A flexible approach

    Roberta Galli and Giovanni Gennari, GE/Nuovo Pignone,
    Peter Grima, Enemalta Corporation

    Delimara power station is Malta’s first combined cycle power plant. It is a 116 MW combined cycle plant based on two gas turbines, each exhausting into a heat recovery steam generator (HRSG), and one steam turbine.

    Since summer 1999, after completing a 30 day trial run during which daily starts were successfully tested, Delimara has been running in parallel with the existing plants of the island, started every day in the morning and shut-down during night with an operating time of around 18 hours per day.

    The Maltese national electrical grid is totally isolated and so is subject to wide load fluctuations, primarily between night and day. This can be managed with difficulty by the existing conventional steam cycle power plants, which have a low load flexibility. Therefore an essential design requirement for Delimara was the ability to respond to the needs of the Maltese grid, demonstrate a high degree of load flexibility and maintain a high efficiency.

    Power island

    Delimara power station phase IIB is a 116 MW (ISO) combined cycle power plant based on two GE MS6001B gas turbines with heat recovery boilers and one single pressure steam turbine.

    The turbines are fired by a light distillate fuel. Each has an associated HRSG which delivers steam to a steam turbine. The combined cycle is in multi-shaft configuration with each gas and steam turbine having their own generator. The three generators are connected to one step-up, two winding, main transformer through circuit breakers. This is connected to an existing 132 kV switchboard through one 132 kV circuit breaker. Each gas turbine is equipped with a steam injection system for NOx emission control.

    STAG 206-B combined cycle schematic
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    Each gas turbine has an exhaust gas by-pass system to enable start-up and operation in simple cycle, independently of the rest of the plant’s systems. The exhaust gas bypass system includes a diverter damper and a stainless steel bypass stack. In normal operation, the gas turbines operate in combined cycle mode, exhausting into the HRSGs.

    Each HRSG is a simple recovery, vertical type (assisted circulation), two pressure level boiler with integral deaerator. The HRSG produces high pressure steam for feeding the steam turbine and low pressure steam for heating and degasing feedwater. The HRSG is designed for operation in sliding pressure mode; heat transfer is by convection from the turbine exhaust gas to feedwater/steam; feedwater/steam flows inside horizontally orientated finned tubes, arranged in rows, and the exhaust gas flows in the vertical direction across these tube rows.

    The GE steam turbine is a condensing type, axial exhaust flow with high pressure steam admission (130 t/h at 503à‚°C/44 bar[a]) and one medium pressure bleed (9 t/h at 410à‚°C/22.5 bar[a]) to be used for NOx emission control.

    Both gas and steam turbines are controlled by Mark V Triple Modular Redundance (TMR) panels; the whole plant is controlled by a plant Distributed Control System (DCS).

    Maltese demands

    Click here to enlarge image

    The Maltese national electrical grid is completely isolated, not being connected to any other European electrical grid. It therefore needs to be highly reliable and capable of covering the electrical requirements, which are typically subject to wide load fluctuations, primarily between night and day. Both average winter and summer workdays are characterized by high load fluctuation; the load varies from 135-150 MW in the early hours of the morning up to 260 MW at midday (summer workday) or in the late afternoon (typical winter workday).

    Malta has a total installed capacity of 551 MW: the majority of the electrical power generation (330 MW) is based on conventional steam plants which typically have a very low load flexibility; the remaining electrical production is split between medium size gas turbines in open cycle (111 MW), which are suitable for peak and emergency operation, and the Delimara plant.

    To be consistent with the needs of the electrical grid and the characteristics of the existing Maltese fleet, the new plant has been designed for operation on a two-shift basis and with a load cycle varying between 20 per cent and 100 per cent of the total combined cycle output. The combined cycle is started every day in the early morning and shut down in the evening.

    The plant can operate in several different configurations, providing a high degree of flexibility in power output while maintaining high efficiency:

    • Full bock with both gas turbines in combined cycle mode
    • Half block, with only one gas turbine in combined cycle mode
    • Gas turbines in open cycle mode, with the diverter dampers open to the atmosphere.

    In normal operation the plant is required to cover the grid load demand fluctuations (load following plant) and so it operates mostly at partial load.

    Typical daily load following at Delimara
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    In order to achieve the maximum plant efficiency in any allowed operating mode, great attention has been addressed to the regulation of the high pressure (HP) steam system. The steam turbine operates in sliding pressure mode, which determines pressure changes in the main steam header as a function of total boiler steam production.

    During start-up, shut-down and normal operation, the plant can be operated under either automatic or manual mode. When in manual control, the operator is responsible for loading and unloading the plant and for connection or disconnection of generating units other than HRSGs. The operator may also choose to control steam pressures directly by manually adjusting the relevant set points of the various steam pressure control valves. The operator’s requests override the automatic control, unless they are restricted in order to safeguard the plant; permissives and interlocks are used to prevent unsafe operation.

    Cycling components

    Daily start/stop is a heavy operating mode for major plant components, as well as for other items in the hot gas path and high temperature steam piping system.

    The ability to cover this service – with an expected minimum life of 20 years – had a strong impact on design and material selection at Delimara. The areas of concern were the exhausted gas path and the components of the high pressure steam system. The exhaust duct, diverter damper, expansion joints and bypass stack have been designed to have a minimum impact, while on boiler components, a dedicated calculation on expected life has been performed.

    The ducts are internally insulated in order to have an external skin temperature not higher than 30à‚°C above ambient temperature; this prevents large thermal displacements and high stress values.

    Click here to enlarge image

    The diverter damper is also internally insulated and the manufacturer has verified the expected lifetime in light of the operating mode. For this component, the concerns were not only about box insulation and casing thermal displacements, but also about the internal arms and the mechanisms for blade driving other than the sealing system.

    Expansion joints, flanged connections and their protections were selected and designed to avoid external overheating, which could cause local stress increase. The expansion joints are textile type and are equipped with insulated protection covers and/or booster bags against hot gas flow effects. Flanged connections have been designed to avoid inconsistencies in the internal insulation layer such as thermal bridges, which could collect heat from internal side to external.

    The bypass stack is not insulated. Thermal displacements are accomplished by sliding supports, on which the stack base is supported.

    The HRSG has been investigated and a dedicated assessment has been performed by the designer and manufacturer, Stork Ketels, to determine fatigue and creep effects on the lifetime of the internal components.

    Grid interface

    The combined cycle plant may be operated as a full bock with both gas turbines in combined cycle mode; or as a half block, with only one gas turbine in combined cycle mode and the other gas turbine either shut down or in open cycle mode. In order to optimize plant efficiency, the normal operation modes are either full block or half block when load demand does not justify continued operation of the full block. Operation of the gas turbines in open cycle is usually transient during start-up or shut-down sequences, although they are capable of continuous operation independent from their associated HRSG.

    Under normal operating mode, the combined cycle plant output is controlled through the load coordinator, which considers the combined cycle plant as a single generating unit and shares the load request among the individual generating units (coordinated control).

    The plant is required to act as a load following plant. For any given configuration and for any actual loading, the plant efficiency is optimized by the load coordinator. The load coordinator setpoint is variable (given by the operator) on a droop characteristic line (four per cent). If frequency deviation rises, the plant responds according to a four per cent droop characteristic and stabilizes at a different frequency. Manual action or action by another unit is required to bring the frequency back to 50 Hz.

    The load coordinator has been designed with the following criteria:

    • Maximizing operational flexibility of the combined cycle plant
    • Maximizing plant efficiency, subject to actual operational configurations and loading
    • Controlling complete combined cycle block output as a single generating unit
    • Supporting the electrical system in the event of major system disturbance; plant response shall be either through the droop function or through frequency control, within the limits of plant capability
    • Automatic actions restricting the operator control in order to safeguard the plant
    • Change-over from automatic to manual control and vice-versa is automatic following operator request, and is ‘bumpless’ to the electrical system
    • All changes in load setpoints, either as a result of system changes or operator action are restricted by maximum loading ramps, based on the maximum permitted thermal change or rate of change.

    Operator requests override the automatic control, unless it is restricted in order to safeguard the plant. During plant operation, the operator may select either coordinated load control or manual load control. Coordinated load control is available only when both gas turbines are in operation (regardless of HRSG) or when a complete half-block is in operation. When in load control (either coordinated or manual) the only mode of operation for the plant is droop control with the load setpoint being given by the operator.

    The load coordinator may operate either in droop control or in frequency control. When load changes on the electrical grid, while operating in droop control, the load coordinator shall respond according to the droop characteristic and stabilize at a new load consistent with the system demand and with the response of the other connected units, at a slightly different frequency. The operator shall either take manual action to bring the frequency back to normal value or another unit operating in frequency control shall carry out this function. In frequency control mode, the unit attempts to maintain the system frequency at 50 Hz.

    With both gas turbines and HRSGs and the steam turbine in service, the function of the coordinated load control is to equalize the gas turbine loads. When a half-block is in combined cycle operation with the other gas turbine in open cycle, the coordinated load control maximizes the output of the combined cycle half-block. The half-block combined cycle gas turbine (CCGT) is at base load and the open cycle gas turbine (OCGT) has a variable load from minimum load (4 MW) to maximum load.

    If total load required from this plant arrangement is less than the minimum load, which is half-block in combined cycle at base load plus one gas turbine at minimum load, then the plant shall exit coordinated load control and revert to manual load control. If both gas turbines are in open cycle, then the coordinated load shall load both gas turbines equally.

    There is no coordinated load control during plant start-up or shut-down. However the load coordinator droop functions are always active. In other words, the block output may vary according to the grid demand while connection or disconnection operations are in progress for either one or both HRSGs and the steam turbine.

    If the plant is in coordinated load control and an external system disturbance occurs so that the system frequency changes by à‚±1 Hz (with a short time deadband), the control automatically switches in frequency control, in order to support the electrical network within the permitted limits of maximum and minimum continuous output of the plant configuration.

    If the plant is in manual load control, or if it has been switched to manual control as a result of a plant trip, then the plant shall respond according to its droop function to support the electrical system within the limits of minimum and maximum load. Plant load control may be switched to coordinated load control or frequency control, if required by the operator and allowed by plant configuration.

    Normal shut-down of the plant is started by an operator command. Other plant shutdowns occur as a result of the intervention of the plant protective system. During shut-down of the whole or part of the plant, load control shall switch from coordinated load control to manual load control. The operator may command either an auto (sequential) or a manual shut-down.

    During the plant shutdown the operator setpoints are fixed, however the plant shall respond to any system changes according to the individual gas turbine’s droop characteristics.

    Delimara power station started commercial operation in September 1999. Since then, it has been operating as a load following plant with typical daily load demand fluctuating between 66 per cent and 96 per cent of plant base load and with plant efficiency varying between 36.4 per cent (at 40 per cent load) and 46.9 per cent (at 100 per cent load). Delimara is a good example of combined cycle technology successfully applied for cycling duty operation mode.

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