Download Economic load dispatch

Unit-5&6
Lecture 45
Automatic Generation Control for a Power System
Automatic generation control and economic load dispatch are two principal areas
of concern for generation control on large, interconnected power systems. The role
and aim of each is quite different, but both act on the generator through controlling
the prime mover, and both vary the generation relatively slowly. overall system
frequency and the net tie-line load exchange between the power companies in the
interconnection. The common practice is to carry out generation control on a
decentralized basis; that is, each individual area [1] tries to maintain its scheduled
interchange of power.
Economic load dispatch is also an on-line computer control, whose function is to
supply the existing system load demand from all currently operating generators in
the most economical manner in terms of minimal fuel cost. A variety of conditions,
such as the presence of hydro along with fossil-fuel power stations, a multiarea
structure for the interconnected power system, or even pollution control, can be
features of an economic dispatch operation.
Usually, neighboring power companies are interconnected by one or more
transmission lines called tie lines, as depicted graphically in Figure 1.7. The
electrical areas are separate power systems under the control of an AGC in a
central digital computer. The boundaries of an area are the points on the tie lines
where a utility’s ownership, maintenance, and loss accounting ends and those of its
neighbors begin. There are very few isolated power systems that are not connected
to neighbors by means of tie lines.
The power systems employ tie lines for the following reasons:
Dept. of EEE, NIT-Raichur
Page 1
Unit-5&6
Lecture 45
1. Tie lines allow a local or “pool” exchange and sale of power between the power
companies on a predetermined schedule. 2. Tie lines allow areas experiencing
disturbances to draw on other areas for help.
3. Tie lines provide a long-distance transmission line for the sale and transfer of
power (e.g., on an interstate or international basis). Interconnections are made so
that operating areas can share generation and load. This sharing is normally on a
scheduled basis as forced by the AGC. However, during times of disturbance,
when an area is unable to meet its own regulating requirements, unscheduled
sharing occurs, as dictated by generator governing responses and by the
contributory function of the AGC frequency bias of each area. Such unscheduled
interchanges persist until either the disturbed area can itself fully respond to its
local requirement, or normal schedules are reset so that the contribution is taken
out of the unscheduled class and put into the scheduled class.
1.4.1 Area Control Error
To maintain a net interchange of power with its area neighbors, an AGC uses real
power flow measurements of all tie lines emanating from the area and subtracts the
scheduled interchange to calculate an error value. The net power interchange,
together with a gain, B (MW/0.1 Hz), called the frequency bias, as a multiplier on
the frequency deviation, is called the area control error (ACE)
Here Pk is an MW tie flow defined as positive out of the area, Ps is the scheduled
MW interchange, and f0 is the scheduled base frequency. When a system is not
interconnected, only the frequency term is used. The interchange power Ps is
generally scheduled for periods of the day and is changed as blocks of MWh are
bought or sold to neighboring utilities.
Dept. of EEE, NIT-Raichur
Page 2
Unit-5&6
Lecture 45
A positive ACE or positive net exchange of power represents a flow out of the
area.
Dept. of EEE, NIT-Raichur
Page 3