Subject: ancillary services
Date: 2010-12-17 (at 16:41:22)
In order to ensure the reliable operation of the power system, power companies contract for a number of ancillary services. These include demand response, black-start capability, regulation service as well as reactive support and voltage control.
Types of Contracts:
Certified Black Start Facilities: These help system reliability by being able to restart their generation facility with no outside source of power. In the event of a system-wide blackout, a black start facility would help to re-energize other portions of the power system. While this is usually accomplished by on-site diesel generators, maintaining these are a net cost to the generator. The better method is large banks of batteries.
Reliability Must-Run (RMR) Contracts: These are negotiated with a generating facility that gives the utility the power to direct that facility to generate power to maintain system reliability.
Regulation Service represents the ability of a generator to control its generation output on a second-by-second basis so that the IESO can adjust any imbalances between load and generation by using the generator's automatic generation control (AGC) capacity. Ancillary services concerned with balancing power supply and demand over short time intervals throughout the power system
The regulation services types used are:
Automatic Generation Control (AGC)
AGC regulates the power output of electric generators within a prescribed area in response to changes in system frequency, tie-line loading, and the relation of these to each other. This maintains the scheduled system frequency and established interchange with other areas within predetermined limits. In normal operation, the AGC subsystem adjusts the power of the generating units automatically. This keeps the area's actual net interchange approximate to the scheduled interchange and the actual frequency near the scheduled frequency.
Rapid Generator Unit Loading/Unloading Protection
The ability to assume load or provide load quickly enough to prevent generator stall or over-speed. Demand Response participants contract to reduce load on short notice to avoid brown outs, and during an Emergency Operating State, just prior to power outages in order to maintain reliability of the grid.
The types of demand response methods are:
load shedding is accomplished through automated systems connected to industrial, commercial and residential users that can reduce consumption at times of peak demand, essentially delaying or advancing draw marginally. The process may involve turning down or off certain loads and, when demand is unexpectedly low, potentially increasing usage.
Spinning reserve is the extra generating capacity that is available by increasing the power output of generators that are already connected to the power system.
Reactive Support and Voltage Control Service:
Reactive Support and Voltage Control Service is a service provided by generators that allow the IESO to maintain consistent reactive power and voltage levels on the grid. This service is affected by two types of compensation, static and dynamic
VAR Compensation adjusts reactive power on electricity transmission networks. It is very important in markets that have a lot of inductive loads, (like air conditioners) switching in and out. Essentially a VAR Compensator is an automated impedance matching system designed to bring the system closer to unity power factor.
Static VAR Compensator systems
If the power system's reactive load is capacitive (leading), the it will use reactors (coils) to consume VARs from the system. Under inductive (lagging) conditions, capacitor banks are automatically switched in.
Dynamic VAR Compensator systems
This features a device called a synchronous compensator which is a specialized synchronous motor whose shaft is not attached to anything, but spins freely. Its purpose is not to produce mechanical power, as other motors do, but to either generate or absorb reactive power as needed to support the grid's voltage thus dynamically maintaining the grid's power factor at a specified level.
Most modern VAR compensators are of the former, static type, as very fast silicon controlled rectifiers (SCR)and Thyristors can respond quicker than a rotating mass.