Author Topic: Improvement of Power System Stability by Simultaneous AC-DC Power Transmission  (Read 4373 times)

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Author : T.Vijay Muni, T.Vinoditha, D.Kumar Swamy
International Journal of Scientific & Engineering Research, IJSER - Volume 2, Issue 4, April-2011
ISSN 2229-5518
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Abstract— This paper presents the concept of simultaneous ac-dc power transmission.Long extra high voltage (EHV) ac lines cannot be loaded to their thermal limits due to this instability occurs in the power system.With the scheme proposed in this paper,it is possible to load these lines very close to their thermal limits.The conductors are allowed to carry usual ac along dc superimposed on it.The advantage of parallel ac-dc transmission for improvement of transient stability and dynamic stability and dampout oscillations have been established.Simulation study is carried out in MATLAB software package.The results shows the stability of power system when compared with only ac transmission.

Index Terms— Extra high voltage (EHV) transmission, flexiable ac transmission system (FACTS), HVDC, MATLab, simultaneous ac-dc transmission, Power System Stability, Transmission Efficiency.

HVDC transmission lines in parallel with EHV ac lines are recommended to improve transient and dynamic stability as well as to damp out oscillations in power system. Long EHV ac lines can not be loaded to its thermal limit to keep sufficient margin against tran-sient instability. But for optimum use of transmission lines here is a need to load EHV ac lines close to their thermal limits by using flexible ac transmission system (FACTS) components .Very fast control of SCRs in FACTS devices like state VAR system (SVS), controlled series capacitor (CSC), static phase shiftier (SPS) and controlled braking resistors oscillations as well as to control the voltage profile of the line by controlling the total reactive power flow.  Only the basic idea is proposed along with the feasibility study using elementary laboratory model.  The main object is to emphasize the possibility of simultaneous ac-dc transmission with its inherent advantage of power flow control improves stability and damps out oscilla-tions in power system.
EHV ac line may be loaded to a very high value if the conductors are allowed to carry superim-posed dc current along with ac current.  The added dc power flow does not cause any transient instability.

Fig 1.  ( Download Full Paper For Fig. View )

This paper presents a simple scheme of simultaneous EHV ac-dc power flow through the same transmission line with an object to achieve the advantages of parallel ac-dc transmission.  Simultaneous ac-dc transmission may also claim advantages in some specific applications LV (low voltage) and MV (Medium voltage) system.
The flexible ac transmission system (FACTS) con-cepts, based on applying state-of-the-art power elec-tronic technology to existing ac transmission system, improve stability to achieve power transmission close to its thermal limit. Another way to achieve the same goal is simultaneous ac–dc power transmission in which the conductors are allowed to carry superim-posed dc current along with ac current. Ac and dc power flow independently, and the added dc power flow does not cause any transient instability.

The circuit diagram in Figure1 shows the basic scheme for simultaneous ac-dc transmission. The dc power is obtained through the rectifier bridge and injected to the neutral point of the zigzag connected secondary of sending end transformer, and again it is reconverted to ac by the inverter bridge at the receiving end.  The inverter bridge is again connected to the neutral of zigzag connected winding of the receiving end transformer. Star connected primary windings in place of delta-connected windings for the transformers may also be used for higher supply voltage.  The single circuit transmission line carriers both 3 –phase ac and dc power.  It is to be noted that a part of the total ac power at the sending end is converted into dc by the tertiary winding of the trans-former connected to rectified bridge. The same dc power is reconverted to ac at the received end by the tertiary winding of the receiving end transformer connected to the inverter bridge.  Each conductor of the line carries one third of the total dc current along with ac current Ia .The return path of the dc current is through the ground. Zigzag connected winding is used at both ends to avoid saturation of transformer due to dc current flow. A high value of reactor, X d is used to reduce harmonics in dc current.
In the absence of zero sequence and third harmon-ics or its multiple harmonic voltages, under normal operating conditions, the ac current flow will be re-stricted between the zigzag connected windings and the three conductors of the transmission line.  Even the presence of these components of voltages may only be able to produce negligible current through the ground due to high of Xd.
Assuming the usual constant current control of rectifier and constant extinction angle control of in-verter, the equivalent circuit of the scheme under normal steady state operating condition is shown in Fig.2.

Fig.2. ( Download Full Paper For Fig. View )
The dotted line in the figure shows the path of ac re-turn current only. The ground carries the full dc cur-rent Id only and each conductor of the line carries Id/3 along with the ac current per phase

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