Table of Contents
SSSC
The Static Synchronous Series Compensator (SSSC) is a series-connected FACTS controller based on VSC and can be viewed as an advanced type of controlled series compensation, just as a STATCOM is an advanced SVC. An SSSC has several advantages over a TCSC such as
- (a) elimination of bulky passive components – capacitors and reactors,
- (b) improved technical characteristics
- (c) symmetric capability in both inductive and capacitive operating modes
- (d) possibility of connecting an energy source on the DC side to exchange real power with the AC network.
However, an SSSC is yet to be installed in practice except as a part of UPFC or Convertible Static Compensator (CSC). An example of the former is a 160 MVAR series connected converter as part of the Unified Power Flow. The controller was installed at the Inez station of American Electric Power (AEP). An example of the latter is the two, 100 MVA series connected converters at Marcy 345 kV substation in Central New York belonging to NYPA. In both cases, 24 pulse three-level converters are used. This topology reduces the injected harmonic voltages considerably and no harmonic filters are needed.
Operation of SSSC
The schematic of an SSSC is shown in Fig (a). The equivalent circuit of the SSSC is shown in Fig (b). The magnitude of Vc can be controlled to regulate power flow. The winding resistance and leakage reactance of the connecting transformer appear is series with the voltage source Vc. If there is no energy source on the DC side, neglecting losses in the converter and DC capacitor, the power
balance in a steady state leads to
Re[VCI∗] = 0
The above equation shows that VC is in quadrature with I. If VC lags I by 90◦, the operating mode is capacitive and the current (magnitude) in the line increases with a resultant power flow increase. On the other hand, if VC leads I by 90◦, the operating mode is inductive, and the line current is decreased. Note that we are assuming the injected voltage is sinusoidal (neglecting harmonics). Since the losses are always present, the phase shift between I and VC is less than 90◦ (in steady state). In general, we can write
VˆC = VC(cos γ − j sin γ)ejφ
= (VCp − jVCr)ejφ
where φ is the phase angle of the line current, and γ is the angle by which VˆC lags the current. VCp and VCr are the in-phase and quadrature components of the injected voltage (concerning the line current). We can also term them as active (or real) and reactive components. The real component is required to meet the losses in the converter and the DC capacitor.
Modelling of SSSC
Neglecting harmonics, we can express the system equations (including SSSC) in D–Q variables (referred to as a synchronously rotating axis). The advantage of using these variables is that in a steady state, the D–Q components are constant and can be expressed as rectangular coordinates of phasors. For stability studies involving phenomena of frequency below 5 Hz, it is adequate to express the network equations using phasors by neglecting network transients.
However, for phenomena involving higher frequencies, one cannot ignore network transients (even for studies involving subsynchronous
frequency oscillations). We can illustrate the derivation of the network equations by considering the single line containing an SSSC shown in Fig. Neglecting, zero sequence components, we can express the network equations (using two-phase variables, α and β) in the complex form given below.
L . dˆi / dt + Rˆi = vˆS− vˆC − vˆR
where
iˆ = (iβ + jiα), vˆS = vSβ + jvSα,
vˆC = vCβ + jvCα, vˆR= vRβ + jvRα,
Transforming from α, β to D − Q components which are related as,
where θ = ω0t + θ0. There is no loss of generality in assuming θ0 = 0. A similar transformation as given above applies to the variables vSα, vSβ and vSD, vSQ, and so on.
We can also express Eq. as a complex equation given below.
(iβ + jiα) = (iQ + jiD)ejω0t = ˆIejω0t
SSSC with an Energy Source
If an energy source such as a battery or fuel cell is provided on the DC side of the VSC, it is possible to exchange real (active) power with the AC network. Here, the real (active) voltage component can be controlled in addition to the reactive voltage. This enables two degrees of freedom to control active
and reactive power flow in the line. Alternatively, it is possible to compensate for the line resistance to increase the power transfer capacity in the line.
Instead of an energy source, it is possible to provide the real power from a shunt-connected VSC which is connected to the series-connected SSSC on the DC side. This arrangement is known as Unified Power Flow Controller (UPFC) which is described in the next chapter in detail.
Applications of SSSC
An SSSC is an advanced version of controlled series compensation, that is based on VSC and the use of GTOs instead of thyristors. There are many technical advantages of an SSSC over a TCSC. However, the application of an SSSC would depend on the techno-economic evaluation and proven reliability based on operating experience. A major drawback with SSSC is the need for a coupling transformer (and an intermediate transformer if multipulse converters are used).
In contrast, TCSCs don’t require any magnetic devices or coupling transformers. However, the harmonics are better controlled with an SSSC. An SSSC requires protection against overcurrents. A high-speed electronic Thyristor Bypass Switch (TBS) is installed in parallel with the converter terminals. When an overcurrent is detected, it operates quite fast.
In case the system fault is not cleared by primary protection, then, the TBS is protected by a parallel connected low voltage breaker (LVB) which passes the TBS in about 6 cycles. If LVB fails to close when required, then its breaker failure protection closes the high side breaker (HSB) that bypasses the SSSC. After the fault is cleared, the SSSC is reinserted into the line by opening the LVB. The improvements in the power (semiconductor) device characteristics and the reduction in the costs would spur the applications of SSSC in place of TCSCs.
Frequently Asked Questions (FAQs)
What is the full form of SSSC?
Static Synchronous Series Compensator
What is the purpose of SSSC?
The purpose of the Static Synchronous Series Compensator (SSSC) is to control power flow in transmission lines, improving system stability and enhancing power quality in electrical grids.
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- STATCOM | Static Synchronous Compensator
- SSSC | Operation, Modelling, and Applications
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