updating the design specifications. Conduct the tests and update the system design and draft procurement specification for the production system based on lessons learned from testing. Once all testing has been completed, deliver the prototype system and all design and test report documentation to the Government. PHASE III DUAL USE APPLICATIONS: Support the Navy in transitioning the MVAC System Grounding Circuit for Navy use. Any naval ship with a MVAC distribution system (including CVN-68 class, CVN-78 class, LHD-8, LHA-6 class, DDG-51 flight III, FFG-62, and DDG(X)) is a candidate for transitioning the technology. Any commercial ship with an MVAC distribution system (such as cruise ships, ice breakers, etc.) is also a candidate for technology transition. Any terrestrial process control industrial application (oil refining, plastic production, chemical manufacturing, etc.) also can benefit from this MVAC System Grounding Circuit. REFERENCES: 1. Bapat, A.; Hanna, R. and Panetta, S. Advanced concepts in high resistance grounding. 2015 61st IEEE Pulp and Paper Industry Conference (PCIC), Milwaukee, WI, USA, 2015, pp. 1-9. https://doi.org/10.1109/PPIC.2015.7165854 2. da Costa, L.A.; Mohammadi, Y.; Leborgne, R.C. and da Silva Gazzana, D. "Impact Evaluation of the Neutral-Grounding Resistance on Short-Duration RMS Voltage Variations." 2020 19th International Conference on Harmonics and Quality of Power (ICHQP), Dubai, United Arab Emirates, 2020, pp. 1-6. https://doi.org/10.1109/ICHQP46026.2020.9177891 KEYWORDS: High Resistance Grounding; Medium Voltage Alternating Current; Ground Fault; Grounding Resistor; Grounding Transformer; Power Continuity