WO2013152482A1

WO2013152482A1 - Master control method for a series mtdc system and element thereof

Info

Publication number: WO2013152482A1
Application number: PCT/CN2012/073796
Authority: WO
Inventors: Dawei YAO; Xiaobo Yang; Chengyan YUE
Original assignee: Abb Technology Ltd.
Priority date: 2012-04-11
Filing date: 2012-04-11
Publication date: 2013-10-17
Also published as: CN103718413A; CN103718413B

Abstract

A master control method for a series multi-terminal direct current (MTDC) system and elements thereof. The method comprises: choosing one terminal as a current setting terminal (CST) and defining other terminals as voltage setting terminals (VST) (302); configuring a current reference of the series MTDC system as the input of the CST converter, generating current values for each VST converter (304) based on the current reference and different margins thereof respectively and making the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side (306). The method for series MTDC system and elements thereof can regulate and optimize the active power and reactive power, reduce the power losses, and control the distribution of the reactive power.

Description

MASTER CONTROL METHOD FOR A SERIES MTDC SYSTEM AND ELEMENT

THEREOF

FIELD OF THE INVENTION

The invention relates to the series multi-terminal HVDC system (i.e. series MTDC system), and more particularly to master control methods for a series MTDC system and elements thereof.

BACKGROUND OF THE INVENTION

According to IEC (International Electrotechnical Commission) standard 'International Standard Terminology for high-voltage direct current (HVDC) transmission', IEC 60633, Edition 2.1 , 2009-07, the multi-terminal HVDC transmission system (MTDC) is defined as 'HVDC transmission system consisting of more than two separated HVDC substations and the interconnecting HVDC transmission lines'. As two basic connection structures, a parallel connected MTDC system is shown in Fig.1 and a series connected MTDC system is shown Fig.2. It should be noted that these two kinds of MTDC system are just taken as examples. The skilled person in art acknowledges that other derivative structures based on such classic structures will not be excluded by the present invention. In the following description, the transmission line, a part of the MTDC system, refers to overhead lines, cables and/or other medium, which can transfer electricity bulky. Furthermore, the MTDC system includes both the bipole and monopole structures. Therefore, in the present invention, 'master control' refers to the general concept for control coordination of a MTDC system at system level (bipole and/or pole level).

The series MTDC system is attractive due to its lower cost of converter stations for some application cases. However, the series MTDC system has the same current for each terminal and transmission line; thus the power losses of series MTDC are higher than that of parallel MTDC. By far, how to control the series MTDC and coordinate all the terminals haven't been practically developed. In the paper "The Control and Performance of a Series Connected Multiterminal HVDC Transmission System" (R.L.Vaughan, etc., IEEE, Transactions on Power Apparatus and System, Vol. PAS-94, No. 5, 1975), current margin for terminals is discussed to propose an idea on implement a central control in series connected HVDC system. However, when the amount of terminals is increased further, or if the series TDC system is upgraded from classic point to point HVDC, new methods need to be developed. In this invention, current order grouping is introduced, which can provide more flexibility to series MTDC system operation.

In the paper "Basic Regulation Methods and Features of Multi-terminal

HVDC Transmission System" (HVDC Transmission Research Group of Power Generation R&E Group in ZheJiang University, High Voltage DC Transmission Engineer and Technology (CN), Chapter 3-8, p155-163, 1982), a current margin shared among VSTs is introduced. However, the method for current margins for rectifiers or inverters is not discussed.

Further, for power control of series MTDC, most solutions of prior arts propose to adjust the firing angle (a) or the extinguishing angle (γ) with coordination control of OLTC (on-!oad tap-charger) by the local control (center or module), which will result in larger firing angles and possibly unacceptable OLTC ranges. No optimized coordination among terminals is implemented in the stages such as start-up, stop/bypass and reactive power balancing. For both series type and parallel type MTDC systems, a current setting terminal (CST) and voltage setting terminals (VSTs) should be identified. In series MTDC system, it is common sense that one terminal should be classified as the CST and others should be classified as the VSTs. However, no existing solution is introduced to choose the CST for series MTDC system.

Thus, existing solutions including above mentioned method cannot be essentially used as the solution to control series MTDC system. Due to the above mentioned problems, the present invention is to propose master control methods for a series MTDC system and elements thereof with regulated active power and reactive power, optimized power losses and other specialized functions. SUMMARY OF THE INVENTION

The present invention provides master control methods for a series MTDC system and elements thereof.

According to an aspect of the present invention, a method comprises: choosing one terminal as a current setting terminal (CST) and defining other terminals as voltage setting terminals (VST); configuring a current reference of the series MTDC system as the input of the CST converter, generating current values for each VST converter based on said current reference and different margins thereof respectively and making the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side.

According to a preferred embodiment of the present invention, the terminal chosen as said CST is the terminal with maximum rated power, the terminal with strongest AC grid which has the biggest short circuit capacity, the terminal with the biggest short circuit ratio (SCR), the terminal with larger rated power to increase reactive power consumption, or the terminal with lower rated power to optimize reactive power consumption.

According to a preferred embodiment of the present invention, said method further comprises: calculating the ratio of the actual active power order to the rated power capacity for each terminal, choosing the maximum ratio and calculating the current reference of the series MTDC system based on said maximum ratio and the rated current.

According to a preferred embodiment of the present invention, the terminals can be preset different priorities in advance or dynamically configured based on the power of the terminals, the load/generation of the terminals, the AC system connected by the terminals or other features defined by the system configuration.

According to a preferred embodiment of the present invention, the terminal with a large power, an important load/generation and/or a weak AC system will be preset a higher priority.

According to a preferred embodiment of the present invention, said method further comprises regulating the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike.

According to a preferred embodiment of the present invention, said method further comprises blocking or de-blocking at least one converter, at least one terminal, one pole or even the whole TDC system in sequence by modifying the current reference of the corresponding converters) followed with other switching actions.

According to a preferred embodiment of the present invention, to smoothly block an inverter terminal, said modifying the current reference of converter is to increase the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

According to a preferred embodiment of the present invention, to smoothly block a rectifier terminal, said blocking/de-blocking module decreases the current reference of the objective rectifier so as to be smaller than the maximum current reference of inverters.

According to a preferred embodiment of the present invention, said method is applicable in the case that said series MTDC system is split into subsystems, each of which includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively.

According to a preferred embodiment of the present invention, each subsystem forms a series MTDC system or a classic point to point HVDC system in monopole or bi-pole.

According to a preferred embodiment of the present invention, said method is applicable in an unbalancing condition comprising at least one of the following conditions: losing at least one converter bridge if the converter consists of more than one 12-pulse bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole.

According to another aspect of the present invention, a master control element comprises: a choosing module, configured to choose one terminal as a current setting terminal (CST) and define other terminals as voltage setting terminals (VST); a configuration module, configured to configure a current reference of the series MTDC system as the input of the CST converter, generate current values for each VST converter based on said current reference and different margins thereof respectively and make the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side.

According to a preferred embodiment of the present invention, said element further comprises: a calculating module, configured to calculate the ratio of the actual active power order to the rated power capacity for each terminal, choose the maximum ratio and calculate the current reference of the series MTDC system based on said maximum ratio and the rated current.

According to a preferred embodiment of the present invention, said element further comprises a regulating module, configured to regulate the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike.

According to a preferred embodiment of the present invention, said element further comprises a blocking/de-b!ocking module, configured to block or de-block at least one converter, at least one terminal, one pole or even the whole MTDC system in sequence by modifying the current reference of the converters) followed with other switching actions.

According to a preferred embodiment of the present invention, to smoothly block an inverter terminal, said blocking/de-blocking module increases the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

According to a preferred embodiment of the present invention, said element is applicable in the case that said series MTDC system is split into subsystems, each of which includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively.

According to a preferred embodiment of the present invention, said element is applicable in an unbalancing condition comprising at least one of the following conditions: losing at least one converter bridge if the converter consists of more than one 12-pulse bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole.

According to a preferred embodiment of the present invention, said element can function well by other communication methods when a default communication network breaks down.

Embodiments of the present invention provide master control methods for a series MTDC system and the master control elements thereof, which can fulfill the fundamental criterions of a series MTDC system and regulate active power and reactive power in the series MTDC system. With the P/Q regulation, further functions of series MTDC can be developed accordingly.

BRIEF DESCRIPTION OF THE DRAWINGS The subject matter of the invention will be explained in more details in the following description with reference to preferred exemplary embodiments which are illustrated in the drawings, in which: Fig .1 illustrates a Multi-terminal Bipolar HVDC Transmission System with parallel connected HVDC substations;

Fig.2 illustrates a Multi-terminal Bipolar HVDC Transmission System with series connected HVDC substations;

Fig.3 illustrates a master control method for a series MTDC system according to an embodiment of the present invention;

Fig.4 illustrates a master control method for a series MTDC system according to another embodiment of the present invention;

Fig.5 illustrates the function blocks of a master regulator in system level for calculating reference value of each terminal to control the power flow;

Fig.6 illustrates a master control element for a series MTDC system according to an embodiment of the present invention;

Fig.7 illustrates a block diagram of the master control element for a series MTDC system according to another embodiment of the present invention;

Fig.8 illustrates a block diagram of the master control element for a series

MTDC system according to another embodiment of the present invention;

Fig.9 illustrates a block diagram of master control element for a series MTDC system according to another embodiment of the present invention; and

Fig.10 illustrates a block diagram of a four-terminal DC system with a master control element according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are described in conjunction with the accompanying drawings hereinafter. For the sake of clarity and conciseness, not all the features of actual implementations are described in the specification.

Fig.3 illustrates a master control method for a series MTDC system according to an embodiment of the present invention.

As shown in Fig.3, the master control method 300 for a series MTDC system comprises:

Step 302, choosing one terminal as a CST and defining other terminals as VSTs. When a series MTDC system is in operation, one terminal should be set as the current setting terminal configured to regulate direct current (DC) in the series MTDC system. Others are defined as voltage setting terminals. Especially operated in the rated voltage, the value of firing angle for CST's converter shall be a default value based on the specific system configuration; because the CST is responsible to regulate the current in system level. The CST's current reference should be hold to avoid disturbance to the system. As to VSTs' converters, firing angle (or extinguishing angle) can be the minimum value according to the requirement of the series MTDC system in rated operation. It shall be noted that the CST mode can be shifted to any other terminals if necessary. The voltage of a VST can be also adjusted at a certain γ angle (for the inverter) or a angle (for the rectifier), or can be controlled by at least one voltage regulator.

According to an embodiment of the present invention, the terminal chosen as the CST is the terminal with maximum rated power, which can increase the operation of the series MTDC system (i.e. maximized control margin of DC side). To realize the minimum influence to the connected AC grid, the CST can be the terminal with strongest AC grid which has the biggest short circuit capacity. The CST can be the terminal with largest short circuit ratio (SCR); and

SCR for each terminal is defined as following: SCR = ^ ^Pscc,t ; in which, Pterminau '^{s tne rate}d power of terminal i, and P_{scc i} is the short circuit capacity of terminal i connected the AC grid. In a series MTDC system, terminals can share the same AC grid or solely connected to different AC grid. To increase reactive power consumption of MTDC system, the terminal, which has higher rated power, should be chosen as the CST. On the contrary, if a decreased Q consumption is wanted, then terminal with lower rated power capacity should be chosen.

Step 304, configuring a current reference of the series MTDC system as the input of the CST converter, and generating current values for each VST converter based on said current reference and different margins thereof respectively. la_ref,sys _> the current reference of a series MTDC system is the input for CST converter; meanwhile, different current references (I«i_re/,i) ^are generated as the inputs for VSTs' converters with different margins (Al_d __ref_ti) to the system current reference as following:

Step 306, making the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side. In a normal operation condition, the minimum value of the current reference (l_{d Teii}i) in a rectifier side should be higher than the maximum value of the current reference {I_d __refii) in an inverter side, i.e.

Min(l_{d re}f Rectifier) >

Fig .4 illustrates a master control method for a series MTDC system according to another embodiment of the present invention.

As shown in Fig.4, the master control method for a series MTDC system comprises: steps 402-410; in which steps 402-406 are the same or similar to the corresponding steps 302-306 in Fig.3. In order to keep the description brief, the same or similar steps will not be described again.

Step 408, calculating the ratio of the actual active power order to the rated power capacity for each terminal and choosing the maximum ratio.

Step 410, calculating the current reference of the series MTDC system based on said maximum ratio and the rated current.

For example, figure 5 illustrates the function blocks of a master regulator in system level for calculating reference value of each terminal to control the power flow. In this system, n terminals are connected in series. Desired power orders of different terminals [P_R1*, P_R2*... P_Rx* PJ1 * PJ2*... P_ly*; in which x+y=n] are inputs of the master controller. To achieve the desired current reference of series MTDC system [l_Sys_ref], firstly, calculating the ratios of the practical active power [P_R1*, P_R2*... P_Rx*; PJ1*, P_I2*... PJy* to the rated power capacity [P_R1_n, PJR2_n... P_Rx_n; P_l1_n, PJ2_n... P_ly_n] for each terminal respectively; then compare such ratios and select the maximum one. Hence, the desired current reference of series MTDC system [l_Sys_ref\ for the whole system can be achieved based on the multiplied result of the maximum ratio and rated system current. It's obvious to the skilled person in art, if the desired current reference [l_Sys_ref is not applied to the series MTDC system, some terminal might not transfer enough power and suffer over voltage or over current, which will conflict with the normal operation criterion. According to above logic, reference value [ld_sys_ref, P R1_ref, P_R2_ref... P_Rx_ref, P_l2_ref, P_l2_ref... PJy_ref\ will be generated. The desired current reference [ld_sys_ref\ is sent to the CST and others power reference [P_R1_ref, P_R2_ref... P_Rx_ref, P_l2_ref, P_l2_ref... P_ly_ref are sent to the VSTs. It is assumed that typical converter control and protection are always implemented in every terminal.

According to another embodiment of the present invention, the terminals of a series MTDC system can be preset different priorities in advance or dynamically configured based on the power of the terminals, the load/generation of the terminals, the AC system connected by the terminals or other features defined by the system configuration. For example, the terminal with a larger power, the terminal with an important load/generation, the terminal with a weak AC system or other features defined by a system operator will be preset a higher priority. When such series MTDC system is operated in light load, the terminals with low priority for long term can be blocked. This function can reduce the power loss and the reactive power required in the series MTDC system and converters.

The master control method further comprises regulating the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike. When a series MTDC system is not in rated operation, the reactive power consumption of each terminal and the whole system will deviate from the desired/designed working point. By means of the master control method, the desired reactive power in a terminal (or several terminals, or the whole system) can be realized. In the mean time, the master control is still able to realize the regulation on the active power. For some cases, the master control method of the present invention can improves the Q regulation speed while prolong the life time of devices with mechanical components, such as on load tap changer.

The master control method further comprises blocking or de-blocking at least one converter, at least one terminal, one pole, or even the whole MTDC system in sequence by modifying the current reference of the VST converter, and necessary switching actions. The coordinated actions in orders from master control element can effectively prevent the failure of blocking/de-blocking operation. By modifying the current reference of converter, we can smoothly block/de-block the designated terminals; for example, to increase the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

The person skilled in art acknowledges that above mentioned master control methods are applicable in the case that a series TDC system is split into subsystems, each of which still forms a series MTDC system or a classic point to point HVDC system in monopole or bt-pole. According to the above mentioned embodiments of the present invention, each subsystem also includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively. Hence, there shall be a CST and its calculated current reference for each subsystem of the split MTDC system.

According to above mentioned embodiments of the present invention, the master control method is further applicable in an unbalancing condition, which includes at least one of the following conditions: losing at least one Converter Bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole. It shall be noted that only one system current reference shall be calculated in the unbalancing condition.

Fig.6 illustrates a master control element for a series MTDC system according to an embodiment of the present invention.

As shown in Fig.6, the master control element 600 for a series MTDC system comprises: a choosing module 602 and a configuration module 604; in which the choosing module 602 is configured to choose one terminal as a current setting terminal (CST) and define other terminals as voltage setting terminals (VST); the configuration module 604 is to configure a current reference of the series MTDC system as the input of the CST converter, generate current values for each VST converter based on said current reference and different margins thereof respectively and make the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side.

According to previously explanation on the present invention, the terminal chosen as said CST is the terminal with maximum rated power, the terminal with strongest AC grid which has the biggest short circuit capacity, the terminal with biggest SCR, the terminal with larger rated power to increase reactive power consumption, or the terminal with lower rated power to optimize reactive power consumption, for example to decrease Q.

Fig.7 illustrates a block diagram of the master control element for a series MTDC system according to another embodiment of the present invention.

As shown in Fig.7, the master control element 700 for a series MTDC system comprises: a choosing module 702, a configuration module 704 and a calculating module 706; in which the choosing module 702 and the configuration module 704 are the same or similar to the corresponding choosing module 602 and configuration module 604 in Fig.6. In order to keep the description brief, the same or similar steps will not be described again.

The calculating module 706 is configured to calculate the ratio of the actual active power order to the rated power capacity for each terminal, choose the maximum ratio and calculate the current reference of the series MTDC system based on said maximum ratio and the rated current.

According to an embodiment of the present invention, the terminals of the series MTDC system can be preset different priorities in advance or dynamically configured based on the power of the terminals, the load/generation of the terminals, the AC system connected by the terminals or other features defined by the system configuration. For example, the terminal with a large power, an important load/generation and/or a weak AC system will be preset a higher priority.

Fig.8 illustrates a block diagram of the master control element for a series MTDC system according to another embodiment of the present invention.

As shown in Fig.8, the master control element 800 for a series MTDC system comprises: a choosing module 802, a configuration module 804, a calculating module 806 and a regulating module 808; in which the choosing module 802, the configuration module 804 and the calculating module 806 are the same or similar to the corresponding choosing module 702, the configuration module 704 and the calculating module 706 in Fig.7. In order to keep the description brief, the same or similar steps will not be described again.

The regulating module 808 is configured to regulate the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike. Fig.9 illustrates a block diagram of the master control element for a series TDC system according to another embodiment of the present invention.

As shown in Fig.9, the master control element 900 for a series MTDC system comprises: a choosing module 902, a configuration module 904, a calculating module 906, a regulating module 908 and a blocking/de-blocking module 910; in which the choosing module 902, the configuration module 904, the calculating module 906, the regulating module 908 are the same or similar to the corresponding choosing module 802, the configuration module 804, the calculating module 806 and the regulating module 808 in Fig.8. In order to keep the description brief, the same or similar steps will not be described again.

The blocking/de-blocking module 910 is configured to block or de-block at least one converter, at least one terminal, one pole or even the whole MTDC system in sequence by modifying the current reference of the VST converter and holding firing/extinguishing angle as well as other switching actions. According to the present invention, to smoothly block a terminal, the blocking/de-blocking module increases the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

According to the present invention, the master control element is applicable in the case that said series MTDC system is split into subsystems, each of which includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively. Moreover, each subsystem individually forms a series MTDC system or a classic point to point HVDC system in monopole or bi-pole.

According to another embodiment of the present invention, the master control element is also applicable in an unbalancing condition comprising at least one of the following conditions: losing at least one converter bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole.

It should be noted that the master control method for a series MTDC system of the present invention can function well when the default communication network breaks down. To maintain the system control element, a system operator in the control room can view the calculated references based on the above mentioned methods and send them to the terminal operators by other communication means, such as telephone or mobile phone. According to the present invention, all fundamental requirements for operating the series MTDC system can be implemented in the master control element. Furthermore, each terminal in a series MTDC system has two operation modes: CST and VST. On the basis of such premise, the working point of terminals can be settled when the system is in rated operation. It's obvious to the skilled person in art that determination of a CST may be dependent on the purpose of practical application or system operator. 'Decreased reactive power consumption of series MTDC system' is taken as an example. A series MTDC system with three rectifier terminals and two inverter terminals is assumed. Its rated operation point is listed as below:

1 ) System current: 4kA

2) Rectifier 1 : 100kV/400MW

3) Rectifier 2: 200kV/800MW

4) Rectifier 3: 400kV/1600MW

5) Inverter 1 : 400kV/1600MW

6) Inverter 2: 300kV/1200MW

7) Alpha of CST: 15°

8) Alpha of VST in rectifier side: 5°

9) Gamma of VST in inverter side: 17°

The active power reference of three rectifier terminal is:

PRi ord = 300 W

PR2 ord = 700 W

P_R3.ord = 1500MW

In this case, the CST is in the rectifier side. Therefore, when the CST is changing from one rectifier terminal to another, the reactive power consumption in the inverter side is constant; and reactive power requirement in rectifier is simulated and its result is listed in the following Table 1.

Table 1 Q requirements of the rectifier side in different CST cases

Case 1 Case 2 Case 3

Rectifier 1 as CST Rectifier 2 as CST Rectifier 3 as CST

CIRI 35.77° 33.19° 33.19°

CIR2 21.50° 25.57° 21.47°

lR3 5° 5° 16.2°

Q 701.1MVar 740.3MVar 907.6MVar It's obvious to the person skilled in art that it's better to choose the terminal with a decreased capacity as the CST, if decreased reactive power requirement is required. Moreover, the disordered operation mode of terminals can be prevented by introducing the current reference margin, and the concept of the current reference group brings enhanced feasibility and reliability to order the blocking/de-blocking terminals.

According to the present invention, the desired working point of series MTDC system can be achieved in the system level. The desired working point is determined by Transmission System Operator (TSO) such as minimum power loss, required transferred active power, desired reactive power and etc.

As shown in Fig.10, an 800kV monopole DC system is taken as an example. In this system, four terminals with 400kV rating voltage (0~400kV or 400-800kV) and 1.6GW rating power capacity per terminal are installed in series. The rated DC current of this system is 4kAand the total resistance of the transmission line is about 13Ω. If the powers of four terminals shown in Fig.10 are defined as 1.5GW, 1.3GW, 1.5GW, 1.3GW respectively, the calculated working points including the DC current of whole system and DC voltage of each terminal are listed in the following Table 2.

Table 2 the power Loss Comparison of this Invention

Compared with an alternative solution shown in the Table 2, the loss of the transmission line can be saved 0.9% by utilizing the master control method according to the present invention. Table 3 brief values of Q consumption in different tap positions

According to the above description, the present invention can also regulate the reactive power consumption in system level by coordinating the actions of terminals. In the following section, a further analysis is carried out based on the case 3 listed in the previous Table 1. In this condition, only the OLTC in rectifier side is taken into consideration. It is assumed that, in rated operation, there are 17 positions for every converter transformer and central tap position of all is 0. It is assumed that, all the active power references of terminals are constant. Then brief values of the reactive power consumption for different tap position are calculated and listed in the Table 3.

Now it is assumed that, reactive power consumption should be decreased by 90MVar. Then the master control will send the following commands to the corresponding terminals simultaneously:

1) Change tap position of terminal R1 to '+1'

2) Change tap position of terminal R2 to -1 ' 3) Change tap position of terminal R3 to '+1 '

Alternative control might be:

1 ) Change tap position of terminal R2 to

2) Wait until tap position is ready

3) Change tap position of terminal R2 to '+2'

By means of the master control methods proposed in the present invention, the desired reactive power regulation can be realized as soon as possible. It's noted that the present invention can also reduce the cost of transformers of series MTDC system. The DC voltage of each terminal is maintained as high as possible. Previous example is taken to calculate the needed voltage in AC side briefly by the following equation:

π

AC.R X (U'_diR + d_x X I_d)

3 2 cos

UACJ = ₃^ ^x (^u'd,i + d_x x l_d)

It should be noted that there are two series connected bridges in each terminal, which means U_d = \ u_d. It is assumed that d_x = 5.1375Ω, ¾_{C n} =

YJOkV, U_d and I_d of each terminal is from the Table 2. Table 4 lists the needed tap number by using the present invention or not. It can be observed that, in the same desired working point, the tapping range of transformers with this invention can be reduced, which leads to a highly reduced investment.

Table 4 AC Voltage Comparison

According to the description of the present invention, other specific operations such as blocking/de-b!ocking of terminals, subsystem operation, unbalancing operation, operation without communication and so on, can be essentially realized. Such functionalities enhance the reliability and flexibility of series MTDC system. I S

Compared with the existing prior arts, the proposed solution of the present invention is much more practical and easier for implementation on the series MTDC system. Referring to the description of the exemplary embodiments, those skilled in the art appreciate the advantages of the present invention:

1 , According to the master control methods for a series MTDC system and elements thereof provided in the present invention, the series MTDC system can regulate and optimize the active power and reactive power, reduce the power losses, control the distribution of the reactive power and realize other specialized functions described in the embodiments.

2, Compared with the method proposed by R.L.Vaughan, the master control methods for a series MTDC system and elements thereof provided in the present invention enhance the ability of the series MTDC systems to follow the frequent power variation. Optimal working point of the whole system can be realized at any time by the proposed invention, which even considers the power fluctuation in some terminals.

3, According to the master control methods for a series MTDC system and elements thereof provided in the present invention, the system current of a series MTDC system can be maintained as low as possible in system level of view. Therefore system power losses (for example the losses in the transmission line) can be reduced.

4, According to the master control methods for a series MTDC system and elements thereof provided in the present invention, it's obviously different from the prior arts mentioned in the background. R.L.Vaughan thought the first terminal to attain its maximum voltage rating will determine the limit to which the current may be reduced. It is a low speed control method, which might lead to undesired variance when power regulation command varies frequently. The paper published by ZheJiang University assumed that all the converter stations are operated under rated voltage, the minimum current is calculated by P_{d ref}/ U_{d n}, and then the maximum one is the system DC current reference. While in the present invention, the reduced current value is determined by the maximum desired power per unit in certain terminal.

Though the present invention has been described on the basis of some preferred embodiments, those skilled in the art should appreciate that those embodiments should by no means limit the scope of the present invention. Without departing from the spirit and concept of the present invention, any variations and modifications to the embodiments should be within the apprehension of those with ordinary knowledge and skills in the art, and therefore fall in the scope of the present invention which is defined by the accompanied claims.

Claims

1. A master control method for a series MTDC system, wherein said method comprises:

choosing one terminal as a current setting terminal (CST) and defining other terminals as voltage setting terminals (VST);

configuring a current reference of the series MTDC system as the input of the CST converter, generating current values for each VST converter based on said current reference and different margins thereof respectively and making the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side.

2. The master control method according to claim 1, wherein the terminal chosen as said CST is the terminal with maximum rated power, the terminal with strongest AC grid which has the biggest short circuit capacity, the terminal with the biggest short circuit ratio (SCR), the terminal with larger rated power to increase reactive power consumption, or the terminal with lower rated power to optimize reactive power consumption.

3. The master control method according to claim 1, wherein said method further comprises: calculating the ratio of the actual active power order to the rated power capacity for each terminal, choosing the maximum ratio and calculating the current reference of the series MTDC system based on said maximum ratio and the rated current.

4. The master control method according to any one of preceding claims, wherein the terminals can be preset different priorities in advance or dynamically configured based on the power of the terminals, the load/generation of the terminals, the AC system connected by the terminals or other features defined by the system configuration.

5. The master control method according to claim 4, wherein the terminal with a large power, an important load/generation and/or a weak AC system will be preset a higher priority.

6. The master control method according to claim 1 , wherein said method further comprises regulating the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike.

7. The master control method according to claim 1 , wherein said method further comprises blocking or de-blocking at least one converter, at least one terminal, one pole or even the whole MTDC system in sequence by modifying the current reference of the corresponding converterfs) followed with other switching actions.

8. The master control method according to claim 7, wherein to smoothly block a terminal, said modifying the current reference of converter is to increase the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

9. The master control method according to claim 7, wherein to smoothly block a rectifier terminal, said method further comprises decreasing the current reference of the objective rectifier so as to be smaller than the maximum current reference of inverters.

10. The master control method according to any one of preceding claims, wherein said method is applicable in the case that said series MTDC system is split into subsystems, each of which includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively.

11. The master control method according to claim 10, wherein each subsystem forms a series MTDC system or a classic point to point HVDC system in monopole or bi-pole.

12. The master control method according to any one of preceding claims, wherein said method is applicable in an unbalancing condition comprising at least one of the following conditions: losing at least one converter bridge if the converter consists of more than one 12-pulse bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole.

13. A master control element for a series MTDC system, wherein said element comprises:

a choosing module, configured to choose one terminal as a current setting terminal (CST) and define other terminals as voltage setting terminals (VST); a configuration module, configured to configure a current reference of the series MTDC system as the input of the CST converter, generate current values for each VST converter based on said current reference and different margins thereof respectively and make the minimum value of current reference in the rectifier side larger than the maximum value of current reference in the inverter side.

14. The master control element according to claim 13, wherein the terminal chosen as said CST is the terminal with maximum rated power, the terminal with strongest AC grid which has the biggest short circuit capacity, the terminal with the biggest short circuit ratio (SCR), the terminal with larger rated power to increase reactive power consumption, or the terminal with lower rated power to optimize reactive power consumption.

15. The master control element according to claim 13, wherein said element further comprises: a calculating module, configured to calculate the ratio of the actual active power order to the rated power capacity for each terminal, choose the maximum ratio and calculate the current reference of the series MTDC system based on said maximum ratio and the rated current.

16. The master control element according to any one of claims 13-15, wherein the terminals can be preset different priorities in advance or dynamically configured based on the power of the terminals, the load/generation of the terminals, the AC system connected by the terminals or other features defined by the system configuration.

17. The master control element according to claim 16, wherein the terminal with a large power, an important load/generation and/or a weak AC system will be preset a higher priority.

18. The master control element according to claim 13, wherein said element further comprises a regulating module, configured to regulate the reactive power in the system level by coordinating the reactive power source of at least one terminal, in which said reactive power source includes at least one of a transformer, a filter, a converter and alike.

19. The master control element according to claim 13, wherein said element further comprises a blocking/de-blocking module, configured to block or de-block at least one converter, at least one terminal, one pole or even the whole MTDC system in sequence by modifying the current reference of the corresponding converter(s) followed with other switching actions.

20. The master control element according to claim 19, wherein to smoothly block an inverter terminal, said blocking/de-blocking module increases the current reference of the objective inverter so as to be larger than the minimum current reference of rectifiers.

21. The master control element according to claim 19, wherein to smoothly block a rectifier terminal, said blocking/de-blocking module decreases the current reference of the objective rectifier so as to be smaller than the maximum current reference of inverters.

22. The master control element according to any one of claims 13-21 , wherein said element is applicable in the case that said series MTDC system is split into subsystems, each of which includes a master control element used to choose a terminal as the CST and calculate the current reference of the corresponding subsystem respectively.

23. The master control element according to claim 22, wherein each subsystem forms a series MTDC system or a classic point to point HVDC system in monopole or bi-pole.

24. The master control element according to any one of claims 13-23, wherein said element is applicable in an unbalancing condition comprising at least one of the following conditions: losing at least one converter bridge if the converter consists of more than one 12-pulse bridge, losing at least one converter, losing at least one terminal, losing at least one transmission line and losing one pole.

25. The master control element according to any one of claims 13-24, wherein said element can function well by other communication methods when a default communication network breaks down.