research highlights
Advancements in Modular Multilevel Coverters Facilitate Deployment of Off-Shore Wind
The Modular Multilevel Converter (MMC) is a preferred topology for HVDC transmission, especially multiterminal HVDC grids. DC grids can potentially be more cost effective than AC grids, especially for offshore wind farms, where DC cables are much cheaper. DC grids can also provided stability benefits when used to connect with AC grids. One drawback of the MMC topology is the requirement for larger capacitors in each module and inductors in each arm. These passive components constitute a significant portion of the overall MMC cost, and therefore should be minimized. Researchers in the power electronics research group at UTK have been working to characterize the minimum arm inductance and module capacitance required to meet the demands of HVDC transmission, during normal operation as well as fault conditions. Theoretical relationships have been derived for the first time and are currently being verified with a scaled-down prototype built in the CURENT lab.
P
Switching frequency (Hz)
400 350 300 250 200 150 100 50 0 0
AC to DC circulating current ratio
Vdc/2
SM
SM
SM
Csub ? A B C
+
2
Larm
Larm
Larm
20%
Fault current lim it C irculating current lim it
1.5 1
Larm Vdc/2
SM SM
Larm
SM SM
Larm
SM SM
L im its
4% 8% 12% 16% 20% 24%
10%
0.5 0.2 0.25 0.3 0
C apacitor V oltage R ipple
0
0.1
N
Fig. 1 MMC topology with arm inductors and submodules capacitors highlighted
Fig. 2 Submodule capacitor selection principle
Fig. 3 Arm inductor selection principle
A rm inductance L (m H )
0.15
Identifying Vulnerability Against Cyber Attack on State Estimators
Power system state estimation is one of the key control center applications since its results are used by other functions necessary for operating the system and power markets. Hence, having a state estimator that is robust against intentional errors (or so called cyber-attacks), as well as other types of errors, is critical. Estimated system states, namely bus voltage magnitudes and angles, are used to calculate the “estimated branch flows” in the system. These estimated branch flows are used for static security assessment as well as for determining loading and congestion levels which in turn will impact energy pricing. Therefore, any type of manipulation of these estimated flows due to cyber-attacks will have to be avoided. It has long been known that by manipulating a finite number of measurements, a hacker could bias the results of state estimation without being detected. These measurements are known as the “critical measurements” in state estimation. In this work we investigate a certain type of cyber-attack that can be carried out by manipulating a strategically selected subset of critical measurements. We illustrate the risks associated with having such critical measurements and how to identify these types of “vulnerable measurements” so that they can be transformed. This will require investments in new meters which need to be strategically placed in order to transform the existing critical measurements, making them no longer vulnerable to cyber-attacks.
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