Problems With the ARCNET Communication Between Mark V and Ex-2000 System

We are operating a big petro chemical plant having self power generation with two 2x25mw ge frame 5 Gas turbines.

This refers to the subject matter , seeking your help in resolution of the problem as described below.

Presently we are facing problems with the ARCNET communication between mark V and Ex-2000 system with our Gas Turbine # 2 (GE MS-5001, Turbine Serial # 296835 and Generator Serial # 335 x 868). The alarm / fault code appeared on EX-2000 display is as under:

418 - LARCRECO

EX2000 details are as follows:

EX2000R
G Frame
GE Requisition - A88G464418
Shop Order - XKV593
D.M. No. R05157
Drawing No.: 259B1399AA

Details of the problem is as under:

On last Thursday, April 29, 2010 we experienced alarm messages on the Mark - V HMI as per following:

- STATUS-S COMMUNICATION LINK FAILED 0389 &
- EX-2K DIAGNOSTIC ALARM 0426

At the moment of alarm, no fault data of EX-2K was available on HMI and the link status was indicated as " Failed". However, 418 LARCRECO fault code was available on EX-2K local display. Therefore, fault resetting was attempted from the local EX-2K keypad twice along with physical tightness of the arcnet connecting cable at EX-2K end. As a result, the fault data appeared on HMI and was then reset from the same.

On the next day i.e April 30, 2010, the fault appeared again, this time EX-2000 control mode was also shifted from "AUTO" TO "MANUAL (FVR)" mode. The restoration attempt from Mark V HMI was not successful and the mode was restored to auto via EX-2K local key pad. Moreover, frequent switching of "Link OK" and "Failed" was also observed for a short interval of time.

Based on the above problems and in the light of recommendations mentioned in the GE manual, the interconnecting coaxial cable (with BNC Type connectors)between Mark V and Ex-2K was replaced.

After replacement of cable the system run smoothly for around 16 Hrs but the problem again appeared repeatedly on May 01, 2010. On this occasion also , the fault was only possible to reset from EX-2K local key pad as described above, followed by resetting from Mark-V HMI.

Moreover, on May 02, 2010 the system communication again failed and has not been restored to-date. However, the control mode is on "AUTO". Presently, as a next step to resolve the problem we are planning to replace the ARCNET card (DS200ACNA) of Ex-2K.

Your expert opinion / recommendations and clarifications are requested for the following:

- Other probable causes of communication failure and recommended actions for resolution of fault.
- Reason for shifting of AVR mode from AUTO to MANUAL.
- With the communication link failed, is it possible to control the voltage ( or reactive power sharing in case of parallel operation) from EX- 2 K keypad? If yes, please provide details.
- Is there any hazard in operation of one gas turbine with communication link failed in parallel operation with other similar gas turbine?

Moroever, please note that we have experienced the same problem several times in the last years with the same 418 (LARCRECO) fault message but in those cases the fault data was visible at Mark V HMI and was successfully resetted from the same.

Your expert opinion & cooperation regarding the above problem is requested, please.

Appreciate early response.

Thanks

Best Regards,