Statement_of_Work.pdf

STATEMENT OF WORK (SOW)

FOR

INSTALL STANDBY GENERATOR AND AUTOMATIC TRANSFER SWITCH

BLDG 1712

FAIRCHILD AFB, WA

9 February 2015

1. Scope of Work. This Statement of Work (SOW) defines the scope for this task order to include installation and connection of a standby generator and all associated components at building 1712, Fairchild AFB, WA. Work consists of providing a diesel, standby generator rated to provide full backup power for Building 1712, estimated to be 100 kilowatts at 200 amp service, complete with automatic transfer switch (ATS), all electrical connections, all-weather enclosure, and capacity/future connection for an uninterruptible power supply (to be provided by others). Work shall be in accordance with technical and regulatory requirements, and following instructions and recommendations of the manufacturer(s) and this Statement of Work (SOW).

2. General Work Description. This Statement of Work (SOW) defines the project as follows:

the contractor shall perform all demolition and construction activities listed below.

2.1. Design and provide generator and automatic transfer switch to meet full backup power requirement of Building 1712 (load test data attached as Appendix B).

2.1.1. Generator shall be designed and sized to provide full backup power to entire facility as well as an uninterruptible power supply (UPS) planned to be installed at a future date (NOTE: UPS will only power rooms 145, 146, and 147, also known as the “Brain Stem”). The government estimates a minimum of a 100 kilowatt (kW) generator must be provided to meet all requirements.

2.1.2. Generator shall be diesel-engine powered three-phase output and include an automatic transfer switch specifically designed to meet the requirements of the generator and facility electrical infrastructure. Contractor shall provide and install all associated circuit and network boards required for safe and normal operation whether in test/load bank or emergency. Automatic transfer switch shall be designed and sized by contractor, but at a minimum shall be a four-pole switch to meet Fairchild AFB design and maintenance standards.

2.1.3. Contractor shall connect the generator and all associated electrical components to the facility. Connections may be made directly at the transformer adjacent to the facility or routed to the primary distribution panels in the main electrical room of the facility. This determination shall be based on design and installation cost as well as maintenance and operation of the system, and part of the design-build process.

2.1.4. Generator must have capability to be run for 72 hours consecutively without refueling – this may be met by on-board tank capacity plus addition of an auxiliary tank. Contractor shall provide and install all necessary components to pump and automatically control fuel from auxiliary tank.

2.1.5. Generator and all components must be installed in weather enclosures or in other approved standard installation to prevent system degradation by exposure to weather.

2.2. Contractor shall perform all site work to prepare generator placement, utility/duct and connections, and existing facilities/infrastructure modifications.

2.2.1. Site generator adjacent to existing facility and electrical infrastructure. In general, generator and accompanying equipment shall be located in the vicinity of the existing transformer and nearby electrical room on the north west corner of the facility. Siting shall be outside the training yard and not obstruct existing pedestrian or vehicular pathways.

2.2.2. Prepare site: remove any existing vegetation or other obstructions limiting placement of generator or infrastructure.

2.2.3. Prepare generator foundation: excavate generator placement site, place and compact gravel. Install utility duct bank/transmission lines as necessary for connection to facility/transformer, automatic transfer switch, future UPS, and day tank. Backfill and compact additional layers of gravel as required to prevent crushing by concrete or exposure.

2.2.4. Design and install longitudinal steel reinforcement as necessary to prevent tension failure of concrete foundation. Design, form and place Portland Cement Concrete foundation – concrete shall be capable of holding the compressive load of the generator, as well as live loads caused by generator vibration while operational. Concrete shall also be treated and placed in accordance with American Concrete Institute standard 318 and subparts to mitigate freeze-thaw cycle degradation and other location/weather hazards.

2.2.5. Contractor shall return site to previous condition (grass, rock, dirt backfill, etc.)

around concrete foundation and enclosure, as well as covering all duct work and utility runs, at completion of installation.

2.3. Contractor shall connect generator and associated equipment in accordance with National Electric Code and National Fire Protection Association provisions, as well as all applicable federal, state, and local regulations.

2.3.1. Connections shall be made at termination points – no splices or connections shall be made within conduit/duct between the generator and facility/other infrastructure.

2.3.2. All connections requiring a utility outage must be accomplished Friday, Saturday, Sunday, or Monday to prevent disruption of ongoing training operations.

2.3.3. Contractor shall perform all trenching, duct/conduit runs, and cable pulls necessary to connect generator, ATS, future UPS, and day tank.

2.3.4. Final connections must be tested and approved by Fairchild AFB Civil Engineering prior to covering work (Other tests and inspections may be stipulated by Fairchild AFB Civil Engineering at any time as well).

2.4. Contractor shall provide test run of generator for all foreseeable power loss scenarios including prime power shutdown (emergency/unplanned outage), planned utility shutdown, emergency bypass, and operating sequence in event of power “bump” or

“spike” scenarios. All performance will be inspected/reviewed by Fairchild AFB Civil Engineering.

3. Period of Performance. The performance period shall not exceed 150 calendar days. The performance periods shall be broken down into three phases. The contractor shall have up to 90 calendar days for completion of Phase I –Design and Material Submittals/Procurement; up to 30 calendar days for completion of Phase II - Physical Work; and up to 30 days for Phase III - Closeout. A Notice to Proceed (NTP) will be issued for each performance period.

Material submittal process must be complete and submittals approved prior to issuance of NTP and start of Phase II.

4. Hours of Operation. Normal hours of operation at the Resistance Training Lab are seven days per week, 0730 – 1600 hours.

4.1. Due to impact to student training and operations, contractor hours will be limited to

Friday through Monday, 0730 – 1600 unless otherwise coordinated with the Contracting Officer and end user.

4.2. All utility outages must occur on Friday or Monday, and be coordinated with 92d Civil Engineer Squadron no less than 10 business days in advance.

5. Statement of Work Attachments.

5.1. Attachment A – As-built Drawings

5.2. Attachment B – Generator Load

Test Data

Attachment A

Application Manual -- Liquid Cooled Generator Sets -- Ver.G.EN

5--22 5 -- ELECTRICAL DESIGN

Vibration Isolation All generator sets vibrate during normal operation, a fact that must be addressed. They are either designed with integral isolators or the entire skid is mounted on spring isolators to allow movement and to isolate vibrations from the building or other structures. Greater movement can also occur upon sudden load change or fault event and during startup or shutdown. Therefore, all connections to the generator set, mechanical and electrical, must be able to absorb the vibration movement and startup/shutdown movements. Power output, control function, annunciation, and accessory circuits all require stranded flexible leads and flexible conduits between the generator set and the building, mounting structure, or foundation.

Large stiff cables may not provide sufficient ability to bend even though they are considered flexible. This is also true of some conduit types, for example certain liquid---tight conduits that are quite stiff. Cables or conduits are not compressible along their length so flexibility in that dimension must be accommodated with sufficient length, offsets or bends.

Further, the electrical connection points on the generator set --- bushings, bus---bars, terminal blocks, etc. --- are not designed to absorb these movements and related stresses. (This is again especially notable for large stiff cables or stiff “flexible” conduits.

Failure to allow sufficient flexibility will result in damage to enclosures, leads, cables, insulation, or connection points.

Note: Simply adding flex conduit or cabling may not result in sufficient capability to absorb the vibratory movement of a generator set. Cables and flexible conduits vary in flexibility and will not stretch or compress. This condition can be addressed by including at least one bend between the generator output enclosure and the structure (cement floor, raceway,wall, etc.) to allow for three dimensional movement.

Note: Control wiring should include strain relief to prevent motion at the point of connection that could potentially cause wiring failure.

Seismic Areas In seismic risk areas, special electrical installation practices are required, including seismic mounting of equipment. The mass, center of gravity, and mounting dimensions of the equipment are indicated on the outline drawings.

Control Wiring AC and DC control wiring (to the remote control equipment and remote annunciators) must be run in separate conduit from the power cables to minimize power circuit interference in the control circuit. Stranded conductors and a section of flexible conduit must be used for connections at the set.

Accessory Branch Circuits Branch circuits must be provided for all accessory equipment necessary for operation of the generator set. These circuits must be fed either from the load terminals of an automatic transfer switch or from the generator terminals. Examples of accessories include the fuel transfer pump, coolant pumps for remote radiators, and motorized louvers for ventilation.

Branch circuits, fed from the normal power panelboard, must be provided for the battery charger and coolant heaters, if used. See Figure 5---10.

Electrical Connections General

Attachment B

5--235 -- ELECTRICAL DESIGN

AC POWER TO

EMERGENCY LOADS

AC POWER TO REMOTE

VENT OR RADIATOR FAN2

DC START

SIGNAL

FROM ATS

DC SIGNALS

TO REMOTE

ANNUNCIATOR

NORMAL

AC POWER

TO CONTROL

BOX HEATER

SET RUNNING

SIGNALS

NORMAL AC POWER

TO GENERATOR

HEATER

NORMAL AC

POWER TO LUBE

OIL HEATER

NORMAL AC

POWER TO

BATTERY

HEATER

NORMAL AC

POWER TO

BATTERY

CHARGER

NORMAL AC POWER

TO COOLANT HEATER

DC SIGNALS

TO GENERATOR

CONTROL AND

REMOTE

ANNUNCIATOR

DAY TANK

FUEL PUMP

DC POWER

TO BATTERY

AC POWER TO

DAY TANK

FUEL PUMP2

NOTES:

1. WHEN A CUMMINS POWER GENERATION ATS (AUTOMATIC TRANSFER SWITCH) IS USED, THE BATTERY CHARGER CAN BE SUPPLIED

WITH THE ATS. ATS MOUNTED BATTERY CHARGERS CANNOT BE USED IN PARALLELING APPLICATIONS.

2. THESE LOADS CAN BE POWERED DIRECTLY OFF THE GENERATOR (WITH APPROPRIATE OVERCURRENT PROTECTION) OR FROM THE

LOAD SIDE OF THE FIRST PRIORITY ATS.

3. THE ITEMS IN ITALICS ARE NOT ALWAYS USED.

4 NETWORK INTERCONNECT MAY REPLACE SIGNALS FOR SOME CONTROL INTERCONNECTIONS.

REMOTE

EMERGENCY

STOP

DC SIGNALS

TO REMOTE

ANNUNCIATOR

BATTERY

CHARGER1

NETWORK

INTERFACE

Figure 5---10. Typical Generator Set Control and Accessory Wiring

Verify a proper match of the number of conductors per phase and their size with the published lug capacities of the equipment (circuit breakers and transfer switches).

A main disconnect device (circuit breaker/switch) should be supervised and arranged to activate an alarm when it is open. Some suppliers will initiate a “not in auto” alarm when the CB is open.

AC Power Connections at Generator

5--24 5 -- ELECTRICAL DESIGN

Connection options at the generator can include the following:

Generator--Mounted Molded Case Circuit Breakers (Thermal–Magnetic or Solid– State) Connections can be made to a generator---mounted circuit breaker. The circuit breaker selected must have adequate interrupting capability based on the available short circuit current. With a single generator set the maximum available first cycle symmetrical short circuit current is typically in the range of 8 to 12 times the rated current. For a specific generator it equals the reciprocal of the generator per unit subtransient reactance, or 1/X′′d. Use the minimum tolerance of subtransient reactance from the specific generator manufacturer’s data for the calculation.

Generator--Mounted Disconnect (Molded Case) Switch Connections can be made to a generator---mounted disconnect switch. This is allowable where the generator includes an inherent means of generator overcurrent protection, such as Power Command. The switch is not intended to interrupt fault level currents, having an interrupting rating sufficient only for the load currents.

Generator Terminals Connections may be made to the generator terminals where no generator---mounted circuit breaker or disconnect switch is required and where the generator includes an inherent means of generator overload protection.

The generator set AC output connects to field--- installed conductors sized as required by the load currents, the application, and applicable codes. The conductors from the generator terminals to the first overcurrent device are considered tap conductors. A generator circuit breaker may be provided at the load end of the generator supply conductors (for example, paralleling breakers in the paralleling switchboard or main breaker in a distribution panel) and still provide overload protection for the conductors.

If the generator set is not factory---supplied with a main---line circuit breaker, the ampacity of the field--- installed AC phase conductors from the generator output terminals to the first overcurrent device should be at least equal to 115 percent of the rated full --- load current, without temperature or altitude de---ratings. The ampacity of the conductors may be 100 percent of rated full --- load current if the generator set is equipped with Power Command. The generator set manufacturer will specify line---ampere ratings of a specific generator set at the specific voltage required. If unknown, calculate using one of the following formulae:

ILINE =

kW • 1000

VL--L • 0.8 • 1.73

OR

kVA • 1000

VL--L • 1.73

ILINE =

Where:

ILINE = Line Current (amps).

kW = Kilowatt rating of the genset.

kVA = kVA rating on the genset.

VL---L = Rated line---to--- line voltage.

See schematics (a) and (b) in Figure 5---11. The length of run for generator tap conductors to the first overcurrent device should be kept as short as possible (generally not more than 25 --- 50 feet).

NOTE: If the generator is supplied with leads, the size of the leads may be smaller than required for field–installed conductors because generator leads have type CCXL or similar, high temperature insulation rated at or above 125_ C.

AC Power Conductors

5--255 -- ELECTRICAL DESIGN

GEN

GEN

GEN *

115% OF GENERATOR FULL-LOAD AMPERES

MAY BE 100% GENERATOR FLA WITH POWER

COMMAND

115% OF GENERATOR

FULL-LOAD AMPERES

EQUAL TO OR GREATER

THAN GENERATOR BREAKER

RATING

* -- FACTORY MOUNTED

CIRCUIT BREAKER

(a) No Main-Line Circuit Breaker

(b) Remote Main-Line Circuit Breaker

(c) Generator Mounted Main-Line Circuit Breaker

EQUAL TO OR GREATER

THAN REMOTE BREAKER

RATING

TO THE AUTOMATIC TRANSFER SWITCHES

TO THE AUTOMATIC TRANSFER SWITCHES

TO THE AUTOMATIC TRANSFER SWITCHES

Figure 5---11. Feeder Ampacity

If the generator set is factory---equipped with a main–line circuit breaker, the ampacity of the field--- installed AC phase conductors connected to the load terminals of the circuit breaker should be equal to or greater than the circuit breaker rating. See Schematic (c) in Figure 5---11.

The minimum ampacity of the neutral conductor is generally permitted to be equal to or greater than the calculated maximum single---phase unbalance of the load. Where a significant portion of the load is non---linear, the neutral should be sized in accordance with anticipated neutral current but never less than 100 percent rated. The generator neutral supplied by Cummins Power Generation is equal in ampacity to the phase conductors.

Note: Medium voltage cable (greater than 600 VAC) must be installed and terminated exactly as recommended by the cable manufacturer, by persons who have learned the procedures through training and practice under close supervision.

Voltage Drop Calculations Conductor impedance due to resistance and reactance causes voltage to drop in an AC circuit. To obtain the performance expected of load equipment, conductors usually should be sized so that voltage does not drop more than 3 percent in a branch or feeder circuit or more than 5 percent overall between the service drop and the load equipment.

While exact calculations are complex, reasonably close approximations can be made using the following relation:

VDROP = (IPHASE • ZCONDUCTOR)

VRATED

For Example: Calculate percentage voltage drop in 500 feet of 1/0 AWG copper cable in steel conduit supplying a 3–phase, 100 kW, 480 volt, (line–to–line) load imposing a 0.91 PF (Power Factor).

5--26 5 -- ELECTRICAL DESIGN

Z(ohms) = L [(R • pf)+X (1--pf2)]

(1000 • N)

Where:

Z = Impedance of conductor R = Resistance of conductor X = Reactance of conductor L = Conductor length in feet N = Number of conductors per phase pf = Power Factor R = 0.12 ohms/1000 feet (NEC Chapter 9, Table 9, Resistance for

1/0 AWG copper conductors in steel conduit.)

X = 0.055 ohms/1000 feet (NEC Chapter 9, Table 9, Reactance for

1/0 AWG copper conductors in steel conduit.)

Z = 500 [0.12 • 0.91) + 0.055 (1--0.912)] (1000 • 1)

= 0.066 percent

IPHASE = kW

= 120.3 amps

0.48 • 1.73kV • 1.73

VDROP (%) =

120.3 • 0.066

100• 480

= 1.65 percent

Allowable Single--Phase Load Unbalance Single---phase loads should be distributed as evenly as possible between the three phases of a three---phase generator set in order to fully utilize the rated capacity (kVA and kW) of the set and to limit voltage imbalance. Figure 5---12 can be used to determine the maximum permissible percentage of unbalanced single–phase load, as shown in the example.

Single phase power can be taken for up to 67 percent of the three---phase rating on Cummins Power Generation generator sets, up through 200/175 kW.

Generally, the larger the generator set, the lower the percentage of single---phase power that can be taken. Figure 5---12 includes single–phase percentage lines for Cummins Power Generation intermediate---size Frame---4 and Frame---5 generators. Confirm the frame size by referring to the applicable Alternator Data Sheet referenced by the generator set Specification Sheet. Single–phase load unbalance should not exceed 10 percent.

For Example: Find the maximum single---phase load that can be powered in conjunction with a total three---phase load of 62 kVA by a generator set rated 100kW/125 kVA.

1. Find the three--phase load as a percentage of the generator kVA rating:

125 kVA Three--Phase

Load Percentage 62 kVA • 100% = 50%

5--275 -- ELECTRICAL DESIGN

2. Find the percentage of allowable single--phase load, as shown by the arrows in Figure 5--12. In this case, it is approximately 34 percent of the three–phase rating.

3. Find the maximum single--phase load:

100% Maximum Single

Phase Load 125 kVA • 34% = 42.5 kVA

4. Note, as follows, that the sum of the three--phase and maximum permissible single--phase loads is less than the kVA rating of the generator set:

62 kVA + 42.5 kVA = 104.5 kVA and

104.5 kVA < 125 kVA Rating of the Generator Set

NOTE: Unbalanced loading of a generator set causes unbalanced phase voltages. The levels of load imbalance anticipated by these guidelines should not result in harm to the generator set itself. The corresponding levels of voltage imbalance, however, may not be acceptable for loads such as three–phase motors.

Because of unbalanced phase voltages, critical loads should be connected to the phase that the voltage regulator uses as the reference voltage (L1–L2 as defined in the generator set schematic) when only one phase is used as a reference.

0 10 20 30 40 50 60 70 SINGLE-PHASE LOAD AS PERCENTAGE OF THREE-PHASE kVA RATING

T H

R E

E -P

H A

S E

LO

A

D A

S P

E R

C E

N T

A G

E O

F T

H R

E E

-P H

A S

E kV

A R

A T

IN

G use THIS LINE FOR 200 Kw and less

Use this line for frame-4 generators

Use this line for frame-5 generators

Figure 5---12. Allowable Unbalanced Single-Phase Load (Typical Three---Phase Generator From Cummins Power Generation)

5--28 5 -- ELECTRICAL DESIGN

Three phase generator sets are rated for continuous operation at 0.8 PF (lagging) and can operate for short periods of time at lower power factors, such as when starting motors. Reactive loads that cause leading power factor can provide excitation power to the alternator, and if high enough, can cause alternator voltage to rise uncontrollably, damaging the alternator or loads or tripping protective equipment. Figure 5---13 is a typical alternator curve of reactive power (kVAR) capability. A reasonable guideline is that a generator set can carry up to 10 percent of its rated kVAR capability in leading power factor loads without being damaged or losing control of output voltage.

Note: The reasonable guideline is based on the alternator rating, not the genset rating, and in critical applications the alternator supplier should be consulted and the exact reactive capability curve should be used to make engineering decisions, rather than estimates, because actual performance can vary considerably from this estimate.

The most common sources of leading power factor are lightly loaded UPS systems with input filters and power factor correction devices for motors. Loading the generator set with lagging power factor loads prior to the leading power factor loads can improve stability. It is also advisable to switch power factor correction capacitors on and off with the load. It is generally impractical to oversize a generator set (thus reducing the percentage of nonlinear load) to correct for this problem.

Figure 5---13. Typical Steady State Alternator Reactive Power Capability Curve

The following is a general description of system and equipment grounding for AC generators permanently installed within a facility. While this is intended as a guide, it is important to follow local electrical code.

Leading Power Factor Load

System and Equipment Grounding

5--295 -- ELECTRICAL DESIGN

3-POLE ATS

3-POLE ATS

4-POLE ATS

GENERATOR SET

GENERATOR SET

GENERATOR SETSERVICE ENTRANCE

SERVICE ENTRANCE

SERVICE ENTRANCE

LOAD

or

LOAD

LOAD

THREE-PHASE, THREE-WIRE UTILITY, THREE-POLE ATS

Generator Neutral may be solidly grounded, resistance grounded or ungrounded with a three-wire system

THREE-PHASE, FOUR-WIRE UTILITY, THREE-POLE ATS

Generator Neutral is grounded at service entrance only with a three-pole ATS

THREE-PHASE, FOUR-WIRE UTILITY, FOUR-POLE ATS

Generator Neutral must be solidly grounded when a separately derived source with a four-pole ATS

Figure 5---14. Typical One-Line Diagrams of Alternative System Grounding Methods

5--30 5 -- ELECTRICAL DESIGN

System Grounding (Earthing) System grounding (earthing) is the intentional grounding of the neutral point of a wye---connected generator, the corner of a delta---connected generator, or the mid–point of one---phase winding of a delta---connected generator, to ground (earth). It is most common to ground the neutral point of a wye---connected generator and bring out the neutral (grounded circuit conductor) in a three–phase, four–wire system.

A corner–grounded delta system has a grounded circuit conductor that is not a neutral.

It also has a “wild leg” that must be identified by orange color coding and be connected to the middle pole of three–phase equipment.

Solid Grounding A solidly grounded system is grounded directly by a conductor (the grounding electrode conductor) with no intentional impedance to earth (grounding electrode). This method is typically used, and required by electrical code on all low voltage systems (600 volts and below) with a grounded circuit conductor (most often a neutral) that serves L---N loads.

Correct grounding in standby systems that are solidly grounded is a function of the transfer switch equipment used (solid neutral or switched neutral). See Fig-ure 5--14.

In some regions, the neutral terminal of a Cummins Power Generation generator is not bonded to ground. If the generator is a separately derived power source (i.e. 4---pole transfer switch) then the neutral will have to be bonded to ground and a grounding electrode conductor connected to the grounding electrode system by the installing electrician. However, when ground fault protection is provided in the generator, the neutral is factory---bonded to ground.

If the generator neutral connects to a service---supplied grounded neutral, typically at the neutral block of a 3---pole transfer switch, then the generator neutral should not be grounded at the generator. In this case, the electrical code may require a sign to be placed at the service supply indicating that the generator neutral is grounded at that location.

Impedance (Resistance) Grounding A grounding resistor is permanently installed in the path from the neutral point of the generator to the grounding electrode. This method is occasionally used on three---phase, three---wire systems (no grounded circuit conductor) operating at 600 volts or below, where it is desirable to maintain continuity of power with the first and only accidental ground fault. However, this practice is not permitted by regulatory code in some regions. Delta---wye transformers may be used in the distribution system to derive a neutral for line---to---neutral load equipment.

Typically, a high---resistance grounded, low voltage system uses a grounding resistor sized to limit ground fault current, at line---to---neutral voltage, to 25, 10, or 5 amps nominal (continuous time rating). The resistance grounding is done for the system, to enable sustained operation with a ground fault in place; or to protect the generator.

Generator protection is used on MV and especially HV systems to limit the impact of groundfault on the alternator: over volts and very short duration for thermal damage.

Ground fault detection and alarm systems are also typically installed.

Select a grounding resistor based on:

1. Voltage Rating: Phase--to--phase voltage (system voltage) divided by the square root of three (1.73).

2. Current Rating: Low enough to limit damage but high enough to reliably operate the pro-tective relaying.

3. Time Rating: Most often 10 seconds for protective relayed systems, and extended time for non--relayed systems.

5--315 -- ELECTRICAL DESIGN

Solid or Effectively Grounded Neutral Systems Solid or effective grounding is defined as the intentional connection of a system conductor to a ground connection or connections of sufficiently low impedance and having sufficient current---carrying capacity to prevent the build---up of voltages that may result in undue hazards to connected equipment or to persons. This method is used worldwide. The effective grounding system reduces the maximum line to ground voltage during a fault since the system remains referenced and therefore the cost of insulating the system is reduced.

Due to the magnitude of currents flowing under earth fault conditions with this type of system, protection settings are relatively straightforward to coordinate and the effect of faults can be localized to the system or part of the system where they occur.

NOTE: Low--resistance grounding is recommended on generator systems operating from 601 through 15,000 volts in order to limit the level of ground fault current (most often 200--400 amps) and permit time for selective coordination of protective relaying. See Figure 5--15 and Medium Voltage Grounding.

Ungrounded No intentional connection is made between the AC generator system and earth. This method is occasionally used on three---phase, three---wire systems (no grounded circuit conductor) operating at 600 volts or below, where it is required or desirable to maintain continuity of power with one ground fault, and qualified service electricians are on site.

An example would be supplying a critical process load. Delta---wye transformers may be used in the distribution system to derive a neutral for line---to---neutral load equipment.

Equipment Grounding (Earthing) Equipment grounding (earthing) is the bonding together and connection to ground (earth) of all non---current carrying (during normal operation) metallic conduit, equipment enclosures, generator frame, etc. Equipment grounding provides a permanent, continuous, low---impedance electrical path back to the power source. Proper grounding practically eliminates “touch potential” and facilitates clearing of protective devices during ground faults. A main bonding jumper at the source bonds the equipment grounding system to the grounded circuit conductor (neutral) of the AC system at a single point. A grounding connection location is provided on the alternator frame or, if a set---mounted circuit breaker is provided, a grounding terminal is provided inside the circuit breaker enclosure. See Figure 5---16.

Selective coordination is the positive clearing of a short circuit fault at all levels of fault current by the overcurrent device immediately on the line---side of the fault, and only by that device. “Nuisance clearing” of a fault by overcurrent devices upstream of the one closest to the fault causes unnecessary disruption of unfaulted branches in the distribution system and may cause the emergency system to start unnecessarily.

Electrical power failures include external failures, such as utility outage or brownout and internal failures within a building distribution system, such as a short circuit fault or overload that causes an overcurrent protection device to open the circuit. Because emergency and standby generator systems are intended to maintain power for selected critical loads, the electrical distribution system should be designed to maximize continuity of power in the event of a fault within the system. The overcurrent protection system should therefore be selectively coordinated.

Overcurrent protection for the equipment and conductors that are part of the emergency or standby power system, including the on---site generator, should follow applicable electrical codes. However, where the emergency power system serves loads that are critical to life safety, as in hospitals or high---rise buildings, more priority should be given to maintaining the continuity of power than to protecting the emergency system. For example, it would be more appropriate to have an alarm---only indication of an overload or ground fault than to have a circuit breaker open to protect the equipment if the result would be the loss of emergency power to loads critical for safety of life.

Selective Coordination

5--32 5 -- ELECTRICAL DESIGN

Resistance grounding can be done for the system to enable sustained operation with a ground fault in place; or to protect the generator. Generator protection is used on MV and especially HV systems to limit the impact of groundfault on the alternator: over volts and very short duration for thermal damage

G

ES2092---4c

G

TO LOADS

MEDIUM VOLTAGE

SWITCHGEAR

INSULATED NEUTRAL

(ISOLATED FROM GROUND)

EQUIPMENT

GROUND

NEUTRAL

GROUNDING

RESISTOR

CURRENT

SENSING

GFP

UTILITY SUPPLY TRANSFORMER

(SOLIDLY GROUNDED)

MEDIUM VOLTAGE

GENERATOR

GROUNDING

ELECTRODE

51G

L

L

Figure 5---15. Typical Low-Resistance Grounding System for a Medium Voltage Generator Set and Load Transfer Equipment

5--335 -- ELECTRICAL DESIGN

G

UTILITY

SUPPLY TRANSFORMER

(SOLIDLY GROUNDED SYSTEM)

SERVICE

DISCONNECT

UNGROUNDED

CIRCUIT

CONDUCTOR

(PHASE)

GROUNDED

CIRCUIT

CONDUCTOR

(NEUTRAL)

SYSTEM

GROUNDING

ELECTRODE

GROUNDING

ELECTRODE

CONDUCTOR

EQUIPMENT

GROUNDING

CONDUCTOR

MAIN

BONDING

JUMPER

L N G NL

3-PHASE, 4-WIRE

SERVICE

UTILITY

SERVICE

EQUIPMENT

Figure 5---16. Typical System and Equipment Grounding Connections at the Utility Service Equipment

For the purposes of coordination, the available short circuit current in the first few cycles from a generator set is important. This current is independent of the excitation system and is solely dependent on the magnetic and electrical characteristics of the generator itself. The maximum first cycle bolted three---phase, symmetrical short circuit current (Isc) available from a generator at its terminals is:

ISC P.U. =

X” d

5--34 5 -- ELECTRICAL DESIGN

EAC is the open circuit voltage and X′′d is the per---unit direct axis subtransient reactance of the generator. A typical Cummins Power Generation generator set will deliver 8 to12 times (instantaneous) or 3 times (sustained) its rated current on a three---phase bolted fault, regardless of the type of excitation system used. (Refer to the generator set Specification Sheets and alternator data sheets for X′′d.)

Generator reactances are published in per unit to a specified base alternator rating.

Generator sets, however, have various base ratings. Therefore, to convert per unit reactances from the alternator base to the generator set base use the following formula:

P.U.Znew=P.U.Zgiven base kVgiven base kVnew base kVAgiven base kVAnew

Example Calculation: Find X′′d (alternator subtransient reactance) for Cummins Power Generation Model 230DFAB diesel generator set rated 230 kW/288 kVA at 277/480 VAC.

Bulletin S---1009a for this model references Alternator Data Sheet No. 303. ADS No. 303 indicates that X′′d = 0.13 for the alternator at a full --- load rating point of 335 kW/419 kVA and 277/480 VAC (125_C temperature rise). Substituting these values into the preceding equation:

X”d(Genset) = X”d(ADS) kVADS kVGenset kVAADS kVAGenset

X”d(Genset) = 0.13

0.48

0.48 = 0.089

Equipment Location Recommendations It is recommended for selective coordination that transfer switches be located on the load side of the branch circuit overcurrent device, where possible on the line side of a branch circuit panel board. With the transfer switch located on the load side of the branch circuit overcurrent device, faults on the load side of the transfer switch will not result in unfaulted branches of the emergency system being transferred to the generator along with the faulted branch.

This recommendation is consistent with the recommendations for overall reliability to physically locate transfer switches as close to the load equipment as possible, and to divide the emergency system loads into the smallest circuits practical using multiple transfer switches.

A second recommendation is to use a sustaining generator (separate or PMG excitation) to positively clear molded case branch circuit breakers. A sustaining generator can provide an advantage in clearing molded case circuit breakers of the same current rating but different time–current characteristics.

One of three approaches described below are usually followed when sizing a main---line generator circuit breaker:

The most common approach is to size the circuit breaker equal to or the next rating up from the generator full --- load current rating. For example, an 800---ampere circuit breaker would be selected for a generator with a 751---ampere full load current rating. The advantage in this approach is one of cost; the cables and distribution panel or transfer

Fault and Overcurrent Protection with Generator Sets

Sizing a Main--Line Generator Circuit Breaker

5--355 -- ELECTRICAL DESIGN

switch can be sized to the breaker rating of 800 amperes. If the circuit breaker is standard rated (80% continuous) it may open automatically at levels below the generator full --- load current rating. However, the generator set is not likely to be run near or at full kW load and at rated power factor long enough to trip the breaker in actual use.

Alternatively, a 100% rated 800---ampere circuit breaker may be used that will carry 800 amperes continuously.

A second approach using standard (80% continuous) rated circuit breakers is to oversize the circuit breaker by 1.25 times the generator full load current. For example, a 1000---ampere circuit breaker would be selected for a generator with a 751---ampere full load current rating (751 amperes x 1.25 = 939 amperes, the next higher standard breaker rating equals 1000 amperes). A breaker selected this way should not trip under full kW load at rated power factor (rated kVA). The disadvantage of this approach is that the cables and distribution panel or transfer switch would need to be sized up to at least 1000 amperes.

Yet a third approach is to size the circuit breaker as the result of the design calculations for a feeder and its overcurrent device, recognizing that the principal purpose of the circuit breaker is to protect the feeder conductors. Feeder ampacity and overcurrent device rating are calculated by summing the load currents of the branch circuits multiplied by any applicable demand factors (DF) that are allowed by applicable electrical codes. Without allowing for future capacity, the minimum required feeder ampacity for a typical generator set application involving both motor and non---motor loads must equal or exceed:

• 1.25 x continuous non---motor load current, plus

• 1.00 x DF (demand factor) x non---continuous, non---motor load current, plus

• 1.25 x largest motor full --- load current, plus

• 1.00 x sum of full --- load currents of all other motors.

Add sizing gensets for transformers.

Because the generator set is sized for both starting (surge) and running load, and may also be sized to include future capacity, the generator set full --- load current may be greater than the calculated ampacity of the generator feeder conductors and circuit breaker. If this is the case, consider increasing both the feeder conductor ampacity and the circuit breaker rating so that the breaker will not trip at full generator nameplate current. This would also provide future capacity for the addition of branch circuits.

NOTE: Feeder conductor ampacity is regulated and determined by codes, such as NFPA or CSA.

While it is based on generator and CB capacity, other critical factors are also applied. Refer to applicable codes for correct feeder conductor sizing.

NOTE: Extended full--load testing may trip a main--line circuit breaker sized at or below the full--load current rating of the generator set.

When the energy for the emergency system is provided by a generator set, it is necessary to provide branch circuit breakers (usually of the molded case type) with a high probability of tripping, regardless of the type of fault which could occur in a branch circuit.

When a generator set is subjected to a phase---to---ground fault, or some phase---to---phase faults, it will supply several times more than rated current, regardless of the type of excitation system. Generally, this trips the magnetic element of a branch circuit breaker and clears the fault. With a self---excited generator set, there are instances of three---phase faults and certain phase---phase faults where the output current of the generator will initially rise to a value of about 10 times rated current, and then rapidly decay to a value well below rated current within a matter of cycles. With a

Generator Set Sources

5--36 5 -- ELECTRICAL DESIGN

sustaining (PMG) generator set, the initial fault currents are the same, but the current decays to a sustained short circuit current ranging from about 3 times rated current for a three---phase fault to about 7---1/2 times rated current for a phase---to---ground fault.

The decay in fault current of a self---excited generator requires that branch circuit breakers unlatch and clear in the 0.025 seconds during which the maximum current flows. A branch circuit breaker that does not trip and clear a fault can cause the self---excited generator to collapse, interrupting power to the un---faulted branches of the emergency system. A sustaining (PMG) generator does not collapse and has the advantage of providing about three times rated current for several seconds, which should be sufficient for clearing branch circuit breakers.

Using the full load current ratings of the generator set and of the branch circuit breaker, the following method determines if a branch breaker will trip on a three---phase or phase---to---phase symmetrical fault. The method only determines if tripping is possible under short circuit conditions with the available fault current, and does not guarantee tripping for all values of fault current (in arcing faults, for instance, where fault impedance is high).

Because most circuit breaker charts express current as a percentage of the breaker rating, the available fault current must be converted to a percentage of the circuit breaker rating. Use the following formula to determine the available fault current as a percentage of the circuit breaker (CB) rating for an AC generator capable of delivering 10 times rated current initially (X′′d = 0.10), ignoring circuit impedance between the generator and the breaker:

Rated CB Amps Fault Current as % of CB rating

10•Rated Generator Amps •100%

Consider the effect of a fault (short circuit) on a 100 ampere branch circuit breaker when power is supplied by a generator set having a rated current of 347 amperes. In this example, the fault current available for the first 0.025 seconds, regardless of excitation system, is:

Fault Current as % of CB rating 10 • 347 • 100% = 3470%

If the AC generator is of the type that can sustain three times rated current, use the following formula to determine the approximate current available as a percentage of the circuit breaker rating:

Sustained Current as % of CB rating

3 • 347 • 100% = 1040%

Figure 5---18 and Figure 5---19 show the results with two 100 ampere thermal---magnetic molded case circuit breakers having different trip characteristics, “A” and “B.” With trip characteristic “A” (Figure 5---18), the initial fault current of 3470% will trip the breaker within 0.025 seconds. With trip characteristic “B” (Figure 5---19), the breaker may not trip with the 3470% current available initially, but will trip in approximately three seconds if fault current is sustained at 1040% of the breaker rating (three times the generator rating). The conclusion is that a sustaining (PMG) generator offers an advantage in providing sufficient fault current to clear branch circuit breakers.

The application of the generator, its excitation system, and operating voltage, determine the extent of overload protection provided for generators and the protective devices used.

NOTE: The following discussion applies for single--unit installations, 2000 kW and below. Refer to Cummins Power Generation publication T–016, Paralleling and Paralleling Switchgear, for protection requirements of multiple generators in parallel.

5--375 -- ELECTRICAL DESIGN

Generator Decrement Curves For the purposes of protection coordination, and to determine the magnetic and mechanical stresses on the electrical system components, it is necessary to know the available short circuit current during the initial few cycles from a generator set. The generator decrement curve represents the fault current during Sub transient, Transient and steady state periods. A typical Generator decrement curve is shown in Figure 5---17.

10000

100000

0.001 0.01 0.1 1 10 TIME (secs)

C

UR

R E

NT

(A m ps

SYMMETRICAL

ASYMMETRICAL

Figure 5---17 Generator Decrement Curve

When the Generator is provided with separate excitation, (e.g. a Permanent Magnet Generator or PMG) the sustained short circuit current will flow until the inbuilt protection operates after about 8---10 seconds. For self excited machines, the initial current decay continues without the recovery illustrated and tends to a very low value in approximately 1 second.

Where Cummins PCC (PowerCommand Control) controls are provided with AmpSentryt the sustained short circuit current will be limited by excitation control to approximately three---times full load current for both symmetrical and asymmetrical cases. In cases where PCC controls fitted with AmpSentryt are not used, it is essential that an independent protection device such as a circuit breaker is fitted to limit the current / time to within the alternator thermal damage curve (see below), particularly for Line---to---Line and Line---to---Neutral faults. This may require the fitting of additional protection such as ground---fault.

Multiplication factors for various voltage ratings and for 50 and 60 Hz are provided in the data sheet, together with factors representing the symmetrical, 2---phase and single phase cases with waveform asymmetry. The multiplication factors listed in Table 5---2 should be used to adjust the values from curves between 0.001 seconds and the minimum current point in respect of nominal operating voltage.

5--38 5 -- ELECTRICAL DESIGN

Voltage Factor

380 x 1.00

400 x 1.03

415 x 1.05

440 x 1.07

Table 5--2 Multiplication Factors for Minimum Current Point

NOTE: The sustained current value is constant in Table 5---2, irrespective of Voltage Level.

The multiplication factors listed in Table 5---3 should be used to convert the values calculated in accordance with Table 5---2 to those applicable in various types of short circuit.

3 Phase 2 Phase L---L 1 Phase L---N

Instantaneous x 1.0 x 0.87 x 1.3

Minimum x 1.0 x 1.80 x 3.20

Sustained x 1.0 x 1.50 x 2.50

Max Sustained Duration 10 sec 5 sec 2 sec

Table 5--3 Multiplication Factors for Short Circuit Types

NOTE: All times other than those listed in Table 5---3 are unchanged.

In low voltage (600 volts and below) emergency/standby applications where critical loads are being served and the generator set runs a relatively small number of hours per year, the minimum protection requirements of applicable electrical codes should be met.

Beyond that, the specifying engineer should consider the tradeoff between equipment protection and continuity of power to critical loads, and may decide to provide more than the minimum level of protection.

In low---voltage prime power or interruptible applications, the loss of power that would result from operation of the protective devices may be tolerable and, therefore, a higher level of equipment protection would be appropriate.

Protection Zone The zone of protection for generators includes the generator and the conductors from the generator terminals to the first overcurrent device; a main---line overcurrent device (if used), or the feeder overcurrent device bus. Thermal overcurrent protection is achieved by comparing the thermal damage curve of the alternator to the trip or operation curve of the protective device. Overcurrent protection for the generator should include protection for short circuit faults anywhere within this zone.

Overload Protection of Generators

5--395 -- ELECTRICAL DESIGN

ALTERNATOR THERMAL DAMAGE THRESHOLD

Figure 5---18. Fault Effect on a 100 Ampere Breaker with Trip Characteristic “A”

On the downstream side of the feeder bus, standard practice for overcurrent protection of conductors and equipment applies. The ratio of generator rated current to the rating of downstream overcurrent devices, multiplied by the short circuit current available from the generator in the first few cycles, should be sufficient for tripping these devices within one to two cycles.

Emergency/Standby Systems 600 Volts and Below The minimum generator overload protection required by applicable electrical codes is recommended for Emergency/Standby applications 600 volts and below. Typically, this means the generator should be provided with phase overcurrent devices such as fuses or circuit breakers, or be protected by inherent design, such as PowerCommand AmpSentryt. In some applications, the electrical code may also require ground fault indication.

5--40 5 -- ELECTRICAL DESIGN

ALTERNATOR THERMAL DAMAGE THRESHOLD

Figure 5---19. Fault Effect on a 100 Ampere Breaker with Trip Characteristic “B”

Generator Circuit Breaker Conventional practice on generators without inherent overcurrent protection is to provide a molded case circuit breaker (MCCB), either thermal---magnetic or solid---state, sized to protect the generator feeder conductors, in order to satisfy electrical code requirements for generator overload protection. However, a typical thermal---magnetic MCCB sized to carry generator rated current does not provide effective generator protection. Generally, if circuit breakers are used for generator protection, a solid---state circuit breaker with full adjustments (Long time, Short time and Instantaneous, LSI) will be required to coordinate the breaker protection curve within the generator thermal capability curve.

Where the generator is protected by inherent design, as generators with PowerCommand Amp Sentryt, the use of a main---line circuit breaker for generator overload protection is not required.

Inherent Design, Balanced Faults A self---excited (Shunt) generator may be considered to be protected by inherent design since it is not capable of sustaining short circuit current into balanced three---phase faults long enough for serious damage to occur to the generator. Considering the need for high reliability of power to critical loads, use of shunt excitation is sometimes considered sufficient to meet the minimum generator protection required by electrical code by inherent design and make generator overcurrent protective devices (fuses or circuit breakers) unnecessary.

5--415 -- ELECTRICAL DESIGN

A generator with PMG excitation, but without PowerCommand, is capable of sustaining short circuit current with an unbalanced or balanced fault. If overcurrent devices downstream of the generator should fail to clear a balanced three---phase short circuit fault, the PMG excitation system includes an over---excitation shutdown function that will serve as “backup”. This over---excitation function will shut down the voltage regulator after about 8 to10 seconds.