A fast real and reactive power flow regulation algorithm for a single-phase grid-connected photovoltaic inverter is developed. The algorithm is fast and has the advantage of a quick response to the change in the required real and reactive power output of the PV inverter. The voltage (both magnitude and phase angle) is assumed to be the same as the mains voltage.
The comparison is made for different power change cases, such as only real power change while the reactive power is fixed or vice versa, etc. It can provide over-frequency protection and support in case of real power mismatch in the grid. 45 Fig. 4.3: (a) Real and reactive power for case-III using proposed. b) Real and reactive power for case-III using iterative algorithm proposed by Chang.
53 Fig. 4.7: (a) Real and reactive power for case-VII using proposed. b) Real and reactive power for case-VII using iterative algorithm proposed by Chang. 55 Fig. 4.8: (a) Real and reactive power for case-VIII using proposed. b) Real and reactive power for case-VIII using iterative algorithm proposed by Chang…..……….…….
Introduction
Statement of Problem
One of the most important features is the absence of energy storage devices such as batteries. Page | Four needs from both the generated side and the consumer side must be met to ensure the stability of the system. It can be used when tension suddenly collapses. It can provide the required reactive power to support the mains voltage.
When the load is low, if the actual power output does not match the consumption, the system frequency will increase. Again, PV inverters can be used to supply only reactive power at night, when sunlight is not available. Therefore, in different cases, the real and reactive power must be adjusted to guarantee the stability of the network.
In addition to that, more efficient and simple control means more reliability and lower cost of the system. If the response time of such a system can be improved, the sudden collapse of voltage, and also the instability of the system can be prevented much faster.
Objectives
Thesis Organization
Literature Review
Background
A reactive power controller based on the instantaneous power theory was also realized by Libo et al. Grid-tied PV systems must supply reactive power as well as real power to the grid. This feature of the PV system plays an important role in triggering more research into the control of real and reactive power injection into the grid [12].
In [13], the possibility of using grid-connected PV inverters as a supplier of reactive power was analyzed. The concept of instantaneous power in three-phase circuit is not new and can be used to estimate instantaneous real and reactive power without any limitation. It also performs reactive power compensation and keeps the grid current nearly sinusoidal.
The second one is the generation of the fundamental reference current corresponding to real and reactive power generation. Page | 16-phase H-bridge inverter injects real power to the grid and is able to compensate the reactive power [16]. The step size of phase shift is determined by the signs of the fictitious active and reactive power components, and is used to correct the sine reference generated in the grid voltage supply network.
Discrete Fourier transform PLL is used by Liu in [18] to achieve more accurate reactive power control. Extracting the real and reactive power components of the grid voltage is essential for power allocation. The real output power and reactive power of that inverter can be controlled more easily than with the previous methods.
To further reduce the computational burden, Chang [21] proposed an effective output power calculation method known as - the simplified reactive power control strategy for single-phase grid-connected PV inverter. From these two sample currents, the real and reactive power of the inverter output are easily calculated. Page | 20 calculation method is able to measure the active and reactive power injected into the network admirably.
![Fig 2.2: Implementation of Hilbert Transform and the current control [15].](https://thumb-ap.123doks.com/thumbv2/filepdfnet/10888869.0/26.918.163.829.599.943/fig-2-implementation-hilbert-transform-current-control-15.webp)
Methodology
Basic Blocks
- Output power Calculation
- Power Adjustment Block
- CMASDM
For this method, the line voltage is assumed to be purely sinusoidal, so that the output power can be directly measured by the injected sinusoidal current. Thus, the actual power output (POUT) of the grid-connected inverter can be determined. So, from the POUT and QOUT equation above, the active and reactive power of the inverter can be calculated using two selected sampled current values taken at the peak voltage point and when the voltage crosses zero.
From the previous block, the output power (P & Q) is calculated, and compared to the required P & Q within this block. In the proposed iteration method [21] these steps are definite, i.e. the difference between two adjacent arcs, or radial lines is constant. To ensure the stable system, it is ensured that at least one AC cycle is required to adjust the output power.
And let the change in magnitude of the current due to the change in operating point ΔIM, be limit. 80% of the limits are then set as the ΔIM and Δθ for different levels of output power and reference power difference. The equation used to find the magnitude of the current reference (since the current control system is used, the output current will follow the generated current reference).
Here, IM (n+1) and IM (n) are the amplitude of the reference current for the main cycle n+1 and n of AC. Then the phase angle of the current reference (Since current controller system is being used, the output current will follow the generated current reference.) will be. Likewise, θ(n+1) and θ (n) are the phase angle of the reference current for the main cycle n+1 and n of AC.
Page | 32 This equation shows that the output current of the inverter is determined by the reference signal. The reference signal for the grid-tied inverter must never be in phase with the AC mains voltage because of the imaginary term. To obtain the desired output current, the reference signal of the CMASDM must be carefully determined.
Flow Chart
Result & Discussion
Finding the minimum and maximum step size
Page | 36 To determine the smallest step size, the minimum allowed change in the reference power was set to 1%. In order to find the maximum step size for different power changes, several levels of difference between output power and reference power were determined. The tables below show ΔIM, MAXIMUM and Δθ MAXIMUM for different power changes.
Parameters Set for the Simulations
Parameters used for eight different cases
Results
The proposed power adjustment method changes the power after 15 AC cycles (Fig. 4.2 a). But for the fixed difference iterative method, the output power is adjusted to the reference power after almost 20 cycles (Fig. 4.2 b). The proposed method changes the power after 18 AC cycles (Fig. 4.3 a). But the previous iterative method needs 82 cycles to adjust the power to the reference power (Fig. 4.3 b). For this case, the proposed method adjusts the power just after 4 cycles (Fig. 4.4 a). The iterative fixed difference method requires 12 cycles to adjust the output power (Fig. 4.4 b).
In this case, the proposed method adjusts the power after 10 cycles (Fig. 4.5 a), but the fixed-difference iterative method takes almost 71 cycles to adjust the power (Fig. 4.5 b). In this case, the proposed method adjusts both the real power and the reactive power after 29 cycles (Fig. 4.6 a), the fixed difference iterative method takes more than 110 cycles to adjust (Fig. 4.6 b). At this point, the adaptive method adjusts both the real power and the reactive power after 19 cycles (Fig. 4.7 a), and the fixed-difference iterative method takes more than 140 cycles (Fig. 4.7 b).
The proposed method adjusts the real and reactive power after 10 cycles (Fig. 4.8 a), and the iterative fixed difference method requires 42 AC cycles to adjust the output power.
Comparison
Observation
Conclusion
Further work
![Fig 3.3: Conceptual diagram for operating point of PV inverter [21].](https://thumb-ap.123doks.com/thumbv2/filepdfnet/10888869.0/37.918.245.602.500.877/fig-conceptual-diagram-operating-point-pv-inverter-21.webp)
![Fig 3.6: Circuit diagram of typical grid-tied single phase inverter [22].](https://thumb-ap.123doks.com/thumbv2/filepdfnet/10888869.0/43.918.236.637.305.506/fig-circuit-diagram-typical-grid-single-phase-inverter.webp)
