ECE1750, Spring 2017

Week 13 – Reliable Power for Critical LoadsCritical Loads

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Reliability and Availability • Reliability applies to components. Once they fail, they cannot be y pp p y , y

repaired.

• Reliability, R, is defined as the probability that an entity will operate without a failure for a stated period of time underoperate without a failure for a stated period of time under specified conditions.

• Unreliability is the complement to 1 of reliability (F = 1 – R)

• For electronic components the most common way of mathematically defining reliability is

( ) tR

where λ is the failure rate (from a large sample of equal

( ) tR t e

where λ is the failure rate (from a large sample of equal components operating under the same conditions and for an equal time interval, it indicates how many of these components are expected to fail within the given specified time interval)

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p g p )

• Mean time to failure (MTTF): It is the expected operating time to (first) failure. Mathematically, it is the inverse of λ.

Reliability Estimation• The failure rate of a circuit is the sum of the failure rate of its

components.

• General form for calculating failure rate (from MIL-Handbook g (217):

adj base Q T E O

Production quality

Thermal stress

Electrical stress

Other factors (power and operational

environment factors)

• Aluminum electrolytic capacitors tend to be a source of reliability concern for PV inverterssource of reliability concern for PV inverters. Although their base failure rate is low (about 0.50 FIT), the adjusted failure rate is among the highest (about 50 FIT). Compare it with a MOSFET

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adjusted failure rate of about 20 FIT.

• NOTE: FIT is failures per 109 hours.

Reliability Estimation• The temperature factor is given by the Arrhenius rate model:

1 1aE

Failure activation R Sk T T

T e energy:

Depends on failure mechanism

(e.g., 0.6 eV)Stress temperature

( g , )Boltzman constant:

8.167x10-5 eV/KReference

temperature

• Electrical stress is calculated from tables:

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Reliability and Availability • Availability applies to systems (hence, it involves dealing with

several components each serving a given function).

Expected time operating “normally”

• System components can be repaired. So, in addition to a failure

Availability = p p g y

Total time (“normal” operation + off-line time)

rate λ we can define a repair rate μ that is the analogous concept to that of the failure rate but applied to repairs.

• Not all the components in the system need to be operating forNot all the components in the system need to be operating for the system to operate

• Ways of improving availability

• Modularity

• Redundancy (parallel operation of same components)

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• Diversity (use of different components for the same function

• Distributed functions

Availability Calculation• The expected time operating “normally” can also be called

“mean up time” (MUT). It equals the inverse of the failure rate.

• The expected “off-line time” can also be called “mean downThe expected off line time can also be called mean down time” (MDT). It equals the inverse of the repair rate.

• The total time can also be called “mean time between failures” (MTBF) It equals the sum of the MUT and MDT(MTBF). It equals the sum of the MUT and MDT.

MUTAMTBF

• If in a system all components need to be operating in order to have the system operating normally, then they are said to be connected in series This “series” connection is from a reliability

MTBF

connected in series. This series connection is from a reliability perspective. Electrically they could be connected in parallel or series or any other way. The availability of a system with series connected components is the product of the components

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connected components is the product of the components availability.

Availability Calculation• If in a system with several components, only one of them need to be

operating for the system to operate, then they are said to be connected in parallel from a reliability perspective. The system unavailability

l th d t f t il bilit h thequals the product of components unavailability, where the unavailability, q, is the complement to 1 of the availability (q = 1 – a).

• The most common redundant configuration is called n + 1 redundancy in which n elements of a system are needed for the system to operate, so one additional component is provided in case one of those n necessary elements fails.

• n +1 redundant configuration. But more modules is not always better:

a = 0.97

1( 1) n nA n a q a

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• Availability decreases when n increases to a point where A < a

Background

• For some loads service interruption is unacceptable.

• Utility grade power is not good enough.

E l• Examples:

• Telecommunication sites

• Hospitals• Hospitals

• Military bases

• Aircrafts / SpacecraftsAircrafts / Spacecrafts

• Banks / Financial institutions

• Considerations:

• Life or death matter

• Lots of $$$

8• Case Study: Telecommunication sites – critical service: 911

Land-line Telecommunications Network

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Land-line Telecommunications Network

• Power infrastructure is for telecommunication networks as cardiovascular system is for humans.

• Power needs to be provided to the switch (i e a “big computer”)

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Power needs to be provided to the switch (i.e., a big computer ) and sometimes to remote terminals.

Wireless Telecommunications Network

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Wireless Telecommunications Network

• Power needs to be provided to the switch (called Mobile

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Power needs to be provided to the switch (called Mobile Telecommunications Switching Office or MTSO) and to the remote terminals called based stations.

P l t 99 99 %Telecommunications power plant availabilities

“3-nines”

Ac mains: 99.9 % Power plant: 99.99 %

(without batteries)

3 nines

- 48 V

Genset: 99 4 % (includes TS) E h tifi 99 96 %Genset: 99.4 % (includes TS)(failure to start = 2.41 %)

Each rectifier: 99.96 %n+1 redundant configuration is used for

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improved availability

Uninterruptible Power Supply (UPS)

RECTIFIER + DC-DC CONVERTER

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Power for communication networks• In Japan, 385 land-line communications buildings or sites lost service. Only 18 p , g y

of them were demolished and 23 were flooded by the tsunami. The rest lost service due to lack of power.

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Power for communication networks• In Japan, 385 land-line communications buildings or sites lost service. Only 18 p , g y

of them were demolished and 23 were flooded by the tsunami. The rest lost service due to lack of power.

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Power for communication networks• In Japan, less than 10 % of NTT Docomo’s 6720 base stations that lost service, p , ,

were demolished or flooded by the tsunami, or were affected by destroyed transmission links. Most of the base stations lost service due to lack of power.

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Katrina: land-line networks

One of the few destroyed central offices

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Outage cause

Katrina: wireless networks

19Predominant cell sites condition after Katrina Cell site geographical distribution

C ti l id f il t

Power grids Reliability Performance• Conventional power grids are very fragile systems

• Why are them fragile systems?Why are them fragile systems?• Because they have

centralized control andpower generationp gstructures.

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• Transmission congestion points

Reliability issues with the grid Transmission congestion points.

• Lack of diverse power alternatives (ultimately, there is only one grid).

• Lack of redundancy in sub-transmission and distribution power paths.y p p

21www.ferc.gov/industries/electric/gen-info/transmission-grid.pdf

Traditional Electricity Delivery Methods: Reliability

Example of lack of diversity for a critical load

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Traditional Electricity Delivery Methods: Reliability

Example of lack of diversity for a critical load

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Power sub-transmission and distribution reliabilityreliability

**

24• There are no redundant power paths

Effects of Ike on the Power Grid

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Telecom central office power plant

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Telecom central office power plant

TelecomPower Plant

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• 13 x 200 Amps. Rectifiers• 11 x 1400 Ah Batteries

Telecom central office power plant

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Typical telecom rectifier schematic

Isolationbarrier

High-frequencytransformer

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Batteries

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Distribution frames

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Distribution frames

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Inverters

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Base station power plant

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Base station power plant

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Base station power plant

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OSP Elements Power Consumption» DLC RTs may provide service up to 500 subscribers in average.y p p g» Local backup is usually provided by batteries with 8 hrs of autonomy» Significant variations in power consumptions:

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Remote land-line nodeRECTIFIERSRECTIFIERS

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Highly available power during disasters

• Common concept of damage to the electric grid during disasters:

• Real sustained damage in more than 90 % of the area:

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Highly available power during disasters

• Main problem: electric grid extreme fragility

Outage

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Outage incidence

Extreme events DamageExtreme events DamageE th k d di t ib ti i i h it ff t tl t» Earthquake damage distribution is inhomogeneous: it affects mostly at

sub-transmission and distribution levels where damage depends on construction practices, terrain, and disaster characteristics.

Orion

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There was some direct damage….

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There was some direct damage….

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And then, there was this…..

46What is wrong with these sites?

There was nothing wrong with those sites, in the same way there was nothing wrong with 99 % of the sites. Yet, their main g gissue is that without an alternative source of power, they lost service after they discharge their batteries.

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Highly available power during disasters

• Power electronic enable micro-grids may be the solution to reliable power during disasters

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E l P ft di t

One possible solution: use of distributed generation

• Example: Power after disasters• In this case energy storage is replaced by diverse power sources.

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Wh t i i id?

MicrogridsMicrogrids• What is a microgrid?

• Microgrids are considered to be locally confined and independently controlled electric power grids in which a distribution architecture integratescontrolled electric power grids in which a distribution architecture integrates loads and distributed energy resources—i.e. local distributed generators and energy storage devices—which allows the microgrid to operate connected or isolated to a main gridconnected or isolated to a main grid

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Residential solar power during disastersResidential solar power during disasters

• The focus is not in these relatively few cases:

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• The focus is on these many cases

Residential solar power during disastersResidential solar power during disastersThe focus is on these many cases

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Photovoltaic systems integration

• Grid-tied (utility centered)( y )

• Most widely used PV integration approach• Most widely used PV integration approach.• PV and home operation subject to grid operation: Due to IEEE 1547, the inverter cannot power the home when the

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grid is not present.

Photovoltaic systems integration

• Customer centered approaches• Less common or inexistent approaches:pp

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