Abstract : Based on the concept of power quality composite control, this paper proposes a composite compensation strategy for harmonic current, negative sequence current, and reactive current to address the problems of harmonic current, load imbalance, and low power factor in smart distribution networks. The design method for key parameters is also presented. Related APF-STATCOM simulations, experimental verification, and field operation test results validate the correctness and feasibility of the composite control concept and compensation strategy.
Keywords : Smart distribution network; Active power filter; Static synchronous compensator; Unbalanced load
Although traditional passive filtering and capacitor switching compensation can solve the above problems and are low in cost, they cannot be adjusted continuously in real time and may cause overcompensation, reactive power backfeeding or even induce distribution network resonance [1-3].
To ensure high-quality customized power supply for end users of smart distribution networks, with the development of instantaneous power theory and power electronic devices, the main circuit topology and design, harmonic current detection, compensation methods, control and modulation strategies, and startup characteristics of devices that replace passive filters and capacitor reactive power compensation devices are all hot topics in industry research and industrial applications [2-6].
In today's smart distribution networks, power quality issues are no longer isolated problems but rather complex systemic issues. As shown in Figure 1, a certain public utility distribution system simultaneously suffers from harmonic currents, load imbalance, and low power factor. Power quality composite control technology is increasingly being researched by both academia and industry [7-8].
Figure 1. Power quality problems in actual distribution networks
Fig.1Powerqualityissueinarealdistributedgrid
This paper studies a composite compensation strategy for harmonic, negative-sequence, and reactive currents in a smart distribution network environment, simultaneously addressing time-varying harmonic currents, unbalanced loads, and reactive power issues. The design method for its key parameters is presented. Relevant simulation, experimental verification, and field operation test results validate the correctness and feasibility of this control strategy.
APF-STATCOM Circuit Structure and Working Mechanism
Figure 2 Block diagram of parallel APF-STATCOM
Fig. 2 AnAPF-STATCOM diagram
As shown in Figure 2, this parallel APF-STATCOM adopts a two-level three-phase four-arm voltage source inverter topology. The first three arms provide harmonic and reactive power compensation, while the fourth arm is used independently to control the neutral current. This is because in a three-phase four-wire system, when the load is unbalanced, a large zero-sequence current often flows through the neutral line, unlike in a three-phase three-wire system. Therefore, the fourth arm, decoupled from the first three arms, provides a zero-sequence current path. At this time, the APF-STATCOM generates a compensation current iC,abc that is opposite to the sum of the harmonic, fundamental negative-sequence, and zero-sequence components in the load current iL,abc. This ensures that the power supply current iS,abc only provides the fundamental positive-sequence component of the load current, guaranteeing symmetrical three-phase current output and improving the power factor.
Neutral current separation detection, phase-locked loop (PLL), harmonic current detection, DC voltage control, current control, and PWM modulation are key to achieving a high-performance APF-STATCOM. The PLL and DC voltage control are the same as in a three-phase three-wire system and will not be described in detail here.
Key Issues Analysis
1. Detection and control of centerline current separation in the fourth bridge arm
Considering that the zero-sequence component iN contained in the load current iL, abc in an unbalanced three-phase four-wire circuit is equal, both are...
As shown in Figure 2, the sampled value of the neutral current iN and the compensation command iNref for the zero-sequence current component of the neutral line are used as inputs to the fourth bridge arm current controller. The modulation signal is obtained through the PI regulator to obtain the switching signal of the fourth bridge arm.
At the same time
The formula contains only positive and negative sequence components, which facilitates the subsequent use of the ip - iq harmonic current detection algorithm in a three-phase three-wire system.
2. Harmonic current detection
Figure 3. Schematic diagram of harmonic detection using dq transform.
Fig.3Theschematicdiagramoftheharmonicsdetectingmethodbasedond-qrotatingcoordinationtransformation
Traditional methods for detecting harmonic current based on pq instantaneous reactive power theory are greatly affected by voltage distortion and asymmetry, and are not applicable in practical situations [9]. In practical situations, the ip-iq instantaneous reactive power theory detection method with added phase-locked loop (PLL) circuit is often used, as shown in Figure 3. The relevant transformation is as follows:
Extract the current without zero-sequence component, transform the fundamental component to 0Hz in the dq-0 coordinate by Park transformation (or transform it first and then dq transformation), and extract the fundamental positive-sequence component by low-pass filter [5].
In Figure 2, the DC voltage regulator output value generates an active current command to stabilize the DC bus voltage and compensate for power losses. To improve the power factor, reactive current can be compensated simultaneously; in this case, the fundamental negative-sequence reactive current command value is set to 0. Finally, by subtracting the fundamental positive-sequence component from the load current, the command current values for compensating the harmonic components of the load current, the negative-sequence and zero-sequence components of the current caused by load imbalance, and the command current for the positive-sequence reactive current are obtained, thus realizing the APF-STATCOM function.
3. Design of Current PR Resonant Controller
Since the current command tracked by APF-STATCOM is a superposition signal of sinusoidal quantities of multiple frequencies, the traditional SPWM modulation using PI control will inevitably have steady-state error and phase shift, and the compensation effect is not good. Current hysteresis modulation is often used, but frequency conversion modulation inevitably brings filter design and noise control problems [9].
A sinusoidal signal can be converted into a DC signal through coordinate transformation, allowing a PI controller to be used in the new coordinate system. However, in the APF-STATCOM control field, coordinate transformations must be performed at multiple frequencies, resulting in complex calculations and hindering practical applications. In recent years, PR controllers proposed for sinusoidal signals have emerged. While avoiding coordinate transformations and significantly reducing computational load, they achieve the same control performance as PI controllers in synchronous coordinate systems: they can track sinusoidal signals of a specific frequency without steady-state error, and more importantly, they can selectively compensate for harmonics at specified frequencies.
In the formula, is the resonant frequency.
From equation (7), it can be seen that for a DC system, due to the presence of the integrator, the gain at 0 Hz is extremely high, thus enabling the system to achieve zero steady-state error regulation. For an AC system, for harmonics of 50 Hz and their multiples, the gain of equation (7) is limited, while equation (8) has a higher gain in the corresponding frequency band due to the introduction of the resonant element. If the target being tracked is the fundamental frequency rad/s; if compensation is required for the 5th harmonic with a higher amplitude, then rad/s is required. Usually, the highest harmonic order to be compensated is 20 or 50, especially for odd harmonics with higher amplitudes. Therefore,
Figure 4 shows the Bode plot of the PR resonant controller used for fundamental and third, fifth and seventh harmonic compensation. It can be seen that the current controller has a high gain in the corresponding frequency band, which helps to reduce tracking error.
Figure 4 Bode plot of PR resonant controller
Fig. 4 PR controller bode plots
Simulation and experimental verification
To verify the proposed composite compensation strategy for harmonics, negative sequence, and reactive current, a simulation platform was established in the Matlab Simulink environment. The relevant parameter settings are as follows: input three-phase four-wire voltage 380V/50Hz, three-phase diode rectifier nonlinear load DC-side filter inductor 1mH, resistance 3.2Ω, three-phase diode rectifier AC reactance 0.4mH, APF-STATCOM grid-connected reactance 0.4mH, DC-side supporting capacitor 4000μF, AC-side unbalanced RL load star connection, inductance values of 8mH for all loads, resistance values of 5Ω, 50Ω, and 500Ω respectively, and switching frequency 10kHz.
Figure 5, taking phase A as an example, shows that the APF-STATCOM injected current after compensation effectively cancels the harmonic current of the load current, resulting in a better sinusoidal grid current and realizing the APF harmonic compensation function. Simultaneously, the grid current and grid voltage are in phase and frequency, with a power factor close to 1, realizing the STATCOM reactive power compensation function. Figure 6 shows the three-phase compensation results; the symmetrical three-phase current waveforms verify its good ability to suppress unbalanced loads.
Figure 5. Voltage and current waveforms after phase A compensation (from top to bottom: grid voltage/V, grid current/A, compensation current/A, load current/A; time axis : t/ s).
Fig. 5 Phase A wave forms after compensation
Figure 6 shows the three-phase voltage and current waveforms of the power grid after compensation (from top to bottom: three-phase power grid voltage/V, three-phase power grid current/A, time axis t/s).
Fig.6Threephasewaveformsaftercompensation
Figure 7 further shows the DC-side bus voltage waveform. It can be seen that after the APF-STATCOM completes harmonic compensation, the bus voltage fluctuates slightly, but stabilizes near the set value of 750V.
Figure 7 DC bus voltage / V (time axis t/s)
Fig. 7 Dclinkbus voltage
Figures 8 and 9 further show the internal test results of the industrial prototype. Due to experimental limitations, the load at this time was only a rectifier nonlinear load, so the load current and compensation current differed from the simulation. This mainly reflects the APF compensation function. Figure 10 shows the results of the product's commissioning in the field. Compared with Figure 1, the neutral current decreased from 37A to 5A, the maximum three-phase current THD did not exceed 3.4%, and the symmetry was good, fully verifying the APF-STATCOM composite compensation function.
Figure 8 shows the grid-side current and load current after compensation for phases A and B (from top to bottom: phase A voltage, phase B current, phase A load current, phase B load current).
Fig.9PhaseA&Bgrid&loadcurrentaftercompensation
Figure 9A shows the grid-side current, reverse harmonic current, and load current after phase compensation.
Fig.9PhaseAgrid,inverseharmoniccurrent&loadcurrentaftercompensation
Figure 10. Results after APF-STATCOM compensation in actual field.
Fig.10APF-STATCOMCompensationeffectsinpractice
Conclusion
Based on the concept of power quality composite control, this paper proposes a composite compensation strategy for harmonic current, negative sequence current and reactive current to address the problems of harmonic current, load imbalance and low power factor in smart distribution networks.
Simulation, engineering prototype testing, and field operation results verified the APF-STATCOM composite compensation function implemented based on this strategy.
Source : Electrical Applications, Issue 6, 2014
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