Load Shedding and Stability Studies
PCE were engaged to carry out transient stability studies on a standalone grid to be used as the basis for a Load Shedding Scheme. The image is of a generator similar to those in this study.
Engine Loading & Disturbance Events
Standalone grids in Australia supply power to facilities such as remote mine sites. The generator engines are usually reciprocating gas or diesel fuelled units. Large quantities of fuel are used by these systems and this has a high cost and requires complex transportation and storage logistics. As a result, it is very desirable to run these power plants as efficiently as possible.
Internal combustion engines on generator units run most efficiently at a high load – typically 80% or more. Diesel engines must also be run hot enough or actual engine damage occurs.
Larger, slower units are also more efficient so it is better to have a few large units than many small units (though it is more costly in capital outlay).
For these reasons, there is a fine balance between running as few generators as possible (at high load) and having enough spinning reserve to cope with unscheduled disturbances. Insufficient spinning reserve usually means that a disturbance, such as a large motor starting, causes all generators to slow down to the point that the under frequency protection trips them all, leaving the site blacked out.
Load Shedding to Recover
If all disturbances were scheduled there would not be a problem, as prior to any start-up enough generation could be brought online to cope with the load increase. However, some disturbances such as a generator tripping are not foreseen. Since the trend is to fewer, larger units, loss of one machine causes a large increase in load on the remaining machines which rapidly slow down and also trip.
Since instability occurs within a few seconds at most and starting a generator takes several minutes at least, it is not possible to deal with generator trips this way – instead, loads must be tripped quickly to enable remaining generators to cope. This gives time for other generation to be restored.
How much load to shed and how quickly must then be determined for all foreseeable scenarios. This is where the transient studies are used.
Key inputs to the studies are engine and alternator inertia, alternator excitation system details and parameters such as time constants, Voltage Regulator type and tuning constants and Governor type and constants such as PID tuning constants and engine response time constants.
Such studies involve modelling the entire system, in a particular load scenario, then modelling a disturbance event such as a generator trip, and a load shed event to see if generators recover their initial speed and voltage. By varying the load shed time and load shed kVA (determined by what actual loads are available to be tripped) the response can be optimised.
The aim is to minimise how much load must be shed and find a maximum limit for trip time. Tripping in practice can require multiple communications paths in series that can prevent trip times less than 250ms.
Load Shed System Design
The output of multiple studies enables the LSS to be set up. The LSS is a dedicated processor-based system similar to a PLC and with good communications and fast processing. Load shedding can then be implemented as a look up table, or with dedicated logic. IEC61850 communication is often required.
The main difficulty with stability studies is to get sufficient information for voltage regulators and governors, and then to verify modelling with recorded real-life events. Setting up of initial conditions for each study is also critical to successful computation.