Research on Step-down Resonance Suppression and Filtering Device

In recent years, offshore wind farms have developed rapidly, and their installed capacity is getting bigger and bigger, and the offshore distance is getting farther and farther. In order to meet the needs of large-capacity and long-distance offshore wind power transmission, the AC submarine cable has the characteristics of long distance, high voltage and large cross-section. Along with the significantly improved transmission capacity, its equivalent capacitance is also increasing, which further reduces the resonance frequency of the offshore wind power AC transmission system. If the parallel resonance frequency of the offshore wind power transmission system is close to the main harmonic current frequency band of wind turbines, it will cause harmful system resonance and harmonic amplification, significantly increasing the harmonic current of grid-connected lines and the voltage distortion rate of nodes in nearby power grids, and seriously affect the safety of equipment and the safe and stable operation of offshore wind power transmission system. A step-down resonance suppression filtering device applied to the offshore wind power transmission system is proposed. The AC cable is connected to the low-voltage 35 kV side of the compensation winding of the centralized control center to eliminate the resonance risk of the 220 kV system and filter out the harmonic current. Compared with the existing technical scheme, the device has the advantages of simple control, reliable operation, stable resonance suppression effect, low operation loss and the like, and has great engineering popularization value.


Introduction
Limited by the site environment of the offshore wind farm, its current collector line and outgoing line must adopt submarine cable.At present, the common configuration scheme in China is that the current collector line of the offshore wind farm adopts the submarine cable with a voltage level of 35 kV, and the outgoing line adopts the submarine cable with a voltage level of 220 kV [1][2][3].The submarine cable has the characteristics of long distance, high voltage and large cross section.Along with the significantly improved transmission capacity, its equivalent capacitance is also increasing, which further reduces the resonance frequency of the offshore wind power AC transmission system.However, the main harmonic currents of wind turbines are generally low-frequency harmonics such as 5th and 7th.Therefore, the extensive use of submarine cables makes the transmission system appear parallel resonance, which greatly increases the risk of harmonic current amplification and voltage distortion rate increase, endangering the safety of equipment and affecting the safe and stable operation of the offshore wind power transmission system [4][5].Harmonic control of the offshore wind power transmission system, especially the resonance suppression of submarine cables, has become one of the common engineering problems in the field of offshore wind power technology [6][7].A step-down resonance suppression 2.1.Network characteristics of offshore wind power transmission system The offshore wind power system includes a wind turbine, 35kV submarine cable collector line, 220 kV submarine cable, 220 kV overhead line, 35/0.69 kV step-up transformer, 220/35 kV main transformer, offshore/landside shunt reactor and static var generator (SVG).The harmonic source is the wind turbine, and other components constitute the collection and transmission system.
Due to the use of the submarine cable, the capacitance C of the offshore wind power system to the ground is relatively large, which forms resonance with the inductive reactance L at a specific frequency [8], and the frequency is: As can be seen from the above equation, the larger the capacitance C is, the lower the resonant frequency is.
Because of the large number of devices and the complexity of the network, the method of computer modeling and simulation is generally used to output the impedance-frequency characteristic curve, find the parallel resonance point, and then study the system resonance and harmonic amplification characteristics.

Existing Harmonic Control Schemes and Their Deficiencies
When the problem of harmonic overrun occurs in the offshore wind power project, the existing control scheme is as follows [9][10][11]: 1) 220 kV high voltage filter device is installed.This scheme needs to occupy 220 kV intervals, and its investment and land occupation are large, so it is rarely used in the project at present.
2) The SVG device is used for harmonic absorption.SVG device is a current source, which cannot change the impedance characteristics of the system and cannot eliminate the resonance problem.And the SVG device compensates for the amplified harmonic current, so the required equipment capacity is large.

Access system schema
According to the typical configuration of the offshore wind power system, the best access point of the step-down harmonic elimination filter device is the 35 kV side of the reactive compensation transformer in the centralized control center, as shown in Figure 1.

Equipment parameter calculation
The step-down harmonic elimination filter device needs to form a series resonance at the H harmonic frequency of the 220kV bus to destroy the parallel resonance and absorb the harmonic current I .The calculation process of the device parameters is as follows.
Step 1: The equivalent reactance X of the transformer is calculated according to the following formula.
where, U is the rated voltage at the low-voltage side of the transformer, kV; U % is the percentage of short-circuit voltage drop of transformer, %; S is the rated capacity of the transformer, MVA.
Step 2: The overall impedance X of the harmonic elimination filter branch is calculated according to the following formula.
where K is the ratio of branch fundamental current I to harmonic current I ; I is harmonic current, A.
And step 3: The capacitive reactance  value of the harmonic elimination filter branch is calculated according to the following formula.

𝑋 = 𝑋
(4) where H is the number of series resonance of the branch.
Step 4: The value of the branch capacitor C is calculated according to the following formula.
Step 5: The branch inductive reactance  value is calculated according to the following formula.

𝑋 = − 𝑋 (6)
Step 6: The  value of the branch reactor is calculated according to the following formula.

𝐿 = (7)
Step 7: The branch rated current  value is calculated according to the following formula. = 1.3 ×  +  (8) This current is also used as the rated current of the reactor and capacitor.
Step 8: The rated voltage  value of the capacitor bank is calculated according to the following formula. =  ×  (9) where  is the rated current value of the filter branch.

Simulation analysis of the expected effect
After the technical parameters are determined according to the above process, the model is built to simulate the harmonic power flow and impedance frequency characteristics.The criteria for determining whether the parameter setting of the step-down harmonic elimination filtering device is correct and reasonable are as follows:  After the device is connected, the series resonance point of the high-voltage power grid in the near area is set correctly. The dangerous parallel resonance risk under various operation modes has been eliminated. The harmonic current injected into the point of common coupling of the power grid conforms to the national harmonic standard GB/T 14549. The voltage distortion rate of the point of common coupling in the power grid conforms to the harmonic national standard GB/T 14549.The device shall be free of overvoltage, overcurrent, and overload.

Project overview
A 500 MW offshore wind power project in Hainan supports the construction of a 220 kV offshore booster station and an onshore centralized control center.The electric energy generated by 50 10 MW wind turbines is connected to the offshore booster station through 20 loops of 35 kV collecting submarine cables, and the offshore booster station is equipped with two 230/36.75kV main transformers with 250 MVA.After being boosted by the main transformers, it is connected to the onshore centralized control center through two loops of 220 kV submarine cables (with a cross section of 3×1000 mm 2 and a length of 2×42 km), and then through an overhead line (with a cross section of 4×500 mm 2 and a length of 23 km).

Harmonic problems on site
Since the wind farm was put into operation at the end of 2020, the wind turbine has been disconnected from the grid and the parts have been damaged many times under the small mode operation condition.
The wind turbine shows that the fault type is "excessive harmonic value of the power grid" and "frequency converter fault".The measured 5th harmonic current of the grid-connected line of the wind farm is 61.60 A, far exceeding the allowable value of 22.12 A specified in the national standard.220 kV voltage waveform and frequency spectrum of the offshore booster station are shown in Figure 2.
(A) Voltage waveform (B) Harmonic spectrum Figure 2. Voltage waveform and harmonic spectrum of 220 kV bus in an offshore booster station

Analysis of harmonic power flow
The rated power of the wind turbine in this project is 10 MW, and the harmonic current is mainly the 5th harmonic current (the content is 2.03%).The superposition value of the harmonic current at the 220 kV side of the wind farm is only 16 A, which is much lower than 61.60A at the grid-connected point, indicating that the network has resonant amplification (about 3.85 times) on the 5th harmonic current.The model built in DigSILENT software is shown in Figure 3.It can be seen from Figure 4 that the system generates parallel resonance near the 5th harmonic, resulting in current amplification of the 5th harmonic.
The simulation results of voltage, current waveform and frequency spectrum of the grid-connection point are shown in Figure 5.It can be seen from Figure 5 that the simulated value of the 5th harmonic current is 61.498A, which is basically consistent with the measured value of 61.60 A; the 5th voltage distortion rate is 5.124%, which is slightly lower than the recorded value (no background harmonics are superimposed in the simulation).The voltage simulation waveform is basically consistent with the recorded waveform.

Parameter calculation of step-down harmonic elimination filter device
According to the parameter calculation method and process of the step-down harmonic elimination filter device proposed in this paper, the parameters used in this project are shown in Table 1.The main electrical wiring of the step-down harmonic elimination filter device is shown in Figure 6.

Simulation analysis of treatment effect
After the device is connected, the frequency characteristic of system impedance is shown in Figure 7.It can be seen from Figure 7 that the parallel resonance point of the step-down harmonic elimination filter device on the 35 kV bus is 5.810 times, while the parallel resonance point of the 220 kV bus is reduced to 4.976 times, which has the same resonance suppression effect as the 220 kV bus direct connection scheme.
After the step-down harmonic elimination filter is connected, the voltage, current waveform and spectrum simulation results of the grid-connected point are shown in Figure 8.It can be seen from Figure 8 that the harmonic current at the grid-connected point is reduced from 61.498A to 3.609 A, the voltage distortion rate at the grid-connected point is reduced from 5.124% to 0.301%, and the system resonance suppression and harmonic filtering effects are significant, with an improvement rate of 94.13%.
The current waveform and frequency spectrum flowing into the step-down harmonic elimination filter device is shown in Figure 9.It can be seen from Figure 9 that the current flowing into the step-down harmonic elimination filter device is mainly 5 times, which is 131.62 A, converted to 20.93 A on the 220 kV side, which is lower than the measured value of 61.498 A. After eliminating the resonance phenomenon, the current injected into the filter device will also be significantly reduced.

Measured data of treatment effect
After the step-down harmonic elimination filter device is put into operation, the voltage and current distortion rate of the wind farm is significantly reduced; the system and equipment operate stably; there are no off-grid and device damage problems such as "excessive harmonic value of the power grid" and "frequency converter failure"; and the harmonic problem has been effectively solved.The 220 kV bus voltage waveform and spectrum of the centralized control center after the device is put into operation are obtained through the power quality online monitoring device, as shown in Figure 10.

Conclusions
 Due to the use of the submarine cable, the capacitance C of the offshore wind power system to the ground is large, which forms a parallel resonance with the inductive reactance L at a specific frequency.The longer the submarine cable is, the larger the capacitance C is, and the lower the resonance frequency is. According to the typical configuration of the offshore wind power system, the best access point of the step-down harmonic elimination filter device is the 35 kV side of the reactive compensation transformer at the centralized control center. According to the parameters calculated by Formulas (2)~( 9) in this paper, the step-down harmonic elimination filter device can form parallel resonance at the H harmonic frequency of 220 kV bus, thus destroying the resonance and absorbing the harmonic current I of the corresponding order. Compared with the existing harmonic control scheme, the step-down harmonic elimination filter device has technical and economic advantages such as simple structure, saving investment and land occupation, remarkable control effect, and low operation loss, which has great engineering promotion value.

Figure 1 .
Figure 1.Schematic diagram of system access of step-down harmonic elimination filter device

Figure 3 .
Figure 3. Simulation analysis model of harmonic power flow

Figure 5 .
Figure 5. Simulation waveform of voltage and current of grid-connection point (before treatment)

Figure 6 .
Figure 6.Electrical main wiring diagram of step-down harmonic elimination filter device

Figure 8 .
Figure 8. Simulation waveform of voltage and current (after treatment)

Figure 9 .
Figure 9. Current waveform and frequency spectrum flowing into step-down harmonic elimination filter.

Figure 10 .
Measured value of voltage waveform and frequency spectrum after step-down harmonic elimination filter device is put into operation

Table 1 .
Parameter calculation results of step-down harmonic elimination filter device