Static Synchronous Series Compensator
(SSSC) is a modern power quality FACTS device that employs a voltage source converter
connected in series to a transmission line through a transformer. The SSSC
operates like a controllable series capacitor and series inductor. The primary
difference is that its injected voltage is not related to the line intensity
and can be managed independently. This feature allows the SSSC to work
satisfactorily with high loads as well as with lower loads.
The Static Synchronous Series Compensator has three basic components:
a.
Voltage
Source Converter (VSC) – main component
b.
Transformer
– couples the SSSC to the transmission line
c. Energy
Source – provides voltage across the DC capacitor and compensate for device losses
Static Synchronous Series Compensator (SSSC) Diagram |
Operation and Capabilities
Static synchronous series compensator
works like the STATCOM, except that it is serially connected instead of shunt. It
is able to transfer both active and reactive power to the system, permitting it
to compensate for the resistive and reactive voltage drops – maintaining high
effective X/R that is independent of the degree of series compensation.
However, this is costly as a relatively large energy source is required.
On the other hand, if control is
limited to reactive compensation then a smaller supply should be enough. In
this case only the voltage is controllable because the voltage vector forms 90ยบ
with the line intensity. Subsequently, the serial injected voltage can advance
or delay the line current, meaning, the SSSC can be uniformly controlled in any
value.
The SSSC when operated with the proper
energy supply can inject a voltage component, which is of the same magnitude
but opposite in phase angle with the voltage developed across the line. As a
result, the effect of the voltage drop on power transmission is offset. In
addition, the static synchronous series compensator provides fast control and is
inherently neutral to sub-synchronous resonance.
Modes of Operation
Generally, the line reactance is
constant but its net effect can be controlled through voltage injection. For
instance, the line current decreases as the inductive reactance compensation
level increases from 0% to 100%. Meanwhile, the line current increases with the
capacitive reactance compensation level from 0% to 33%.
SSSC Modes of Operation |
It can be noted that the static
synchronous series compensator does not only increase the transferable power
but it can also decrease it, by simply reversing the polarity of the injected voltage.
This reversed polarity voltage is fed directly to the line voltage drop as if
the line impedance was increased.
In short, the effects of reactance compensation
on normalized power flow in the transmission line are as follows:
· When
the emulated reactance is capacitive, the active and reactive power flow
increases and the effective reactance decreases as the reactance compensation
increases in the positive direction.
· When
the emulated reactance is inductive, the active and reactive power flow
decrease and the effective reactance increases as the reactance compensation
increases in the negative direction
SSSC and DVR
The SSSC’s counterpart is the Dynamic VoltageRegulator (DVR). Although both are utilized for series voltage sag
compensation, their operating principles differ from each other. The static
synchronous series compensator injects a balance voltage in series with the
transmission line. On the other hand, the DVR compensates the unbalance in
supply voltage of different phases. Also, DVRs are usually installed on a
critical feeder supplying the active power through DC energy storage and the required
reactive power is generated internally without any means of DC storage.
Applications and Advantages
The SSSC is typically applied to correct
the voltage during a fault in the power system. However, it also has several
advantages during normal conditions:
· Power
factor correction through continuous voltage injection and in combination with
a properly structured controller.
·
Load
balancing in interconnected distribution networks.
·
It
can also help to cover the capacitive and reactive power demand.
·
Power
flow control.
·
Reduces
harmonic distortion by active filtering.
References:
El-Zonkoly A. (2006). Optimal Sizing
of SSSC Controllers to Minimize Transmission Loss and a Novel Model of SSSC to
Study Transient Response.
Gupta S., Pachar, R., and Tiwari, H.
(2010). Study on Major Issues and Their Impact on DVR System Performance.
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