The Public Utilities Commission of Sri Lanka has told the CEB by letter dated 15 March 2016 that “It is the responsibility of the CEB to maintain an uninterrupted power supply throughout the country, which has been violated three times in the last six months by way of island wide power failures”.
It asked the CEB management to inform the PUCSL on action CEB has taken to prevent repetition of islandwide power failures after the first recent one in 27 September 2015.
PUCSL recommended the CEB maintain the system steady state voltage within plus or minus 5%, which is the international practice as the present CEB practice of ±10% tolerance for voltage control on its transmission lines is “risky considering that this limit is very close to over current protection limit of many system devices.”
The PUCSL said that it hired the Manitoba HVDC Research Centre (MHRC) to do a study after the September 2015 failure, identify shortcomings and make recommendations ro prevent recurrence of power cuts. The study states:
The follow are the main observations and conclusions of the study.
1. Based on MHI’s extensive experience on similar situations, it is our opinion that the ‘adjusted’ model sufficiently represent the Sri Lankan system for the purpose of examining the event of September 2015. However, the ‘adjusted’ model should not be used for general purpose system planning studies. MHI strongly recommends the following:
- The model dynamic data be reviewed and updated based on accurate name plate information, test results and data gathered through site visits.
- The existing dynamic model is not suitable for long term event analysis such as the event under consideration. Proper model update including transformer tap changers and specific load modeling is recommended (i.e. induction motor models and composite load models representing actual load behavior should be used instead of simple static load model used in existing models).
2. Based on generator ratings provided to MHI (steady state limits provided in PSSE data) , it is very likely that the full reactive power capability of generators are not being utilized. The generator reactive power limits as well as the actual settings that are currently in place should be readily obtained from the respective generating stations. If the reactive power limits are not at their respective maximum values, this should be reviewed and adjusted where possible.
3. As per the load flow data provided to MHI, the system steady state voltages on many 230 kV and 132 kV buses were above 105% (1.05pu). The International practice is to maintain the steady state voltage on the High Voltage network at or below 105%. Allowing 110% voltage at steady state will bring the system devices dangerously close their overvoltage protection limits.
4. It is recommended to maintain the system steady sate voltage within ± 5%. The present CEB criteria is ±10%. This can be achieved by mechanically switched shunt devices (reactors and capacitors). Simulation verified that this is a feasible solution to prevent a system instability under similar events in future. The size and location of such shunt devices should be determined through system studies once the model is updated and validated.
- As an interim measure, the steady state voltage during low load periods can be improved by ‘tap staggering’ of parallel transformers (though not recommended as a permanent solution)
- Taking selected transmission lines out of service during low load periods (though not recommended as a permanent solution as this action impacts overall system reliability)
5. Shunt reactors are an effective means of controlling steady state voltage as well as improving the system dynamic response. The effectiveness of shunt reactor additions are demonstrated through simulations.