ATS, Grounding Issues & Installation Considerations

transcript

Automatic Transfer Switches , Grounding Issues & Installation Considerations for Standby Power Systems

Paul O’Hara, GM

Cummins Cal Pacific

May 15, 2014

Automatic Transfer Switches

NFPA110 Overview

Grounding Discussion Agenda

General Requirements & Terminology

The Two Big Rules

Applications

Hardware Requirements

Recommendations

Terminology

3-phase 3-wire System (Neutral Not Used)

Generator May Be Solidly-Grounded (Shown), Resistance Grounded, or

Ungrounded.

Generator side of system is separately derived

– No neutral connection to the neutral which is bonded at the service entrance

3P ATS GenSet

To Loads

SERVICE

ENTRANCE

GEC GEC

Grounding Electrode System

BONDING

JUMPER

UTILITY

Terminology

3-Phase/4-wire system & loads

Not Separately Derived

– Common neutral for entire system

– (NEC 250-20 (d) FPN No. 1)/(CEC 10-204 (4))

3P/4W ATS GenSet

To 3-Phase/4wire

UTILITY

The Two Big Rules for grounding and bonding low voltage generator systems

There shall be one, and only one neutral-to-ground

bond on any neutral bus

– There are some exceptions, such as impedance-grounded

systems and floating systems, but these don’t allow use of

neutral to serve loads.

When ground fault equipment is used, the bonding

point must be between the sensor and the source.

– During all operation modes…more on that later

Grounding Rule #1

There can be only one neutral to ground bonding

jumper on any neutral bus

– 4-pole switches or 3 phase/3-wire loads

ATS GenSet

To Loads (3-phase/4W with gnd)

TOUTILITY

GECGEC

EGC SYSTEM

BONDING

JUMPER

BONDING

JUMPER

Breaking the First Big Rule…

Parallel Path for IGF on the Neutral

GFP Does Not Sense All Fault Current

Solution: Remove Bond on Generator

ATS GenSet

To Loads

Single Bonding Jumper

Utility Ground Fault Accurately Sensed

ATS GenSet

To Loads

Breaking the 2nd Big Rule…

The GF sensor must be downstream from the bonding/grounding point

– GF sensor for a specific source must have bonding point between the sensor and the source.

ATS GenSet

GNDFAULT

Function of 4-Pole Switches

Fourth Pole Opens the Path on Neutral, isolating utility neutral

from generator neutral

Allows Accurate GFP Sensing, Both Sides

ATS GenSet

To Loads

Multiple ATS Applications

2 levels of GFP and 2 or more 3-pole ATS

Neutral Current May Nuisance Trip Feeder GFP

3P ATS

GenSet

3P ATS

UnbalancedLoad

Conclusion: 4-Pole Switches

Should Be Used on Any 3-Phase 4-Wire System

– Especially when 480VAC with ground fault

– When Used, Generator Is Separately-Derived

Assures Proper GFP Sensing

– (NEC 230-95 FPN No. 3) (CEC 14-102)

– Solidly Grounded Wye

– More Than 150 Volts to Ground (277/480

347/600VAC)

– OCD Rating 1000A or More (CEC 120/208VAC &

2000A)

Used If Outdoor Generator conductors pass

through a service entrance into the building

4-pole Switches

Used With GF Indication on Generator (NEC 700-7(d))

Used With Multi-Level GFP and Multiple ATS

Not Used With Existing 3-Pole Switches

– Exception: Second Separately-Derived Normal Source

– Exception: All 3-Pole Switches Serve 3-Wire Loads

Other Possible Methods

– 3-Wire System Feeding Transformers

– Use OCD Less Than 1000 A

SUSE Breaker or Connection Box Requirements

Ground Bus

Electrically solid to alternator

frame & genset frame

Neutral to Gnd Link

Ground Fault

Provisions

Adequate Lug Space

Required Labels

UL, Protection, Etc.

Wire Bend Space

(Top or Bottom Connect)

Bracing & SpacingSupply Side Barriers for

SUSE (Not Shown)

Overcurrent function: here

provided by a breaker

Article 250 Grounding and Bonding

Grounding (Outdoor) Separately Derived Sources

NFPA 70 250.30

Grounding Electrode

connection must be made

at the service disconnect

or first disconnect means

for outdoor generators

GES Physical Provisions (Outdoor Generators)

Grounding Electrode System

– Ground Rod at Generator Disconnect

• Must be Suitable for Use as Service Equipment (SUSE)

– Ground Rod at Generator set

• New requirement for 2011

Parallel Generators

Not specifically

addressed in code

Many opinions on

best practice

Best to consider

the BUS as the

source, and apply

rules from there

– As long as the AHJ

agrees…

GF SENSOR

GenSet GenSet

51G 51G

Zero Sequence Detected Zero Sequence Detected

Utility Service 1 Utility Service 2

GM1 U2GM2

GFI on Utility Mains and Generator Breakers

Recommendations

Keep in mind proper terminology

Remember & use the two big rules:

– Single neutral to ground bonding connection on any neutral bus

• This is for safety and reliability of the distribution system

• Bond is usually in the switchgear for low voltage generator systems—

NOT at each generator

– For ground fault sensing to work, the bonding connection must

be between the source and the GF sensor

Recommend the use of 4-pole (switched neutral)

switches for any system requiring ground fault alarm or

protection

For multiple source systems, design considering the bus

as the source, and follow the two big rules

Hospital –Gen 2 Short Circuit(Actual incident June 22, 2010)

Ground

Occurs

Ground

Current

(Relay Trips

turned off)

Generator

Control Shuts

down genset on

Short Circuit Fault

(>175%)

Neutral

Grounding

Resistor Fails

(>200 Amps)

NFPA110 Overview

NFPA 110

Requirements to achieve

maximum on-site power

system reliability

System Focus

Practical, but not cheap

See appendix for some

practical advice on power

system design and

operation

Typical Emergency Power System

5/15/2014 Cummins Confidential27

Typical Paralleled Emergency Power System

Chapter 3 - Definitions

Chapter 3 – Definitions (continued)

Chapter 4 - Class, Type and Level of EPSS

Class – minimum time

emergency power is to

operate

Type – maximum time load

won’t have power

Level – importance of

system to human life

– 1 – Critical to human life

– 2 – Less critical to life & safety

Typical systems for us are Type 10, and Level 1,

and Class based on fuel capacity desired

Chap 5 – Energy Sources, Converters & Acc’s

Ref: Page 120, T-030

Electrical Interconnections

Each installation

different

Ampacity of

Supply Circuit is

GenSet Specific

Note Some

Circuits Fed by

GenSet, others by

Utility

NFPA110 Overview

Fuel Systems

Why Gas?

– Fewer fuel storage concerns

– Lower Emissions

Why not Gas?

– Seismic shutoff valves

– State code for on-site fuel storage

– Cost (Gensets >100 kW)

Why Diesel?

– Not as dependent on outside fuel source

– Fast starting with proper fuels

– Long life

– Usually better transient performance

– Usually better frequency stability

Large Gas vs. Diesel Gensets with Tier 4 final Aftertreatment

Most Data Centers

have standardized

on Diesel Fuel

Most use Tier 2

and are limited to

emergency use

Some use Tier 4 to

limit emissions

NFPA110 Overview

Criteria Nat Gas T4f Diesel

Comparitive Models (Cummins) C1700N6 1500DQGAE

Performance

10 sec to start and full load no yes

Transient performance poor good

Meets CEC for life safety loads no yes

UL2200 Listed no yes

Seismic Certified no yes

On site storage required per code yes yes

Exhaust Emissions (gm/hp-hr)

Oxides of Nitrogen (NOx) 1.73 0.38

Particulate Matter (PM) negligble 0.00

Unburned Hydrocarbons (NMHC) negligble 0.02

Carbon monoxide (CO) 3 1.02

Installation Related

Physical Characteristics

Length (inches) 522 370

Width (inches) 144 129

Height (inches) 166 154

Weight (lbs - less fuel tank) 56493 38747

Approximate costkW* 1,276,000$ 872,414$

*Cost includes 75dbA enclosure and fuel storage tank for diesel

Diesel Fuel Systems

Reliable fuel supply depends on:

– no air in fuel

– fuel temperature

– proper volume delivered to engine

– fuel quality

System Design Greatly Affected by Local Codes and

Interpretation

System Design Should Meet NFPA 37

(kW)*(57) BTU/Min

Assume 140,000 Btu/Gal diesel fuel, and 35%

overall efficiency

Estimating Diesel Fuel Consumption

Rule of Thumb:

Multiply the standby KW times .07…

that’s the fuel consumption (gph)

Mechanical Energy

Fuel (BTU) In

Power Out 35%

What Size Fuel Tank?

Decision based on:

– GenSet Fuel Consumption

– Application Type

– Expected Duration of Outage

– Priority and time to Re-Fuel

Recommendations

– Plan for Fuel Maintenance

• Fuel testing

• Fuel polishing

12 and 24 hour capacity unit mounted (sub-base)

24 hour tanks are very common

High Rise Buildings (CFC)

Major fuel storage issues to pay attention to

Stairs and platforms

are required when sub-

base fuel tanks raise

controls and output

breakers to over 78”

from ground

Although not in state

code, overfill prevention

(valves) and spill

protection are required

by several authorities

Genset Control Functions

Transfer Switches

Transfer Switches (continued)

Installation

Indoor vs Outdoor Sets

Outdoor

– Lower Cost

– Modular

– Less engineering

– Ease of Monitoring

– Noise - airborne

– Weather

– Security

Indoor

–More stable environment

–Better security

–More difficult service

access

–Air flow issues

Most Data Center owners have

standardized on outdoor generators

Indoor Data Center

NFPA110 Overview

Standard Outdoor

Larger Outdoor Genset

Outdoor Generators at Data Center

NFPA110 Overview

Tips for Outdoor Installations

Location

– Access for fueling and maintenance

– Security

– Property line clearances

– Grounding

Orientation

– Consider Prevailing Winds (recirculation)

– Shortest conduit runs

Corrosion (aluminum near coast)

Installation (cont)

Installations (continued)

Seismic concerns

All emergency equipment for legally required

systems need to be seismically certified to meet

IBC/CBC seismic withstand requirements. Can be

by analysis

All emergency equipment California OSHPD

overseen installations need to be listed with an OSP

by OSHPD. These items need to be shake table

tested

– OSP-0028 and 268 Generators

– OSP-0029 Automatic Transfer Switches

– OSP-0030 Paralleling Controls

NFPA110 Overview

NFPA 110 “Explanatory Material”

Airborne Noise

130 Pneumatic Riveter (130)

100 Jet @ 1000ft (103)

90 Power Mower (96)

80 Heavy Street Traffic (85)

60 Normal Conversation (65)

50 Light Traffic @ 100ft (55)

40 Library (40)

20 Broadcast Studio (20)

• Primarily a problem in outdoor

gensets

• A System is Too Noisy IF:

– Local Codes Exceeded (may be

in 40-50 dBA range)

• Someone thinks it is

• Aftertreatment is

EXPENSIVE

– Hearing Protection Required in

Generator Rooms per OSHA

• Amount and Perception

Depends on Background Noise

• Logarithmic Basis is Hard for

Laymen to Understand

Typical City Ambient Noise Levels

NFPA110 Overview

Adding Noise Levels

DIFFERENCE IN dB(A) BETWEEN VALUES BEING1 2 3 4 5 6 7 8 9 10

Difference in dB(A) between values being addedDIFFERENCE IN dB(A) BETWEEN VALUES BEING

1 2 3 4 5 6 7 8 9 10

Sound Attenuation StrategiesTotal Noise Level is SUM of all the Sources

• Mechanical Engine Noise

• Fan Noise

• Exhaust

Consider all the parts operating together to get to desired result.

Exhaust 94

Fan 86 dB(A)

Engine 80

89 dB(A)

79 dB(A)

87 dB(A)

Install 15 dB

Std. Muffler

Sound Attenuation Features

Insulation (3-4 dBA)Inlet Silencer (3-4 dBA)

Door Seals

Exhaust Air Silencer

Look for Measured sound performance data

NFPA110 Overview

Reducing the Noise by Site Design

Increase Distance from Receiver

Insert High Mass, Absorptive Barriers

Direct Noise Away From Sensitive

Locations

Watch for Hard, Reflective Surfaces

Rule of Thumb:

Sound power drops 6dBA at 2 times distance.

Rule of Thumb:

Sound power increases 3dBA for two equal sources.

+5 dBA

Effect of Reverberation

The noise source is effectively duplicated by

hard walls.

+3 dBA

For further information

New On-Line Library – PowerSuite 5.0

https://powersuite.cummins.com/

For additional help contact:

Guy Shullerts, Territory Manager, (510) 347-6664

John McWilliams, Application Engineer, (510) 347-6673

Paul O’Hara, GM Mission Critical/Tech Comm (949) 337-5393

ATS, Grounding Issues & Installation Considerations

Engineering