CN112467777B - Method for controlling asymmetric direct current by using modular combined direct current transformer - Google Patents
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- CN112467777B CN112467777B CN202011283040.3A CN202011283040A CN112467777B CN 112467777 B CN112467777 B CN 112467777B CN 202011283040 A CN202011283040 A CN 202011283040A CN 112467777 B CN112467777 B CN 112467777B
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J1/00—Circuit arrangements for dc mains or dc distribution networks
- H02J1/06—Two-wire systems
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]
Abstract
The invention discloses a method for controlling asymmetric direct current by utilizing a modular combined direct current transformer, which solves the problem of possible asymmetry of positive and negative direct currents in a symmetric bipolar power grid through the control of the modular combined direct current transformer, thereby canceling the setting of a neutral point of the symmetric bipolar direct current power grid and solving the problems of high cost and large occupied area of a grounding device. In addition, the invention improves the operation stability of the system by controlling the direct current balance through the modularized combined direct current transformer.
Description
Technical Field
The invention belongs to the technical field of power system control, and particularly relates to a method for controlling asymmetric direct current by utilizing a modular combined direct current transformer.
Background
Aiming at the urgent situations of shortage of traditional energy, aggravation of environmental pollution and serious ecological damage at present, all countries seek a new way which can take energy utilization and environmental protection into consideration, and people gradually aim to develop pollution-free, renewable and environment-friendly clean energy such as wind power, photovoltaic and the like. However, the conventional power grid is not free from the acceptance of large-scale and low-quality renewable energy, and the direct current power grid has attracted wide attention at home and abroad by virtue of the advantages of new energy grid connection and consumption.
With the continuous construction of large-scale direct current transmission projects in the world, more and more direct current transmission projects and direct current power grids are put into use, and the voltage grade of the direct current power grid is determined by the connected alternating current power grid, the transmission capacity and the actual project requirements. Because no unified standard exists for the construction of the direct-current power grid at present, the voltage levels of the direct-current power grids are different, the development of the multi-voltage-level flexible direct-current power grid is severely restricted, and great challenges are brought to the interconnection of the direct-current power grids; an effective solution is to use dc transformers to interconnect dc networks of different voltage classes.
Currently, the mainstream dc transformers can be roughly classified into conventional large-capacity dc transformers, resonant dc transformers, and MMC dc transformers. The traditional high-capacity direct current transformer comprises two types, namely a bilateral active bridge topology and a phase-shift control full-bridge converter, but the bilateral active bridge topology has larger circuit loss when in heavy load, and the phase-shift control full-bridge converter has reduced power efficiency when in light load; the resonant type direct current transformer can realize direct current transformation, bidirectional energy transmission and electrical isolation functions, but high power density and high transformation efficiency are difficult to realize. The voltage-resistant level of a single full-control power device limits the voltage-resistant level of the single full-control power device, and the traditional direct-current converter is not suitable for high-voltage occasions; in order to realize the application of the low-voltage power device in high-voltage electric energy conversion, a power device series connection technology, a converter submodule series-parallel connection technology and a multi-level technology can be adopted; however, as the voltage level increases, the complexity of the dc converter circuit topology and control increases, the reliability decreases, and the switching frequency and loss increase. The direct current Modular Multilevel Converter (MMC) has the advantages of low requirement on voltage sharing of devices, good expansibility, good quality of output voltage waveform, low running loss and the like, the topology can output high-quality voltage waveform while the economy is ensured, and the MMC becomes a research hotspot in the current medium-high voltage direct current Converter due to the characteristics. The modular combined type direct current transformer consists of a plurality of MMC modules, and has 4 outlets connected with an external direct current power grid, wherein the 4 outlets are respectively connected with the positive electrode and the negative electrode of the direct current high-low voltage power grid.
The direct-current power grid with the symmetrical bipolar structure has the advantages of large transmission capacity, strong reliability, easy expansion and the like, and is easy to gradually become a new trend of direct-current power grid construction. In the direct current power grid with a symmetrical bipolar structure, a negative outlet of the positive pole MMC is connected with a positive outlet of the negative pole MMC to form a neutral point, the neutral point is connected with the grounding pole through the grounding lead, and when the positive and negative direct currents are unequal, the neutral point can flow through the ground current. The common mode is that a neutral point is grounded through high impedance, but the problems of high equipment cost and large occupied area exist in the arrangement of the grounding device, and the grounding device is not suitable for occasions such as offshore flexible direct current transmission and the like.
Disclosure of Invention
In view of the above, the present invention provides a method for controlling asymmetric dc current by using a modular combined dc transformer, which can control unbalanced positive and negative dc currents in a dc power grid through the dc transformer, thereby eliminating the need for a grounding device.
A method of controlling an asymmetric direct current with a modular combined dc transformer for dc voltage conversion between high and low voltage side converter stations, the method comprising the steps of:
(1) acquiring and obtaining positive direct current I at time t of high-voltage side or low-voltage side converter stationdcp(t) and a negative DC current Idcn(t), t represents time;
(2) determining a control strategy of each MMC in the direct-current transformer;
(3) judgment of Idcp(t) and Idcn(t), if the two are not equal, further calculating the anode direct-current voltage command value U of the direct-current transformer at the time of t + delta tdcp_ref(t + Δ t) and negative dc voltage command value Udcn_ref(t + Δ t), Δ t being; if the two are equal, no additional adjustment is needed;
(4) at time t + Δ t, according to Udcp_ref(t + Δ t) and Udcn_refAfter the (t + delta t) is controlled to be applied to the direct current transformer, acquiring and obtaining the anode direct current I at the t + delta t momentdcp(t + Deltat) and a negative direct current Idcn(t+Δt)；
(5) And (4) repeatedly executing the steps until the positive direct current and the negative direct current at the new moment are equal.
Further, the modular combined dc transformer consists of 4 MMCs, wherein: the positive pole of first MMC direct current side links to each other with the positive pole of high pressure side converter station, the negative pole of first MMC direct current side links to each other with the positive pole of second MMC direct current side and the positive pole of low pressure side converter station, the negative pole of second MMC direct current side links to each other with the positive pole of third MMC direct current side and ground, the negative pole of third MMC direct current side links to each other with the positive pole of fourth MMC direct current side and the negative pole of low pressure side converter station, the negative pole of fourth MMC direct current side links to each other with the negative pole of high pressure side converter station, the interchange side of 4 MMCs links to each other with public alternating current bus through converter transformer.
Further, the air conditioner is provided with a fan,in the step (1), when the high-voltage side converter station adopts constant direct-current voltage control and the low-voltage side converter station adopts passive control or constant active power control, acquiring and obtaining the positive direct current I at the moment t of the low-voltage side converter stationdcp(t) and a negative DC current Idcn(t); when the low-voltage side converter station adopts constant direct-current voltage control and the high-voltage side converter station adopts passive control or constant active power control, acquiring and obtaining the positive direct current I at the time t of the high-voltage side converter stationdcp(t) and a negative DC current Idcn(t)。
Further, in the step (2), when the high-voltage side converter station adopts constant direct-current voltage control and the low-voltage side converter station adopts passive control or constant active power control, the second MMC and the third MMC in the direct-current transformer adopt constant direct-current voltage control, and the first MMC and the fourth MMC adopt passive control or constant active power control; when the low-voltage side converter station adopts fixed direct-current voltage control and the high-voltage side converter station adopts passive control or fixed active power control, the first MMC and the fourth MMC in the direct-current transformer adopt fixed direct-current voltage control, and the second MMC and the third MMC adopt passive control or fixed active power control.
Further, in the step (3), the positive dc voltage command value U is calculated by the following equationdcp_ref(t + Δ t) and negative dc voltage command value Udcn_ref(t+Δt)；
Udcp_ref(t+Δt)＝Udcp-k[Idcp(t)-Iav(t)]
Udcn_ref(t+Δt)＝Udcn-k[Idcn(t)-Iav(t)]
Wherein: u shapedcpAnd UdcnThe method comprises the steps that original direct-current voltage instruction values of two MMC adopting a constant direct-current voltage control strategy in a direct-current transformer are respectively adopted, and k is a preset control coefficient (between 0 and 1).
Further, when the control is applied to the dc transformer in the step (4), the second step isThe two MMCs and the third MMC adopt constant direct current voltage control, and then U is controlleddcp_ref(t + Δ t) as a DC voltage command value, U, of the second MMCdcn_ref(t + Δ t) as a direct-current voltage command value of the third MMC; if the first MMC and the fourth MMC adopt constant direct-current voltage control, the U is controlleddcp_ref(t + Δ t) as a DC voltage command value, U, of the first MMCdcn_ref(t + Δ t) is the dc voltage command value of the fourth MMC.
Compared with the prior art, the invention has the following beneficial technical effects:
1. the invention solves the problem of asymmetric direct current of positive and negative poles possibly existing in a symmetrical bipolar power grid through the control of the modularized combined direct current transformer, thereby canceling the setting of a neutral point of the symmetrical bipolar direct current power grid and solving the problems of high cost and large occupied area of a grounding device.
2. According to the invention, the operation stability of the system is improved by controlling the direct current balance through the modularized combined direct current transformer.
Drawings
Fig. 1 is a schematic structural diagram of a modular combined dc transformer.
Fig. 2(a) is a schematic diagram of a simulation waveform of the transmission power of each MMC in the dc transformer.
Fig. 2(b) is a schematic diagram of a simulation waveform of a phase current at the ac side of each MMC in the dc transformer.
Fig. 2(c) is a schematic diagram of a simulated waveform of the common ac bus voltage in the dc transformer.
Fig. 2(d) is a schematic diagram of a simulation waveform of the dc voltage of each MMC in the dc transformer.
Fig. 2(e) is a schematic diagram of simulation waveforms of the positive and negative currents on the low-voltage side of the dc transformer.
Fig. 2(f) is a schematic diagram of simulation waveforms of the high-voltage side positive electrode voltage and the low-voltage side positive electrode voltage of the direct-current transformer.
FIG. 2(g) shows a DC voltage command value U of the DC transformerdcp_ref(t)、Udcn_ref(t) waveform change diagram.
Detailed Description
In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.
The invention provides a method for controlling asymmetric direct current by utilizing a modular combined direct current transformer, the targeted modular combined direct current transformer comprises 4 MMCs (modular multilevel converters) as shown in figure 1, the j-1 th MMC cathode and the j-th MMC anode on the direct current side are sequentially connected in series, the j-2, 3 and 4 MMC is a three-phase six-bridge arm structure, and each bridge arm is formed by cascading a bridge arm reactance and a plurality of sub-modules; wherein, the positive pole of the 1 st MMC and the negative pole of the 4 th MMC correspond with positive, negative pole of direct current high voltage electric network respectively and are connected, and the positive pole of the 2 nd MMC and the negative pole of the 3 rd MMC correspond with positive, negative pole of low pressure direct current electric network respectively and are connected.
In order to be applied to a symmetrical bipolar direct current power grid, a connecting point between the cathode of the 2 nd MMC and the anode of the 3 rd MMC in the modular combined direct current transformer is grounded and serves as a grounding point of the whole modular combined direct current transformer.
The asymmetric direct current control method comprises the following steps:
(1) acquiring the value I of the positive and negative direct currents on a certain direct current side (high voltage side or low voltage side) at the time tdcpi(t) and IdcniAnd (t), t is a natural number, Δ t is a control period of the system, and when i is 1, the dc high-voltage side is indicated, and when i is 2, the dc low-voltage side is indicated.
When the direct current high-voltage side adopts constant direct current voltage control and the direct current low-voltage side adopts passive control or constant active power control, acquiring positive and negative direct currents of the low-voltage side; when the direct current low voltage side adopts constant direct current voltage control and the direct current high voltage side adopts passive control or constant active power control, the positive and negative direct currents of the high voltage side are collected.
(2) Judgment of Idcpi(t) and Idcni(t) when they are not equal, according to t at time Idcpi(t) and Idcni(t) calculating a direct-current voltage command value U at the time of t + delta tdcp_ref(t + Δ t) and Udcn_ref(t + Δ t); otherwise no additional adjustment is necessary.
For the modular combined direct-current transformer, the direct-current voltage command value at the moment t + delta t is calculated and determined in the following way:
Udcp_ref(t+Δt)＝Udcp0-k[Idcpi(t)-Iav(t)]
Udcn_ref(t+Δt)＝Udcn0-k[Idcni(t)-Iav(t)]
wherein: k is a control parameter, Udcp0、Udcn0For MMC DC voltage command value without considering DC voltage unbalance amount, Iav(t) is time tdcpi(t) and Idcni(t) average value.
(3) At time t + Δ t, the DC voltage command value UdcP_ref(t + Δ t) and UdcN_ref(t + Deltat) is applied to the control system to control the positive and negative DC currents I in the systemdcpi(t + Δ t) and Idcni(t + Δ t) is adjusted.
When the direct current high-voltage side adopts constant direct current voltage control and the direct current low-voltage side adopts passive control or constant active power control, the 2 nd and 3 rd MMC adopt constant direct current voltage control, and at the moment, the UdcP_ref(t + Δ t) and UdcN_ref(t + Δ t) are the MMC direct-current voltage control command values of the 2 nd and the 3 rd at time t, respectively.
When the direct current low-voltage side adopts constant direct current voltage control and the direct current high-voltage side adopts passive control or constant active power control, the 1 st MMC and the 4 th MMC adopt constant direct current voltage control, and at the moment, the U is controlled by the constant direct current voltagedcP_ref(t + Δ t) and UdcN_ref(t + Δ t) are the MMC direct-current voltage control command values of the 1 st and 4 th at time t, respectively.
(4) Detecting the value I of the positive and negative direct currents again at the moment t + delta tdcpi(t + Δ t) and Idcni(t + Δ t), judgment Idcpi(t + Δ t) and Idcni(t + delta t) and repeating the steps (2) and (3) until the value I of the positive and negative direct currents at a certain momentdcpiAnd IdcniUntil they are equal.
In the following, simulation verification is performed by taking a double-end direct-current power grid with a symmetric bipolar structure as an example, a converter station with a primary side of an MMC type direct-current transformer being a +/-600 kV high-voltage direct-current power grid and a secondary side of a low-voltage +/-300 kV is connected to an alternating-current synchronous power grid. Under the normal operating condition, the rated direct current power of high-voltage side to low-voltage side transmission is 600MW, the parameter of the main loop of a single converter in the secondary low-voltage converter station is shown in table 1, an alternating current bus in the MMC type direct current transformer is grounded through a 3H reactance and a 3k omega resistor, the parameter of the main loop of the MMC is shown in table 2, the control strategy of each MMC is shown in table 3, and the control parameter k is 0.1.
TABLE 1
| Parameter name | Numerical value |
| Rated capacity/MVA of |
150 |
| DC voltage/ |
300 |
| Rated capacity/MVA of connection transformer | 180 |
| Voltage ratio of connecting transformer | 220/160 |
| Short-circuit impedance of connecting transformer (%) | 15 |
| Bridge |
200 |
| Sub-module rated voltage/kV | 1.3 |
| Sub-module capacitance value/. mu.F | 666 |
| Bridge arm reactance/H | 0.076 |
| Converter station outlet smoothing reactor/H | 0.1 |
TABLE 2
TABLE 3
When the system operates in a steady state, the high-voltage direct-current power grid transmits 600MW direct-current power to the low-voltage direct-current power grid, and the system is already in the steady state in 0 second; at time t-0.5 s, the transmission power is decreased from 600MW to 400MW, and the corresponding simulated waveforms are shown in fig. 2(a) to fig. 2 (g). It can be seen that when the transmission power is changed, the positive and negative dc currents at the dc low voltage side are equal, which shows that the asymmetric dc current control method of the present invention has a better effect.
The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.
Claims (6)
1. A method of controlling an asymmetric direct current with a modular combined dc transformer for dc voltage conversion between high and low voltage side converter stations, the method comprising the steps of:
(1) acquiring and obtaining positive direct current I at time t of high-voltage side or low-voltage side converter stationdcp(t) and a negative DC current Idcn(t), t represents time;
(2) determining a control strategy of each MMC in the direct-current transformer;
(3) judgment of Idcp(t) and Idcn(t), if the two are not equal, further calculating the anode direct-current voltage command value U of the direct-current transformer at the time of t + delta tdcp_ref(t + Δ t) and negative dc voltage command value Udcn_ref(t + Δ t), Δ t is; if the two are equal, no additional adjustment is needed;
(4) at time t + Δ t, according to Udcp_ref(t + Δ t) and Udcn_refAfter the (t + delta t) is controlled to be applied to the direct current transformer, acquiring and obtaining the anode direct current I at the t + delta t momentdcp(t + Deltat) and a negative direct current Idcn(t+Δt)；
(5) And (4) repeatedly executing the steps until the positive direct current and the negative direct current at the new moment are equal.
2. The method of claim 1, wherein: modular combined type direct current transformer comprises 4 MMC, wherein: the positive pole of first MMC direct current side links to each other with the positive pole of high pressure side converter station, the negative pole of first MMC direct current side links to each other with the positive pole of second MMC direct current side and the positive pole of low pressure side converter station, the negative pole of second MMC direct current side links to each other with the positive pole of third MMC direct current side and ground, the negative pole of third MMC direct current side links to each other with the positive pole of fourth MMC direct current side and the negative pole of low pressure side converter station, the negative pole of fourth MMC direct current side links to each other with the negative pole of high pressure side converter station, the interchange side of 4 MMCs links to each other with public alternating current bus through converter transformer.
3. The method of claim 1, wherein: in the step (1), when the high-voltage side converter station adopts constant direct-current voltage control and the low-voltage side converter station adopts passive control or constant active power control, acquiring and obtaining the positive direct current I at the moment t of the low-voltage side converter stationdcp(t) and a negative DC current Idcn(t); when the low-voltage side converter station adopts constant direct-current voltage control and the high-voltage side converter station adopts passive control or constant active power control, acquiring and obtaining the positive direct current I at the time t of the high-voltage side converter stationdcp(t) and a negative DC current Idcn(t)。
4. The method of claim 2, wherein: in the step (2), when the high-voltage side converter station adopts constant direct-current voltage control and the low-voltage side converter station adopts passive control or constant active power control, the second MMC and the third MMC in the direct-current transformer adopt constant direct-current voltage control, and the first MMC and the fourth MMC adopt passive control or constant active power control; when the low-voltage side converter station adopts constant direct-current voltage control and the high-voltage side converter station adopts passive control or constant active power control, a first MMC and a fourth MMC in the direct-current transformer adopt constant direct-current voltage control, and a second MMC and a third MMC adopt passive control or constant active power control.
5. The method of claim 1, wherein: in the step (3), the positive direct-current voltage command value U is calculated by the following formuladcp_ref(t + Δ t) and negative dc voltage command value Udcn_ref(t+Δt)；
Udcp_ref(t+Δt)＝Udcp-k[Idcp(t)-Iav(t)]
Udcn_ref(t+Δt)＝Udcn-k[Idcn(t)-Iav(t)]
Wherein: u shapedcpAnd UdcnThe method comprises the steps that original direct-current voltage instruction values of two MMC adopting a constant direct-current voltage control strategy in a direct-current transformer are respectively adopted, and k is a preset control coefficient.
6. The method of claim 4, wherein: when the direct current transformer is controlled in the step (4), if the second MMC and the third MMC adopt constant direct current voltage control, the U is enableddcp_ref(t + Δ t) as a DC voltage command value, U, of the second MMCdcn_ref(t + Δ t) as a direct-current voltage command value of the third MMC; if the first MMC and the fourth MMC adopt constant direct current voltage control, the U is enableddcp_ref(t + Δ t) as a DC voltage command value, U, of the first MMCdcn_ref(t + Δ t) is the dc voltage command value of the fourth MMC.
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