Load shedding

These industries try to locate near a source of hydropower (Niagara Falls or Hoover Dam) or near a source of excess nuclear power. They generally work out arrangements to get power at a reduced cost based on being the first one cut off when electric load shedding is required. 

If a system disturbance requires extra generating capacity, the generator may drop speed, reducing the frequency proportionately. The under-frequency relay 81-G will detect such a condition and operate a systematic load shedding in a programmed manner in order to meet the load demand. If the abnormal condition persists, the generator is taken off the main supply system. 

This chapter discusses the most popular features and controls that are critical to turboexpander operations. Among those discussed are magnetic bearings, squeeze film dampers, radial fit bolts, valve arrangements for FCC expanders, FCC expander load shedding technology, and surge detection and avoidance. 

Load sharing or selective load shedding is of interest to many users of hot gas expanders. A particularly successful European FCC application is illustrated in Figure 6-43. The addition of an expander-generator set to the FCC unit at a major refinery presented a challenge because a trip of the expander could upset the process. The company that is the subject of this application case study, GHH Borsig, solved this problem with the installation of a computerized control system and through computer simulation of trips. 

Source Expander Load Shedding Control, " Turbomachinery International, January/Fehruary 1998. 

In the dynamic simulation run, the pressures and flowrates at the input and output of each module are known. It is, therefore, possible to perform non-linear correction of the control mode, such that the changes in regenerator pressure in the event of load shedding are minimized. In a test performed with a correspondingly corrected controller structure, the pressure drop after load shedding was reduced from 46 mbar to 19 mbar. The subsequent pressure rise of 27 mbar is just below the specified threshold. 

DC powcr -New batteries, chargers, iavenors (p) -Alierri. iic battery chargtnj capabilities (b) Increased bus load shedding (b,p)... 

Workers carry out alternative (higher priority) operations (e.g., load shedding). 

Fit an automatic load-shedding maximum demand limiter. 

The utility company has to turn on idle equipment to meet peak demand, and that is expensive. Therefore, the cost of operation of a renewable energy plant can be reduced if nonessential electricity is turned off when a peak is approaching, and turned back on when the peak has passed. Load shedding is the simplest, and usually the first, method employed in managing energy use and controlling energy costs. 

The PREPA BESS was a large facility selected by a public utility as the best solution to a costly operating problem, i.e., insufficient spinning reserve and an attendant necessity to resort to load shedding to maintain system stability. The specifications and design were intended to meet the requirements of a commercial plant. No incentive funds or apparatus were involved in its construction. 

However, it is also possible to introdnce a high-speed load shedding scheme into the power system when Fg is fonnd to be above 105%. Snch a scheme will compute in an anticipatory manner how much consumption should be deleted in the event of a loss of one generator. The designer will be able to predetermine enough low priority consumers to achieve the necessary corrective action. See Chapter 16. 

This approach is the most economical in terms of the hardware necessary for load shedding. The PMS can be arranged to detect the fault at the 86-G and 86-T relays, in one of two methods ... 

If the load factor F , is less than or equal to F c then load shedding will not be necessary. Most plants that have their own power generation nse two, three or fonr generators. With only two... 

The consumers in the high-voltage network can be examined for their importance in the operation of the whole plant. Hence each consumer can be placed in a table that identifies its order of importance. This is called the load shedding priority table. Such a table will vary considerably from plant to plant because of the nature of the processes therein. In some exceptional situations it may be necessary to include some of the low-voltage loads. 

The table as shown applies to a fully loaded platform with all four 4 MW generators running at a load factor of approximately 80%. At first sight it may appear that too many items are included in the table. However, as the platform becomes loaded from its start-up condition the nmnber of items in their on state increases from a small number, and each item may be only partially loaded. Once the total load requires two generators to be on-line, then the PMS can be enabled to take load shedding action. 

Load shedding priority (I = low) Description Rated load (kW) Consumed load (kW) Status (ON or OFF) Remaining load (kW) No. of running generators (approx.)... 

---