Design and Verification of Cascaded Multilevel STATCOM Active Filter Function

STATCOM can be used as both dynamic reactive power compensation device and active power filter to improve the power quality of a power grid. This paper describes the active filter function implemented into an existing STATCOM control. The paper explains firstly the filtering control system of the STATCOM, starting from the introduction of active power filter principle, harmonic current detection and tracking control. After that, the parameter design, open-loop characterization and simulation verification of the proposed filtering control scheme are carried out. The PSCAD simulation results show that the active filtering control scheme proposed in this paper can effectively reduce the harmonic content in the system and has good stability.


Introduction
Due to a high proportion access of power electronic equipment to the power grid, harmonic pollution of the power grid is becoming increasingly significant [1], and active harmonic suppression is an important means of realizing high-quality operation of the power system.STATCOM is a new type of power electronic equipment based on the fully-controlled semiconductor switching devices, which can suppress harmonics while providing reactive power compensation [2].
Different countries have formulated their own standards to limit the harmonic content of the power grid access equipment [3].The industry has also formed a variety of harmonic suppression techniques.Reference [4] utilizes a band-pass filter to extract the nth harmonic current component from the concerned load current, which is then used to generate the harmonic control voltage.In order to minimize the complexity of the control system, some studies choose to compensate specific frequency harmonics only.In reference [5], the SVG output current is used to track the load current.The error between them is separated from the specified low harmonics using multi-synchronous rotating coordinate systems.The voltage components are obtained by multiple PI controllers.In the αβ coordinate system, the inner loop of reference [6] uses a quasi-ROGI resonance controller for the error current to realize the fundamental and harmonic separation.For specific scenarios, reference [7] separates the controlled current from the specified frequency harmonic currents using multisynchronous rotating coordinate systems, and generates the corresponding voltage components directly after being amplified.In order to reduce the burden of multiple coordinate transformation calculations of a SVG control system, references [8] and [9] use a combination of repetitive controllers and PI controllers to generate voltage components for load harmonic currents with full complement to participate in carrier modulation.All the above methods improve the power quality and provide the technologies for engineering implementation.
In order to design the active filter function on an existing STATCOM device, this paper takes a 35kV/150MVar STATCOM as an example.The active filter function was designed by using the full complementary method for the harmonic currents.The open-loop stability analysis and simulation verification of the proposed design were performed, which shows that the method proposed in this paper can effectively suppress the harmonic currents of the loads.

STATCOM Control System
The control system of STATCOM is shown in figure 1.It contains phase-locked loop (PLL), coordinate transformation, outer loop control, inner loop control, and phase-shifted carrier modulation.Among them, the PLL provides the angle for the dq coordinate transformation and inverse transformation of the control parameters.The d-axis component of the outer loop provides split-phase control of the DC capacitor voltage, and the q-axis component is uses constant reactive power control to form the d and q current reference components needed for inner loop control, respectively.The d and q components are amplified using the inner loop proportional resonant (PR) controller and output a modulating voltage, which is modulated by a PWM carrier to generate pulses to control the switching state of the IGBTs in different H-bridge sub-modules in the converter chain.In the figure, v is the system line voltage.i is the converter chain phase current.θ is the electric angle of the PLL output.i dref + , i qref + , i dref − , i qref − are the positive and negative reference sequence dqaxis current components of the inner loop.v aref , v bref , v cref are three-phase modulated voltage signals.U grid is the grid voltage.

Active Power Filter
Active Power Filter (APF) can suppress the harmonics of different amplitudes and frequencies, and in the meanwhile provide the reactive power compensation for a load.In most cases, a shunt APF can compensates harmonic current sources, the structure of which is shown in figure 2. In the figure, i s is the grid current, i L is the load current, and i C * is the STATCOM current.For APF function, The harmonic current detection circuit of the STATCOM first detects the load current and separates harmonic current components.Then the STATCOM is controlled to output a compensation current of equal amplitude but in opposite direction to the corresponding harmonic current of the load, and inject the compensation current into the grid to offset the harmonic current generated by the load.In this way, the grid only provides the load with the fundamental active current, thus achieving the purpose of active filtering.

Harmonic Current Detection
The effectiveness of harmonic compensation is closely related to the harmonic detection method.The harmonic and reactive current detection method based on the instantaneous reactive power theory has good dynamic performance, and the i p − i q operation has high detection accuracy, of which the principle is shown in figure 3. The output of the harmonic detection unit will be used as the command current of the active filter to adjust the output of the STATCOM.
i β , i p , i p are the load current in different coordinate systems.i p , i q , i Lαf , i Lβf , i Lfa , i Lfb , i Lfc are the DC component of the load current in different coordinate systems.i Lha 、i Lhb 、i Lhc are the harmonic components of the load current.

Harmonic Current Tracking Control
The harmonic current tracking control is shown in figure 4, in which, the detected harmonic current is used as the command signal.The difference between the command signal and the harmonic compensation current provided by the STATCOM is filtered and amplified and then added as a superimposed signal to the modulation voltage of the PWM comparator.are the target current values of the dq-axis components of the inner loop.i abref , i bcref , i caref are the target current values of the abc-axis of the inner loop.i ab_SVG , i bc_SVG , i ca_SVG are the STATCOM fundamental-frequency components of the compensation current.i cha , i chb , i chc are STATCOM harmonic components of the compensation currents.U a_grid , U b_grid , U c_grid are the STATCOM output voltages.

Design of Open Loop Gain and Bandwidth
On one side, the magnitude of the control error reflects how well the harmonic component of the STATCOM output current compensates the load harmonic current.On the other side, the relative value of the control error is inversely related to the open-loop gain of the control system.Therefore, the active filter performance can by controlled by choosing an appropriate open-loop gain of the control system, or the coefficient of the proportional amplifier.
To ensure that the STATCOM can effectively suppress harmonic currents in the load within the harmonic frequency range specified in relevant standards, the bandwidth of the STATCOM active filter control system should be designed to be 100Hz-1250Hz.
When the bandwidth and gain are determined, it should be ensured that it can operate stably and recover quickly in transient, steady state and fault recovery by adding filters in the control loop or designing overrun and hysteresis corrections.The commonly used criterion for control system stability evaluation is that for a given frequency range, the phase shift between the output and the input of the control system should be less than 180 electric degrees as long as the open-loop gain of the control system is more than 1.Practically, a certain margin, saying 30 electric degrees, is typically allowed for the safe operation of the control system.

Test for Open Loop Characterization
In the open loop characterization test, a perturbation signal of fixed amplitude and adjustable frequency is first used as the harmonic reference, and the expression of the perturbation signal phase A is The signal amplitude is selected to be 1% of the STATCOM fundamental current amplitude, and the frequency is varied from 100 Hz to 1100 Hz.
The simulation reveals that the bandwidth of the low-pass filter has a big impact on the performance of the control system.When the bandwidth is wide, the error is large and there are more high harmonics in the compensation current.On the contrary, the error is small and the harmonics are less.
The filtering performance is better when the cutoff frequency of the low-pass filter is selected as 200 Hz and the amplification is 17.In order to avoid the instability of the control system caused by other uncertainties during the start-up period, an active filter delay module is added to during simulation.The obtained amplitude-frequency and phase-frequency characteristic curve of the active filter control system is shown in figure 5.
From this figure, it can be seen that the open loop gain of the active filter control system remains stable below 200 Hz, the open-loop gain being close to 20 dB.As the frequency increases the open loop gain of the control system decreases while the phase shift increases.The trend line shown in red line indicates that the frequency for unity open-loop gain of the control system is about 1100 Hz, the phase shift of the open-loop is about -130 degrees.The stability margin of the system is about 50 degrees, which is in line with the general design principle.The sudden drop of the open-loop gain for the frequency ranged from 600Hz to 900Hz is due to the measurement errors of the simulation system.

Experimental Simulation Verification
The STATCOM control system is built using the PSCAD simulation platform, and the equivalent circuit is shown in figure 6.A system voltage of 110kV is assumed.The nonlinear load is represented using a controllable current source.The STATCOM and the current source are connected to the same 110kV bus via two step-down transformers.The operation voltage of both STATCOM and nonlinear load are 35kV.The STATCOM is based on the cascaded multilevel technology and has a power rating of 150MVAr.The primary windings (110kV side) of the step-down transformers are configured in the Y-connection while the secondary windings of the transformers are configured in the Δ-connection.The STATCOM is set at the constant reactive power operation mode.Considering the three-phase system is symmetrical, only the harmonic current of phase A of the 110kV bus is taken as an object for analysis.The compensation performance is expressed as the ratio between the change of harmonic current content (HCC) before and after compensation and the before-compensation harmonics, measured at the 110kV bus. (2)

Active Filter Performance Test with Different Amplitudes of Load Harmonics
Keeping the frequency of the load harmonics at 300Hz (6th harmonic), the harmonic compensation performance of the STATCOM is measured for different load harmonic amplitudes and different STATCOM reactive power targets Q ref .
The test results are shown in figure 7.
In figure 7 (a), Q ref = 1p.u., which corresponds to a STATCOM branch current of about 1.15 kA.In figure 7 (b), Q ref = 0.1p.u., which corresponds to a STATCOM branch current of about 0.12 kA.'Original' is the harmonic distortion (THD) of the 110kV bus without active filtering control.'APF' is the THD of the 110kV bus after applying the active filter control function of STATCOM.'Total' is the compensation performance of the total harmonics after applying the active filter control function of STATCOM, and "6X frequency" is the compensation performance for the 6th harmonic frequency injected after applying the active filtering control function of STATCOM.From the left figure, it can be seen that as the amplitude of the load harmonic current increases, the THD of the 110kV bus current increases.This is mainly due to the fact that the reference current used in the THD calculation is the fixed fundamental current output from the STATCOM, while the harmonic currents are supplied by the load harmonic current source.
As can be seen from the figure on the right, the compensation performance for the test frequency is generally about 90%.However when the amplitude of the load current is small, the total harmonic compensation performance after applying the compensation function is lower than that of the test frequency.This indicates that there are other harmonic components in addition to the test frequency injected in the system.Further diagnosis reveals that these additional harmonic components are generated by the STATCOM.

Active Filter Performance Test with Different Load Harmonic Frequencies
Amplitude of the load harmonic current is set to a constant value of 0.2kA, and the frequency is changed between 100Hz~1000Hz.The harmonic compensation performance of the STATCOM is measured at Q ref = 1p.u.The simulation results are shown in figure 8. From the left-hand side figure, it can be seen that when no active filter control is added, the THD of the 110kV bus current for different frequency injections is almost the same.However, after the active filter control is in operation, there is a significant decrease in the THD, and the decrease in the lowfrequency band below 400Hz is in the range of 70-90%.As the load harmonic frequency increases, the 110kV bus current THD tends to rise, due to the limited cut-off frequency of the active filter control.As the frequency continues to rise beyond the cut-off frequency of 200Hz, the gain of the active filter control starts to decrease, resulting in a bigger error in the harmonic current control.
It can be seen from the on the right-hand side figure that the total harmonic compensation performance of the STATCOM is slightly lower than that of the test frequency, indicating that the STATCOM system itself still produces fewer other frequency harmonics.

Comparison of Different Compensation Methods
Table 1 compares the compensation performances provided in different literatures.It is seen that all the proposed harmonic compensation methods based on the STATCOM device can reduce the harmonic content in the system.Among them, literature [6] gives the best performance by compensating different frequency harmonics individually.In a follow-up work, this individual frequency control strategy could be optionally adopted to further improve the harmonic suppression performance by combining it with the full compensation approach presented in this paper.[5] 28.25 1.47 94.80 [6] 8.54 5.28 38.17 [7] 9.82 3.04 69.04 [8] 10.03 1.28 87.24 [9] 11 3.2 70.90

Conclusion
In this paper, an active filter control function is added to the existing STATCOM design for specific scenarios, and the following conclusions are obtained: STATCOM can be used as an active filter to suppress the harmonic current injected into the power grid by outputting a compensation current equal in size and opposite in direction to the harmonic current of a nonlinear load; The harmonic current tracking scheme based on the superimposed voltage component method can reduce the harmonic content of the grid current and has good harmonic suppression characteristics; The stability of the control system can be determined by analysing the open-loop amplitude-frequency and phasefrequency characteristics of the control system.Due to the use of separate control paths, the stability of the active filter control system of a STATCOM can be improved by adding amplifiers and low pass filter links without impacting the performance of the STTCOM existing functions; Although STATCOM can compensate harmonic currents caused by nonlinear loads, the harmonics generated by itself will also have an impact on the stability and power quality of the grid.Compensation performance of the full compensation approach can be improved by combining with the individual frequency compensation strategy.

Figure 4 .
Figure 4. Harmonic current tracking control of superimposed voltage component method.

Figure 6 .
Figure 6.Main circuit single line diagram of STATCOM active filter control system.

Figure 5 .
Figure 5. Characteristic curves of open-loop amplitude frequency (a) and phase frequency (b).

Figure 7 .
Figure 7. Harmonic Compensation for Different Load Harmonic Current Amplitudes.

Figure 8 .
Figure 8. Harmonic compensation effect for different load harmonic frequencies.

Table 1 .
Comparison of harmonic compensation schemes in different references.