Flexible control strategy for HVDC transmission system adapted to intermittent new energy delivery

0 Introduction

According to the plan, China will generate 50% of its electricity from nonfossil energy sources by 2050 [1].To realize the consumption of clean energy such as hydropower, photovoltaic, and wind power in western China,China is building many long-distance ultra-high voltage (UHV)power transmission projects to solve the space-time imbalance of energy distribution and consumption [2-3].UHV direct current (DC) transmission technology plays an important role in long-distance transmission owing to its advantages of adjustable power and asynchronous networking [4-7].As of June 2020, the State Grid Corporation of China has built 11 UHVDC projects, forming a large alternating current AC/DC hybrid grid.

New energy generation introduces numerous difficulties in the operation of power grids [8-10].The output of new energy generation is random, which leads to frequent modulation of the DC power [11-12].Consequently, a DC transmission system requires frequent switching of the converter tap-changer.Since 2018, numerous serious accidents caused by UHV converter tap changers have occurred in China.In January 2019, the tap changer at the Changji Station caught fire [13].Subsequently in March, the same fault occurred at the Yinan Station.Equipment safety of tap changers has become the main bottleneck restricting the secure and stable operation of DC systems.The converter tap changer is a key component of a DC system,and is responsible for the regulation of DC power and DC voltage.To solve the above problems, it is necessary to improve the technology to enhance the reliability of a high-point device [14].Concurrently, it is essential to improve the DC control method to reduce the operation frequency of the tap changer.Based on statistics, a UHVDC transmission project tap changer is operated approximately 200,000 times/year [15].Frequent change of a tap changer significantly reduces the service life of the corresponding equipment.Therefore, reducing the operation frequency of the tap changer of a DC system by improving the DC control strategy is an important approach to ensure the safety and reliability of the system.

The mainstream tap changer control strategy is typically to operate the control angle of the converter valve at the rated value [16].Under this limitation, a DC system strongly depends on the tap changer to adjust both the power and voltage.Ref.[17] compared and analyzed three different tap changer control strategies for a practical back-to-back DC project.It was found that no-load DC voltage control at the fixed-valve side could effectively reduce both the number of tap changer positions and the number of operation adjustment times of the tap changer.Ref.[18] employed the aforementioned control method on the inverter side of a long-distance high-voltage DC (HVDC) transmission system.Recently, in DC transmission systems, such as Lugu and Zhaoyi, a dynamic voltage control method has been adopted at their inverter stations with an increased voltage dead zone.Although these methods reduce the operation frequency of a discrete voltage regulator, it is at the cost of a small loss of the DC operation efficiency and a reduction in the power range of the DC system.

In recent years, with the improvement of equipment manufacturing technology, a converter valve need not necessarily strictly operate at the rated angle.Under a reduced-voltage operation condition, the control angle of a DC system can be above the rated value.The angle control of a converter is also used for transient stability control,such as rapid reactive power control [19-20] and active power modulation [21].Therefore, this study relaxes the control angle constraint of a DC system.By expanding the working range of the steady-state control angle, full power regulation of a DC system with fixed converter tap changer positions at both feed and receiver ends is realized.

In this paper, the steady-state model of the converter for DC power control is introduced and a DC power control method based on a fixed tap changer position is proposed.A power control strategy for achieving constant power on the rectifier side and constant DC voltage on the inverter side is established.Subsequently, based on the above strategy, the initial tap position of the converter transformer is optimized to realize DC full power modulation within the allowable range of the control angle.The effects of the new control strategy on the control angle, reactive power compensation,and commutation failure are analyzed.Finally, the above optimization methods and their effects are verified by steady-state and electromagnetic transient simulations,respectively.

1 Analysis model

1.1 Steady-state model of converter

Ref.[22] introduces the topological structure of a typical six-pulse bridge converter.In a practical DC system,it is possible to increase the DC voltage and restrain the fluctuation of DC current by increasing the number of bridges and valves.The steady-state model of a six-pulse converter is introduced in this paper.

The steady-state model of the above converter is simplified based on the ideal commutation process in the case of three-phase symmetry; therefore, the DC voltage can be expressed as equation (1).

where kT is the ratio of the converter transformer, UL is the voltage of the converter bus, θd is the control angle (the firing angle on the rectifier side, and the extinction angle on the inverter side), Xc is the reactance of the convertor transformer, and Id is the DC current.

1.2 Model of DC power

For two-end DC systems, the DC current directly determines the DC power at a given DC voltage.The DC current is determined by the voltage of the two converter stations, as expressed in equation (2).

where subscript r represents the rectifier, i represents the inverter, α represents the firing angle of the rectifier,and γ represents the arc extinction angle of the inverter.The equivalent admittance of the DC line is expressed in equation (3).

2 Relationship between power control and tap changer

2.1 Comparison of constant-extinction control and constant-DC voltage control

An important advantage of HVDC systems is the ability to adjust power flexibly.Fig.1 shows the traditional DC power control strategy, including fast and slow control.The slow control needs the coordination of the tap changer, whose operation logic varies with the control mode of the inverter.

Fig.1 Traditional control strategy of DC power

The rectifier of a DC transmission system generally adopts constant power control (or constant-current control).The rectifier station controls the DC current to the reference value using the firing angle.The tap changer ensures the firing angle within the range of normal operation (12.5°-17.5°).Once the firing angle exceeds the allowable range,the tap changer switches until the firing angle returns to the normal range.

The control mode of the inverter can be mainly divided into fixed-extinction angle control and fixed-DC voltage control [23].If the inverter station adopts the fixedextinction angle control, the inverter tap changer control chooses the DC voltage as the reference.Once the DC voltage exceeds the allowable range, the tap changer switches until the DC voltage returns to the normal range(17.5°-21.5°).If the inverter station adopts the fixed-DC voltage control, the inverter tap changer control chooses the extinction angle as the reference.Once the extinction exceeds the allowable range, the tap changer switches until the extinction angle returns to the normal range.

2.2 DC power control with fixed tap changer position

Fixed-extinction angle control cannot directly control the voltage of a DC system.If the tap changer does not switch and the DC voltage is less than the rated value, the system cannot deliver the rated power.Therefore, the optimization space of tap changer control is very limited in the fixedextinction angle control mode.To reduce the frequency of the DC tap changer switch, this paper suggests optimizing the control method based on the constant-voltage control of the inverter station.

DC voltage control ensures that a DC transmission system can operate at the maximum designed power.Simultaneously, because the control angle under DC voltage control has a certain adjustment range, it can be realized to increase the operation dead zone appropriately to avoid the frequent switch of the tap changer.However, owing to valve-based electronics firing/extinction reliability, the control angle cannot be extremely small; therefore, there is a minimum allowable value θdMIN.Restricted by the stress of the valve, a DC system cannot operate for a long time under a large control angle; thus, the maximum allowable range of the control angle of the valve, θdMAX, also needs to be specified.

With the improvement of the technology, the converter valve has been able to withstand a large firing/extinction angle.When the allowable ranges of the firing and extinction angles are sufficiently large, the tap changer of an HVDC system can be kept inactive, which is equivalent to the HVDC transmission system operating at a fixed ratio of the converter transformer.A fixed ratio inevitably leads to a large firing angle/extinction angle under certain DC power or AC bus voltage.Therefore, it is important to take note whether in extreme cases the firing angle/extinction angle can meet the engineering requirements.

3 Control strategy

3.1 Basic scheme

Based on the concept proposed in Section 2.2, the control strategy, as shown in Fig.1, is changed in this study,and a DC power control method with a fixed tap changer position is proposed.Similar to the conventional DC power control, the new control strategy can be divided into fast control and slow control.

As shown in Fig.2, when the DC power command is issued, the dynamic control of the DC system starts first,forming the rapid control part of the DC power control.The rectifier station operates with constant-power control,whereas the inverter station with constant-DC voltage control.Simultaneously, the filter control strategy table calculated in advance is queried to match the filter quantity under the current power, which forms the slow control part of DC system.The filter control drives the fast control to act again, which eventually causes the DC system to reach to a new operating condition.

Fig.2 Control strategy of DC power under fixed-converter transformer tap changer position

Compared with the traditional DC control as shown in Fig.1, the voltage control on the inverter is no longer dependent on the action of the tap changer, instead it is realized by the firing angle control and the filter control.Because of the continuity of the firing angle control, the new control strategy improves the control precision and avoids the frequent switching caused by improper matching between the filter control and the tap changer control.

3.2 Dynamic controller

Because the DC system is no longer operating at the rated angle, the DC dynamic controller is correspondingly modified to the constant-voltage control mode.As shown in Fig.3, the DC current order contains the VDCOL control and is sent to both the rectifier and inverter sides.The inverter side maximizes the firing angle generated by the current and voltage orders.Generally, a DC system operates in the constant-power (current) mode on the rectifier side and the constant-voltage mode on the inverter side.When the power regulation capacity of the rectifier side is insufficient, the firing angle of the inverter side is also involved in the regulation of DC power, and the inverter side operates in the constant-current state.

Fig.3 Diagram of DC dynamic controller under fixed converter transformer tap changer position

4 Optimization of initial position of tap changer

4.1 Relationship between control angle and DC power

DC power control with the original control strategy requires participation of the converter tap changer.When the DC power is low, the DC current, overlap angle, and voltage drop caused by the commutation overlap are small.For the inverter side, when the DC voltage and the extinction angle are determined, the no-load DC voltage of the inverter is frequently not extremely high, corresponding to the low position of the tap changer.For the rectifier side,the DC current is small and the voltage drop on the line is also small; thus, the no-load DC voltage on the rectifier is not extremely high, which also corresponds to the low position of the tap changer.On the contrary, when the DC power is high, the DC current is large, and the overlap angle increases.At this time, the voltage drop caused by the commutation increases.For the inverter side, when the DC voltage and the extinction angle are determined, the no-load DC voltage of the inverter increases, and the tap changer is upgraded.Similarly, for the rectifier side, the tap changer is upgraded accordingly.The main reason the transformer operates with a low position of the tap changer at low DC power is that the DC power is proportional to the no-load DC voltage.Under the new control method, the DC power depends on the control angle of the converter valve, instead of on the action of the tap changer.Because the no-load DC voltage is inversely proportional to the control angle, the DC power is also inversely proportional to the control angle.When the DC control angle is small, the DC power is large.With the increase in the DC control angle, the DC power gradually decreases.

4.2 Optimization considering regulation range

Under a fixed ratio, in extreme cases, the initial tap position of the converter tap changer directly determines the control angle of the converter valve.To ensure reliable triggering and extinction of the converter valve, the control angle should not be less than a certain minimum value.To avoid excessive reactive power consumption and reduce the valve stress, the control angle should not increase unlimitedly.Considering that the control angle cannot be extremely large, the ratio of the transformer should be small to the maximum extent.It also must satisfy the restriction of the minimum control angle when the DC transmission system runs with rated power.To ensure reliable triggering of the rectifier, the actual firing angle of a 50 Hz system is required to be no less than 5° during the dynamic process.For steady-state operation,an extra angle margin is also required.Similarly, to ensure reliable extinction of the inverter, the extinction angle will need some margin.Assuming that the DC current range is[ I d min , I dmax] and substituting minimum firing angle αmin,minimum extinction angle γmin, and maximum DC current Idmax into Equations (4) and (5), ratios Kr and Ki can be calculated.

5 Influence analysis of proposed strategy

5.1 Control angle

For a DC transmission system, the DC voltage of the rectifier is typically set as the rated value, Udr.DC power Rd is set by the dispatch center based on the operation condition.Equivalent commutation resistances Rr and Rci are determined by the leakage reactance of the converter transformer.Assuming that the filter and other reactive power equipment in the converter station can maintain bus voltage ULr and ULi on both converters constant, the firing angle of the rectifier and the extinction angle of the inverter can be calculated for any given tap changer position using Kr and Ki.

If the inverter side of a DC system operates at the rated voltage, Udi is the set value.The DC current, Id, can be calculated using equation (6).

By substituting the DC current, Id, into equation (1), the extinction angle of the inverter can be determined based on the DC current order as

The rectifier DC voltage, Udr, can be calculated using equation (8).

Similarly, based on equation (1), the firing angle of the rectifier, α, can be calculated.

5.2 Reactive power compensation

The power factor on the rectifier side can be approximately expressed as equation (10).

The power factor on the inverter side can be approximately expressed as equation (11).

The reactive power consumed by the converter station can be calculated based on the power factor angle, as expressed in equations (12) and (13).

To maintain the AC voltage of the converter bus, a DC control system needs to realize reactive power control of the converter station using the filter in the switching station [24].In general, a DC transmission system should ensure the reactive power exchange at the commutator bus is approximately zero(generally less than the capacity of one set of filter).

Using equations (10) and (11) it is easy to find that the power factor of the converter station is only related to the converter bus voltage and the converter transformer ratio.Under a fixed tap changer position, once a DC system is controlled by the constant-voltage mode on the inverter side and the converter bus voltage is maintained constant by reactive power compensation, the power factor of the inverter side becomes constant.

In the traditional method, a DC system operates with a low tap-changer position at low power and has a higher power factor than that at high power, which reduces both the line loss and reactive power consumption.The power factor with the proposed method is constant.It is not as economical as the original method when the power is low.However, because the DC power is low in this scenario, the reactive power compensation device at the station can still meet the reactive power demand of the DC system.

5.3 Commutation failure

Compared with the traditional DC power control strategy, the proposed method with a fixed tap changer position changes the steady-state operation condition(extinction angle) of the DC system.In addition, to adapt to the nonrated angle operation condition of the converter station, the dynamic control system of the DC inverter side must be transformed into the constant-DC voltage mode.Therefore, it is necessary to analyze the influence of the control strategy on the DC commutation failure from two aspects: steady-state extinction angle and control mode.Refs.[25,26] analyze the commutation failure when the DC inverter side adopts constant-DC voltage control and constant-extinction angle control.The research showed that the effects of the control modes on the commutation failure were not remarkable.Therefore, this study mainly analyzes the influence of the steady-state extinction angle on the commutation failure.

Based on the DC power control strategy proposed in this article, a DC system has a larger control angle under a low power order, and the inverter valve operates with a large extinction angle.The commutation security can be expressed by equation (14).

where γ represents the actual extinction angle of the DC system and γmin represents the minimum commutationsuccess extinction angle.Clearly, the proposed method can improve the commutation margin and reduce the risk of commutation failure in a DC system with a low power.

6 Case study

6.1 Initial tap changer position

Taking the CIGRE benchmark DC model in Ref.[27]as an example, the power regulation ability of a DC system with a fixed tap changer and its influence are calculated and verified.In this study, the number of converter transformer tap changer positions is assumed as 19.For each gap, the voltage is 1.25% p.u.The 12th tap changer position is the rated ratio, and the voltage adjustment range is from -8.75% to 13.75%.

To ensure reliable trigger (turn-off) of the valve, the rectifier firing angle is taken as αmin=17° and the inverter extinction angle as γmin=17°.In addition, the maximum DC power of the system, Idmax=1000 MW.The following values are substituted into equations (8) and (9): the rectifier is at the seventh tap-changer position, whose ratio is 1.0625,whereas the rectifier is at the seventh tap-changer position too, whose ratio is also 1.0625.

6.2 Control angle analysis

In general, DC power can be regulated from 10% of the rated current to the maximum power.Taking the CIGRE benchmark system as an example, the DC power should be regulated within [100,1000] MW.

Figs.4(a) and (b) show the changes in the firing and extinction angles with the DC power order respectively.The maximum firing/extinction angle is approximately 30°.Fig.5 shows the DC voltage, DC current, and extinction angle when a DC system runs at the maximum angle.It can be seen that the change trends of the control angles of the two converter stations are similar.The control angle decreases with the increase of DC power.Even when operating at the lowest power, the control angles of the DC system are much lower than the threshold value of a large angle alarm in a DC control and protection system.

Fig.4 Control angle of converter under different DC power orders

Fig.5 Dynamic simulation at maximum angle

6.3 Reactive power analysis

Based on the analysis discussed in Section 5.1, the control strategy with a fixed tap changer has a smaller power factor than the original strategy; therefore, it may lead to higher reactive power consumption.Taking the test system as an example, the DC power order is substituted into equations (8)-(11) to calculate the reactive power consumption from 100 MW to 1000 MW DC power.

As shown in Fig.6, the reactive power consumption of the original control method is less than that of the fixed ratio control method; however, the reactive power consumption remains the same at the maximum power.As described in Section 5.1, although the DC power control strategy with a fixed ratio increases the reactive power during low-power operation, the existing reactive power compensation device in the station can still meet the requirements of the DC system.

Fig.6 Reactive power consumption of converter under different DC power orders

Under the reactive power exchange control of a DC system, the total exchange power between the DC and AC systems is close to zero.Therefore, the line loss will not increase with the proposed method.

6.4 Commutation failure analysis

Based on the dynamic control model of the CIGRE standard DC system in PSCAD 4.5, this study analyzes the influence of the control strategy on the commutation failure.In this study, the critical transition resistance, Rc, of the commutation failure caused by a single-phase grounding fault of the commutation bus on the inverter side is used to quantitatively evaluate the ability of two control strategies to resist commutation failure under different DC powers.A fault occurs in 1.5 s and lasts for 0.05 s.Based on the electromagnetic transient simulation results of both the method proposed in this paper and traditional method,the abilities to resist the commutation failure with a fixed tap changer position and traditional fixed extinction angle control are compared.

Fig.7 shows the extinction angle curve under DC power Pd = 800 MW and transition resistance R = 80 Ω.Because of the fixed tap changer position, the extinction angle of the inverter is approxiwmately 20°, which is larger than the angle of the traditional method.No commutation failure occurs using the proposed method.

Fig.7 Extinction angle–time curve under single-phase ground fault

Fig.8 shows the critical transition resistances of the commutation failure of the two control strategies under different DC powers.With the decrease in the DC power, the extinction angle increases, and the critical transition resistance of the commutation failure decreases significantly.The results of the electromagnetic transient simulation show that the DC power control strategy with a fixed tap changer position is beneficial for suppressing the DC commutator failure.

Fig.8 Comparison of commutation failure critical transition resistance of two control strategies under different DC powers

7 Conclusion

Intermittent new energy generation needs a flexible power transmission strategy.For the traditional AC system, the on-load tap changer is the key component,which is difficult to be replaced during power and voltage regulation.However, for a DC system, the control angle of the valve plays a complementary role to the on-load tap changer.In this paper, a DC power control strategy based on constant-DC voltage control of the inverter is proposed.By optimizing the initial tap position of the tap changer, DC full-range power regulation without tap changer control is realized.The simulation results of a test system show that the DC control angle is approximately 30。 under the new control strategy.It only slightly increases the reactive power consumption of the converter station and helps to suppress the commutation failure.

The proposed strategy will inevitably cause the valve to run above the rated angle for a long time.Large-angle operation demands higher requirements on the performance of the converter valve.Therefore, it is necessary to further study the valve stress and loss when the thyristor operates at a large angle.

Acknowledgements

This study was supported by an independent research project from the Shandong Electric Power Research Institute, “Research on the control method of DC power under fixed converter transformer tap-changer position”(ZY-2020-01).Based on the achievement, a national invention patent (No.2020112240143) has been applied.

Declaration of Competing Interest

We declare that we have no conflict of interest.