Cascade tripping of CPP for grid side faultsCase of AV Cell, Canada
Problem

AV Cell is one of the Pulp Fibre mill owned by Aditya Vikram group in AtholVille, NB, Canada. Plant has in-house thermal based generation of 20MW at 13.8kV through Contra Rotating type Turbine.  Mill is also having the local utility power support at 138kV from Hydro Quebec and NB, Power. Mill had all electromagnetic type of protective relays of Westinghouse Make. In February 2015, EEPL got a call from AV, Cell, describing of their typical problem of cascade tripping of CPP in event of grid side disturbances and resulting in to complete dark out of the plant and a huge production loss. This was more prone in bad weather conditions.

As it was mutually decided EEPL Senior Executives visited of all three mills of AVG in Canada namely AV Cell, AV Nackawic and AV Terrace bay in March 2015. They submitted their report based on their site visits to all three mill narrating their observations and recommendations for improvement of system reliability.

Solution

In April 2015, EEPL was awarded with Service Order for Power System Study mainly for TG Islanding Scheme and Load Shedding Scheme. EEPL conducted the Transient Stability Analysis, Engineering for Grid Islanding and Load Shedding Scheme and Recommendations for TG, Grid TR and Line Protection Schemes. EEPL suggested to design these vital protection schemes using accurate and faster numerical protection relays. It was mutually decided to go for numerical protection relays. EEPL worked out settings, logics and detailed protection engineering for all four schemes considering numerical protective relays. EEPL did the detailed engineering for proposed new Relay Panels to accommodate the numerical protection relays for Grid Islanding, Generator Protection, Grid TR Protection and EHV Overhead Line Protection. EEPL also did the detailed engineering for proposed Load Shedding Scheme which was designed based on Simple Open Loop Logic having faster operation and very cost effective solution.

AVC got these relay panels and load shedding panel manufactured and assembled locally based on panel drawings, BOQ, Schematics and Wiring Diagrams submitted by EEPL. Engineering support for installation of these panels in control room and control cable schedule to hook-up these panels with existing system was also extended by EEPL.

Implementation

EEPL prepared the Task Schedule, SOP and check list for the testing and commissioning work to be completed within 48 Hours of mill shutdown. In November 2016 Relay Panels were Installed and Commissioned at site by the team work of AVC Staff and local contracting companies under the supervision of EEPL team. In broad spectrum it includes Settings and Configuration of Relays, Testing of CTs and Relays, Sensitivity and Stability Checks of Differential and REF Schemes, Simulation of Tripping and Annunciation Circuits, Simulation of Load Shedding Scheme. EEPL extended their support for plant start-up after completion of afore mentioned work and observations for next 2 days till plant got running on its full capacity. Also AVC team was trained for basic operating of these relays and for trouble shooting and start-up procedure followed by tripping of relays, if any.

Afterward Challenges

AVC faced two cascade tripping of the plant after implementation. Trip incident details and relay event records were sent to EEPL for their study. After their study EEPL established that Grid Islanding Scheme and Load Shedding Schemes in both these incidents had operated successfully and as per the expectations from these schemes. It was observed that Turbine got tripped due to over speeding followed by these incidents. It was required to fine tune the governor of TG to get the faster response from it in such eventualities. The governor was finally fine-tuned by the OEM engineer at site in subsequent opportunity.

Result

Unfortunately though AVC continued facing light to sever nature of grid disturbances after above implementations at site which is beyond mill control, however, end result now got changed drastically and is now more favourable to mill. Now followed by any such grid disturbance, mill is getting islanded from grid successfully, surplus load shedding also take place in time and TG sustains with balance of critical plant load. As a result effective plant down time has reduced drastically and plant production loss is prevented. The most importantly this gives complete peace of mind to the operation staff and electrical team in particular.