Application: General Electric MS5001, 6001 and 7001
Controls: Speedtronic Mark I or Mark II

Gas turbines with early Speedtronic™ control and protection systems, namely Mark I or Mark II, can be modified to allow “isochronous” operation.  Most were installed originally with only “droop” capability.  Some plant operators have implemented changes to their operating plans to accommodate “island” operation.  Should they be severed from the main power grid, as a consequence of a transformer or transmission line failure, they could temporarily operate one of their gas turbines in the “isoch” mode until repairs are done.  Sometimes it takes hours or even days to rectify the situation.  The “isoch” mode allows them to maintain the frequency of the system at 60.00 cycles per second.

Fig. 1: Simplified Speed Control Circuit for Speedtronic Mark I

GE gas turbine/generators installed in the 1970s and early 1980s were equipped with Mark I or Mark II controls.  A simplified schematic of a Mark I system is shown in Fig. 1 above.  This system had a transistor circuit (not shown) that could be turned “on” or “off” depending upon whether the operator wanted “droop” operation (default) or “isochronous” operation .  Most gas turbines of this generation, however, did not come with the needed switch for changing between the two modes.

Lately, Pond and Lucier, LLC has been asked to install this switch with the necessary features to allow for “selectable” operation between the two modes.  Clients in the states of Washington and Massachusetts, both with MS7001E gas turbines with Mark II controls, have desired this feature on at least one of their gas turbine/generators.  It seems to be a new operating option required in the post 9/11 and Hurricane Katrina era.

The modification involves a “lockable” switch with the default mode in “droop.”  When “isoch” is selected, that turbine speed circuit sets the system frequency.  In the case of the GE frame 7, the speed set is 3600 rpm which holds the power output frequency at exactly 60 cps (Hz).  This “isoch” machine would then react instantaneously to changes in load demand on the “island” system, always correcting frequency back to 60 Hz.  Once selected, this turbine/generator reacts to all the increases/decreases in demand without the operator changing load setpoint.  In Fig. 2 below, the “isoch” mode is represented by a horizontal line at 100% speed.  The 4 percent droop line is for all other generators on the system to allow load sharing.

Fig. 2: Speed Droop and Isochronous Operation Curves

Minor wiring modifications to the input of the SSZA card are required.  A selector switch installed on the panel to turn on/turn off a relay on the circuit board (or transistor in Mark II) by tying DCOM, as applicable.  This is a relatively easy installation.  On-line generator testing is required thereafter to simulate “island” operation of the small power system.  A portable load bank may be required with as much as 5 megawatts capacity.

Fig. 3: Speed Control Card SSZA and its operational amplifier AOAC.

Contact Dave Lucier of PAL Engineering if you would like a turnkey proposal converting your controls to allow for either “droop” or “isochronous” operation.  Tel: 518-371-1971 or cellular: 518-330-4801