Isolated network stability control method and device based on multi-type embedded direct current

Abstract

The invention discloses an isolated network stability control method and device based on multi-type embedded direct current, and relates to the technical field of power systems and automation thereof. A multi-type embedded direct current-based isolated network stability control method comprises the following steps of: monitoring an alternating current line fault in an isolated network by a control main station, and triggering an isolated network voltage control strategy when the alternating current line fault is monitored: judging whether the isolated network is low in voltage according to bus voltage acquired by the control main station and each voltage monitoring station; when the isolated network is in low voltage, calculating the reactive power control total amount according to the reactive power loss amount of the control master station; a reactive power control quantity distribution model is established, and according to the types of the embedded direct current stations, different control schemes are adopted to distribute the total reactive power control quantity to the embedded direct current stations; and the stability control device sends the control scheme command to the control protection device to perform isolated network stability control. The problem of voltage stability of the regional power grid is effectively solved, and the voltage stability of the isolated network system is improved.

Description

Technical Field

[0001] This invention relates to a method and apparatus for islanded grid stability control based on multiple types of embedded DC, belonging to the field of power system and automation technology. Background Technology

[0002] Guided by the "dual carbon" goal, the transformation of the new power system has entered an accelerated phase. The large-scale integration of new energy sources such as wind and solar power has become the main driver of incremental power generation, propelling the energy structure towards a cleaner and lower-carbon model. The spatial mismatch between power supply concentrated in resource-rich areas and electricity demand concentrated in load centers has become prominent, exacerbating the imbalance between supply and demand in terms of timing and space, posing new challenges to the safe and stable operation of the power grid and the optimal allocation of resources. Embedded DC, by transforming existing key transmission channels within the regional power grid into "embedded" high-voltage DC transmission systems, significantly improves the transmission capacity of key sections of the regional power grid, enhancing its controllability and flexibility. However, the significantly increased coupling depth and interaction strength between embedded DC and the AC power grid not only complicate the grid stability mechanism but also give rise to dynamic interaction problems not prominent in traditional power grids. The manifestations and evolution paths of various stability risks exhibit new characteristics, posing higher requirements for power grid stability analysis and control strategies. Summary of the Invention

[0003] The purpose of this invention is to provide a method and device for islanded grid stability control based on multiple types of embedded DC stations. By setting different control schemes for different types of embedded DC stations, the total reactive power control is allocated to each embedded DC station to improve the voltage stability of the islanded grid system.

[0004] To achieve the above objectives, the present invention is implemented using the following technical solution.

[0005] On one hand, the present invention provides an islanded grid stability control method based on multiple types of embedded DC, including:

[0006] The control master station monitors AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered.

[0007] Based on the bus voltage collected by the control master station and each voltage monitoring station, determine whether the isolated grid is undervoltage;

[0008] When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station;

[0009] A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station.

[0010] The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

[0011] Optionally, the process of judging whether the isolated network is low voltage is:

[0012] The control master station collects the local bus voltage and receives the highest voltage sent by each voltage monitoring station, and according to the formula:

[0013] ;

[0014] In the formula, is the bus voltage collected by the control master station, is the highest voltage sent by the i-th voltage monitoring station, is the set system stable voltage threshold, is the number of voltage monitoring stations; represents the isolated network voltage state; When any voltage satisfies the formula condition,

[0015] the isolated network is in a low voltage state; otherwise the isolated network is in a normal voltage state. Optionally, the reactive power loss calculation formula of the control master station is:

[0016]

[0017] ;

[0018] In the formula, is the current calculated reactive power, is the reactive power calculated 200 ms before the AC fault starts.

[0019] Optionally, the embedded DC station type includes SLCC type DC station (multi-source adaptive commutated converter station) and VSC type DC station (voltage source converter type DC station).

[0020] Optionally, the process of establishing the reactive power control amount allocation model is:

[0021] With the core goal of maximizing the system voltage support effect, the voltage improvement degree is positively related to the product of voltage sensitivity and reactive power output, and the objective function is defined as:

[0022] ;

[0023] In the formula, is the voltage sensitivity of the i-th DC station, The greater the voltage support priority is higher; is the reactive power output of the i-th DC station, is the total number of embedded DC stations;

[0024] ​​The sum of reactive power output of each embedded DC station is equal to the system reactive power loss That is,

[0025] ;

[0026] In the formula, is the reactive power control amount of the SLCC type embedded DC station No. is the reactive power control amount of the VSC type embedded DC station No. is the number of VSC type embedded DC stations, is the number of SLCC type embedded DC stations.

[0027] Optionally, the distribution of the total reactive power control amount to each embedded DC station comprises:

[0028] The VSC type DC station adopts a fixed AC voltage control mode, and the output reactive power is maximum, that is,

[0029] ;

[0030] In the formula, is the maximum reactive power control amount of the VSC type embedded DC station No. is the voltage improvement degree;

[0031] The SLCC type DC station output reactive power needs to meet the device reactive power limit, only considering the output reactive power, considering the converter, transformer and other device constraints, the reactive power output of each DC station does not exceed its maximum reactive power capacity, that is,

[0032] ;

[0033] In the formula, is the maximum reactive power control amount of the SLCC type embedded DC station No. is the active power transmission power of the SLCC type embedded DC station No. is the active power transmission power of the SLCC type embedded DC station No.

[0034] Sort all DC stations according to the voltage sensitivity from large to small:

[0035] ;

[0036] Cumulatively add the upper limit of the reactive power of the first DC station, until the cumulative value approaches the total reactive power loss :​​​​​​

[0037] ;

[0038] forward Among the DC stations, the reactive power output of the VSC type DC station is the maximum limit of the DC station, and the remaining reactive power... Assigned to SLCC type DC stations:

[0039] ;

[0040] In the formula, For the front Number of VSC-type embedded DC stations in a DC station;

[0041] For the remaining SLCC type DC stations, based on active power The inverse distribution of residual reactive power is expressed by the formula:

[0042] ;

[0043] In the formula, The number of SLCC-type embedded DC stations; For the first SLCC type embedded DC station reactive power control quantity.

[0044] Optionally, the process by which the stability control device sends the control scheme command to the control and protection device is as follows: the stability control device sends the control scheme command to the control and protection device through the standardized FT3 protocol, thereby changing the DC station control mode or reactive power output.

[0045] Secondly, the present invention provides an islanded grid stabilization control device based on multiple types of embedded DC, comprising:

[0046] The fault triggering module is used to control the master station to monitor AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered.

[0047] The control quantity allocation module is used to determine whether the isolated grid is undervoltage based on the bus voltage collected by the control master station and each voltage monitoring station.

[0048] When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station;

[0049] A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station.

[0050] The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

[0051] Thirdly, the present invention provides a computer-readable storage medium having a computer program / instructions stored thereon, which, when executed by a processor, implements the steps of any of the methods for islanded grid stabilization control based on multi-type embedded DC as described in the first aspect.

[0052] Fourthly, the present invention provides a computer device / equipment / system, characterized in that it comprises:

[0053] Memory, used to store computer programs / instructions;

[0054] A processor for executing the computer program / instructions to implement the steps of the islanded grid stability control method based on multi-type embedded DC as described in any of the first aspects.

[0055] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0056] 1. This invention can detect the voltage instability of isolated grids in real time, calculate the reactive power loss of core substations, set appropriate control schemes based on the control characteristics of each embedded DC station, and reasonably allocate reactive power control quantities.

[0057] 2. This invention provides a new approach to islanded grid stability control, effectively solving the problem of regional power grid voltage stability and improving the voltage stability of islanded grid systems. Attached Figure Description

[0058] Figure 1 The flowchart shows the islanded grid stability control method based on multi-type embedded DC of the present invention.

[0059] Figure 2 This is a schematic diagram of the islanded network stability control system of the present invention. Detailed Implementation

[0060] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of the present invention and the specific features in the embodiments are detailed descriptions of the technical solution of the present invention, rather than limitations thereof. In the absence of conflict, the embodiments of the present invention and the technical features in the embodiments can be combined with each other.

[0061] Example 1:

[0062] This embodiment introduces an islanded grid stability control method based on multiple types of embedded DC, such as... Figure 1 As shown, it includes:

[0063] The control master station monitors AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered.

[0064] Based on the bus voltage collected by the control master station and each voltage monitoring station, determine whether the isolated grid is undervoltage;

[0065] When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station;

[0066] A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station.

[0067] The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

[0068] like Figure 2 As shown, this is the islanded grid stability control system of this embodiment, which includes a control master station and multiple voltage monitoring stations and embedded DC stations deployed at different sites;

[0069] The control master station implements a method for multiple types of embedded DC stations to participate in islanded grid stability control.

[0070] Voltage monitoring stations are used to measure the voltage at each station and send the highest voltage in the station to the control master station.

[0071] Embedded DC stations are used to execute reactive power control commands issued by the control master station.

[0072] I. Fault Strategy Trigger

[0073] The control station monitors the opening and closing of high-voltage AC sections and AC line faults within the islanded network. Based on the opening and closing of the interconnection sections between the islanded network and the main network, the operational status of the interconnection line or main transformer is commonly used to determine the islanded network's operating mode. Faults in the core AC lines within the islanded network are identified, including single-phase permanent faults, phase-to-phase faults, three-phase permanent faults, and fault-free tripping. Tripping criteria adopt conventional stability control tripping criteria. When an AC line fault is detected, a voltage control strategy is triggered.

[0074] II. Island voltage control

[0075] Determine if an isolated network is experiencing low voltage:

[0076] The control station collects the local bus voltage and receives the highest voltage transmitted from each voltage monitoring station, according to the formula:

[0077] ;

[0078] In the formula, To control the bus voltage collected by the master station, For the first The highest voltage transmitted by each voltage monitoring station The set system stability voltage threshold, The number of voltage monitoring stations; Indicates the islanded grid voltage status;

[0079] When any voltage satisfies the formula condition, The isolated network is in a low-voltage state; otherwise The isolated grid is under normal voltage conditions.

[0080] When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station;

[0081] The formula for calculating the reactive power loss of the control master station is:

[0082] ;

[0083] In the formula, This is the currently calculated reactive power. It is the reactive power calculated 200ms before the start of an AC fault.

[0084] Different control schemes are adopted based on the characteristics of different embedded DC stations. For SLCC type DC stations, a reactive power capacity control scheme is adopted; for VSC type DC stations, a bumpless switching control mode scheme is adopted.

[0085] The process of establishing the reactive power control quantity allocation model is as follows:

[0086] With maximizing the system voltage support effect as the core objective, the degree of voltage improvement is positively correlated with the product of voltage sensitivity and reactive power output. The objective function is defined as follows:

[0087] ;

[0088] In the formula, Let be the voltage sensitivity of the i-th DC station. The larger the value, the higher the priority of voltage support; For the first Reactive power output of a DC station; Total number of embedded DC stations; To the degree of voltage improvement;

[0089] The sum of the reactive power outputs of each embedded DC station equals the system's reactive power loss. ,Right now:

[0090] ;

[0091] In the formula, For the first One SLCC type embedded DC station reactive power control quantity; For the first One VSC type embedded DC station reactive power control quantity; This refers to the number of VSC-type embedded DC stations. This refers to the number of SLCC-type embedded DC stations.

[0092] Allocating the total reactive power control amount to each embedded DC station includes:

[0093] The VSC type DC station adopts a constant AC voltage control mode, with maximum reactive power output, i.e.:

[0094] ;

[0095] In the formula, For the first Maximum reactive power control quantity of a VSC type embedded DC station;

[0096] The reactive power output of SLCC type DC substations must meet the reactive power limit of the equipment. Considering only the output reactive power and taking into account the constraints of equipment such as converters and transformers, the reactive power output of each DC substation shall not exceed its own maximum reactive power capacity, that is:

[0097] ;

[0098] In the formula, For the first Maximum reactive power control of an SLCC type embedded DC station For the first Active power transmission of an SLCC type embedded DC station For the first The ratio of active power to reactive power in an SLCC type embedded DC station;

[0099] All DC stations are classified according to voltage sensitivity Sort by largest to smallest:

[0100] ;

[0101] Before sequential accumulation The reactive power limit for each DC station is set until the accumulated value approaches the total reactive power loss. :

[0102] ;

[0103] forward Among the DC stations, the reactive power output of the VSC type DC station is the maximum limit of the DC station, and the remaining reactive power... Assigned to SLCC type DC:

[0104] ;

[0105] In the formula, For the front Number of VSC-type embedded DC stations in a DC station;

[0106] For the remaining SLCC type DC stations, based on active power The inverse distribution of residual reactive power is expressed by the formula:

[0107] ;

[0108] In the formula, This refers to the number of SLCC-type embedded DC stations.

[0109] The stability control device sends commands to the control and protection device via the standardized FT3 protocol, thereby changing the DC station control mode or reactive power output. For example, the FT3 protocol is:

[0110] Table 1 Communication Interface Protocol between DC Station Control System (DSC) and Stabilization Control Device

[0111]

[0112] Example 2:

[0113] Based on the same inventive concept as Embodiment 1, this embodiment introduces a multi-type embedded DC-based islanded grid stabilization control device, comprising:

[0114] The fault triggering module is used to control the master station to monitor AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered.

[0115] The control quantity allocation module is used to determine whether the isolated grid is undervoltage based on the bus voltage collected by the control master station and each voltage monitoring station.

[0116] When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station;

[0117] A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station.

[0118] The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

[0119] The specific functions of each module described above are explained in the relevant content of the method in Embodiment 1, and will not be repeated here.

[0120] Example 3:

[0121] Based on the same inventive concept as other embodiments, this embodiment introduces a computer-readable storage medium storing a computer program / instructions thereon, which, when executed by a processor, implements the steps of the islanded grid stabilization control method based on multiple types of embedded DC as described in any of Embodiment 1.

[0122] Example 4:

[0123] Based on the same inventive concept as other embodiments, this embodiment introduces a computer device / apparatus / system, characterized in that it includes:

[0124] Memory, used to store computer programs / instructions;

[0125] A processor is configured to execute the computer program / instructions to implement the steps of the islanded grid stability control method based on multi-type embedded DC as described in any of Embodiment 1.

[0126] Example 5:

[0127] This embodiment describes the simulation experiment process of an islanded grid stability control method based on multiple types of embedded DC:

[0128] First, an islanded grid stability control system is constructed, with one control master station, three voltage monitoring stations, and three embedded DC stations. The embedded DC stations include two SLCC type DC stations and one VSC type DC station.

[0129] When an AC line fault is detected, the bus voltage collected by the control master station is 125kV (the primary side rated voltage Un is 127kV). The voltages sent by each voltage monitoring station are 112kV, 120kV, and 122kV, respectively. The system stable voltage threshold is 95%Un, which is 120.65kV. It is determined that the isolated grid system is in a low voltage state.

[0130] For SLCC type DC stations, a reactive power capacity control scheme is adopted; for VSC type DC stations, a disturbance-free switching control mode scheme is adopted.

[0131] After an AC line fault occurs in an isolated network, the system is in a low-voltage state. At this time, the reactive power of the bus is 230 Mvar, the reactive power is 340 Mvar 200 ms before the line fault, and the reactive power loss is 110 Mvar.

[0132] The configuration parameters for each embedded DC station are: voltage sensitivity. These correspond to the first SLCC type DC station, the second SLCC type DC station, and the VSC type DC station, respectively; the DC output reactive power is the maximum limit of DC, which are as follows:

[0133] 60Mvar;

[0134] In the formula, , and These are the maximum reactive power control values ​​for the first SLCC type DC station, the second SLCC type DC station, and the VSC type DC station, respectively.

[0135] The reactive power of VSC type DC station 60 Mvar; Remaining reactive power = 50Mvar;

[0136] The active power transmitted by the SLCC embedded DC station 200ms before the line fault is as follows:

[0137] 100MW; 80MW;

[0138] In the formula, and The active power transmission power of the first SLCC embedded DC station and the second SLCC embedded DC station are respectively.

[0139] The reactive power of the first SLCC type DC station =22.2 Mvar;

[0140] The reactive power of the second SLCC type DC station =27.8Mvar.

[0141] It meets the system voltage stability requirements for isolated grids.

[0142] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0143] This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1A device that provides the functions specified in one or more boxes.

[0144] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0145] These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0146] The embodiments of the present invention have been described above with reference to the accompanying drawings. However, the present invention is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of the present invention without departing from the spirit and scope of the claims. All of these forms are within the protection scope of the present invention.

Claims

1. A method for islanded grid stability control based on multi-type embedded DC, characterized in that, include: The control master station monitors AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered. Based on the bus voltage collected by the control master station and each voltage monitoring station, determine whether the isolated grid is undervoltage; When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station; A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station. The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

2. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The process for determining whether an isolated network is undervoltage is as follows: The control station collects the local bus voltage and receives the highest voltage transmitted from each voltage monitoring station, according to the formula: ； In the formula, To control the bus voltage collected by the master station, For the first The highest voltage transmitted by each voltage monitoring station The set system stability voltage threshold, The number of voltage monitoring stations; Indicates the islanded grid voltage status; When any voltage satisfies the formula condition, The isolated network is in a low-voltage state; otherwise The isolated grid is under normal voltage conditions.

3. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The formula for calculating the reactive power loss of the control master station is as follows: ； In the formula, This is the currently calculated reactive power. It is the reactive power calculated 200ms before the start of an AC fault.

4. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The embedded DC station types include SLCC type DC station and VSC type DC station.

5. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The process of establishing the reactive power control quantity allocation model is as follows: Define the objective function as: ； In the formula, Let be the voltage sensitivity of the i-th DC station. The larger the value, the higher the priority of voltage support; For the first Reactive power output of a DC station; Total number of embedded DC stations; To the degree of voltage improvement; The sum of the reactive power outputs of each embedded DC station equals the system's reactive power loss. ,Right now: ； In the formula, For the first One SLCC type embedded DC station reactive power control quantity; For the first One VSC type embedded DC station reactive power control quantity; This refers to the number of VSC-type embedded DC stations. This refers to the number of SLCC-type embedded DC stations.

6. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The process of allocating the total reactive power control amount to each embedded DC station includes: The VSC type DC station adopts a constant AC voltage control mode, with maximum reactive power output, i.e.: ； In the formula, For the first Maximum reactive power control quantity of a VSC type embedded DC station; The reactive power output of SLCC type DC substations must meet the equipment reactive power limit. Considering only the output reactive power, the reactive power output of each DC substation shall not exceed its own maximum reactive power capacity, that is: ； In the formula, For the first Maximum reactive power control of an SLCC type embedded DC station For the first Active power transmission of an SLCC type embedded DC station For the first The ratio of active power to reactive power in an SLCC type embedded DC station; All DC stations are classified according to voltage sensitivity Sort by largest to smallest: ; Before sequential accumulation The reactive power limit for each DC station is set until the accumulated value approaches the total reactive power loss. : ； forward Among the DC stations, the reactive power output of the VSC type DC station is the maximum limit of the DC station, and the remaining reactive power... Assigned to SLCC type DC stations: ； In the formula, For the front Number of VSC-type embedded DC stations in a DC station; For the remaining SLCC type DC stations, based on active power The inverse distribution of residual reactive power is expressed by the formula: ； In the formula, The number of SLCC-type embedded DC stations; For the first SLCC type embedded DC station reactive power control quantity.

7. The islanded grid stability control method based on multi-type embedded DC as described in claim 1, characterized in that, The process by which the stability control device sends the control scheme command to the control and protection device is as follows: the stability control device sends the control scheme command to the control and protection device through the standardized FT3 protocol, thereby changing the DC station control mode or reactive power output.

8. A network stabilization control device based on multi-type embedded DC, characterized in that, include: The fault triggering module is used to control the master station to monitor AC line faults within the isolated network. When an AC line fault is detected, the isolated network voltage control strategy is triggered. The control quantity allocation module is used to determine whether the isolated grid is undervoltage based on the bus voltage collected by the control master station and each voltage monitoring station. When the isolated grid is low voltage, the total reactive power control amount is calculated based on the reactive power loss of the control master station; A reactive power control allocation model is established, and different control schemes are adopted according to the type of embedded DC station to allocate the total reactive power control amount to each embedded DC station. The stability control device sends the control scheme command to the control and protection device to perform islanded network stability control.

9. A computer-readable storage medium having a computer program / instructions stored thereon, characterized in that, When the computer program / instruction is executed by the processor, it implements the steps of the islanded grid stabilization control method based on multi-type embedded DC as described in any one of claims 1 to 7.

10. A computer device / equipment / system, characterized in that, include: Memory, used to store computer programs / instructions; A processor for executing the computer program / instructions to implement the steps of the islanded grid stability control method based on multiple types of embedded DC, as described in any one of claims 1 to 7.

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