Apparatus for converting electrical energy to heat, method for operating the apparatus, and a converter arrangement

Abstract

The present invention relates to an apparatus for converting electrical energy to heat comprising a plurality of modules connected in series arranged between a first and a second de pole, wherein each of the modules has a controllable switchoff type semiconductor switching element. The invention is characterized in that the modules are varistor modules (VM) comprising a varistor (14) arranged in parallel to the switching element (12), and wherein the apparatus (10) further comprises a control system (CS) for controlling the energy conversion, wherein said control system (CS) is configured to actively control a current flowing through the apparatus (10) to equal a preset current reference value by selectively switching the switching elements (12). The present invention further relates to a converter arrangement with the apparatus and a method of operation of the apparatus.

Description

[0001] 2024PF00359

[0002] Description

[0003] Apparatus for converting electrical energy to heat , method for operating the apparatus , and a converter arrangement

[0004] The present invention relates to an apparatus for converting electrical energy to heat comprising a plurality of modules connected in series being arranged between a first and a second de pole , wherein each of the modules has a controllable switch-of f type semiconductor switching element .

[0005] Known HVDC systems that connect of fshore wind farms to onshore power grids (HVDC Grid Access ) are usually required to absorb transmitted active power of the wind farm during a fault in the onshore AC grid . The goal is to maintain the energy generation of the of fshore wind farm, even i f its active power cannot be fed into the AC grid . I f the active power generated by the of fshore wind farm cannot be fed into the connected AC grid, it must be absorbed until the AC fault is resolved .

[0006] An aforementioned apparatus for converting electrical energy to heat ( also denoted as DC chopper ) is known from WO 2010 / 023127 A2 . The known apparatus comprises a converter arm consisting of a plurality of chopper modules , that is installed between the DC terminals of an HVDC station . Each of the chopper modules comprises several semiconductor switches , a capacitor, and a braking resistance . The capacitors of the series-connected chopper modules are charged via input diodes . When the semiconductor switches of the chopper modules are blocked, the series connection of the capacitors keep a DC voltage value present at the DC terminals , and only leakage currents flow through the series-connected chopper modules . When one of the IGBTs in the modules is switched on, current flows through one of the braking resistors of the chopper module converting electrical energy into heat energy . The capacitor voltage of the respective module decreases , 2024PF00359

[0007] 2 thereby reducing the total voltage of the series-connected chopper modules .

[0008] The obj ect of the present invention is to provide an aforementioned apparatus that is reliable and cost-ef fective .

[0009] The obj ect is achieved by the apparatus according to claim 1 .

[0010] According to the present invention the modules of the apparatus are varistor modules , wherein each of the varistor modules has a varistor arranged in parallel to the switching element , and the apparatus further has a control system for controlling the energy conversion, wherein said control system is configured to actively control a current flowing through the apparatus to equal a preset current reference value by selectively switching the switching elements . The varistor can e . g . be a metal oxide varistor (MOV) . A varistor is generally characteri zed by a non-linear voltage-current characteristics . Such characteristic ( function) preferable means that a current flowing through the varistor increases (with increasing voltage across the varistor ) but remains low below a certain voltage threshold (varistor threshold) and it increases to a high level for voltages above the voltage threshold . None of the varistor modules of the apparatus comprises an energy storage ( e . g . in form of a capacitor element ) . The number of varistor modules can be chosen in accordance with the requirements of a particular application, suitably depending on a DC voltage between the DC poles .

[0011] When the IGBTs are blocked, the series connection of the power modules can be considered as a conductor that allows only a very small leakage current to flow through the series connection . I f active power is to be absorbed by the HVDC chopper, individual varistors are bridged by switching on the parallel switching elements ( e . g . , IGBTs , IGTCs , MOSFETS or any other controllable semiconductors configured to be selectively switched on and of f ) . By bypassing a suf ficient number 2024PF00359 of varistors , current begins to flow through the series connection ( through some of the varistor modules of the series connection) . Since the voltage between the DC poles is given, bypassing a varistor in a varistor module means that the voltage across non-bypassed varistors increases . As soon as enough varistor modules are bypassed, the voltages across the non-bypassed varistors increase above the respective varistor thresholds , and the current begins to flow . In the bypassed varistor modules the current flows through the switching elements , whereas in the non-bypassed modules the current flows through the varistors , where electrical power is converted to heat energy .

[0012] By actively choosing which modules to bypass and which ones not to , the conductivity of the series connection can be actively adj usted . The chopper can thus be used as a control element for regulated power absorption . After an activation of the apparatus , the current through the apparatus is actively controlled . It means that a closed-loop control is used to regulate the power absorption . To control the apparatus , a suitable current reference can be set . The control system controls the current by switching on and of f the switching elements of the varistor modules according to a di f ference between a measured current through the series connection of the varistor modules and the current reference , so that the di f ference is minimi zed . By exchangeably selecting the bypassed and non-bypassed varistor modules ( or their switching elements ) , the energy input into the di f ferent varistors can be evenly distributed across all modules .

[0013] The apparatus of the present invention has signi ficantly fewer components ( compared to the prior art ) . Consequently, its costs are lower . The usually large capacitor is not needed . Therefore , the challenge of handling of stored energy in the event of short circuits is not relevant . Consequently, the apparatus also has an increased reliability . 2024PF00359

[0014] In some applications it may be advantageous i f each of the varistor modules comprises a plurality of varistors arranged in parallel . The current through the respective varistor module can be distributed among the varistors . This allows to absorb more energy per given time .

[0015] In one embodiment , each of the varistor modules comprises a plurality of semiconductor switching elements arranged in parallel . This increases the current capacity of the apparatus .

[0016] In some of the applications , especially HVDC applications , it can be advantageous i f the switching elements ( 12 ) of the varistor modules each have a nominal voltage lying between

[0017] 1 . 5 kV and 5 kV . A voltage of 500 kV between the DC poles requires then at least 100 varistor modules in series .

[0018] According to an embodiment of the invention the varistors are arranged to conduct a current of more than 100 A at a voltage between 3 kV and 4 kV . This defines a suitable rate of energy conversion for most applications .

[0019] Suitably, the apparatus further comprises an inductance arranged in series with the varistor modules . The inductance serves as a current smoothing reactor, limiting inrush currents flowing into the varistor modules .

[0020] The present invention further relates to a converter arrangement with a converter having six converter arms , wherein each converter arm is arranged between a de pole and an ac terminal of the converter, each of the converter arms comprising a series of converter modules , each converter module having semiconductor switches and an energy storage , wherein the converter arrangement further comprises an apparatus for converting electrical energy to heat comprising a plurality of varistor modules connected in series connected to a DC side 2024PF00359 of the converter, wherein each of the varistor modules has a controllable switch-of f type semiconductor switching element .

[0021] The obj ect of the present invention is to provide such converter arrangement that allows a reliable and cost-ef fective operation .

[0022] The obj ect is achieved by a converter arrangement according to claim 8 .

[0023] The converter can be a modular multilevel converter (MMC ) comprising six converter arms with series connections , each comprising a plurality of converter modules , as well as an arm inductance . The converter modules can be for example hal f-bridge or full-bridge converter modules ( or any combination thereof ) .

[0024] The converter arrangement can on its AC side be connected to a wind park and on its DC side via an HVDC link to another converter station ( for example onshore ) . In other applications , the converter arrangement can be connected on its AC side to an AC supply high-voltage grid and on its DC side to an HVDC system with an of fshore converter station .

[0025] The present invention further relates to a method for converting electrical energy to heat by means of an apparatus comprising a plurality of varistor modules connected in series arranged between a first and a second de pole .

[0026] The obj ect of the invention is to provide such a method that allows a reliable and cost-ef fective energy conversion .

[0027] According to the invention each of the varistor modules of the apparatus has a controllable switch-of f type semiconductor switching element and a parallel varistor, and a control system for controlling the energy conversion . The method comprises the steps of 2024PF00359 activating the apparatus i f a measured voltage across the apparatus reaches / exceeds a predefined voltage threshold, actively controlling a current flowing through the apparatus to equal a preset current reference by selectively switching the switching elements . Upon activation the apparatus starts converting the electrical energy to heat . The active control of the current flow is performed as described earlier .

[0028] The voltage threshold is preferably given by a sum of a voltage reference value and a preset constant . The voltage reference value can be given by a voltage reference value of a converter arrangement being part of an HVDC system . The HVDC system may be configured to transmit energy from an energy source ( for example a wind park) to a supply grid . The preset constant represents a voltage margin that prevents an unintentional activation of the energy conversion .

[0029] Figures 1 to 4 describe an embodiment of the present invention .

[0030] Figure 1 schematically shows a converter arrangement of the present invention;

[0031] Figure 2 schematically shows a varistor module ;

[0032] Figure 3 schematically shows a voltage-current characteristics of a varistor ;

[0033] Figure 4 schematically shows a flow diagram of a method according to the invention .

[0034] In Figure 1 , a converter arrangement 1 with a converter 2 is shown, which is often referred to as a modular multilevel converter (MMC ) . The converter 2 comprises six converter arms CAI- 6 with series connections SC1-SC6 , each comprising a plurality of converter modules SMl-SMn, as well as an inductance 2024PF00359

[0035] Lconv (arm inductor) . In the example shown, the MMC 2 is configured to convert an alternating voltage from an alternating current network, to which the MMC 2 can be connected, for example, via ac terminals A1-A3 and a network transformer 3, into a direct voltage UDC = UdHp - UdHn (or vice versa) . The converter 2 can be connected to a direct voltage network or a direct voltage line via connections at de poles DI, D2. Furthermore, the converter 2 comprises a converter control CON, which is set up for a control of the converter operation. Current, voltage, power, and frequency can be controlled by the control device CON. For example, an arm voltage Ulp-U3p, Uln-U3n can be regulated by the converter control CON.

[0036] In the example shown in Figure 1, all converter modules SMl-n are of the same type. However, it is also possible to use differently configured converter modules in the same converter, for example, half-bridge converter modules and fullbridge converter modules.

[0037] Figure 1 also shows an example of a converter module SMI that can be used in the converter arrangement 1 described above. The converter module SMI is a half-bridge converter module. The converter module SMI comprises bridge branch 4 and an energy storage branch 5. The energy storage branch 5 comprises a first semiconductor switch SI with an antiparallel freewheeling diode F and an energy storage (capacitor) C arranged in series with the ( anti- ) parallel arrangement of the switch SI and diode F. The bridge branch 4 is arranged between terminals XI, X2 of the converter module SMI, and comprises a second semiconductor switch S2 with an antiparallel freewheeling diode F. By suitable control of the two semiconductor switches SI, S2, a converter module voltage USM1 can be generated at the terminals XI, X2. The converter module voltage USM1 corresponds to a capacitor voltage Uc or to a zero voltage. The semiconductor switches SI, S2 are power semiconductor switches with a rated voltage of 4kV. In the example shown in figure 1, the semiconductor switches SI, S3 are IG- 2024PF00359

[0038] BTs , but other switchable semiconductor switches , such as IGCTs , MOSFETs , or so-called wide-gap semiconductor switches , are also conceivable .

[0039] The converter arrangement 1 of figure 1 further comprises an apparatus 10 for converting electrical energy to heat . The apparatus 10 is arranged between the DC poles DI and D2 and comprises a plurality of controllable varistor modules VM connected in series together with two inductances LDCC . The varistor modules VM are described in figure 2 in more detail . According to the embodiment of figure 1 the apparatus 10 comprises a control system CS that is configured to actively control a current flowing through the apparatus to equal a preset current reference value . In particular, the control system CS is configured to selectively switching on and / or of f semiconductor switching elements of the varistor modules VM . The control procedure is shown in figure 4 in more detail . The control system CS can in some application be part of the converter control CON .

[0040] Figure 2 shows a varistor module 11 that can for example be used as the varistor module of the arrangement of figure 1 . The varistor module 11 comprises a first and second terminal Yl , Y2 to connect to other varistor modules or for example to other components of the apparatus 1 of figure 1 . The varistor module 11 further comprises a switching element 12 that in the example shown in figure 2 is an IGBT . However, the switching element can also be any other suitable controllable switching element that can be switched on and of f . A freewheeling diode 13 is connected in parallel to the switching element 12 .

[0041] The varistor module 11 further comprises a varistor 14 that is connected in parallel to the switching element and the freewheeling diode 13 . According to the example shown in figure 2 the varistor 14 is a metal oxide varistor . The characteristics of the varistor are shown in figure 3 in more de- 2024PF00359 tail . A dashed line 15 indicates that it is possible to arrange one or more further varistors in parallel to the varistor 14 to increase the energy conversion capability of the apparatus . Another dashed line 16 shows the possibility to arrange further switching elements in parallel to the switching element 12 to increase the current carrying capacity of the apparatus .

[0042] Any or all of the varistor modules 15 may have a bypass switch (not shown in the figure ) connected in parallel to bypass the varistor module 15 in case of a fault . The bypass switch suitably is a mechanical switch that can be controlled to close and to remain in a closed-contacts position . The series connection of the varistor modules 15 may contain additional ( redundant ) modules (more modules than necessary for the normal operation) , so that in case of a fault and a bypass of a varistor module , the operation of the apparatus can be maintained .

[0043] Figure 3 shows an example of the characteristics of the varistor 11 of figure 2 . A diagram 17 has on its x-axis a current I through the varistor and on its y-axis a voltage Umov across the varistor . A function 18 shows the characteristic for a low temperature of the varistor, whereas function 19 shows a high temperature characteristic . Both characteristics are non-linear and are for example such that the varistor conducts a current of more than 100 A at a voltage between 3 kV and 4 kV . In the diagram 17 a voltage of 0 . 6 pu corresponds in a possible application to an absolute voltage of 500 kV .

[0044] Figure 4 shows a mode of operation of an arrangement of figure 1 . In a normal mode of operation of the arrangement 1 the entire power provided at its DC side is converted to AC power . However, under certain circumstances the active power provided at the DC side of the arrangement cannot be fed into 2024PF00359 the AC grid . In such situations a DC voltage at the DC poles increases .

[0045] In a first step 101 the DC voltage between the DC poles of the arrangement is monitored by a suitable measuring system .

[0046] In a second step 102 , i f the DC voltage exceeds a predefined voltage threshold, the apparatus 10 is activated to convert electrical energy to heat to dissipate the surplus of energy .

[0047] In a third step 103 the control system CON ( or, alternatively, the control system CS ) actively controls a current flowing through the apparatus 10 to equal a preset current reference (value ) . It is of course possible to vary the current reference (value ) . To control the current the control system selectively switches the switching elements of the varistor modules . Wenn a switching element of a varistor module is switched on, then the corresponding varistor is bypassed . By bypassing a suf ficient number of varistors , current begins to flow through the series connection of varistor modules . In the bypassed varistor modules the current flows through the switching elements , whereas in the non-bypassed modules the current flows through the varistors according to its voltagecurrent characteristic function, converting electrical power to heat .

[0048] As soon as enough energy is dissipated, the arrangement returns to normal operation by deactivating the apparatus 10 in a fourth step 104 .

Claims

2024PF00359Patent Claims1. Apparatus (10) for converting electrical energy to heat comprising a plurality of modules connected in series arranged between a first and a second DC pole (DI, D2 ) , wherein each of the modules has a controllable switchoff type semiconductor switching element (12) , characterized in that the modules are varistor modules (VM) each comprising a varistor (14) arranged in parallel to the switching element (12) , and wherein the apparatus (10) further comprises a control system (CS) for controlling the energy conversion, wherein said control system (CS) is configured to actively control a current flowing through the apparatus (10) to equal a preset current reference value by selectively switching the switching elements (12) .

2. Apparatus (10) of claim 1, wherein the varistors (14) each have a non-linear voltage-current characteristics.

3. Apparatus (10) according to any of the preceding claims, wherein each of the varistor modules (VM) comprises a plurality of varistors arranged in parallel.

4. Apparatus (10) according to any of the preceding claims, wherein each of the varistor modules (VM) comprises a plurality of semiconductor switching elements arranged in parallel.

5. Apparatus (10) according to any of the preceding claims, wherein each of switching elements (12) of the varistor modules (VM) has a nominal voltage lying between 1.5 and 5 kV.

6. Apparatus (10) according to any of the preceding claims, wherein the varistors (14) are arranged to conduct a2024PF0035912 current of more than 100 A at a voltage between 3 kV and 4 kV.

7. Apparatus (10) according to any of the preceding claims, wherein the apparatus (10) further comprises an inductance (LDCC) arranged in series with the varistor modules (VM) .

8. Converter arrangement (1) with a converter (2) having six converter arms (CAI-6) , wherein each converter arm (CAI-6) is arranged between a de pole and an ac terminal of the converter (2) , each of the converter arms comprising a series of converter modules (SMl-n) , each converter module (SMl-n) having semiconductor switches (S1,S2) and an energy storage (C) , wherein the converter arrangement (1) further comprises an apparatus (10) according to any of the preceding claims, wherein said apparatus (10) is arranged at a de side of the converter .

9. Method for converting electrical energy to heat by means of an apparatus (10) comprising a plurality of varistor modules (VM) connected in series arranged between a first and a second de pole (DI, D2 ) , wherein each of the varistor modules (VM) has a controllable switch-off type semiconductor switching element (12) and a parallel varistor (14) , and a control system (CS) for controlling the energy conversion, wherein the method comprises the steps of- activating the apparatus (10) if a measured voltage across the apparatus (10) exceeds a predefined voltage threshold,- actively controlling a current flowing through the apparatus (10) to equal a preset current reference by selectively switching the switching elements (12) .2024PF003591310 . Method of the preceding claim, wherein the voltage threshold is given by a sum of a voltage reference value and a preset constant .

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