SYSTEM PLC REPLACEMENT

In December 2013, IEA completed a project that involved the replacement of the main Programmable Logic Control (PLC) for its ten-generator, 16 MW standby generation system.

Problem

The prior PLC was over 20 years old, its hardware was obsolete and its software lacked many of the enhancements desired by IEA’s operations staff. The PLC had been diagnosed with a failed input module and attempts at hardware replacement or input re-assignment proved unsuccessful. The failure resulted in the inability to automatically shutdown. This required that IEA personnel be on-site to manually shut down the system and restore it to standby readiness following the operation.

Solution

A replacement of the obsolete system PLC was determined to be the optimal solution. The old PLC was installed inside of the generating system switchgear cabinets, which were overcrowded and hard to access. The new control system would be located in a separate cabinet, utilizing the location of the old PLC as a marshalling cabinet (a convenient place to connect the new controls to the old field devices) The updated PLC was an Allen Bradley Compact Logix model that allowed for additional functionality to be included that was beyond the capability of the old control system.

Process

IEA issued a technical specification for the new control system in May 2013. Quotes were received in June and a purchase order issued in July. IEA worked through significant constraints imposed by the customer to arrive at a final installation and testing schedule.

Hardware procurement and panel construction commenced in August in order to meet a Dec 31 startup deadline. Software development began in earnest in September and continued into November.
The specification included a long list of acceptance tests. To enable the testing, a simulator was developed that was used to provide real world responses to the new control system. IEA provided testing procedures and participated in final acceptance testing.

The simulation process aided software engineers in working out bugs during software development and reduced the amount of actual testing needed in the field. Simulator testing was completed in late November, and the panel was shipped to the site.

Installation

Installation pre-work started in early December with the installation of conduit and pulling of nearly 200 control wires from the old PLC location to the new control cabinet. This was done to minimize system downtime.

The week before Christmas, the system was shutdown, and all of the old control system hardware was removed. Interconnect wires that had been pulled into place were terminated at the new control cabinet, and at the location where the old PLC had been installed. All of the interconnect wiring was verified from end to end in order to assure that no installation errors had occurred. At this point, the new control cabinet was powered up to verify that no damage had occurred during shipment to the field location. Each input and output was verified electrically before proceeding to startup.

Prior to shutting down the old control system, IEA developed detailed manual operating instructions in the event the system was required to run during the time that the automatic control system was out of service.

Startup & Testing

Following the holidays, final acceptance testing began. The customer had been prepared for the fact that testing could result in numerous power interruptions of varying severity and duration. Except for the failure of a DC power supply circuit, with caused a 7 second outage affecting half of the customer’s electrical distribution, acceptance testing proceeded flawlessly. The project schedule called for three full days of operational testing, but was completed in less than 9 hours, thanks in large part to the simulator testing conducted at the factory. The customer was very pleased with the results (i.e., completed in a short time window, little system unavailability and minimal impact on power continuity).

Lessons Learned

Start early giving yourself, vendors, and your customer plenty of time to provide input and ask questions. Prepare a detailed written specification for the software, hardware and documentation package. Select a qualified controls contractor with experience in your type of application. Pay the extra money and take the time to utilize a simulator; you will make it up on the back-end. Control the installation and preserve the ability to operate the equipment in manual mode if at all possible. In order to minimize system down time, develop a thorough startup and testing plan. Keep the customer involved from start to finish in order to minimize communication issues.

Result

This updated PLC included such functionality as the ability to individually start or stop a generator remotely, to manually control generator load from a remote location, to initiate a transfer of station power from one utility source to another, to control the dual air compressors used for the generator air start system, and to select from several possible utility supply configurations upon shutdown. The new control system also utilized a redundant DC power supply system to reduce the probability of failures.

In addition, a new 21” color touchscreen was installed to serve as the local Human Machine Interface (HMI). This replaced an alpha-numeric keypad that had served as the operator interface for the prior system. This new HMI allowed more detailed system performance information to be displayed in real time, aiding in any future troubleshooting and providing the operator with critical performance data during system operation.

All upgrades increased reliability, usability, and overall visibility to the operation of the facility.