Arthur Birchenough

Glenn Research Center, Cleveland, Ohio

David Hervol

Analex Corporation, Cleveland, Ohio

Test Results From a Simulated High-Voltage Lunar 

Power Transmission Line

NASA/TM—2008-215164

January 2008
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Arthur Birchenough

Glenn Research Center, Cleveland, Ohio

David Hervol

Analex Corporation, Cleveland, Ohio

Test Results From a Simulated High-Voltage Lunar 

Power Transmission Line

NASA/TM—2008-215164

January 2008

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Prepared for the

Space Technology and Applications International Forum (STAIF –2008)

sponsored by the Institute for Space and Nuclear Power Studies at the University of New Mexico

Albuquerque, New Mexico, February 10–14, 2008
Acknowledgments

The work in this paper was performed for NASA Exploration Systems Mission Directorate and the Prometheus Power and 

Propulsion Program. The authors would like to thank Vicki Crable, Don Fong, Marcelo Gonzalez, Larry Trase, Mike Chornak, 

Kevin Blake, Dave Hausser, and Tom Balogas for their assistance with testing.

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Level of Review: This material has been technically reviewed by technical management. 

This report is a preprint of a paper intended for presentation at a conference. 

Because changes may be made before formal publication, this preprint is made available 

with the understanding that it will not be cited or reproduced without 

the permission of the author.
NASA/TM—2008-215164 1 

Test Results From a Simulated High-Voltage Lunar Power 

Transmission Line 

Arthur Birchenough 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

David Hervol 

Analex Corporation 

Cleveland, Ohio 44135 

Abstract 

The Alternator Test Unit (ATU) in the Lunar Power System Facility (LPSF) located at the NASA 

Glenn Research Center (GRC) in Cleveland, Ohio was modified to simulate high-voltage transmission 

capability. The testbed simulated a 1 km transmission cable length from the ATU to the LPSF using 

resistors and inductors installed between the distribution transformers. Power factor correction circuitry 

was used to compensate for the reactance of the distribution system to improve the overall power factor. 

This test demonstrated that a permanent magnet alternator can successfully provide high-frequency ac 

power to a lunar facility located at a distance. 

Introduction 

A higher power electrical testbed was desired in order to evaluate the electrical control issues and 

system performance characteristics at representative conditions for Fission Surface Power Systems 

(FSPS) with multiple independently switched user loads. To meet these needs, the Lunar Power System 

Facility (LPSF) was developed and built. The LPSF contains an Alternator Test Unit (ATU), a Main 

Power Distribution Unit (MPDU), user loads, and an alternator controller using a Parasitic Load Resistor 

(PLR). In this form the LPSF was used to determine the alternator electrical performance, the 

effectiveness of the parasitic loading based speed and voltage controller, the power quality delivered to 

the loads, and system responses to transient loading conditions (Birchenough and Hervol, 2007). A 

fundamental requirement for unshielded or lightly shielded surface power is the ability to locate the 

fission power source at a distance from habitable areas. Transmission cables become a major source of 

weight and high-voltage transmission lines are a way of dramatically reducing cable mass (Mason, 2006). 

As a result, the LPSF was modified to include a high-voltage transmission simulator. The goal of this test 

was to demonstrate that the ATU could successfully deliver power to the end user loads with the addition 

of a high power transmission line simulator. 

Nomenclature 

μH microhenrys, unit of electrical inductance 

μF microfarad, unit of electrical capacitance 

Hz Hertz, cycles per second 

krpm thousand revolutions per minute 

emf electromotive force 
NASA/TM—2008-215164 2 

Alternator 

Test Unit 

Parasitic Load 

Resistor (PLR) 

Parasitic Load 

Controller 

Main Power Distribution Unit (MPDU) 

Remote Power Controller 1 25 kWe User Load 

Facility 

Controller 

ATU 

Controller 

Remote Power Controller 7 4.5 kWe User Load

Remote Power Controller 2 18 kWe User Load

Remote Power Controller 3 18 kWe User Load

Remote Power Controller 4 12 kWe User Load

Remote Power Controller 5 4.5 kWe User Load

Remote Power Controller 6 4.5 kWe User Load

Remote Power Controller 8 4.5 kWe User Load

High Voltage 

Transmission 

Line Simulator 

Figure 1.—Overall schematic of the Lunar Power System Facility (LPSF). 

LPSF Description 

The main elements of the LPSF are a 50 kWe ATU, an alternator controller using a Parasitic Load 

Resistor (PLR), a Main Power Distribution Unit (MPDU), eight user loads and a high-voltage 

transmission line simulator. These elements are made of breadboard/brassboard components. The LPSF 

configuration was developed using readily available off-the-shelf components and uses technologies that 

are applicable to FSPS. A block diagram of the test facility is shown in figure 1. 

Alternator Test Unit (ATU) 

The ATU is the power supply for the LPSF. It is representative of a style of alternator that would be 

used on a Brayton dynamic power conversion system. A variable speed 2-pole samarium cobalt 

permanent magnet brushless motor drives the alternator in place of a Brayton cycle power system. The 

alternator is a 6-pole permanent magnet alternator utilizing a Halbach array of samarium cobalt magnets. 

The stator has slots and is fabricated from hiperco 50 laminations that are skewed 1⁄2 slot to minimize 

cogging and rotor can losses. The stator windings are paralleled small gauge copper strands to minimize 

high-frequency losses. A dedicated control panel allows the motor to be operated in either speed or torque 

control. Speed control operation senses rotor speed and corrects any deviation by adjusting motor supply 

current. In torque mode a constant current is commanded for the motor, resulting in nearly constant torque 

output independent of the motor speed. A pressurized oil loop is included in the ATU assembly. Oil jets 

are used to lubricate and cool the individual shaft ball bearings and to cool the end turns of the motor and 

alternator. Additionally, oil passages are used to cool the rotor and stator. This waste heat is transferred 

through a commercial oil-to-water heat exchanger which is connected to the test facility water supply. 

The electronic motor drive controls are located in a separate cabinet on a cold plate which is also 

connected to the facility water supply. There is an emergency over speed shutdown circuit located within 

the ATU cabinet that trips if the shaft speed exceeds 40,000 rpm. ATU vibration health monitoring is 

performed with two sets of 3-axis accelerometers mounted on the alternator and motor housing exteriors. 

The ATU will automatically shutdown should any of the accelerometer readings exceed a predetermined 

g-level that would indicate an abnormal condition. The drive motor magnet health is monitored by 

periodically measuring back emf at shutdown and comparing it to beginning of life test values. The ATU 
NASA/TM—2008-215164 3 

is shown as installed in the LPSF at GRC in figure 2. The oil-to-water heat exchanger, facility water 

pressure gauge, and oil pressure gauge are visible through the motor/alternator housing window. 

High-Voltage Transmission Line Simulation 

A 1 km length of high-voltage transmission cable made up of AWG-14 wire with 1/8 in. line to line 

spacing was simulated by using a matching pair of three phase 12:1 ratio step-up/step-down transformers, 

with 4 Ω resistors and 500 μH inductors representing the resistive and inductive line losses. The highvoltage transmission line simulator is shown in figure 3. 

Motor Drive Electronics

Motor/Alternator

Figure 2.—View of the ATU as installed in the LPSF. 

12:1 High Voltage 

Transformers

Transmission Line 

Simulated Resistance Transmission Line 

Simulated Inductance

Figure 3.—High-voltage transmission line simulator. 
NASA/TM—2008-215164 4 

ATU 

Power Factor 

Correction 

Circuitry 

Transmission line Simulator 

12:1 Ratio Transformers 

100’ Cable to 

Testbed 

Testbed Loads 

 

Figure 4.—Relative placement of power correction circuitry. 

User Load

MPDU Alternator Controller PLR

Figure 5.—MPDU, Alternator Controller, PLR, and User loads as installed in the LPSF. 

The relative placement of the power factor correction circuitry is shown as a simplified schematic in 

figure 4. 

MPDU, Alternator Controller, PLR, and User Loads 

All of these components have been described in greater detail (Birchenough, Hervol, 2007). The 

MPDU contains the mechanical relays or contactors that distribute power from the ATU to the user loads 

and to the parasitic load resistor. The MPDU was sized to handle 50 kW at 400VAC line to line rms, 

3 phase, 1750 Hz (corresponding to 35,000 rpm ATU shaft speed). 

The Alternator Controller and PLR maintain a nearly constant bus voltage and ATU shaft speed 

regardless of user loading. This is accomplished by applying a parasitic load to the alternator output to 

maintain total load as required for the desired output voltage and/or speed. This type of parasitic loading 

was also used very successfully on a 2 kW Brayton cycle power system (Hervol, Mason and 

Birchenough, 2003 and Birchenough, 2006). The User Loads are downstream of the high-voltage 

transmission simulator, and are connected directly to the main distribution bus with a 400VAC line-toline 3 Phase, 1750 Hz feed. In order to process the ac power, each load implements a 3 Phase 

Transformer-Rectifier that changes the ac voltage level, rectifies, and then filters the ac into the required 

dc power. The MPDU, Alternator Controller, PLR, and User loads are shown in figure 5. 
NASA/TM—2008-215164 5 

Results and Discussion

To determine baseline system performance values, the ATU was run without the high-voltage 

transmission line simulator using the PLR to provide the electrical loading. The high-voltage transmission 

line simulator was then installed and the system was retested in the same manner. Based on preliminary 

calculations, implementing high-voltage transmission on a Brayton system high speed alternator was 

expected to be technically simple and straightforward, because the added losses of the transmission line 

were expected to be very low. The transmission line wire resistance was sized for a 1 percent power loss, 

and the inductive reactance was calculated to be ~1 percent per unit impedance. These values would have 

resulted in a 1 percent reduction in power available to the loads if the source was still limited to 50 kW, 

but the impedance would be low enough that only a 1 to 2 percent increase in alternator speed would be 

required to boost the source voltage enough to overcome the voltage drop in the high-voltage line. 

However, it turned out that the losses in the step up and step down transformers were far more significant 

in determining how much power we could transmit. First, the magnetizing (no load) losses in the 

transformers were in the order of 1.2 kW, greater than the expected full load transmission line losses. This 

was not unexpected, but the leakage inductance of the transformers, ~35 μH per transformer (referred to 

the primary), was 10 times the transmission line impedance, and the two transformer leakages are equal to 

60 percent of the alternator inductance. This substantial increase in system impedance limited the system 

transmission power to between 30 and 35 kW with a moderate (~ 10 percent) increase in the alternator 

speed. 

The transformers used for the voltage step up and step down were not specifically designed for low 

leakage inductance, so an improved transformer design could probably have reduced the problem and 

allowed more power to be transmitted, but the leakage inductance was obviously an issue. A more elegant 

solution than simply optimizing the transformer design around that parameter, at the expense of mass or 

efficiency was desired. Series capacitors to cancel the inductive reactance were investigated. At normal 

power line frequencies, 60 Hz, such capacitors would be prohibitively large; however at the 1750 Hz 

operating frequency of the Brayton high speed turbo alternator, the capacitor size was very modest. 

Consequently, the mass penalty for capacitor based reactance compensation is small. To that end, power 

correction factor circuitry was installed. The circuitry consisted of six 30 μF capacitors in parallel per 

phase. This allowed us to tailor the amount of corrective capacitance to optimize the power factor. This 

circuitry is shown in figure 6. The relative physical locations of the high-voltage transmission simulator 

and power correction circuitry are both shown in figure 7. 

Power Factor Correction 

Capacitors

Figure 6.—Power factor correction circuitry. 
NASA/TM—2008-215164 6 

Transmission Line 

Simulator

Power Factor Correction 

Circuitry

Figure 7.—High-voltage transmission line simulator and power correction circuitry. 

33

34

35

36

37

38

39

5 10 15 20 25 30 35 40 45 50 55 60

ATU Power (kW)

ATU speed (krpm)

High Voltage With 

90 μF Capacitors

High Voltage 

Without Capacitors

Baseline

Figure 8.—ATU speed versus ATU power. 

Several values of series capacitance were tried, and the power transmission capability increased 

consistently as we decreased the value of the capacitance to the value which was resonant with the 

transformer leakage inductance. At the resonant value the power transmission parameters were nearly 

identical to the values without the high-voltage line simulation and transformers, other than the additional 

resistive power loss and the magnetizing power loss. The effects of the added inductance were 

successfully cancelled out. The results of ATU shaft speed as a function of ATU power is shown in 

figure 8. Three cases are shown: baseline, high-voltage simulator without capacitors, and high-voltage 

simulator with 90 μF capacitive correction per phase. 

It is clearly shown in figure 8 that 90 μF of capacitance restored the ATU’s ability to deliver full 

power at rotational speeds equal to or less than the baseline case. This correction capacitance improved 

the power factor to the ATU as shown in figure 9. 
NASA/TM—2008-215164 7 

0.85

0.9

0.95

1

5 10 15 20 25 30 35 40 45 50 55 60

ATU Power (kW)

ATU Power Factor

High Voltage With 

90 μF Capacitors

High Voltage 

Without Capacitors

Baseline

Figure 9.—ATU power factor versus ATU power. 

360

365

370

375

380

385

390

395

400

405

410

415

420

425

5 10 15 20 25 30 35 40 45 50 55 60

ATU Power (kW)

ATU Voltage

Baseline

High Voltage 

Without Capacitors

High Voltage With 

90 μF Capacitors

Figure 10.—ATU Voltage versus ATU Power.

The ATU voltage as a function of power level is shown in figure 10. The baseline data demonstrates 

that the ATU output voltage is essentially constant over a broad power band. The slope of the line is the 

voltage drop across the cable between the ATU and the MPDU. When the same test was run with the 

high-voltage transformers and the transmission line simulation, but without the compensating capacitors, 

there was a substantial voltage drop at high power levels, due primarily to the high leakage inductance of 

the transformers. 

Once the capacitors were added to the circuit, the losses across the leakage inductance were largely 

cancelled. The remaining voltage drop corresponds to the IR losses in the transformers and the 

transmission line simulation. The MPDU bus distribution voltage was essentially unaffected in all three 

cases, as shown in figure 11. 
NASA/TM—2008-215164 8 

360

370

380

390

400

410

5 10 15 20 25 30 35 40 45 50 55

Distribution Power (kW)

Distribution Voltage

High Voltage 

Without Capacitors

Baseline

High Voltage With 

90 μF Capacitors

Figure 11.—Distribution (MPDU) voltage versus ATU power.

Conclusions 

These tests demonstrate the practicality of high-voltage and high-frequency ac electrical power 

transmission generated by a permanent magnet alternator compatible with a Brayton cycle engine. Highfrequency power systems such as this one are likely to experience inductive reactance due to restraints in 

alternator windings, transmission lines, step-up and step-down transformers, and distribution 

transformers. For example, these tests revealed a reduced power delivery capability when the highvoltage transmission simulator was first installed. However, this was easily corrected through the use of 

series capacitors. Specifically, the data showed that the baseline distribution bus power output of 50 kWe 

was initially limited by the transmission simulator to 35 kWe. By adding 90 μF of capacitance per phase, 

the distribution bus power was restored to 50 kWe. The amount of reactance capacitance is only 3 to 

4 percent of the value required at 60 Hz to provide the same effect. Adding series capacitance should be 

an effective tool to overcome the effects of inductance in the transmission system, or even tune the 

alternator impedance to control fault currents and system impedances. 

References 

Birchenough, A., “The 2kW Mini-BRU Electrical Controls Concept and Transient Performance,” 

NASA/TM—2006-214043, NASA GRC, Cleveland, OH, 2006. 

Birchenough, A., and Hervol, D.S.,” Experimental Operational Results from a High Power Alternator 

Test Bed,” in proceedings of Space Technology and Applications International Forum (STAIF–

2007), edited by M.S. El-Genk, AIP Conference Proceedings 880, Melville, NY, 2007, pp. 692–699. 

Hervol, D.S., Mason, L.S., and Birchenough, A.,” Experimental Results from a 2 kW Brayton Power 

Conversion Unit,” in proceedings of Space Technology and Applications International Forum 

(STAIF-2003), edited by M.S. El-Genk, AIP Conference Proceedings 665, Melville, NY, 2003, 

pp. 621–628. 

Mason, L., “A Comparison of Fission Power System Options for Lunar and Mars Surface Applications,” 

NASA/TM—2006-214120, NASA GRC, Cleveland, OH, 2006. 
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14. ABSTRACT

The Alternator Test Unit (ATU) in the Lunar Power System Facility (LPSF) located at the NASA Glenn Research Center (GRC) in 

Cleveland, Ohio was modified to simulate high-voltage transmission capability. The testbed simulated a 1 km transmission cable length 

from the ATU to the LPSF using resistors and inductors installed between the distribution transformers. Power factor correction circuitry 

was used to compensate for the reactance of the distribution system to improve the overall power factor. This test demonstrated that a 

permanent magnet alternator can successfully provide high-frequency ac power to a lunar facility located at a distance. 

15. SUBJECT TERMS

Brayton Cycle; High-voltage; Low-voltage; Rectifiers; Energy conversion; Energy conversion efficiency 

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