The Sabiyah Power Plant, where Turbine Technology Services (TTS) re-engineered 60 Hz gas turbines for a 50 Hz environment
 

When the Kuwaiti Ministry of Electricity and Water needed new power generating capacity in a hurry, Turbine Technology Services took on a complex project to re-engineer gas turbine equipment built to operate at 60Hz, to 50 Hz. Patrick Morgan describes the challenges and success of a ground-breaking project.

Patrick Morgan, TTS

In response to serious power shortages in 2007, the government of Kuwait initiated an emergency power procurement effort to install several hundred megawatts of gas turbine fired generation. The project was such that only immediately available equipment could be used.

Three projects were quickly awarded. Two used available aeroderivative gas turbine gensets but the equipment chosen for the third contract became unavailable, forcing the client to urgently seek alternatives.

At the time there was a shortage of available 50 Hz units (the frequency of the Kuwaiti grid), causing this client to look at 60 Hz units. The client eventually picked four GE Frame 7EA gas turbines, each rated at 84 MWe. The units had been operational in the US and the purchase was made on the assumption that these units could easily be converted for use at 50 Hz – which was not the case.

Turbine Technology Services (TTS) was awarded the contract by the plant owner, Combined Group Contracting, Kuwait (CGC), to project manage and engineer the re-application of these units at the Kuwait Ministry of Electricity and Water’s (MEW) Sabiyah Power Plant. This ground-breaking project became the first ever application of Frame 7EA gas turbine generators into a 50 Hz environment.

The conversion required extensive modifications to the gas turbines, including the addition of load gearboxes, as well as generator changes and modifications to all auxiliary systems. As insufficient natural gas fuel was available at site, the units were also modified from dry low NOx gas-only combustion, to dual-fuel standard combustors, with water injection added to meet contract emission requirements.

The fast-track nature of the contract meant no advance engineering or procurement of any kind was performed until the units arrived in Kuwait. All necessary design engineering (frequency conversion, fuel conversion, HV interconnect and protection, gas compression, liquid fuel handling, water treatment, etc.) was performed on site.

The units entered baseload operation on liquid fuel in March 2009 and on natural gas in April 2010. The units have run successfully since commissioning and the plant has been used extensively in the Kuwaiti summer, when temperatures routinely exceed 50 °C.

Frequency conversion

The gas turbines were already on site at the Sabiyah power plant in Kuwait when TTS first assumed responsibility for project engineering in October 2007.

In the first months of the project, TTS mobilized an engineering team to site to review the condition of all parts and generate an action plan. The power plant was required for emergency power, so delivery time was a primary consideration when awarding subcontracts.

GE Frame 7EA gas turbines are direct-drive units, designed for 60 Hz grids. TTS had two choices for electrical frequency conversion of the output power: a reduction gearbox to change generator speed from 3600 rpm to 3000 rpm, or a static frequency conversion using an enormous DC rectifier. Upon investigation, it was found that suitable DC rectifiers were extremely expensive and had unacceptable delivery times. The best choice was to move ahead with the gearbox and system modifications.

As a result of the gearbox addition, the following work and equipment was required:

  • a complete turbomachinery shaft train analysis;
  • new high speed, rigid load coupling;
  • auxiliary gearbox lube oil skid and coolers;
  • generator modifications (replacing bearings and changing cooling fans) in order to accommodate the generator rotor now turning slower and in the opposite direction.

All on-base auxiliaries such as pumps and fans had to be modified to run at 50 Hz, with a slower speed of rotation. TTS implemented changes for all affected components to restore original design performance. Three large diesel generators were also installed to provide 60 Hz power to the starting package.

Gearbox and lube oil module

A single double-helical reduction gearbox was designed to couple the hot end of the turbine shaft, at 3600 rpm, to the open end of the generator shaft, now at 3000 rpm. A new foundation was designed and poured, with ample room between the equipment for the additional load gear, enclosed vented compartment and oil feed and drain piping.

A complete turbo-machinery drive train analysis was performed to calculate the stress limits of the materials and equipment involved to allay concerns about rotor critical speeds and confirm the drive train and the materials could handle the additional dynamic stresses.

A new high-speed rigid load coupling was designed for the turbine side of the gearbox. To protect the gearbox from the high inertial forces and the sensitivity of this new rigid 3.3 metre drive shaft, eight proximity vibration detectors were installed – two at the end of each gearbox shaft.

The new load gearbox required a new auxiliary closed-loop lube oil system, complete with integrated oil-to-air heat exchanging coolers. Each auxiliary skid contained a 6300 litre reservoir, with dual AC powered pumps rated at 1500 litre per minute at nearly 7 bar pressure. The closed-loop system had an emergency DC oil pump to protect the load gear upon loss of power.

Adding the load gear not only slowed generator speed but reversed the direction of rotation. This required new bearing sets and change-out of the rotor cooling fans. The generators were completely disassembled for installation, and the rotors were reinstalled.

Starting modules and auxiliaries

The starting means for the units are 60 Hz, 4.16 kV AC motors. The price, delivery and engineering challenges of rewinding the motors and reconfiguring the motor control, or of installing diesel powered starting means, were unfeasible. Three 1 MW/13.8 kV/60 Hz diesel generators were installed in parallel on site to provide sufficient electrical power to start all four turbines at the same time without replacing any original equipment. Site-wide, all AC pumps were modified to compensate for the frequency change through motor rewind and impeller replacement.

Fuel system

The contract demanded less than 50 ppm NOx on gas and 165 ppm NOx on liquid. The gas requirement was satisfied directly with the dry low NOx (DLN) design, but on liquid fuel, water injection to the combustion system was required to cut emissions.

But the main concern was that the gas-only DLN system is unsuitable when the turbines run for extended periods on liquid fuel, the likely scenario for this plant. After a full engineering analysis, it was decided to convert the combustion system into a standard diffusion combustion system, through rework of all on-base combustion equipment, including the fuel system components, atomizing air system, water injection system, piping and instrumentation and the controls.

The fuel conversion on the units was from a gas-only DLN 1 to a dual-fuel standard diffusion system, using natural gas and distillate liquid fuel, with water injection available for both modes of operation.

The original DLN gas fuel compartment was shipped with the turbines and was modified for the conversion. Of the three gas manifolds in the turbine compartment of the unit, one remained as installed for use as the single gas manifold. A second manifold was kept in place and re-piped to distribute atomizing air. The third manifold was removed, and the space was used to fit the manifold for the added water injection system.

To accommodate the combustion changes, the entire site of combustion hardware was redesigned and replaced: combustion cans, liners, flow sleeves, can covers and fuel nozzles. The original flame scanners were incorporated into the standard diffusion can. Breech loaded nozzles for water injection were not available within a reasonable delivery time and the can covers were completely re-designed. Each cover incorporated a single water injection tap point and an internal manifold, designed to distribute demineralized water to 12 spray nozzles, built into the inside surface of the cover itself.

Balance of plant (BOP) engineering

The plant required identifying, engineering and procuring many new systems – including gas compression, liquid fuel handling and storage, HV interconnects at both 275 kV and 132 kV, and water treatment. Once this equipment had been ordered, TTS had to integrate control systems of varying vintage, complexity and accessibility across a large physical area and provide operational information seamlessly to the plant operation and maintenance (O&M) team.

New BOP fluid systems

Four water injection skids were designed and constructed using TTS’s ElectroFlo valves; one for each unit. The power plant is next to a large desalination plant but its distilled water had to be further treated to obtain demineralized water for injection. A single water deionization plant was designed and constructed with mixed-bed ion exchangers to forward 200 m3/h of demineralized water, with a storage capacity of 227 m3.

Though gas was available at site, the supply pressure of the main header was too low for four 84 MWe engines along with the six existing aeroderivative turbines at an adjacent site. Four gas compressors were purchased and installed, with one to stay in continuous standby. The 1.4 km gas pipeline to the compressor area was integrated into the main gas supply line at site using a hot tap procedure, where the supply pipe was drilled into without shutting down gas to the site.

At the time of construction and startup, there was no distillate fuel supply pipeline to site. Two 1800 m3 fuel oil tanks were designed and built for fuel forwarding supply. The entire liquid fuel system was designed new, incorporating eight fuel forwarding pumps, two for each unit, and two 100 m3 forwarding tanks. One of these horizontal tanks was designed to forward fuel to the three 60 Hz diesel generators used to provide electrical power to the plant’s AC starting means.

HV interconnect and station transformers

MEW allocated two high-voltage (HV) connection locations: 132 kV connection points in switchgear in the desalination plant and a 275 kV point in a single spare bay in the HV switchyard. For the 132 kV connections, two turbines were connected by individual 11.5 kV/132 kV step-up transformers. The 275 kV connection was through a 11.5 kV/11.5 kV/275 kV step-up transformer for two gas turbines. To maintain site flexibility for auxiliary power, back feeds were made available from either the 132 kV or 275 kV system.

All necessary power system engineering was done on site, including a complete system protection study, load flow analysis, switchgear design, new equipment protection design and the modification and integration into existing site protection schemes. A comprehensive set of site one line diagrams were developed from 275 kV to 110 VAC.

Plant-wide control systems

Each Frame 7EA gas turbine is controlled by a GE SpeedTronic MK VI gas turbine control system and features ‘triple modular redundancy’ as well as integrated control, protection and monitoring. Liquid fuel and input/output (I/O) for the new gearbox and auxiliary systems required significant reprogramming and hardware installation.

TTS picked Allen Bradley’s ControLogix Programmable Automation Controller (PAC) for the BOP control system. The TTS design included redundant I/O communications over a fibre network, multiple links using Ethernet TCP/IP, Modbus TCP and Allen Bradley’s Producer Consumer model over Ethernet. An integrated GPS was used to ensure a common plant time stamp.

This complex network could then provide information through OPC to the custom-built central control room for the O&M team. A Modicon PLC was provided for the water treatment plant to control production of desalinated water for water injection. An Allen Bradley CompactLogix system was provided for the gas compressors, with one PLC per compressor and a master PLC for load balancing. For the customer, a RTU interface for monitoring performance parameters at the NCC was provided through a remote OPC interface.

Multiple communications links were interfaced with the Allen Bradley PLC including: Ethernet TCP/IP, Control/Net, Modbus TCP, Modbus RS485, and Allen Bradley’s Producer-Consumer model over Ethernet. As highlighted above an integrated GPS was used to ensure a common plant time stamp for all systems. All the information from these components is then distributed using the OPC protocol to the central control room.

The original plant was designed by GE to have one human-machine interface (HMI) per unit, as well as a ‘master’ HMI which manages the databases for each unit and provides a means for remote control of all units. TTS left this system intact and integrated the BOP system as an additional layer to the existing GE system.

Central control room

TTS provided a custom-built central control room where four servers delivered: a historical workstation; an engineering workstation with licences for changing logic and HMI screens; two HMI workstations, primarily for BOP and unit data.

Each HMI workstation has several independent screens that can display information from any of the four units or from any of the auxiliary BOP systems. The historian workstation has GE Proficy Historian software. The engineering workstation is designed as the central hub for all communications related to control and monitoring for the BOP system. OPC, the primary transport of data, enables TTS to integrate the Allen Bradley PLC with the existing unit HMIs, including alarms and events. The high level of integration lets TTS provide the customer with a complete set of data points for historical logging.

Communications redundancy was provided at the hardware level via multiple Ethernet connections and Ethernet appliances. Each workstation has a highly reliable enterprise-level server chassis with a RAID controller for redundant data storage and the capability to ‘hot-swap’ a bad hard drive. Five large battery backups provide enough power to run the entire system for 30 minutes.

Summary

A groundbreaking 334 MWe project, utilizing GE Frame 7EAs for the first time in a 50 Hz grid, was installed as part of an emergency power project. No advance engineering was performed prior to purchase, leaving TTS to fully engineer, design and procure all required systems. All design work was performed in the field. This extremely complex project was completed successfully and is fully operational. MEE

Patrick Morgan is vice president, engineering, with Turbine Technology Services, Orlando, Florida, US. Email: rmorgan@turbinetech.com

This article is published with the permission of Combined Group Contracting (CGC).
Special thanks are given to Hashim Alusif (CGC project manager) and Humood Al-Enezi (CGC executive manager).

This article was first printed in the November–December 2010 issue of Cogeneration and On-Site Power magazine.

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