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Dynamic Scheduling: The Forgotten Issue

Fortnightly Magazine - April 15 1997

U.S. Department of Energy, Office of Utility Technologies.

What Is It?: The Details of Dynamic-Scheduling

The control area is a critical component

Dynamic scheduling. The electronic transfer from one control area to another of the time-varying electricity consumption associated with a load or the time-varying electricity production associated with a generator.

The FERC, in its Order 888 on open-access transmission, defined dynamic scheduling as:

"[T]he metering, telemetering, computer software, hardware, communications, engineering, and administration required to allow remote generators to follow closely the moment-to-moment variations of a local load. In effect, dynamic scheduling electronically moves load out of the control area in which it is physically located and into another control area."

The FERC decided not to require transmission providers to offer this service. It also did not address the dynamic scheduling of generation.

Control area. The NERC defines a control area as: "An electrical system bounded by interconnection (tieline) metering and telemetry. [Each control area] controls its generation directly to maintain its interchange schedule with other control areas and contributes to frequency regulation of the Interconnection."

Four bulk electric networks (interconnections) exist in North America. Within each interconnection, all the generators are synchronized and therefore operate at the same frequency, and electricity flows freely on AC transmission lines. Electricity flows between the four interconnections are limited and occur only on DC links.

Control areas seek to minimize any adverse effect they might have on other control areas within the interconnection by minimizing their area-control error (ACE). ACE is the instantaneous difference between actual and scheduled interchange, adjusted to take account of any difference between actual and scheduled frequency in the interconnection.

Each control area maintains some generating units on automatic generation control (AGC) "to continuously balance its generation and interchange schedules to its load." AGC refers to equipment in the control center that automatically computes ACE every few seconds and, based on the calculated errors, sends signals to generating units to increase or decrease output to reduce ACE to zero. AGC also refers to equipment at individual generators that respond to the control-center AGC signals.

NERC defines two key control-area performance criteria. The first (A1) requires that, on an instantaneous power basis, the control area be in balance with the rest of the interconnection (i.e., ACE must equal zero) at least once every 10 minutes. The second criterion (A2) requires that the control area's energy imbalance (average ACE) be within a certain limit (roughly 0.2 to 0.5 percent of peak demand) every 10 minutes.

The electronic transfer of a load (or of generation) requires adjustment to either the actual or the scheduled interchange terms in the ACE equations of both the physical-host control area and the electronic-host control area.

Table 1. The Effects of Aggregation

on Regulating Requirements for an Industrial Customer

Load Load Load Sum of Metered

#1 #2 #3 loads total

Mean (MW) 84 98 88 270 270

Standard deviation (MW) 2.4 11 0.9 4.4 2.4

Range (MW) 12.7 5.5 4.8 23.0 13.6

The table shows hourly statistics based on 10-second data for three separately metered components of the