Versatile power flow transformers for compensating power...

Details Versatile power flow transformers for compensating power... Versatile power flow transformers for compensating power...

2000-12-04

2002-07-16

Sterrett, Jeffrey (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using impedance

C323S216000, C323S255000, C323S340000, C323S361000

Reexamination Certificate

active

06420856

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to power flow transformers that compensate power flow in a transmission line. More particularly, the present invention relates to a power flow transformer that is simple, versatile, and relatively inexpensive.
BACKGROUND OF THE INVENTION
Electric power flow through an alternating current transmission line is a function of the line impedance, the magnitudes of the sending-end and the receiving-end voltages, and the phase angle between such voltages, as shown in FIG.
1
. The impedance of the transmission line is typically inductive; accordingly, power flow can be decreased by inserting an additional inductive reactance in series with the transmission line, thereby increasing the effective reactance of the transmission line between its two ends. The power flow can also be increased by inserting an additional capacitive reactance in series with the transmission line, thereby decreasing the effective reactance of the transmission line between its two ends. The indirect way to emulate an inductive or a capacitive reactance is to inject a voltage in quadrature with the prevailing line current.
The direct method of voltage regulation of a transmission line is to add a compensating voltage vectorially in- or out-of-phase with the voltage of the transmission line at the point of connection. The indirect method to regulate the line voltage is to connect a capacitor or an inductor in shunt with the transmission line. A shunt-connected capacitor raises the line voltage by way of generated reactive power. A shunt-connected inductor absorbs reactive power from the line and thus lowers the voltage. The indirect way to implement a shunt capacitor or inductor is to generate a voltage in phase with the line voltage at the point of connection and connect the voltage source to the line through an inductor. Through control action, the generated voltage can be made higher or lower than the line voltage in order to emulate a capacitor or an inductor. Lastly, inserting a voltage in series with the line and in quadrature with the phase-to-neutral voltage of the transmission line can change the effective phase angle of the line voltage.
In order to regulate the voltage at any point in a transmission line, an in-phase or an out-of-phase voltage in series with the line is injected.
FIG. 2
shows the shunt compensating transformer scheme for voltage regulation in a transmission line. The exciter unit consists of a three-phase Y-connected primary winding, which is impressed with the initial line voltage, v
1
′ (i.e., v
IA
′, v
1B
′, and v
1C
′). The shunt-compensating unit consists of a total of six secondary windings (two windings in each phase for a bipolar voltage injection). The line is regulated at a voltage, v
1
by adding a compensating voltage, v
11
, either in- or out-of-phase with the line voltage. The bipolar compensating voltage in any phase is induced in two windings placed on the same phase of the transformer core. To control the shunt compensating unit, a reference voltage V
1
* is fed to a gate pattern logic which monitors the magnitude V
1
′ of the exciter voltage, v
1
, and determines the number of turns necessary on the shunt compensating unit. Based on this calculation, an appropriate thyristor valve is switched on in a tap changer (FIG.
3
), which puts the required number of turns in series with the line.
FIG. 3
shows the schematic diagram of a thyristor-controlled tap changer. A transformer winding is tapped at various places. Each of the tapped points is connected to one side of a back-to-back thyristor (triac) switch. The other sides of all the thyristor switches are connected together at point A. Depending on which thyristor is on, the voltage between points A and B can be varied between zero and the full winding voltage with desired steps in between. In a mechanical version of this arrangement, a load tap changer connects with one of a number of taps to give a variable number of turns between the connected tap and one end of the winding.
A Static VAR Compensator (SVC) consists of a series of inductors and capacitors as shown in FIG.
4
. SVC compensation is achieved by putting either inductance or capacitance in the circuit through a thyristor switch. The level of compensation is determined by adjusting the conduction angle of the thyristor switch.
A static synchronous compensator (STATCOM) is a voltage source converter (VSC) coupled with a transformer as shown in FIG.
5
. Such STATCOM injects an almost sinusoidal current of variable magnitude at the point of connection with a transmission line. Such injected current is almost in quadrature with the line voltage, thereby emulating an inductive or a capacitive reactance at the point of connection with the transmission line.
The STATCOM is connected at BUS
1
of the transmission line, which has an inductive reactance, X
s
, and a voltage source, V
s
, at the sending end and an inductive reactance, X
1
and a voltage source, V
r
, at the receiving end, respectively. The STATCOM consists of a harmonic neutralized voltage source converter, VSC
1
, a magnetic circuit, MC
1
, a coupling transformer, T
1
, a mechanical switch, MS
1
, current and voltage sensors, and a controller. The primary control of VSC
1
is such that the reactive current flow through the STATCOM is regulated.
The STATCOM controller operates the VSC such that the phase angle between the VSC voltage and the line voltage is dynamically adjusted so that the STATCOM generates or absorbs desired VAR at the point of connection.
FIG. 6
shows a simplified diagram of the STATCOM with a VSC voltage source, E
1
, and a tie reactance, X
TIE
, connected to a power system with a voltage source, V
TH
, and a Thevenin reactance, X
TH
. When the VSC voltage is higher than the power system voltage, the system “sees” the STATCOM as a capacitive reactance and the STATCOM is considered to be operating in a capacitive mode. Similarly, when the power system voltage is higher than the VSC voltage, the system “sees” the STATCOM as an inductive reactance and the STATCOM is considered to be operating in an inductive mode.
The effective line reactance is varied directly by using either mechanically switched or thyristor switched inductors and capacitors, such as those found in a Thyristor Controlled Series Compensator (TCSC) as shown in FIG.
7
. The basic implementation of a TCSC consists of one or a string of capacitor banks, each of which is shunted by a Thyristor Controlled Reactor (TCR). In this arrangement, the current through a TCR, which also circulates through the associated capacitor bank, is varied in order to control the compensating voltage and thus the variable reactance. A STATCOM and the STATCOM model are disclosed in more detail in Sen,
STATCOM—STATic synchronous COMpensator: Theory, Modeling, and Applications
, IEEE Pub. No. 99WM706, hereby incorporated by reference.
A Static Synchronous Series Compensator (SSSC) is a Voltage Source Converter coupled with a transformer as shown in FIG.
8
. An SSSC injects an almost sinusoidal voltage, of variable magnitude, in series with a transmission line. This injected voltage is almost in quadrature with the line current, thereby emulating indirectly an inductive or a capacitive reactance, X
q
, in series with the transmission line as shown in FIG.
9
. The compensating reactance, X
q
, has a positive value when emulating a capacitor and a negative value when emulating an inductor. The effective line reactance, X
eff
, has a positive value when being inductive and a negative value when being capacitive.
The SSSC is connected in series with a simple transmission line, which has an inductive reactance, X
s
, and a voltage source, V
s
at the sending-end and an inductive reactance, X
1
, and a voltage source, V
r
, at the receiving-end, respectively. The SSSC consists of a harmonic neutralized Voltage Source Converter, VSC
2
, a magnetic circuit, MC
2
, a coupling transformer, T
2
, a mechanical switch, MS
2
, one electronic switch, ES, current and vo

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