TABLE OF CONTENTS SCOPE...2 REFERENCES ...2 DEFINITIONS ...2 POWER SOURCE ...3 GENERAL ...3
COORDINATION WITH UTILITY...3
SERVICE RELIABILITY ...3
METERING...3
SUBSTATION ...3
POWER DISTRIBUTION SYSTEM TYPE ...4
SERVICE REQUIREMENTS ...4
GENERAL ...4
PRIMARY SERVICE ...4
LOW VOLTAGE SERVICE ...4
VOLTAGE ...4
RATED AND NOMINAL VOLTAGE ...4
VOLTAGE SELECTION ...5
VOLTAGE SPREAD...5
VOLTAGE DROP...6
VOLTAGE DROP CALCULATIONS...6
EQUIPMENT VOLTAGE ...6 SYSTEM CAPACITY ...7 DELIVERY CAPACITY...7 SHORT-CIRCUIT CAPACITY...7 FAULT CALCULATIONS...7 POWER FACTOR...7 CODE COMPLIANCE ...8 SYSTEM GROUNDING ...8 GENERAL ...8
SYSTEM NEUTRAL GROUNDING...8
SOLID GROUNDING ...8
UNGROUNDED SYSTEMS...8
GROUNDING INTERCONNECTION ...9
FIGURE 1 TYPICAL RADIAL DISTRIBUTION SYSTEM ...10
Changes to the prevision revision are marked with an →.
SCOPE
This Design Practice is an overview of basic electrical systems including the source and distribution of electric power utilized in an Exxon Marketing Distribution terminal.
Instrument/computer power supplies and emergency power supplies are not included.
REFERENCES ENGINEERING PRACTICES
EP 16-16-1 Area Classifications and Related Electrical Design EP 16-16-2 Design Criteria for Lighting
EP 16-16-3 Design Criteria for Power Systems EP 16-16-4 Transformers
EP 16-16-5 Low Voltage Control Devices
EP 16-16-6 Wiring Methods and Material Selection EP 16-16-7 Design Criteria for Grounding
EP 16-10-1 Motor Control Centers EP 16-10-2 NEMA Frame Motors
OTHER LITERATURE
ANSI C84.1 Voltage Ratings for Electric Power Systems and Equipment (60 HZ) ANSI/NEMA MG1 Motors and Generators
NFPA 70 National Electrical Code (NEC)
NFPA 30 Flammable and Combustible Liquids Code NFPA 77 Static Electricity
NFPA 78 Lightning Protection Code
ANSI/IEEE Std 141 IEEE Recommended Practice for Electric Power Distribution for Industrial Plants (IEEE Red Book) ANSI/IEEE Std 142 IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
(IEEE Green Book)
ANSI/IEEE Std 242 IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (IEEE Buff Book)
ANSI/IEEE Std 493 IEEE Recommended Practice for Design of Reliable Industrial and Commercial Power Systems (IEEE Gold Book)
DEFINITIONS Base Load: The minimum load over a given period of time.
Demand: The load integrated over a specified interval of time expressed in kW or kVA.
Demand Factor: The ratio of the maximum demand of a system to the total connected load of the system. High Voltage System: Equipment with a normal operating voltage in excess of 69kV.
Load Factor: The ratio of the average load over a designated period of time to the peak load occurring in that period. Low Voltage System: Equipment with a normal operating voltage of 600 volts or less.
Medium Voltage System: Equipment with a normal operating voltage in the range 601 volts to 69kV.
POWER SOURCE GENERAL
→ Terminal facilities are typically served with electric power from a public utility company. The utility's transmission or distribution system is extended to the terminal property and transformation facilities are provided on terminal property to reduce the incoming line voltage to the terminal distribution voltage.
It is generally preferable for the utility to own the substation. However, for some large facilities it may be economically beneficial in some cases for the terminal to own the main substation. Very careful studies should be performed to justify Exxon substation ownership.
The transformer shall be dedicated for the Marketing Distribution facility. This avoids the possible problems which may occur when two or more customers are served from one transformer. These problems include voltage drops from motor starting, generation of harmonics and service interruptions for maintenance.
→ Whether or not a dedicated transformer is supplied, there shall be a single main service disconnect device on Exxon property which deenergizes the complete site except for firefighting equipment. See Figure 1.
COORDINATION WITH UTILITY
Coordination with the utility to define the conditions of electric service should begin during the early design phase of a project. Contractual matters to be resolved include the delivery voltage and kVA capacity, optimum rate schedule, minimum monthly charge and in some cases guaranteed payment of the utility's construction charges.
Design of the terminal power system must be consistent with the type of service available from the utility and the requirements of the utility.
SERVICE RELIABILITY
Terminal facilities are typically served by a single transmission or distribution line. This type of service is subject to occasional power interruptions from lightning and other causes. Service through dual services increases the reliability but is not cost justifiable for normal Marketing Distribution terminals.
METERING
→ Energy usage and power demand is measured by kWH meters owned and maintained by the utility. Generally, these meters are calibrated annually by the utility and an adjustment is made if the meter error is ±1% as compared to a rotating standard whose calibration is traceable to the National Bureau of Standards. Experience over many years has shown the utility meters to be accurate within the required limits. Nevertheless, as main supply equipment is modified or expanded, solid state check kWH meters should be installed to fully satisfy Exxon audit concerns.
SUBSTATION
→ Power is typically brought to the facility by a utility owned distribution line. A utility owned 3-phase transformer is typically located on the terminal property for the exclusive use of the terminal at 480 volts, 3-phase.
→ The transformer shall be delta connected on the primary and solidly grounded wye on the secondary. There shall be three fused disconnects and three lightning arrestors on the primary side of the transformer.
→ From the transformer, 480 volt, 3-phase, 3-wire power is typically run to a meter pole or switch rack. A terminal main disconnect shall be provided at this point. See Figure 1.
POWER SOURCE (continued) POWER DISTRIBUTION SYSTEM TYPE
The electrical system for most truck loading terminals is a simple radial system (see Figure 1) with a single source with a central main Switchboard/MCC. Sometimes there are switchracks at strategic locations throughout the terminal.
Distribution in a radial system is at the utilization voltage. A single primary service and distribution transformer supply all feeders. There is no duplication of equipment. System investment is the lowest of all circuit arrangements.
Operation and expansion are simple. Loss of a cable, primary supply or transformer will cut off service. Equipment must be shut down to perform routine maintenance and servicing.
SERVICE REQUIREMENTS GENERAL
Each terminal must be provided with a service disconnecting means to disconnect the utility supply. Other service facilities should be provided as covered in this section.
PRIMARY SERVICE
This section applies where electric power is supplied to the terminal at the utility's transmission or distribution voltage and the main substation is owned by Exxon. This includes the transformer.
→ The service disconnect in this case shall be located on the primary side of the substation at the point of the incoming lines on Exxon property. The service disconnect should be a ground operated, three-pole disconnect, key-interlocked with the secondary breaker or fused load break switch if the service disconnect is not capable of breaking load.
Electrical equipment on the secondary side of the substation should be provided as described in the Section for Low Voltage Service.
For primary voltage service, the electrical conductors from the secondary side of the substation to the terminal disconnects are "feeder conductors" and not "service conductors". However, the conductors and associated equipment and the function of the conductors and equipment are the same in either case.
LOW VOLTAGE SERVICE
For low voltage electric service, the utility owns the transformer. The service disconnecting means typically should be a circuit breaker or fused disconnect switch mounted on the meter pole below the meter. This protects the main feeder conductors where they receive their supply and provides a convenient disconnecting means.
→ Other arrangements may be acceptable that meet the requirements of the National Electrical Code and the supplying utility if approved by the Technical Support Group.
VOLTAGE RATED AND NOMINAL VOLTAGE
→ The electrical industry has a number of standard voltages all within the same voltage class. This is done to compensate for the voltage drop between the source and the load. As an example, in a system having a "nominal system voltage" of 480 volts, transformers and motor starters would be rated at 480 volts while motors would be rated at 460 volts.
VOLTAGE (continued) VOLTAGE SELECTION
The distribution voltage for a terminal is generally 480 volts.
Some exceptions apply, one example a tank farm having numerous 460 volt NEMA frame motors at separated locations. To reduce distribution losses, power could be delivered to the tank farm at 2,400 or 4,160 volts and distributed at this level to area transformers serving the 460 volt motors.
Other cases might be where there are large barge loading pumps which operate at 4,000 volts.
VOLTAGE SPREAD
Voltage spread is the difference between the maximum and minimum voltages which appear at any location in a power system under normal operating conditions. Voltage spread is caused by variations in the utility supply voltage and by voltage drops in the terminal distribution system. Voltage spread is not intended to cover momentary changes such as those caused by motor starting or switching surges.
Variations in the utility supply voltage are normally limited by the utility using voltage regulators and transformers with automatic tap changers.
The voltage spread of interest in terminal applications is that measured at the terminals of the terminal utilization equipment. The maximum voltages will normally occur under light load conditions and the minimum voltages at full load conditions. Electrical utilization equipment is designed to operate at peak performance when the equipment terminal voltage is equal to the equipment rated voltage. Operating voltages above or below the equipment rated voltage will affect the performance and the operating life of the equipment. The effect will vary with the type of equipment and the amount of the voltage variation. Refer to ANSI/IEEE Std. 141, Section 3.5 for the effect of voltage variations on different types of electrical utilization equipment.
For NEMA frame motors, which are the principal load in terminal applications, the voltage spread at equipment terminals should not exceed minus four percent or plus eight percent of the rated voltage of the equipment served.
For medium voltage systems (2,400 and 4,160 volts), the voltage spread at equipment terminals should not exceed minus four percent or plus four percent of the rated voltage of the equipment served. The closer tolerance for medium voltage systems is to limit the maximum operating voltage for form wound motors.
The recommended limits for voltage spread at utilization equipment terminals are shown in Table 1.
→ TABLE 1
Transformer and Switchgear Operating Voltage Load Equipment Voltage Rating Maximum Voltage Spread Low Voltage 120 115 110 - 125 -4%, +8% 208 208 200 - 225 240 230 220 - 250 480 460 440 - 500 Medium Voltage 2400 2300 2200 - 2400 -4%, +4% 4160 4000 3850 - 4150
VOLTAGE (continued) VOLTAGE DROP
Voltage drop is the difference between the voltage at the power source and the voltage at the point of utilization, caused by the resistance of the connecting conductors.
The calculated voltage drop for a conductor shall not exceed the following: All motor feeders - 3 percent at full-load current
- 10 percent at full voltage locked rotor for starting Lighting feeders - 1 percent
Lighting branch circuits - 2 percent Other power loads - 3 percent
Voltage spread lighting fixtures shall not exceed ±5% of rated lamp or ballast voltage.
VOLTAGE DROP CALCULATIONS
Calculations should be performed for all motor and other significant loads to determine if the above requirements have been met.
EQUIPMENT VOLTAGE
AC voltage levels shall be limited to the following:
Low Voltage (LV), (≤≤600 VAC)
Single phase:
120V Lighting and small power loads 240V Small motor and heating loads
277V Mercury vapor, HID, and fluorescent lighting
480V Outdoor mercury vapor or high pressure sodium lighting and larger loads, such as heaters
Three phase:
208/120V Small motor and heating loads
480/277V General power usage, motors ½ HP and above
Medium Voltage (MV), (601-69k VAC, 3 ∅∅)
4160/2400V Large (≥250 HP) motor loads and distribution of power where economically justifiable
SYSTEM CAPACITY DELIVERY CAPACITY
The utility should be given: a) The total connected load b) The anticipated demand c) Future load growth d) Schedule
→ The utility’s proposal of supply must be reviewed and approved by the Technical Support Group.
If any capacity margin is deemed necessary it should be in the "demand" or else specifically tagged and presented to the utility as such.
SHORT-CIRCUIT CAPACITY
The short-circuit capacity of a power system is a measure of the maximum current that the system will deliver to any specified point. As current is a function of voltage, short-circuit capacities are normally stated in terms of MVA at a stated phase angle. The short-circuit capacity increases with the size and strength of the power system. For any given system, the short-circuit capacity may be represented by a range of values rather than one particular value, depending on the number of transmission lines in service and other factors.
The short-circuit capacity used for terminal design is the capacity at the point of delivery to the terminal from the utility. This is typically on the secondary of the main transformer (or transformer bank) when the main substation is owned by the utility, or on the primary of the main transformer when the main substation is owned by Exxon.
Some utilities furnish the total per unit impedance on a stated MVA base rather than the short-circuit MVA. The per unit impedance can be readily converted to short-circuit MVA if desired.
The available short-circuit capacity is required for short circuit studies pursuant to equipment selection and for calculating voltage drops during motor starting. The maximum available short-circuit MVA is used to determine the required rating of circuit breakers and fused motor starters. The minimum available short-circuit MVA is used to calculate voltage drops during motor starting.
Refer to the General Electric Co. publication Industrial Power System Data Book, Section 1, or the IEEE Red Book for information on short-circuit studies.
FAULT CALCULATIONS
A short-circuit analysis should be performed on all new facilities, to determine the rating of the electrical equipment to be provided.
In addition, short-circuit analysis should be performed whenever changes to the utility supply and/or transformer is contemplated. All existing equipment should then be inspected for adequate short circuit capability, withstand and interrupting.
POWER FACTOR
Without any correction, power factor in the range of 0.8 to 0.86 lagging can be expected for a load consisting of squirrel cage motors operating between half to full load.
There is no incentive to improve the inherent power factor of the load by using special motors or capacitors unless the utility insists, or there is a financial incentive in the power contract.
SYSTEM CAPACITY (continued) CODE COMPLIANCE
Most city, state and federal agencies base their regulations on the latest edition of the National Electrical Code, commonly designated the NEC. The NEC is published by the National Fire Protection Association and is designated NFPA-70. The NEC was created to offer minimum safety requirements, which might not be the optimum for operation and maintenance. It is the designer's responsibility to provide a system which fulfills installation, maintenance and operational requirements while meeting the minimum requirements of the NEC.
→ Note that the local Authority Having Jurisdiction (AHJ) might adopt regulations that are more stringent than the NEC's. Therefore, local codes should always be investigated.
SYSTEM GROUNDING GENERAL
Generally, power systems for new facilities should be solidly grounded at the neutral of the supply transformer.
SYSTEM NEUTRAL GROUNDING
A system neutral ground is a connection from the neutral point of a transformer or generator to earth. Some of the reasons for system neutral grounding are as follows:
a. To limit overvoltages appearing on the system from lightning, line surges, accidental contact with higher voltage lines or restriking ground faults.
b. To stabilize the circuit voltage to ground during normal operation.
c. To limit the potential difference between conductive materials enclosing electrical conductors and equipment. d. To facilitate overcurrent device operation in case of ground faults (solidly grounded or low resistance grounded).
SOLID GROUNDING
In solid grounding, the neutral point of the wiring system is connected directly to ground without the addition of any impedance.
UNGROUNDED SYSTEMS
What is commonly referred to as an ungrounded system is in reality high-reactance capacitively grounded as a result of the capacitive coupling to ground of every energized conductor.
While power systems for new terminal facilities should be grounded at all voltage levels, it is not uncommon for older terminals to have ungrounded delta systems. In the past, ungrounded systems have been installed in order to provide what was thought to be better electric service continuity than that provided by grounded systems. A single line-to-ground fault on an ungrounded system, if sustained, does not cause immediate tripping of the faulted circuit.
SYSTEM GROUNDING (continued)
For the duration of the ground fault on one conductor, the other two phase conductors throughout the entire metallic system are subjected to 73 percent overvoltage. However, a second ground fault on another phase of a circuit other than that where the original fault occurred causes a phase-to-phase fault, large short-circuit current flow, and tripping of both faulted circuits. If the fault at the first failure location is not cleared, sporadic low level arcing or restriking, at the first failure point, produced by the capacitive coupling to ground of the two ungrounded phases, may cause surge voltages four (4) to six (6) times normal voltage (to ground), severely stressing insulation systems.
GROUNDING INTERCONNECTION
Where a terminal is served by a utility transformer or transformer bank having a grounded neutral system, the terminal equipment grounding conductors must be interconnected with the utility system ground. This is necessary to provide a low impedance ground path for ground fault current to return to the neutral of the supplying transformer.
Refer to Section 7.3 of IEEE Std. 141-1986 and to the various requirements of Article 250 of the National Electrical
→
FIGURE 1
TYPICAL RADIAL DISTRIBUTION SYSTEM
NOTE:
1. A separate disconnect for the office building may be installed if there is adequate separation between the building and the rest of the terminal to keep communications equipment in service during a fire.
REVISION HISTORY Date Revision Description
1/94 0 Issued as DP 16-16-3, with minor changes to Revision C, dated 5/93. 3/97 1 Issued as DP 16-16-3, with minor changes to Revision 0, dated 1/94.
