This invention relates to a method to protect a dynamic voltage restorer against thermal or mechanical damage in case of an internal short circuit.
The invention further relates to a dynamic voltage restorer for carrying out that method.
A dynamic voltage restorer is designed to compensate short-term disturbances in an AC system such as voltage dips or voltage swells by injecting a series voltage by means of a voltage source converter. In order to be independent from an external power source during such disturbances a large energy storage device arranged by capacitor banks is required.
xe2x80x9cRequirements and Solutions for Dynamic Voltage Restorer, A Case Studyxe2x80x9d (P. Dxc3xa4hler, R. Affolter; IEEE PES 2000, Singapore) describes a dynamic voltage restorer rated for 4MVA load and a compensation capability specified for a 38% 3-phase dip of 150 ms duration. The converter was based on conventional 2-level topology. Due to the moderate rating an energy storage capacitor bank of 245 mF/2200V was sufficient and a conventional protection concept utilizing fuses was used.
The following design example is based on a dynamic voltage restorer specification with a significantly higher rating:
Load to be protected: SGN=15 MVW cos("PHgr")=0.9
Worst case voltage dip to be compensated: xcex94uG=35% 3-pase dip 500 ms
When focussing on large-scale applications the neutral point clamped (NPC) circuit topology as described in EP 0,969,586 is the preferred solution. To be able to compensate an AC voltage dip of these parameters a DC link with the following main data is required:
DC Link Voltage: Ud=xc2x15200 V (nominal at begin of fault) Ud=xc2x12400 V (minimal at end of fault)
Total Capacitance: CD=237 mF (5200 V)
Stored Energy: ED=3.2 MJ (5200 V)
With such a large capacitor bank the well-known DC link protection problem with voltage source converters is dramatically accentuated. The design of the bank and its protection concept must ensure that no thermal or mechanical damages can occur at the following internal fault events:
Semiconductor failure within the voltage source converter leading to a xe2x80x9cFire Throughxe2x80x9d condition across one side of the DC link (+ or xe2x88x92 to midpoint) or across the total DC link (+ to xe2x88x92),
Short circuit across DC busbar (+ or xe2x88x92 to midpoint/+ to xe2x88x92) within the capacitor bank or the voltage source converter,
Internal or external short circuit of individual capacitor cans, and
Ground faults within the capacitor bank or the voltage source converter.
It is a principle object of the present invention to provide a method to protect a dynamic voltage restorer comprising a voltage source converter and a large energy storage capacitor bank against thermal or mechanical damage in case of a short circuit within the converter or the capacitor bank. It is a further object of the invention to provide a dynamic voltage restorer for carrying out that method.
These and other objects are accomplished with the claimed invention as follows:
A short circuit within the converter or the capacitor bank is immediately detected by a short circuit detection unit which permanently monitors currents and/ or voltages in the dynamic voltage restorer. Upon detection either a normal off sequence of the capacitor bank including normal capacitor bank discharge, or a fast discharge of the capacitor bank including firing of all available semiconductors of the converter (protection firing) and distributing the resulting current stress as evenly as possible within the converter is initiated. The short circuit currents are detected and evenly distributed within the converter so that expensive fuses to interrupt the high short circuit currents are not needed.
Thanks to at least one can resistor connected in series to each string of capacitor cans the current stress resulting of the protection firing is kept within the surge capability of the semiconductors.
The can resistors eliminate the need for costly high current fuses, which had to be replaced after a short circuit within the converter or the energy storage device. They also reduce the risk for very high mechanical forces and therefor help lower costs for construction efforts.