Artificial Intelligence in Power System Optimization

CHAPTER

7

AVAILABLE TRANSFER CAPABILITY

*

7.1 INTRODUCTION

In 1992, the United States Congress developed the Energy Policy Act (EPAct),

which was interpreted by the Federal Energy Regulatory Commission (FERC)

as a mandate to introduce open-access of transmission resources and generation

competition to the electric power industry [1]. In 1995, the development of technical

guidelines for transmission transfer capability defi nitions and determination was

deferred from FERC to the North American Electric Reliability Council (NERC),

an industry group that develops reliability standards and guides for the planning

and operation of electric power systems.

NERC revised the earlier reference documents on transfer capability published

in 1980 to provide additional clarifi cations and examples. The revised document [2]

addresses transmission transfer capability from the perspective of the transmission

systems physical characteristics and limitations. It provides the technical basis

for discussions about transfer capability. Background information on industry

practices related to transfer capability is also presented including defi nitions,

concepts, technical issues, and simulation techniques used to calculate and

report transmission transfer capability. The report also recommends two transfer

capability measures: First Contingency Incremental Transfer Capability (FCITC)

and First Contingency Total Transfer Capability (FCTTC). The FCITC is defi ned

as the amount of electric power, incremental above normal base power transfers

that can be transferred over the interconnected transmission systems in a reliable

manner. The FCTTC is defi ned as the total amount of electric power (normal base

power transfers plus FCITC) that can be transferred between two areas of the

interconnected transmission systems.

In April 1996, FERC issued Order No. 888 [3] and Order No. 889 [4] to promote

electric supply competition and accommodate customer demand for a choice in

*This chapter has been written with assistance from Peerapol Jirapong

356 Artifi cial Intelligence in Power System Optimization

the electric power industry. The Final Rule 888 was intended to promote utility

competition through open-access of transmission resources. This rule required public

utilities to functionally unbundle wholesale generation and transmission services.

The Final Rule 889 mandated the calculation of available transfer capability (ATC)

for each control area and the posting of this value on a communications system

called the open access same-time information system (OASIS). The purpose of this

rule was to enhance the open-access of bulk transmission systems by providing

a market signal of the capability of a transmission system to deliver energy. The

associated rulings of FERC have added considerable emphasis to the interest in

quantifying transmission transfer capability. This interest has led to new defi nitions

and recommended methods for transfer capability determination. In June 1996,

NERC published a technical report regarding ATC defi nitions to provide a uniform

framework for determining ATC and related terms. The report also provided ATC

principles under which ATC values would have to be calculated.

7.2 TRANSMISSION TRANSFER CAPABILITY CONCEPTS

This section provides the key basic concepts of transmission transfer capability.

Other terms related to transfer capability can be explored in detail in [2] and [5].

7.2.1 Transfer Capability versus Transmission Capacity

Electric power systems have used a common terminology to calculate and report

transmission transfer limits to maintain the reliability of the interconnected

transmission networks. These transfer values are called “capabilities” because

they are highly dependent on the generation, customer demand, and transmission

system conditions assumed during the time period analyzed. The electric industry

generally uses the term “capacity” as a specifi c limit or rating of power system

equipment. In transmission, capacity usually refers to the thermal limit or rating of

a particular transmission element or component. The ability of a single transmission

line to transfer electric power, when operated as part of the interconnected network,

is a function of the physical relationship of that line to the other elements of the

transmission network.

7.2.2 Transmission Transfer Capability

Transfer capability is the measure of the ability of interconnected electric systems

to reliably move or transfer electric power from one area to another by way of all

transmission lines or paths between those areas under specifi ed system conditions.

The unit of transfer capability is, in terms of electric power, generally expressed in

megawatts (MW). In this context, area may refer to an individual electric system,

power pool, control area, region, or a portion of any of these. Transfer capability