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The same design criteria apply to the electric production is affected by the different GT load but
power production system. In particular, in the case can be readjusted by resetting the post-fring load.
of the operation of the Plant disconnected from the In normal operation the two existing package
external grid, the new COGEN, integrated with the boilers operate at constant minimum steam
existing steam turbine of the Dow plant, is capable production level set by the operator.
of producing at least 110 MWe, which corresponds
to the frm motors load power demand of Dow
Plant, also when one generator is out of service Rearrangement of electrical
(the largest, i.e. one gas turbine). system
The confguration of the new COGEN in terms of
number of main equipment items and capacity The Plant network is connected to the 220 kV
of the main equipment is dictated not only by external public grid (fed from two 220 kV overhead
the application of the Plant in terms of fexibility lines) via a double busbar switchyard. Five power
of steam and electric power supply. In fact, the transformers are connected to the 30 kV and
integrated COGEN and package boilers have to be (via additional step-down transformers) to 15 kV
able to follow the fuctuating energy demand of the primary distribution systems. Through several
Plant, maintaining in different operating scenarios a secondary substations, all the Dow site process
reasonably high effciency of generation with all the loads are supplied from the primary distribution
generation components in operation. network.
Energy demand variations are: The electrical system design is based on a dual
• from 330 t/h to 430 t/h (380 t/h avg) of steam; source power distribution philosophy (secondary
• from 110 MWe to 150 MWe of electric power. selective scheme), down to the lowest utilization
voltage level and automatic transfer with capability
The confguration resulting from these requirements from one source to the other.
of reliability and fexibility is represented in fgure The generating units are directly connected to
1, showing the main COGEN components and the primary distribution system of the chemical
capacities, the integration with the complex, which is in turn connected
existing Stade facilities and the main The integrated COGEN in parallel with the 220 kV external
elements of the steam and electric and package boilers public grid. The new generation
power balance in normal conditions. have to be able to units and the existing steam turbine
Steam is produced at high pressure follow the fuctuating generator can provide around
in the new HRSGs and in the 165 MW at full load capacity and
existing package boilers. Part of it energy demand of the reference conditions. The remaining
is used in an existing back-pressure Plant, maintaining in difference to the total site power
steam turbine and in a new back- different operating demand is taken from the 220 kV
pressure steam turbine (component scenarios a reasonably public grid.
of the COGEN) to generate energy. high effciency of The power balance is continuously
HP and MP steam are distributed to monitored and controlled by
the Plant users through the existing generation with the existing SCADA system
distribution network. all the generation (Supervisory Control Automation
Electrical and steam production are components in Data Acquisition), which has been
controlled by two master controllers. operation upgraded to manage active and
The electrical production is managed reactive power sharing among all
by automatically setting the gas turbines’ load generating units, to fulfl public grid constraints (i.e.
through the electric power master controller, while do not exceed maximum active and reactive feed-
the steam production is managed by setting the in power/energy fxed by commercial agreement)
HRSG post-fring load through the steam master and also to manage load shedding to ensure
controller. So, the HRSG post- fring assures the safe and reliable operation under exceptional
independence of steam control from electric power and emergency conditions (e.g. during “island
control: if the gas turbines’ load changes because operation”).
of different electric power needs, the steam In particular, in the case of N-2 situation (i.e. island

62 Impiantistica Italiana - Novembre-Dicembre 2013