An Example of a Remedial Action Scheme (RAS) Used to Improve the Reliability of Power Systems, Transmission Operation
The following is an example of a remedial action scheme.
Reference: Design, Implementation and Commissioning of a Remedial Action Scheme, DistribuTECH Conference and Exhibition, San Diego, California January 25-27, 2005
A private company, TeckCominco Metals Ltd (TCML) operates a private network in the southern interior of British Columbia. The network comprises a hydro station of about 400MW capacity at Waneta, and a smelting plant with 220MW of load, approximately 15km away at Trail, connected with a 63kV transmission line. The TCML network is connected to the Western grid by two (2) 230kV interconnectors, at Warfield and Nelway. The TCML network may either import or export power via the interconnectors.
Excessive import or export power may result in the TCML network becoming islanded. The disconnection event may cause significant frequency excursions within the TCML network, with the possibility of a black-out occurring. The role of the remedial action scheme is to limit frequency excursions to acceptable levels and to prevent a black-out, if the TCML network is islanded.
The goal of the RAS system is to protect the MCML 63kV transmission system from excessive frequency excursions and possible black-out.
Software was used to model the WECC system and the TCML network. Within the TCML plant, modelling was done of dynamic motor loads and system inertial characteristics.
It was found that a combination of the rate of change of system frequency (df/dt) and the number of Waneta generators connected, were indicators of the degree of power imbalance in the islanded network. A RAS plan was developed and initial protection settings proposed, measuring df/dt at both the load bus and the generation bus.
A load flow and dynamic stability analysis was made of the proposed RAS system. The findings of the modelling indicated that with fewer than three generators on line at Waneta, immediate load shedding was required, independent of the system df/dt. The RAS could limit frequency excursions and allow the system to reach steady state with three or more generators on line.
The RAS system comprised two schemes which could each operate independently. The first is a load shed scheme located near the load bus and the second is a generator shedding scheme, located at the generator bus. The RAS uses 15 IED relays at three different locations, integrated over 4 communication networks and systems.
The RAS system is armed when the network is islanded. The RAS logic is predefined. Switching is actioned in response to measured changes in frequency. Load shedding occurs in response to negative df/dt measurements. There are 4 load blocks which may be shed. Should islanding occur with less than three generators on line, there is immediate load shedding of all 4 load blocks. Generator shedding occurs in response to positive df/dt measurements. The operators may pre-select a priority list for generator shedding.
The benefits of using df/dt measurements instead of MW calculations included; more accurate RAS response to the system conditions, a faster reaction time and a reduced communication bandwidth requirement.
BESST Engineering is able to provide solutions for remedial action schemes (RAS) through power system analysis, detailed design of the implementation and commissioning.