Commissioning of a twelve-pulse-bridges LCC-HVDC Analog-Hybrid-Simulator

Transcription

1 Commissioning of a twelvepulsebridges LCCHVDC AnalogHybridSimulator Alexander Raab, Christoph Hahn, Klaus Schneider, Gerhard Czerwenka, Johannes Floth Institute of Electrical Energy Systems FriedrichAlexanderUniversity ErlangenNuremberg (FAU) Erlangen, Germany alexander.raab@fau.de, christoph.hahn@fau.de, klaus.schneider@fau.de Abstract The objective of this paper was to describe and commission a twelvepulse LCCHVDC and its appropriate control system to an AnalogHybridSimulator(AHS). The AHS is a system with a digital control and an analog HVDC system with thyristor bridges. The aim was to connect the analog system with a Electronic Control Unit (ECU) to create a realtime model of an HVDC. The current control, the extinction angle control and the DC voltage control of a LCC HVDC were implemented with the software MATLAB/Simulink. The control structure created in MATLAB/Simulink was then transfered to a Processor Board. A highspeed A/D board and an I/O board have been linked with the LCCHVDC system for signal measurements and signal output. With the Processor Board a connection between the hardware and a host computer could be established. The operation of the HVDC was performed with the software DSPACE Control Desk. An operating point was set under steady state operating requirements as a reference. The control model was tested for various operating points. Simulations using Simscape Power Systems in Simulink of the system under equal operating conditions were conducted to verify the operation of the HVDC. Keywords AnalogHybridSimulation; Hardware in the Loop; HVDC; HVDC Control; LCC; Realtime simulation I. INTRODUCTION Due to the increasing challenges of energy transmission, Highvoltage direct current transmission (HVDC) is gaining importance. Since the advancing decentralization of power plants, HVDC often represents the only profitable solution for energy transmission. HVDC is used once threephase AC transmission can no longer be used for power transportation. In this case, the power losses are no longer sustainable from an ecological and physical point of view. There are various fields of application for the use of an HVDC: The energy transmission over long distances with overhead DC lines, the use of submarine and underground cables and the coupling of asynchronus connections [2]. The planning of such HVDC systems requires detailed knowledge about the specifical behavior on the power system. Simulations are necessary to investigate the effects on the electrical grids. Simulation methods have changed considerably over the years. While analog circuits with analog control have been built in the past, digital realtime simulations are mainly used at the present day. A method of simulation, which includes analog and digital systems, is the AnalogHybridSimulator. This paper presents an analog model and a digital control structure of a LCCHVDC and a detailed description of the commissioning. The interaction between the single components of the AHS as well as the functional principles of the converter represent a crucial part. The various operating points of the system are verified with a digital simulation. For this purpose, the fundamentals and the control concepts for a LCCHVDC are explained first. The control concept can be subdivided into the Station Control, the Pole Control and the converter unit controls [6]. Under stationary conditions the the rectifier operates in DC current control mode and the inverter in DC voltage control. Reference values are determined according to the power requirement from the Pole control. Following Section I, the investigated LCCHVDC system, its fundamentals and the control principles are described in Section II. The Station Control is only briefly introduced in this section. The Pole control is subdivided into the rectifier control and the inverter control. Section III gives an overview over the AHS and its components. Section IV shows the commissioning of the AHS. This section includes the simulation results of the AHS and the digital simulation. Section V shows the conclusion of the commissioning. II. FUNDAMENTALS OF LCCHVDC LCCHVDC apllies socalled thyristor valves in bridge circuits. These require grid voltage for firing. The firing of the thyristor can be delayed with a so called firing angle α. Most LCCHVDC systems are operated with twelvepulse converters. These are realized by a series connection of two sixpulse bridges connected via starstar and stardelta transformers. The DC current thus has a lower ripple compared to the application with sixpulse bridges. The converter injects only harmonic currents of orders n = 12k ±1, where k is equal to the integer numbers, into the connected AC system [1]. The twelvepulse configuration is shown in Figure 1. The figure illustrates Station A with one converter of the AnalogHybridSimulator. The ACvoltage is generated by a controlled voltage source (Source A). Source A represents the grid voltage and is connected by a circuit breaker (SA) to the primary sides of the converter transformer (TA). Station B has the same configuration as Station A. Station A represents the

2 Station A Converter A V AC,r ' Source A SA TA Fig. 1. Equivalent circuit diagram of Station A V dr The converters at Station A and Station B can work both as rectifier and inverter. Since a reversal of the DC power flow can only be performed by reversing the DC voltage, due to the thyristors, the following operation area is obtained for the firing angles: Operating area rectifier: 0 α < 90 Operating area inverter: 90 < α 180 1) Commutation: In real converters a current transfer from one phase to the other takes place, while switching from one valve to the next. Figure 3 shows the process for a threephase bridge and illustrates the change from valve 3 to valve 1 and the associated commutation from phase 3 to phase 1. rectifier side and Station B the inverter side for a power flow from A to B. The internal resistance of the voltage sources is neglected L d I d The circuit diagram of the monopolar HVDC system of the AHS is shown in Figure 2. The rectifier and the inverter are in the same configuartion as in Figure 1. V AC V AC ' V DC R d Rectifier I d L d R d Inverter V DC V V DC VAC V dr V di V C Fig. 2. Schematic diagram of the LCCHVDC L d represents the DC smoothing reactor as well as the line inductance. The resistor R d includes the ohmic resistances of the DC circuit. According to [2] the voltages for rectifier and inverter are defined as: V dr = BV d0r cos(α) B 3X c,r π I d (1) V di = BV d0i cos(γ) B 3X c,i π I d (2) where γ = 180 α u (3) B is the number of sixpulse bridges per converter and is equal to two for twelvepulse configuartions. X c,r and X c,i are the commutation reactances at the rectifier and inverter. α is the firing, γ the extinction and u the overlap angle. The ideal open circuit voltage V d0,r/i on the rectifier and inverter are defined as: V d0,r/i = 3 2 π V AC,r/i (4) Where V AC,r/i is the phasetophase AC voltage of the rectifier and inverter sided AC grids. Detailed analyses about the equations are presented in [3] and [1]. I Fig. 3. Voltage and current during commutation of a threepulse bridge V C is the voltage during the commutation; V DC is the resulting DC voltage; V AC is the applied voltage of the threephase system; i 1/2/3 are the currents for the respective valve and I d is the resulting DC current [4]. The overlap angle u indicates the duration where two phases simultaneously conduct current and describes the length of commutation period. According to [1] u can be determined: 2Id X c u = [cos(α) ] α (5) V AC Due to the recovery time of the thyristors and to prevent commutation failures, u has to be considered in equation 3. γ min must not fall below and has to be available after commutation [1]. The commutation has to be observed in the control system of an HVDC... I d

3 2) HVDC Control: The control of an LCCHVDC can be subdivided hierachically: The Station Control coordinates and controls the required power and relays it to the Pole Control. In this paper the required power is set manually, wherefore a Station Control is not implemented. A detailed description of the control can be found in [6] and [2]. The Pole Control determines the firing commands for the converters from the power or DC voltage setpoints. The control of the converters generates the firing pulses of the valves for rectifiers and inverters and sets the value for γmin [6]. The pole control is thus divided into a rectifier and an inverter control. a) Rectifier control: The main task of the rectifier control is to control the DC current Id in the DC circuit. Figure 4 shows the control scheme of the rectfier. Vdr Id,VDCOL Pd* Vdr ΔId Id VDCOL Id* MIN Id Id,VDCOL Vdi MIN Id,inv* CC Id* Imarg Vdi CEC ΔV ΔVdi Vdi* γ Δγ VC MIN CEA γmin* Fig. 5. Block diagram of the inverter control [2] the marginal current control concept. The marginal current Imarg = 0.1 pu is subtracted from Id. The inverter only controls the current once the rectifier is not able to keep Id constant due to voltage drop in the AC system. The operating point of the marginal current control is 0.9 pu for Id [6]. VDCOL Id,rec The extinction angle control observes whether the value of γmin is not fallen below. γ is calculated, according equation 3 and 5, during the operation of the HVDC. A drop of VAC can cause an exceeding of γmin and limits the DC voltage. 1 1 st f Fig. 4. Block diagram of the rectifier control [2] III. A NALOG H YBRID S IMULATOR The control includes a Voltage Dependent Current Order Limitation (VDCOL). At low DC voltages, the DC current and thus the DC power is limited. The VDCOL prevents the occurrence of excessive valve loads and increased reactive power consumption in the event of faults or faults in the applied AC networks [6]. The required power Pd relayed by the Station Control or set manually is divided by the filtered Vdr to determine the reference value for Id. The operating point of Id is 1 pu under stationary operating conditions. If a voltage drop occurs in the network of the inverter, Vdr also drops and a new operation point is set. A reduction in the AC voltage leads to a lowering of the DC voltage while the new operating point of Id is still at 1 p.u due to the current control [6]. The AHS consists of an analog and a digital circuit. In the analog circuit, a monopolar twelvepulse LCCHVDC is built. A part of this configuration is shown in Figure 6. b) Inverter control: The primary task of the inverter control is to control the DC voltage Vdr but it is selecting the minimum value among the outputs of constant current (CC), constant voltage (CV) and constant extinction angle (CEA) controls as illustrated in Figure 5. The reference value Vdi for the volatge control is calculated repective Figure 2 as: Vdi = Id Rd + Vdr (6) The control also includes an VDCOL with a smaller refrence value for the DC current than for the rectifier. This is characterized by the current margin. The current control at the inverter is activated in order to prevent a too strong decrease of Id and thus Pd. It uses Fig. 6. Source A and Converter A of the AHS The frequency and the AC voltage for both stations can be set with Source A and Source B. Converter A consists of thyristor valves as depicted in Figure 1. The whole HVDC system is in the configuartion as shown in Figure 2. The digital circuit is responsible for the control and firing pulse generation. The Electronic Control Unit (ECU) is the digital computing unit of the AHS. All parameters of the HVDC which are relevant for the control are relayed to the ECU by measurements in the HVDC system. Figure 7 shows the schematic structure of the AnalogHybridSimulator as a block diagram.

4 channels of the I/O board are used to generate the pulse signals for the thyristors. ECU GI2 V[k] V I[k] ADC I GI1 AnalogHybridSimulator Fig. 7. Structure of the AHS V I HVDC The measured values of the HVDC are galvanically isolated with optocouplers to enable a potentialfree measurement. The signals I d, V dr, V di, V AC,r and V AC,i are inverted by the galvanic isolation GI1 and scaled down by a fixed factor. Since the ECU is a digital computing unit, the analog signals must be discretized with an analogtodigital converter (ADC). The ADC is the DS2004 highspeed A/D board from dspace. The digital signals are then sent to the ECU. The ECU determines the firing angles for the converters according to the control concepts of Section II2. The firing pulses are then generated by a pulse generator implemented in the ECU. Before the firing pulses are passed to the inverters, a further galvanic separation occurs at GI2. GI2 is a special hardware board developed by the institute of Electrical Energy Systems to secure the ECU. Subsequently, the signal profile of the firing pulses is converted to the voltage required for firing the thyristors. The ECU itself is also a connection of different components. Figure 8 gives an overview of the ECU and its connected components. The DS1006 Processor Board provides the computing power for the realtime system and is therefore the digital computing unit of the AHS. It is possible to program the DS1006 Processor Board from Simulink via RealTime Interface and special DS2004 ADC blocks for signal measurment and DS4004 Bit Out blocks for signal generation. It provides access to the entire range of the DS4004 I/O Board and works as an interface between the I/O board and the host PC [8]. This interface is used to implement the control of the HVDC. The parameters of the HVDC of the AHS and its appropriate control system are listed in Table I and Table II. TABLE I PARAMETERS OF THE TWELVEPULSE HVDC OF THE AHS Parameter Description Value R d Resistor in intermediate circuit 10Ω L d Inductance intermediate circuit 1.161H a 1 Turn ratio transformer A 1 a 2 Turn ratio transformer B 1 R T Resistor of the transformers 0.3Ω L c Commutation inductance of the transformers H f rec Frequency at the inverter side 50Hz f inv Frequency at the rectifier side 50HZ TABLE II CONTROL PARAMTERS OF THE AHS Controler description K PI T PI DC current controler rectifier DC voltage controler inverter Marginal current controler inverter Extinction angle controler inverter HostPC DS1006 α re α inv Processor Board ECU DS4004 Digital I/O Fig. 8. Structure of the AHS U[k] I[k] DS2004 ADC The ECU consists of the DS4004 HIL Digital I/O Board and the DS1006 Processor Board from dspace. Using the appropriate software dspace ControlDesk, it is possible to access the ECU from a computer. This offers the possibility to specify the setpoints for the HVDC or to read out the various parameters of the HVDC and the control system. The DS4004 HIL Digital I/O Board provides signal conditioning and 96 bidirectional digital I/O channels, allowing each channel to be used for both input and output. In general, an interpretation of different physical quantities as current and voltage values is possible with signal conditioning [7]. The IV. COMMISSIONING AND RESULTS The AC voltages at the rectifier V AC,r and at the inverter V AC,i are decisive for the operating state of the HVDC system. In case of weak fluctuations of the AC voltages, the firing angles are adapted according to the control system, where the predetermined DC voltages and the DC current remain unaffected. Only if the AC voltages drop under a certain level, the firing angles are limited and new operation point are set. It should be noted that the parameters listed in the following are average values measured with the software dspace ControlDesk 4.2 and measuring instruments. To verify the AHS, a model of the complete system is built up in MAT LAB/Simulink. The Simulink model operates with the same parameters and control as the AHS itself. A. Steady state operation According to Section II2 the rectifier controls the current and the inverter controls the voltage under stationary operating conditions. Table III shows the paramters for this operation.

5 TABLE III PARAMETER FOR STEADY STATE OPERATION V AC,r 10V 10V V AC,i 10V 10V P d 4.1W 4.1W V dr 41V 41V V di 40V 40V I d 100mA 100mA α rect α inv out. α rec = t f rec 360 (7) The meassured firing angle is equal to the predetermined firing angle, which indicates the accordance of the AHS and digital simulation. The commutation for the valves 1,3 and 5 is shown in Figure 10. The table illustrates, that the HVDC is in steady state operation. The rectifier operates in current control and the inverter in the voltage control mode. The measured firing angles deviate slightly from the simulation. This might be caused by the signal transit time of the measurement and the dspace system. Table III is used as a reference for the other operating points. In order to verify the measured voltage, with respect to the determined firing angle, the control mode is changed to the openloop control. The firing angles are manually set to α rect = 18 and α inv = 140. For the verification of the firing angle, the relationship between firing angle and commutation shown in section II1, Figure 3 is used. Figure 9 shows the oscillograms of the AC voltages and the positve valve currents. Fig. 10. Oscillogramm of the commutation Channel 1 (yellow) is the DC voltage of the deltadelta part of the rectifier while channel 2 (blue), channel 3 (purple) and channel 4 (green) are the valve currents for the valves 1,3 and 5. The oscillogram is similar to Figure 3. It shows the commutation of the positive valves and the DCvoltage connected to the deltadelta part of the rectifier. The measurement in the oscillpgram yields with equation 7 an overlapp angle u = 10, 08. A calculation according to equation 5 with the paramters listed in Table I with I d = 90mA provides an overlapp angle u = 10, 37. The small deviation indicates a correct result. B. Operation during different scenarios Fig. 9. Oscillogram for determining the firing timing at the rectifier Channel 1 (yellow) and channel 4 (green) are the phase voltages while channel 3 (purple) and channel 2 (blue) are the respective valve currents. The intersection of the branch voltages is the transfer from one phase to the next. Subsequently, the beginning of the transition of a currentcarrying valve to the next is considered. The measurement in the oscillogram yields a time of t = 1ms. This time corresponds to the firing angle α rec. With equation 7 and f rec a conversion to α rec = 18 is carried The HVDC operation during different scenarios are enacted by a abrupt symmetrical drop in the three phases of the AC voltages at the rectifier and the inverter. 1) Current control of the rectifier: To verify the operation of the current control, the AC voltage at the inverter is reduced by the controlled voltage source at station B. Table IV shows the paramters for the new operating point. The parameters illustrate the current control mode of the rectifier. The DC current and the DC voltage ramain constant. 2) Marginal current control of the inverter: The AC voltage of the rectifier is reduced by the controlled source at Station A. Table V shows the paramters for the new operating point. The parameters illustrates the marginal current control mode of the inverter. The DC voltages have slightly decreased, whereas

6 TABLE IV PARAMETER OF THE CURRENT CONTROL MODE ON THE RECTIFIER V AC,r 10V 10 10V V AC,i 10V V P d 4.1W W V dr 41V 41V V di 40V 40V I d 100mA 100mA α rect α inv TABLE V PARAMTERS OF THE MARGINAL CURRENT CONTROL ON THE INVERTER V AC,r 10V 9V 9V V AC,i 10V 10 10V P d 4.1W 3.5W 3.5W V dr 41V 38.9V V di 40V 38V I d 100mA 90mA α rect α inv the marginal current control provides a DC current of the required 0.9 pu. 3) Extinction angle control of the inverter: To verify the operation of the extinction angle control, the AC voltage at the inverter is reduced by the controlled voltage source at station B. Table VI shows the paramters for the new operating point. TABLE VI PARAMTERS OF THE EXTINCTION ANGLE CONTROL ON THE INVERTER V AC,r 10V 10V V AC,i 10V 9V 9V P d 4.1W 4W 4W V dr 40V 40V V di 39V 39V I d 100mA 100mA γ α rect α inv and digital components and their functions as a system was introduced. A comparison between the commissioning of the AHS and the digital simulations in Simulink indicated the accordance of boths systems. The commutation was used to verify the firing angles and the overlap angle. According to the previously described specifications, the commissioning of the AHS has shown that the AHS works correctly with the introduced control concepts. The digital simulation with Simulink did not show any significant deviations compared to the realtime operation of the AHS. A correct operation of both systems can be assumed. REFERENCES [1] J. Arrillage, Y. H. Liu and N. R. Watson, Flexible Power Transmission: The HVDC Options., 1st ed. Chichester, England: Hoboken, New Jersey, U.S : John Wiley & Sons, 2007 [2] V. Crastan and D. Westermann, Electrical power supply 3: Dynamics, Control and Stability, Quality of Supply, Network Planning, Operational Planning and Management, Control and Information Technology, FACTS, HVDC, 1st ed. Germany: Springer Berlin Heidelberg, [3] E. W. Kimbark Direct Current Transmission, 1st ed. New York, U.S: John Wiley & Sons, [4] B. Orlik, P. Rose, M. Buttermann, Power Electronics and Converter Technology II, 1st ed. Bremen, Germany: University of Bremen, Institute of Electrical Drives, Power Electronics and Components, Lecture notes, 1996 [5] S. Endruschat, Generation of an electromagnetictransient HVDC simulation model for testing the system behavior and for investigating the second harmonic instability, Erlangen, Germany: University of Erlangen, Institute of Electrical Energy Systems, Diploma Thesis, 2012 [6] K. W. Kanngiesser, HVDC Systems and Their Planning, Germany: Siemens AG, 2007 [7] dspace digital signal processing and control engineering GmbH, Website, DS4004 HIL Digital I/O Board, URL: hardware introduction/i o boards/ds4004 hil digital io board.cfm, Last viewed on May 05, 2017 [8] dspace digital signal processing and control engineering GmbH, Website, DS1006 Processor Board, URL: hardware introduction/processor boards/ds1006.cfm, Last viewed on May 05, 2017 The table shows the extinction control of the inverter by the parameters. The DC voltages have decreased a little bit in order to comply γ min. V. CONCLUSION In this paper, the process of commissioning a twelvepulse LCCHVDC transmission on an AHS was shown. Firstly, the basic physical contexts of an HVDC were explained. Subsequently, the control concepts of the rectifier and inverter control were presented. These concepts served as the basis for the implementation of a digital control model. In order to understand the functioning of the AHS, an overview of the overall system was presented. The interaction of the analog