Voltage Source Converter (VSC) based HVDC transmission systems are available in market for power rated between a few MVA to a few hundreds of MVA. The new system results in many application opportunities and new applications in turn bring up new issues of concern. This paper aims at exploring the HVDC transmission systems and also presents a comprehensive investigation on one of the concerned issues, which is the contribution of HVDC Light? to short circuit currents. Different AC network conditions, load conditions and fault types are considered under different operation conditions and control modes. Results from simulations with complete functions as in real systems are presented and discussed. The associated control and protection strategy is also discussed. Finally a comparison is made between the HVDC Light? and conventional HVDC, the SVC and the STATCOM regarding the impact on the short circuit current. A comprehensive investigation on the issue regarding the contribution of HVDC Light? to short circuit current has been performed. The studies lead to the following conclusions. The HVDC Light?, in contrast to the conventional HVDC that does not contribute any short circuit current, may contribute some short circuit current. The possible maximum short circuit current contribution is determined by the SCR. It is inversely in proportional to the SCR and it occurs when the transmission system is operating at zero active power. Hence, it is comparable to the STATCOM as long as the maximum short circuit current contribution is concerned. The amount of contribution depends on control modes, operation points and control strategies. With the reactive power control mode, the short circuit current contribution will be limited due to the current order limit decreasing with the voltage. With the AC voltage control mode, the short circuit current contribution will be increased with the decreasing of active power, if the current order limit is not changed. If the current order limit is decreasing with voltage, the short circuit current contribution will be small even if the load level is low. The contribution to the short circuit current is irrelevant to the fault location if the fault current is evaluated in per unit with the base value equal to the 3-ph fault current at the corresponding fault location and without HVDC Light? connected. Under the same load and operation condition, the 1-ph fault current is usually smaller than the 3-ph fault current. Finally, it should be noticed that in associated with higher short-circuit current the voltage stability and performance is likely to be improved. If the HVDC Light? contributes a higher short-circuit current, the voltage dip due to distant fault is possibly reduced and thereby the connected electricity consumers may suffer less from disturbances.
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Short Circuit Current Contribution of HVDC Light
TheVoltage Source Converter (VSC) based HVDC
transmission systems are available in market for power rated
between a few MVA to a few 100s of MVA. here, the contribution of hvdc light to short circuit conditions is discussed. Different AC network conditions, load conditions and fault types
are considered under different operation conditions and control
HVDC LIGHT TRANSMISSION SYSTEM:
It consists of two cables and two converter stations. The converter station has voltage source converter (VSC) built up with
IGBTs, phase reactors, ac filters and transformer. the
amplitude and phase angle of the ac voltage is adjusted using PWM. independent active and reactive power control can be applied as the AC side has two degrees of freedom.
INVESTIGATION OF SHORT-CIRCUIT CURRENT:
system considered here has a mixture structure in radial and
mesh connection. The three types of fault namely the close-in fault; the near-by fault and the distant fault are applied at the three buses. the short circuit ratio (SCR) is
defined as the short circuit capacity of the AC system observed.
The Impact of Strength of AC Networks:
the short circuit ratio determines the maximum relative short circuit current
increment and it is is inversely in proportional to the SCR.
The Impact of Control Modes:
the choice of different control modes in respect of the active
power control does not give any impact to the short circuit
The Impact of Operation Points:
the short circuit current would be
also reduced if the current order is also limited with the
The Impact of Fault Type and Location:
Under the same load and operation condition, the 1-ph fault
current is usually smaller than the 3-ph fault current as the average voltage reduction is smaller for single phase.
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Transmitting power at high voltage and in DC form instead of AC is a new technology proven to be economic and simple in operation which is HVDC transmission. The HVDC (High Voltage Direct Current) technology is used to transmit electricity over long distances by overhead transmission lines or submarine cables. It is also used to interconnect separate power systems. A further development in this technology is HVDC light where HVDC light converters are used for faster and efficient conversion of power.
HVDC Light is a fundamentally new power transmission technology developed recently. It is particularly suitable for medium to small-scale power transmission applications. This new transmission and distribution technology, HVDC Light provides an important role to today’s requirements on our network systems and opens up new opportunities for both investors and environmentalist alike.
HVDC light technology for transmission of electric power is introduced in this paper. Its features, advantages and applications are pointed out. HVDC cables, their design, underground laying, their advantages and applications are also added. The paper also gives a note on the advantages of HDVC light cables over AC underground cables.