6 Design Requirements
6.1 General
The generator sets shall be engineered, manufactured and supplied in accordance with Gen Packaged Unit AC Generator Sets.
The generator sets shall be skid mounted, diesel engine driven units suitable for robust oil field service. The generators shall have the following output characteristics; 415 volt, 50Hz, 3 phase, 4 wire. Frequency stability ± 2%. Voltage stability ±5% as per PROJECT REQUIREMENT. Power requirements are given on the AC generator data sheets.
A 10% overload shall be sustainable for one hour in any 12 hour period.
A 24 hour diesel day tank shall be supplied with the diesel engine.
In case of given Mechanical or Electrical fault, the electrical load shall be automatically disconnected, the engine shall shutdown and be locked out. Fault conditions necessitating shut down are given in the requisition and data sheets.
All primary protection devices for the engine and alternator shall be fail-safe.
Exposed surfaces which attain a temperature in excess of 70oC under normal operating conditions and which are a potential hazard to personnel shall be insulated or screened.
6.2 Environmental Conditions
The complete skid-mounted generator will be installed on a flat concrete raft and shall be protected from the environmental conditions by a roofed structure partially clad on three sides. The Vendor is to exercise special care with the design of the equipment to minimise the effect on performance of the desert and saliferous conditions typical of regions.
6.3 Hazardous Area Classification
The generator and diesel engine skid will be installed in a non-hazardous area. The generator set, its equipment and services shall be designed and engineered to safely operate in the generator set area/enclosure as applicable.
The control panel and MCC will be located in a non-hazardous area.
If located in a hazardous area as stated in data sheet, the generator set shall conform to the relevant aspects of IEC 60079.
6.4 Plant Layout
The generator/engine assembly shall be mounted on a common baseplate. The baseplate shall be designed to suit transport, lift and installation conditions.
The Vendor shall supply grating, ladders, platforms and jacking points to facilitate positioning. Vendor shall also supply all davits, insulation rings, lugs, and pipe supports. Adequate access for maintenance shall be provided.
6.5 Diesel Engine
Diesel fuelled reciprocating internal combustion engine for specific duty given in the Requisition and Data Sheets. The engine shall satisfy the requirements of Gen ‘Diesel Engines’ Specification.
6.5.1 Cooling System
Closed circuit jacket water to be cooled with the use of an air cooled heat exchanger. Ambient air temperature set at 58oC as stated in ‘Basis of Design’.
6.5.2 Exhaust System
Exhaust silencer, with suitable spark arrestor. Stainless steel exhaust bellows supplied loose. Exhaust lagging to be supplied complete in mat form covering the exhaust manifold and piping.
Fittings for bellows including gaskets and bolts to be supplied loose with free end flange to ANSI 150 LB RF and a long sweep elbow to be supplied complete with loose flanges for site installation.
6.5.3 Lubricating System
The manufacturers standard system of lubrication on engine moving parts will be acceptable unless specified in the MR.
6.5.4 Starting System
Engine starting shall be by electric motor from a single set of nickel cadmium or lead-acid recombination type batteries, rated for twelve sequential starts. Each battery bank shall be sized for a cold start at minimum ambient temperature.
Batteries shall be charged either by an AC mains charger supplied by the manufacturer or by an engine driven alternator as specified in the MR. Both systems maybe specified, in which case an automatic change over system shall be supplied. The engine shall also be provided with manually operated hydraulic starter.
6.5.5 Fuel System
The fuel system shall be supplied complete with all necessary shut off valves, regulators, pressure gauges and filters. Piping shall be in accordance with the Project Piping Specification.
A fuel oil day tank shall be supplied by the VENDOR if specified in the MR. The tanks shall be manufactured from AISI 316L Stainless steel. The fuel tank shall be arranged to supply the engine by gravity via the pre-filter and the fuel supply pumps. The day tank shall be sized to provide 24 hours of fuel supply at the rated fuel consumption.
6.5.6 Protection Devices
A primary shutdown system operating on the fuel quick acting shut off valve and actuated by one or any combination of the following:-
- Fire/rate of temperature rise/smoke detectors
- High water temperature
- Low oil pressure
- Overspeed
- Overcrank
A secondary shutdown system operating on a ‘rig saver’ with an interlock to prevent engine cranking when tripped.
The rig saver solenoid operated shut off system shall have a complete set of sensors for overspeed, high water temperature and low lube oil pressure; the sensors shall be separate and independent from those used in the primary shutdown system.
Both of the above systems shall incorporate additional contacts to receive signals from the plant fire and gas detection system.
6.5.7 Control and Protection Equipment
The generator control and protection equipment and the control panel shall be in accordance with the requirements.
6.5.8 Fire Detection/ Protection
Vendor shall offer for Purchasers agreement engine fire detection/protection based on the following facilities:
- Fuel slam-shut shut-off valve operated by engine enclosure rate of rise of temperature detector, smoke detector.
- Fuel shut-off valve to also be operable via fusible link or other manufacturers protection device agreed with Purchaser.
- Enclosure to be furnished with fire alarm which will react to rate of rise of temperature detector, smoke detector.
- Smoke and heat detectors to be located to manufacturers standard but with agreement of Purchaser.
6.6 Electrical Systems
6.6.1 Rating and Duty
The alternator shall be a brushless CACA two bearing machine generally in accordance with the Specification. The minimum rated output shall be not less than specified in the data sheets. The alternator shall also be capable of meeting the diesel engine maximum short time capacity in kW at 0.8 power factor, at the specified site maximum ambient temperature.
6.6.2 Short Circuit Requirement
Generators shall be capable of withstanding without damage, a three-phase, line-to-line, or line-to-earth short circuit at generator terminals for a period of at least three seconds.
6.6.3 Sub-transient Reactance
Guaranteed values of the sub-transient, transient and synchronous reactance with their tolerances shall be given by the Vendor. These guaranteed values shall be compatible with the particular Voltage Regulation Grade specified for the machine and shall have a minimum sub-transient value as specified in the data sheet.
6.6.4 Momentary Excess Current
Generating units shall be capable of withstanding a current of 150% of rated value for 10 seconds with the voltage being maintained within 5% of rated value.
6.6.5 Windings
Insulation to earth shall be suitable for full line volts and shall not be graded. Non-hygroscopic materials shall be used to the maximum extent. The insulation shall be Class ‘F’, but temperature rises must not exceed Class ‘B’ as specified in BS 2757 (IEC85).
Both exciter and generator shall have an approved fungus treatment.
The Vendor shall demonstrate to the satisfaction of the Purchaser that hot spot winding temperatures do not exceed the limits stated in BS 2757 (IEC85) for Class ‘B’ insulating materials.
The methods for determining hot spot winding temperatures shall be agreed between the Vendor and the Purchaser prior to the tests taking place.
6.6.6 Winding Terminations
Stator windings shall be star connected with six ends brought out into separate line and neutral terminal boxes.
The neutral end of the windings shall be star connected within the neutral terminal box by means of an adequately sized removable solid copper link and a similar removable copper link shall be provided between the winding star connection and the point of connection of the generator neutral earthing resistor.
Terminal boxes shall be adequately sized to facilitate easy access for connection and removal of cables and sufficient space within the neutral terminal box to permit the inclusion of the current transformers required for load sharing and earth fault detection.
A terminal marking plate shall be fitted at an agreed position near the terminal box to facilitate the correct cabling and checking of the phase rotation in-situ.
Phase sequence shall be in the order U-V-W in accordance with BS 4999 Part 3 (or equivalent IEC Standard), when the machine is driven in the correct direction of rotation by the prime mover.
6.6.7 Cable Terminations
Cable terminations shall be by Raychem. Tapped ISO metric entries shall be provided in the cable box, or alternatively, a removable undrilled plate for the reception of such glands for drilling by the Purchaser shall be provided. Undrilled gland plates shall be suitably marked to facilitate correct line up with cables and terminal lug bolt or stud centres. Gland plates for single core power cables shall be non-magnetic.
6.6.8 Earthing of Star Point
The Star Point of all generators both Main and Black Start shall be connected via links to an earthing resistor supplied by Vendor.
6.7 Excitation System and AVR
6.7.1 Excitation System
A brushless excitation scheme shall be employed. Data shall be submitted with the tender documents to demonstrate the ability to meet the following specific performance requirements:
- Rapid build up of AC generator excitation from black-start conditions.
- The ability of the excitation system under AC generator short circuit conditions to meet the requirement specified in Clause 6.1.2 of this Specification.
The requirements of Clause 40.2 of BS 4999 shall be met for parallel operation.
The excitation system shall comprise a Permanent Magnet Generator (PMG), AC exciter, rotating diode rectifier, automatic voltage regulator (AVR), voltage adjuster, facilities for remote manual voltage control, automatic changeover to hand control incorporated on voltage failure of reference voltage to AVR, automatic adjusting of manual field rheostat by the automatic part of the AVR and generator trip contacts on voltage failure to the AVR. Where applicable the system shall include field discharge resistor, current transformers, field current and voltage metering, minimum frequency relay and diode failure detection.
6.7.2 Automatic Voltage Regulator (AVR)
The AVR shall be of the static type provided with a fine voltage adjuster and a switch for selecting hand or automatic control and shall include frequency sensing circuitry to limit ceiling voltage and prevent damage to components when the generator is driven at reduced speeds such as when starting or idling the prime mover. It shall have a droop of maximum 5%, adjustable over the full range and have a characteristic for reactive load sharing. The voltage level shall be adjustment within ± 5% of the rated value.
The voltage regulation grade will be specified in the data sheet according to BS 4999 Part 40.
The regulator shall be robustly built and shall contain components with adequate design and rating margins to ensure long life of equipment. A small heater shall be provided so as to prevent condensation when the equipment is on standby.
The Generator Vendor shall include in his supply the provision of AVR current transformers for mounting in the neutral cable box, see Clause 6.1.6.
The AVR and associated equipment shall be mounted on the Generating Unit local control panel by the Vendor.
The AVR shall be suitable for local adjustment, alternatively remote adjustment from controlling circuit breaker panel supplied by others according to position of a “Local/Remote” change-over switch on the local control panel.
The Vendor shall guarantee that the AVR is matched to the generator characteristics and is of adequate construction and design so as to fully meet the operational and performance requirements of this specification.
The AVR shall be provided with over-voltage and under-voltage protecting features.
6.8 Maintenance Access
The arrangement on the skid shall make adequate allowance for maintenance access. Any covers shall be designed for quick and simple removal. Access to the rotating excitation rectifier equipment shall permit easy maintenance in this area by a man using both hands only.
In the tender documents, the Vendor shall indicate minimum distance between each skid base and a laydown area if this is necessary for maintenance.
6.9 Rotor Withdrawal
Ease of rotor withdrawal to facilitate rapid inspection and repair in service shall be considered as a highly desirable feature. Vendor shall describe the sequence of operations required to effect rotor withdrawal and shall provide any special tools required.
Vendor shall specify the requirements.
6.10 Anti-Condensation Heaters
Suitably rated anti-condensation heaters shall be installed within each separate enclosure. Location and maximum surface temperatures of the heaters shall be such that no damage can be caused to any insulation when they are switched on. Heaters shall be suitable for the supply specified in the data sheet. The performance of the equipment shall nevertheless be guaranteed in the event that the heaters are not in operation.
6.11 Jacking Bolts
Generator feet shall be provided with jacking bolts or a similar arrangement to achieve alignment of shaft and coupling.
6.12 Rating and Indication Plates
A rating plate of corrosion resistant metal shall be fixed by screws to the AC generator frame and shall give the information set out in BS 4999 Part 4, for the type of machine being offered.
An engraved or embossed, non-corrodible metal plate shall be affixed to the generator stator to show the correct direction of rotation.
An exciter rating plate of corrosion resistant metal shall be fixed by screws to the exciter frame.
All labels and tags shall be made of 316 stainless steel, mechanically fixed in positions.
6.13 Automatic Start-Up on Failure of Mains Supply
In the event of loss of voltage at the busbars of the Essential Services Switchboard, a voltage relay with an adjustable time delay will provide a signal to initiate an automatic start-up sequence of the Generating Unit.
Automatic start-up equipment shall be provided by the Vendor and shall include as a minimum, the following facilities:
- Voltage detection with time delay to initiate automatic start-up.
- Tripping of incoming circuit breakers to the HV Black Start Switchboard to isolate this switchboard from the main supply.
- Automatic start-up and run up to speed of the Black Start Generator.
- After establishing full speed (frequency) and full voltage has been attained, a signal shall be given to the generator circuit breaker to close onto dead busbars. Subsequent loading of the generator shall be in accordance with requirements specified on generator data sheets.
6.14 Manual Start-Up
Provision of an auto/manual start switch on the local control panel will allow the generator start-up sequence to be initiated without detecting loss of voltage and without tripping the incoming circuit breakers or closing the generator circuit break (by others) on the LV essential services.
6.15 Manual Synchronising
It shall be possible to synchronise the black start generator to the main generation and load share. The black start generator should be capable of being ‘brought on-line’ with the main generation running via the load management system (by others) or by manual means by an auto/manual switch on the local control panel. With the black start generator running, it shall be possible to bring the main generation on-line and synchronise.