Series compensator

Details Series compensator Series compensator

2000-02-24

2001-12-18

Nguyen, Matthew (Department: 2838)

Electricity: power supply or regulation systems

For reactive power control

Using impedance

Reexamination Certificate

active

06331765

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATION
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-047983, filed Feb. 25, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an improvement on a series compensator which is constructed by a power converter connected in series to an AC transmission line via a transformer and compensates for an electric quantity of the AC transmission line such as the voltage, current, phase or impedance.
Recently, the capacity of switching devices with intrinsic turn-off capabilities have increased and large-capacity self-commutated converters for high voltage power transmission lines to control the power thereof are being put to a practical use.
A compensator which is connected in series to an AC transmission line via a series transformer and which electrically compensates for the impedance of a power transmission line by generating a compensation voltage on the primary winding of the series transformer, thereby controlling the power flow on the transmission line, or which compensates for a variation in transmission line voltage is known as disclosed in, for example, “Static Synchronous Series Compensator: A Solid-State Approach to Series Compensator of Transmission Lines” (L. Gyugyi et al., IEEE PES 96 WM 120-6 PWRD, 1996).
FIG. 1
is a block transmission line diagram exemplifying the structure of a conventional series compensator of this type.
In
FIG. 1
, “G” is an AC power supply, “X
1
” is the transmission line inductance of an AC transmission line, “Tr
1
” is a series transformer, “CNV” is a power converter, “BP” is a bypass transmission line and “FL” is a harmonic filter.
The power converter CNV is structured by bridge-connecting a switching device with intrinsic turn-off capabilities like a gate turn-off thyristor (hereinafter called “GTO”) and is capable of generating a voltage with an arbitrary amplitude and arbitrary frequency in accordance with the voltage and current of an AC transmission line by controlling the switching of the GTO.
The voltage generated by the power converter CNV is applied to the secondary winding of the series transformer Tr
1
, generating a voltage on the primary winding that is connected in series to the transmission line. The transmission line inductance X
1
of the AC transmission line can be compensated by properly controlling the level and phase of the voltage generated on the primary winding of the series transformer Tr
1
with respect to the voltage and current of the AC transmission line.
FIG. 2
is a vector diagram for explaining the principle of a method of compensating for the transmission line inductance.
In
FIG. 2
, “Vs” denotes the voltage vector of the AC transmission line, “Is” denotes the current vector of the AC transmission line, “Vc” denotes the voltage vector a power converter 4 generates on the primary winding of the series transformer Tr
1
, and “V
1
” and “V2” respectively denote the primary-side terminal voltage vector of the series transformer Tr
1
on the power-supply side and the primary-side terminal voltage vector of the series transformer Tr
1
on the load side.
Given that the transmission line inductance is L and the frequency of the AC power supply is &ohgr;, the relationship between the AC supply voltage vector Vs and the primary-side terminal voltage V
1
of the series transformer Tr
1
is expressed by the following equation.
{overscore (V)}1
={overscore (V)}s−j&ohgr;L{overscore (I)}s
  (1)
The primary-side terminal voltage V
1
of the series transformer Tr
1
has a phase delay of &dgr; and is lower by &Dgr;V with respect to the AC supply voltage Vs due to a voltage drop caused by the transmission line inductance L.
When the power converter CNV generates the compensation voltage Vc advanced by 90 degrees relative to the transmission line current on the primary winding of the series transformer Tr
1
, the primary-side terminal voltage vector V
2
of the series transformer Tr
1
on the load side changes in the direction of Vs and the phase delay and voltage drop with respect to the AC supply voltage Vs are reduced.
This is electrically equivalent to the transmission line inductance L having become smaller, and the transmission line inductance can be changed equivalently by changing the level of the compensation voltage Vc.
In general, given that the voltage at the sending end is Vs, the voltage at the receiving end is Vr and the phase difference between the voltages of the sending end and the receiving end is &thgr;, the maximum active power P that can be transmitted is given by the following equation.
P
=
VsVr
ω
⁢
 
⁢
L
⁢
sin
⁢
 
⁢
θ
(
2
)
Because the maximum power that can be transmitted is inversely proportional to the transmission line inductance, the maximum transmission power can be increased by electrically compensating the transmission line inductance of a transmission line with large transmission line inductance.
In the structure in
FIG. 1
, as the AC transmission line and the power converter CNV are connected in series via the series transformer Tr
1
in whose primary winding the same current as the transmission line current flows, the output current of the power converter CNV connected to the secondary winding of the series transformer Tr
1
is constrained to the transmission line current.
When a large current flows in the transmission line due to a ground fault or the like, therefore, an excess current also flows in the power converter.
Designing the power converter so as to withstand such a large current means that a power converter having a very large capacity is used. However, the output that is needed in the normal state requires a much lower capacity such that its use is not economical.
In this respect, the bypass transmission line BP as shown in
FIG. 1
is connected to the output terminal of the power converter CNV so that in case of a ground fault, the bypass transmission line BP is activated upon detection of the excess current, short-circuiting the output of the power converter. As the current constrained to the transmission line current is shifted to the bypass transmission line, the switching elements of the power converter are all turned off (gate-blocked) to prevent any excess current from flowing into the power converter.
As apparent from the above, the bypass transmission line is essential in the prior art and in case of a ground fault, the power converter should be gate-blocked to stop operation.
When the power converter is a voltage source converter as shown in
FIG. 1
, the current control system is generally structured to detect the output current. In a case of a series compensator, however, the output current is constrained to the transmission line current because of the above-described reason, so that current control cannot be performed.
For the series compensator, the voltage control system is designed to feedback the voltage applied to the winding of the series transformer. Since the voltage control system does not have an ability to suppress excess current that is likely to be induced by a disturbance on the transmission line side, excess current must be separately compensated.
The power converter generates a voltage with an arbitrary amplitude and arbitrary frequency by controlling the switching of the switching device with intrinsic turn-off capabilities but produces harmonics in accordance with the switching operation.
As the series compensator in
FIG. 1
is connected in series to the transmission line via the series transformer, the harmonic voltage generated by the power converter is added directly to the transmission line voltage, making it essential to provide a harmonic filter like FL shown in FIG.
1
.
To reduce the harmonics generated by the power converter, multiple converters should be connected.
The amount of compensation of the series compensator directly corresponds to the capacity of the power converter, so a power converter ha

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