Lessons Learned From Developing Advanced Space … · • 76 cells @ 81 amp*hrs / battery ... • Coordinated DC fault control @ high voltage ... absolute protection for system wiring

National Aeronautics and Space Administration

www.nasa.gov 1

James F. Soeder

Raymond Beach

Larry Trase

NASA Glenn Research Center

Cleveland, Ohio

Lessons Learned From Developing Advanced

Space Power Systems Applied to the

Implementation of DC Terrestrial Micro-Grids

IEEE EnergyTech 2012

Case Western Reserve University

Cleveland, Ohio

May 29-30, 2012

https://ntrs.nasa.gov/search.jsp?R=20150010351 2018-06-18T04:38:53+00:00Z


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Discussion Topics

• Why is NASA interested in Smart Grid?

• Advanced Terrestrial Smart Grids

• ISS Description

• DC Power Challenges

– Fault Control

– Stability

• Wrap-up

2


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Why Intelligent Power / Smart Grid?

• NASA

– Provide a utility–like power

generation and distribution

capability with automated

operation to enable deep

space exploration and

settlement

• Terrestrial

– Increase the power delivery

capability and reliability of

the grid by integrating

renewables and energy

storage without major

increases to the

transmission and

generation infrastructure

3


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x Increased power demands x

x Utilization of diverse power sources (renewables) x

x Incorporation of large amounts of distributed energy storage x

x Seamless accommodation of Variable / Peak load demand x

x Failure diagnostics and prognostics for power components x

x Automated control for operations management, fault detection and system reconfiguration x

x Long term reliability / availability for exploration survivability and terrestrial users x

Exploration Power Terrestrial Power

Commonality of Challenges for Grid Developers

Power Grid Challenges

Exploration Power vs Terrestrial Power


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Advanced Terrestrial Smart Grids

5


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…the Grid of the Future?*

6

Residential

Commercial

Industrial

Storage

Wind Fuel Cell

Solar

*Courtesy of John Schneider, AEP


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Advanced Power Grid Hierarchy

Mini-Grid

Appliances HVAC Entertainment Lighting

S/A Battery Flywheel

Mini-Grid

Transmission Grid

Micro-grid

Mini-Grid Mini-Grid

AC Bus AC/DC

DC Bus

Distribution Grid

FES Flow Battery

Pumped Hydro

Hybrid

Vehicle


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More Advanced Power Grid Hierarchy

Mini-Grid

Appliances HVAC Entertainment Lighting S/A Battery Flywheel

Mini-Grid

Transmission Grid

Micro-grid

Mini-Grid Mini-Grid

DC Bus

FES Flow Battery

Pumped

Hydro

Hybrid

Vehicle

DC Bus AC/DC

AC Bus


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Why DC Micro grids?

• DC powered electrical devices make up 50 to 80%* of the

load in many buildings

– Computing equipment

– LED lighting will become more common

• Variable speed drives are penetrating into the appliance

market.

• Many renewables such as solar / wind / batteries (flow and

non-flow) fuel cells flywheels etc. are already compatible

with DC systems (Have DC outputs)

9

* Nextek Power Systems


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DC Distribution and the International Space

Station

10


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Size


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International Space Station

Power System Characteristics • Power 75 kW average

• Eight independent power

channels -- 9.75 kW

• Solar array power 200+ kW

• Planar silicon arrays

• 18% Efficient

• NiH battery storage – 3 per

channel (2 ORUs)

• 76 cells @ 81 amp*hrs /

battery

• Distribution

– 116 - 170 V primary

– 120 V secondary

• Contingency power > 1 orbit

• System lifetime of 15+ years


www.nasa.gov 13

ISS Primary Grid


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Reg

Reg

Battery

Reg

Reg

Reg

Reg

Reg

Reg

L

L

L

L

•

•

• •

•

•

•

•

Considerations in DC Distribution Systems

• Stability with multiple power converters

• Coordinated DC fault control @ high voltage

Primary Secondary


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Stability

15


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Power System Stability

16

SOURCE LOADZS ZL

• If |ZS| < |ZL| for all frequencies,

then the system is stable

• When |ZS| > |ZL|, further analysis

is needed to determine system stability.

From SAE Spec AS5698


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17

-30

-20

-10

0

10

20

10 100 1000 10000 100000

Frequency (Hz)

Per

Un

it O

hm

s (

dB

)

ACCEPTABLE REGION

Normalized load Input Impedance Limit

• If |ZS| < |ZL| for all

frequencies, then the

system is stable


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18

• When |ZS| > |ZL|, further analysis

is needed to determine system stability

• Using the Nyquist criterion,

small-signal system stability can

be determined by whether

the curve of ZS/ZL circles the

(-1,0) in the S-plane

• The forbidden region on this diagram

establishes a system stability margin.

From SAE Spec AS5698


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Power System Stability Examples

19

Stable System Under damped but stable system


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Stability

• With soft sources it is generally impractical to avoid crossover

of source and load impedances

– Stability is generally determined by phase margin

• Loads should have 3 db Ohms of gain margin, or 30 degrees of

phase Margin with respect to source impedance

• Limit cycles must be avoided when applying loads to current

limited sources and resettable protective devices

• Phase Margin is 180 degrees minus phase


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DC Fault Control

21


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DC Fault Control

• Fault Clearing for DC systems is inherently difficult

because the current never passes through zero.

• Challenges

– Clear the fault quickly with minimum stress on the

system

– Minimize the effect on other branches of the system

• Coordinated fault response

– Switch closest to the fault needs to trip first

• Form factor should be compact and lightweight

22


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DC Switchgear

23

Fuse Mech Relay Hybrid CL Solid

State

Resettable N Y Y Y

Mass B

Losses B

Coordination B

System Impacts B

Complexity B

B – Baseline

Y – Yes

Best

Better

Worse

Worst


www.nasa.gov 24

Mechanical Switch: Kilovac Vacuum Relay

Description

• AP350X “Bubba” -- Largest

space-rated switch

• Voltages:

• 270 Vdc continuous

• 350 Vdc 10 µsec

• Currents:

• 500 A continuous

• <5000 A surge

• Switching time: 10 msec

• Magnetic arc blow-out

• Low loss

• No current limiting

• System transients tend to be high

Kilovac 500 A Vacuum Switch

“Bubba”


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Hybrid Switch

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Schematic of hybrid switch

Description

• Contains both mechanical and

Solid state components

• Solid state switch permits “soft” turn-

on and turn-off

• Mechanical switch provides low loss

in on-state

• Mechanical switch does not need to be

sized for interrupt

• Trip coordination can be tricky


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Current Limited RPC

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Description

• Remote Power Controllers control

and protect electrical system

• Current limited RPCs provide

absolute protection for system wiring

• Can be reset

• Can be paralleled to increase current

handling capability

- Unlike fuses or circuit breakers

• Utilizes an innovative v2t trip curve.

• Can distinguish between sever and

slight overload

• Multi-level trip coordination is easily

implemented

- Avoids ambiguity of using i2t trip

curves

0.1

1

10

100

1000

0.01 0.1 1 10 100 1000Trip Time (sec)

Sw

itch

Vo

ltag

e D

rop


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Switch Voltage Drop

RPCVsource

Load

V load

+ Vsw -

Psw = Load * Vsw

Description

• Utilizes switch voltage drop to determine trip time

• Can distinguish between sever and

slight overload

• Trip curve utilizes semiconductor „s safe

operating area

0.1

1

10

100

1000

0.01 0.1 1 10 100 1000Trip Time (sec)

Sw

itch

Vo

ltag

e D

rop


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Coordination Example

12A

8A

8A

20 W

20 W

6A

6A

120 Vdc

Short


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0.1

1

10

100

1000

0.01 0.1 1 10 100 1000Trip Time (sec)

Sw

itch

Vo

ltag

e D

rop

12A

8A

8A

20 W

20 W

8A

4A

120 Vdc

Short

80 V

40 V

80 V

0 V


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Wrap-Up

• Advanced DC terrestrial micro-grids can learn a great deal

from experience developing the International Space Station

Power System

• The negative impedance of multiple power converters in

series can pose stability challenges

• Fault control with soft sources such as power converters

and solar arrays needs to be accommodated

30


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References

• David Fox -- Hamilton Sundstrand Corp.

• James Soltis – NASA Glenn Research Center

• Nextek Corporation

31


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Back-up

32


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RPC

SSU

1 of 8 power channels

RBIRBI

RBI RBI RBI

DCSU

MBSU

RPC

RPC

DDCU

DDCU

DDCU

B

C

D

U

B

C

D

U

B

C

D

U

B

C

D

U

Challenges

• Evolution to accommodate peak

power

• Variable Loads with constrained

sources

• Automated operation

SSU – Sequential Shunt Unit

RBI – Remote Bus Isolator

DCSU – Direct Current Switching Unit

MBSU – Main Bus Switching Unit

DDCU – dc to dc Converter

RPC – Remote Power Controller

ISS Power Architecture


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12A

8A

8A

20 W

20 W

8A

4A

120 Vdc

Short

80 V

40 V

80 V

0 V


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What is NASA‟s Interest In Smart Grid?

35

Planetary Surface Power

Systems

ISS Automation

Facility Sustainability

NASA‟s interest is in the development of technologies that benefit

space exploration and enable the Terrestrial Smart Grid

Deep Space Habitat

Documents

Lessons Learned From Developing Advanced Space … · • 76 cells @ 81 amp*hrs / battery ... • Coordinated DC fault control @ high voltage ... absolute protection for system wiring