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In the minutes prior to the delivery, a system operator is using the [[power-flow study]] algorithms in order to find the [[optimal power flow]]. At this stage the goal is reliability ("security") of the supply.{{sfn|Conejo|Baringo|2017|p=10}} The practical electric networks are too complex to perform the calculations by hand, so from 1920s the calculations were automated, at first in the form of specially-built [[analog computer]]s, so called ''[[Network analyzer (AC power)|network analyzers]]'', replaced by digital computers in the 1960s.
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The ''secondary control'' is used to restore the system frequency after a disturbance, with adjustments made by the balancing authority control computer (this is typically referred to as [[load-frequency control]] or [[automatic generation control]]) an manual actions taken by the balancing authority staff. Secondary control uses both the [[Spinning reserve|spinning]] and non-spinning reserves, with balancing services deployed within minutes after disturbance (hydropower plants are capable of an even faster reaction).{{sfn|NERC|2011|pp=12-13}}▼
▲==== Seconds-after control ===
The ''primary control'' is engaged automatically withing seconds after the frequency disturbance. Primary control stabilizes the situation, but does not return the conditions to the normal and is applied both to the generation side (where the [[Governor (device)|governor]] adjusts the power of the [[Prime mover (engine)|prime mover]]) and to the load, where:{{sfn|NERC|2011|p=12}}
* induction motors self-adjust (lower frequency lower the energy use);
* under-frequency relays disconnect [[interruptible load]]s;
* [[ancillary services]] are engaged.
=== Minutes-after control ===
▲The ''secondary control'' is used to restore the system frequency after a disturbance, with adjustments made by the balancing authority control computer (this is typically referred to as [[load-frequency control]] or [[automatic generation control]]) an manual actions taken by the balancing authority staff. Secondary control uses both the [[Spinning reserve|spinning]] and non-spinning reserves, with balancing services deployed within minutes after disturbance (hydropower plants are capable of an even faster reaction).{{sfn|NERC|2011|pp=12-13}}
=== Tertiary control ===
The ''tertiary control'' involves reserve deployment and restoration to handle the current and future contingencies.{{sfn|NERC|2011|p=13}}
== Time control ==
The goal of the '''time control''' is to maintain the long-term frequency at the specified value within a [[wide area synchronous grid]]. Due to the disturbances, the average frequency drifts, and a ''time error'' accumulates between the official time and the time measured in the AC cycles. In the US, the average 60 Hz frequency is maintained within each [[Wide area synchronous grid|interconnection]] by a designated entity, '''time monitor''' that periodically forces a grid frequency change to bring the overall time offset within the predefined limits. For example, in the [[Eastern Interconnection]] the action (setting the frequency to 60.02 Hz or 59.98 Hz) is initiated when the time offset reaches 10 seconds and ceases once the offset reaches 6 seconds. Time control is performed either by a computer (''Automatic Time Error Correction''), or by the monitor requesting balancing authorities to adjust their settings.{{sfn|NERC|2011|pp=13-14}}
== References ==
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