ABSTRACT:
The
increasing penetration of power electronics-based distributed energy resources
(DERs) displacing conventional synchronous generators is rapidly changing the
dynamics of large-scale power systems. As the result, the electric grid loses
inertia, voltage support, and oscillation damping needed to provide ancillary
services such as frequency and voltage regulation. This paper presents the
multi-mode operation of a Z-source virtual synchronous generator (ZVSG). The
converter is a Z-source inverter capable of emulating the virtual inertia to
increase its stability margin and track its frequency. The added inertia will protect
the system by improving the rate of change of frequency. This converter is also
capable of operating under normal and grid fault conditions while providing
needed grid ancillary services. In normal operation mode, the ZVSG is working
in MPPT mode where the maximum power generated from the PV panels is fed into
the grid. During grid faults, a low voltage ride through control method is
implemented where the system provides reactive power to reestablish the grid
voltage based on the grid codes and requirements. The proposed system operation
is successfully validated experimentally in the OPAL-RT real-time simulator.
KEYWORDS:
1. Impedance-source
inverter
2. Virtual
synchronous generator
3. Photovoltaic
(PV) systems
4. Low
voltage ride through
SOFTWARE: MATLAB/SIMULINK
SCHEMATIC DIAGRAM:
Figure 1. Proposed ZVSG Converter
Equipped With VSG And LVRT Control Algorithms.
EXPECTED SIMULATION RESULTS:
Figure 2. Rocof Curves With
Different Amounts Of (A) Inertia (H) And (B) Damping Constant (Dp ).
Figure 3. Comparison In Zvsg Current Increase
While (A) The Converter Is Directly Connected (100(Ma)_3:2_10000 D 3200a) And
(B) A Pre-Synchronizing Control Method Is Hired To Decrease The Current
Increment (200(Ma)_1_5000 D 1000a).
Figure 4. Multi-Mode Operation Of The Zvsg During
(A) Normal Operation (C) Voltage Sage Occurrence At T1 And Switching To Lvrt
Mode And (D) Returning To Normal Mode At T2.
CONCLUSION:
This
paper studied the multi-mode operation of an impedance-source virtual
synchronous generator which is comprised of a single-stage ZSI, equipped with
VSG control algorithm and is capable of providing grid ancillary services. Since
the PLL may fail to detect the correct angle in case of harmonic distorted
voltage, a virtual flux orientation control method is hired which can select
the correct angle to be fed to Park transformation. The operation of the system
has been tested while transitioning from islanded to grid-connected mode where,
to protect the system against inrush current while connecting to the grid, a
pre-synchronizing control method is used to minimize the phase difference
between grid and converter. In addition, a solution to survive the system against
voltage faults is embedded in the system which can regulate the reactive power
based on the grid codes. Hence, the control paradigm will switch from MPP generation
to LVRT mode after detecting voltage sag in the system. In this method, the
peak of the grid current is kept constant during LVRT operation mode and
ensures over current protection limit is not violated then. The ZVSG has been
implemented in the OPAL-RT real-time digital simulator and its validity have been
verified by conducting several case studies. The proposed seamless control
frame- work helps to smoothly switch between normal and faulty conditions.
REFERENCES:
[1]
K. Jiang, H. Su, H. Lin, K. He, H. Zeng, and Y. Che, ``A practical secondary frequency
control strategy for virtual synchronous generator,'' IEEE Trans. Smart Grid,
vol. 11, no. 3, pp. 2734_2736, May 2020.
[2]
K. Shi, W. Song, H. Ge, P. Xu, Y. Yang, and F. Blaabjerg, ``Transient analysis
of microgrids with parallel synchronous generators and virtual synchronous
generators,'' IEEE Trans. Energy Convers., vol. 35, no. 1, pp. 95_105,
Mar. 2020.
[3]
J. Chen and T. O'Donnell, ``Parameter constraints for virtual synchronous generator
considering stability,'' IEEE Trans. Power Syst., vol. 34, no. 3, pp.
2479_2481, May 2019.
[4]
H. Cheng, Z. Shuai, C. Shen, X. Liu, Z. Li, and Z. J. Shen, ``Transient angle stability
of paralleled synchronous and virtual synchronous generators in islanded
microgrids,'' IEEE Trans. Power Electron., vol. 35, no. 8, pp.
8751_8765, Aug. 2020.
[5]
H. Nian and Y. Jiao, ``Improved virtual synchronous generator control of DFIG
to ride-through symmetrical voltage fault,'' IEEE Trans. Energy Convers.,
vol. 35, no. 2, pp. 672_683, Jun. 2020.
