[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

• Overloading of lines
• Increased system losses
• Reduction in security margins
• Contractual violations concerning power import/export
• Increase in fault levels beyond equipment rating
Typical power-flow inefficiencies and limitations encountered in modern power systems may be the
result of one or more of the following:
FIGURE 2.9.17 Typical discharge-current-limiting reactor connection.
FIGURE 2.9.18 Typical high-voltage power-flow-control reactor connections.
2.9.2.4 Discharge-Current-Limiting Reactors
High-voltage series-capacitor banks are utilized in transmission systems to improve stability operating
limits. Series-capacitor banks can be supplied with a number of discrete steps, insertion or bypass being
achieved using a switching device. For contingencies, a bypass gap is also provided for fast bypass of the
capacitors. In both cases, bypass switch closed or bypass gap activated, a discharge of the capacitor occurs,
and the energy associated with the discharge must be limited by a damping circuit. A discharge-currentlimiting
reactor is an integral part of this damping circuit. Therefore, the discharge-current-limiting
reactor must be designed to withstand the high-frequency discharge current superimposed on the system
power-frequency current. The damping characteristic of this reactor is a critical parameter of the discharge
circuit. Sufficient damping can be provided as an integral component of the reactor design (de-Q’ing),
or it can be supplied as a separate element (resistor). See Figure 2.9.17.
2.9.2.5 Power-Flow-Control Reactors
A more recent application of series reactors in transmission systems is that of power-flow control
(Figure 2.9.18) or its variant, overload mitigation. The flow of power through a transmission system is
a function of the path impedance and the complex voltage (magnitude and phase) at the ends of the
line. In interconnected systems, the control of power flow is a major concern for the utilities, because
unscheduled power flow can give rise to a number of problems, such as:
• Nonoptimized parallel line impedances resulting in one line reaching its thermal limit well before
the other line, thereby limiting peak power transfer
• Parallel lines having different X/R ratios, where a significant reactive component flows in the
opposite direction to that of the active power flow
• High-loss line more heavily loaded than lower-loss parallel line, resulting in higher power-transfer
losses
• “Loop flow” (the difference between scheduled and actual power flow), although inherent to
interconnected systems, can be so severe as to adversely affect the system reliability
Power-flow-control reactors are used to optimize power flow on transmission lines through a modification
of the transfer impedance. As utility systems grow and the number of interties increases, parallel
operation of ac transmission lines is becoming more common in order to provide adequate power to
load centers. In addition, the complexity of contemporary power grids results in situations where the
power flow experienced by a given line of one utility can be affected by switching, loading, and outage
conditions occurring in another service area. Strategic placement of power-flow reactors can serve to
increase peak power transfer, reduce power-transfer loss, and improve system reliability. The paper, “A
Modern Alternative for Power Flow Control, [4] provides a good case study. The insertion of high-voltage
power-flow-control reactors in a low-impedance circuit allows parallel lines to reach their thermal limits
simultaneously and hence optimize peak power transfer at reduced overall losses. Optimum system
performance can be achieved by insertion of one reactor rating to minimize line losses during periods
of off-peak power transfer and one of an alternative rating to achieve simultaneous peak power transfer
on parallel lines during peak load periods or contingency conditions.
Contingency overload-mitigation reactor schemes are used when the removal of generation sources
and/or lines in one area affects the loading of other lines feeding the same load center. This contingency
can overload one or more of the remaining lines. The insertion of series reactors, shunted by a normally
closed breaker, in the potentially overloaded line(s) keeps the line current below thermal limits. The
parallel breaker carries the line current under normal line-loading conditions, and the reactor is switched
into the circuit only under contingency situations.
2.9.2.6 Shunt Reactors (Steady-State Reactive Compensation)
High-voltage transmission lines, particularly long ones, generate a substantial amount of leading reactive
power when lightly loaded. Conversely, they absorb a large amount of lagging reactive power when heavily
loaded. As a consequence, unless the transmission line is operating under reactive power balance, the
voltage on the system cannot be maintained at rated values.
reactive power balance = total line-charging capacitive VARs – line inductive VARs
To achieve an acceptable reactive power balance, the line must be compensated for a given operational
condition. For details of the definition of reactive power balance, refer to Section 2.9.3.3, Power-Line
Balance. Under heavy load, the power balance is negative, and capacitive compensation (voltage support)
is required. This is usually supplied by the use of shunt capacitors. Conversely, under light load, the
power balance is positive, and inductive compensation is required. This is usually supplied by the use of
shunt reactors.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC