P1567 – December 2013
Implementation of the model of a solar inverter Revision D
Created for
Austrian Institute of Technology GmbH
DIgSILENT GmbH Heinrich-Hertz-Strasse 9 D-72810 Gomaringen Tel.: +49 7072 9168 - 0 Fax: +49 7072 9168- 88 http://www.digsilent.de E-mail:
[email protected]
Contact:
Manuel Castillo Tel.: +49-7072-9168-61 E-mail:
[email protected]
Rev.
Comments
Created
Date
Review
Date
A
Peter Lilje
August 23, 2012
Sabine Seeger
24.08.2012
B
Peter Lilje
September 3, 2012
Slight improvements
Peter Lilje
September 26, 2012
Description of “Qcontrol” and “Pf_Reduction”
C D
M. Castillo
December 12, 2013
Version 2.1
1 Introduction ......................................................................................................................................... 4 2 Model description................................................................................................................................. 4 2.1 Model of the "Converter" plug-in unit ....................................................................................................... 6 2.2 Model of the "Curr_Ctrl" plug-in unit ........................................................................................................7 2.3 Model of the "Udc_Control" plug-in unit ................................................................................................. 10 2.4 Model of the "Power-Freq Reduction" plug-in unit ................................................................................... 12 2.5 Model of the "Protection" plug-in unit..................................................................................................... 13 2.6 Model of the "Q Control Mode" plug-in unit............................................................................................. 13 2.7 Model of the plug-in units of measuring instruments ............................................................................... 14 3 Test simulations ................................................................................................................................. 15 3.1 Test 0: Reduction of available power ..................................................................................................... 15 3.2 Test 1: Symmetric short circuit at 0% voltage (MV) ................................................................................. 15 3.3 Test 2: Symmetric short circuit at 30% voltage (LV) ................................................................................ 15 3.4 Test 3: Symmetric short circuit at 77% voltage (LV) ................................................................................ 15 3.5 Test 4: Asymmetric short circuit at 53% voltage (positive-sequence system, LV) ....................................... 15 3.6 Test 5: Asymmetric short circuit at 65% voltage (positive-sequence system, LV) ....................................... 16 3.7 Test 6: Asymmetric short circuit at 88% voltage (positive-sequence system, LV) ....................................... 16 3.8 Test 7:Change of the prescribed reactive power...................................................................................... 16 3.9 Test 8: Change of the prescribed active power ....................................................................................... 16 3.10 Test 9: Change of the mains frequency ................................................................................................ 16 3.11 Test 10: Rise-in-voltage protection....................................................................................................... 16 4 Instruction for using the model ........................................................................................................ 17 5 Sources ............................................................................................................................................... 18
R e v. D
3
The Austrian Institute of Technology GmbH has contracted DIgSILENT GmbH to produce a model of a 680 kVA solar inverter of the Schneider Electric Company. The model must be appropriate for simulating symmetric and asymmetric errors. The active power (and consequently the active current) is generated by radiation on the PV cells. The DC-link voltage is set to the optimum operating point and regulated by changed in the active power. The inv erter controller sets the reactive current based on the current and prescribed reactive power. In the case of an error (symmetric or asymmetric), current-supporting reactive current is fed in. This additional reactive current is calculated dependent on the current. There is no function for limiting the current to phases, which are not affected by an error. The model has been developed using the standard software PowerFactory Version 14.1. The inverter h as been designed based on a "static generator", which behaves like a power source in the simulation in the time range (RMS). The controller has been developed in the dynamic simulation language DSL (DIgSILENT Simulation Language) integrated in PowerFactory. The model is contained in the PowerFactory file "Schneider_Conext_XC_680kVA_v2_1.pfd". An encrypted version of the model is contained in "Schneider_Conext_XC_680kVA_v2_1_enc.pfd". It contains the control of the solar inverter for normal operation and for error cases. This report describes the model and th e result of a few test simulations.
Figure 1 shows the model of the solar inverter incl. connection to an external transformer and an external electric power system. The results of a load flow calculation are shown as an example. Typical transformer parameters were used. Figure 2 shows the wiring diagram, i.e., the connections of the inverter model, control models and measuring instrument models as they are used for dynamic tests. The blocks are called "plug-in units" and the lines "signals" in PowerFactory. The signals id_ref and iq_ref create the current target values of the static generator model in a dq reference system. The dq reference system is defined by the current angle measurement (block "PLL"), so that current in steady state in the d axis is active current and current in th e q axis is reactive current. These target values are calculated in a controller model, which is contained in the plug-in unit "Curr_Ctrl". A separate model, which is contained in th e plug-in unit "Q Control Mode", calculated the required reactive power for normal operation. This is either the r eactive power Q_user prescribed by the user or the reactive power, which results from a prescribed power factor and the current active power. The controller models require several measuring instruments, which are contained in the plug-in units "PQ_Meas", "Ulv_Meas" and "VT_prot". The plug-in unit "Protection" is designed for a model of the protection relay. This model also requires measurement of the mains frequency, which is measured in "PLL_prot". The "Power-Freq Reduction" plug-in unit is designed for a model, which reduces the active power at excessive mains frequency. The models are described below, which are assigned to the individual plug-in units.
R e v. D
4
Figure 1: Solar inverter and connection to an external electric power supply
R e v. D
5
T N E L I S g I
Control_Frame:
D
Spare
P
0
0
1
id_ref
0
0
PQ_Meas StaPq* 1
0
Power-Freq Reduction ElmPf_* 1
P_f
iqref
2 3
1
4
2
iq_ref
1
2
Ulv_Meas StaVmea*
Converter ElmGenstat* 0
Q_user
0
Pfactor_user
1
Spare
1
Qset Q Control Mode ElmQco*
5
2
Curr_Ctrl 3 ElmCur*
Spare
4
Spare
Puser 2
6
P_dem
0 0
7
1
idref
1Udc_Control 1
0
PLL ElmPhi*
cosref sinref
2
3
8
ElmUdc* Hold 3
2
PLL_prot ElmPhi*
f
0
Protection ElmPro* 1
VT_prot StaVt*
9
Uar;Uai;Ubr;Ubi;Ucr;Uci
Figure 2: Wiring diagram
The inverter has been designed based on the static generator model, which has been configured as power source: cf. Table 1. The power diagram, which is shown in Figure 3, has been saved in the static generator model. The power diagram refers to cu rrent limitation of 100% (i.e., 1.03 kA) and an apparent power limitation of 100%. The reactive power limitation is considered in the load flow calculation (if th e respective calculation option is set accordingly).
Table 1: Solar inverter model Input parameters
Value
Nominal apparent power
680 kVA
Active power
Depending on the operating point
Reactive power
Depending on the operating point
Capability curve
Reference to "ConexctXC_680kVA"
R e v. D
6
1.2
1
. u . p 0.8 , r e w 0.6 o p e v i 0.4 t c A 0.2
0 -0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
0.2
0.3
0.4
0.5
Reactive power, p.u.
The controller, which calculated the current target v alues, has been implemented using the PowerFactory simulation language DSL. Figure 3 displays a diagram of the controller type "Current_Control". The input "u" shows the measured voltage in the positive-sequence system. "Quser" is the required reactive power in normal operation. "Qcap" is not used. "P_dem" is the active power required by the voltage regulator of the DC-link voltage. The active power is limited due to excessive frequency by "P_f". An additional DSL model can be connected to "Puser" to show an active power limitation by the public utility. The outputs idr and iqr are the current target values in p.u., which are connected with the inverter model in the wiring diagram. The block "Fault" detects an error if the voltage falls below "Umin" or increases above "Umax". The block "Fault" calculates additional reactive current based on the K factor, which must be fed in ca se of an error. The required reactive power "Quser" is transmitted as "Qcorr" to the block "Qlim" in normal operation. The reactive current of the previous error is to be considered in case of an error after TC2007. Consequently in case of an error, the signal "Qcorr" receives either the value Qpv*u/Umin or Qpv*u/Umax, depending on whether the voltage is below Umin or above Umax. The block "Qlim" limits the reactive power "Qreq", so that the maximum apparent power "Smax" and the maximum "Imax" or "Imax_unbal" are not exceeded. The maximum apparent power depends on the voltage. This dependency is given in Table 3, in which Smax_x shows the voltage and Smax_y the apparent power limitation. If there are symmetric errors, the reactive power is limited to "Imax", and it is limited to "Imax_unbal" in the case of asymmetric errors. Errors are detected as asymmetric if the difference between maximum phase-to-phase voltage and minimum phase-to-phase voltage is more than 20%.
R e v. D
7
At substantial power dips, phase measurement is blocked. You can find additional information about this blocking in Chapter 2.7. The parameters and typical settings are listed in Table 2. These parameters are entered in the general model "Curr_Control", which refers to "Current_Control". The block "Plim" limits the active power to the lowest of the following signals: "P_dem", "P_f", "Puser" and √(Smax*Smax-Qref*Qref). The block PQ_2_idq calculates the outputs "iqr" and "idr". The signal "iqr" follows the signal Qref/u with PT-1 behavior (time constant "Tcc"), which displays the dynamics of the inverter simplified.
T N E L I S g I
Current_Control:
D
0
-
Qpv
1
0
Qcorr
Quser u
2
Smax
Calc_Smax array_Smax
p a c Q
um
1/(sT+1) Tm
Qreq
Quser_corr Umin,Umax
0
0
Uar Uai Ubr
5
Ubi
6
Ucr
7
Uci
8
0 1
0
2 umin_umax 3Vnom,Tm 4
1
minU
maxU
0
0
f e r Q
1
4
iqr
Qlim Qmax,Imax,Imax_unbal
1
Q d
3
0
0
1
Fault Umin,Umax,Dip,Swell,Tpick,Tdro,K1
PQ_2_idq Tcc,Imax,Tdro,Ram.. 1
Flt
2
2
5
0
1
0
Pr
9 10 11
P_dem P_f Puser
1
3
1
idr
1
Plim Imax
2 3
Figure 3: Controller type "Current_Control"
R e v. D
8
Table 2: Parameter of the general model "Curr_Control”, Part 1
Table 3: Parameter of the general model "Curr_Control”, Part 2
R e v. D
9
Table 4: Parameter of the general model "Curr_Control”, Part 3
The model "Udc_Control" is shown in Figure 4. It calculates the active power "P_PV" from the available power "Psun" and the DC voltage "udc" based on a typical characteristic of a PV panel. The available power "Psun" is initialized based on the load flow calculation, and it is assumed that the inverter is operating at the optimum operating point at the time. The behavior of a maximum power point tracker is not modeled. The same powervoltage characteristic is used for different active powers, and the DC voltage is always initialized and controlled at the value 1.0 p.u. The difference between the power "P_PV" (in p.u.) fed into the DC link and the AC output power "Pout" (in p.u.) is used to calculate the charging or discharging of the DC link capacity. The inverter losses are not considered. The deviation in the DC voltage from the target value "udc_ref" is used in a PI controller to calculate the target active current. The target active power "P_dem" is calculated from this active power based on the the AC voltage. The signal "Hold" can be used to stop the integrator of the PI controller, but it is not used in this model. The target active power is not actually determined by a simple PI controller, but instead in a complicated controller that calculates the saved energy of the capacitor. This controller is not used in this model. Because the inverter has been designed as power source, the dependency between DC voltage and AC voltage is not considered at any rate, which means in the end that the design of the DC link is of little importance in this model.
R e v. D
10
T N E L I S g I
Udc_Control:
0
D
udc_ref
MaxI 1.2
1
Psun
0
PV array_PV
P_PV
dP
-
1
Cap C,Pn,Udcn
udc
-
dUdc
0
Pout 2
{K+1/sT} Kd,Td
Id_dem
P_dem
Hold 3
4
1
u
1/(1+sT) Tm
um
Figure 4: Controller type "Udc_Control" The parameters of the general model, which refer to the controller type "Udc_Control", are listed in Table 5 and Table 6.
Table 5: Parameter of the general model "Udc_Control”, Part 1
R e v. D
11
Table 6: Parameter of the general model "Udc_Control”, Part 2
The "PFC_PowerReduction" model calculates the limiting power "P_f", which takes effect if there is overfrequency. The power is limited if the frequency increases above "f_enable". The maximum power then drops from the initial power with th e slope "Pf_slope" (p.u. Initial power/s). If the frequency falls below "f_reset", the limitation is canceled. Table 7: Parameter of the general model "Power(f)Control"
R e v. D
12
The protection relay model disconnects the inverter from the mains if there is overvoltage, undervoltage, overfrequency or underfrequency. Table 8 lists the typical setting values of the protection relay model. Table 8: Parameter of the general model "Protection"
This model transmits the target reactive power to the "Curr_Control" model. Table 9 lists the typical settings of the model. If the controller mode on "Qcontrol_mode" is set to 0, the reactive power is set to the signal "Q_user". If the controller mode on "Qcontrol_mode" is set to 1, the target reactive power is calculated based on the "Pfactor_user" and the active power delayed with the PT-1 element (time constant Tpw). The signals "Q_user" and "Pfactor_user" are initialized based on the load flow calculation, but can be changed either with a user-defined event of by a system controller (not included).
R e v. D
13
Table 9: Parameter of the general model "Q control"
A total of five PowerFactory measuring instruments are required for the solar inverter model. The voltage measurement "Ulv_measurement" calculates the positive-sequence voltage at the 380 V terminal. The voltage transformer "Protection VT" is in the field, which connects the static generator with terminal "LV". It transmits the phase-to-phase voltage to the protection relay "Protection". The power measuring instrument "PQ_Measurement" measures the power of the inverter in p.u. with respect to 680 kVA (nominal power of th e static generator). The PLL (phase-locked loop) "PhaseMeas" calculates the phase of the voltage at terminal "LV" and consequently defines the dq reference system. The PLL parameters Kp and Ki have been set in such a way that the dynamics of PLL approximate the dynamics of the MATLAB model provided. If the voltage falls below "umin_PLL" (a parameter of the "Curr_Control"), the phase measurement is stopped. When voltage increases again, phase measurement is restarted. The PLL "FreqMeas" calculates the main frequency, which is used in "Protection" and "Power(f)Control".
R e v. D
14
The PowerFactory file contains examples of several test simulations. The test simulations and their results are described below.
Sudden reduction of available power was simulated, in that the signal "Psun" of the "Udc_Control" was set from 0.90 p.u. to 0.40 p.u. with an event. The simulation results show that the active power dips for a short time to 0.39 p.u., but then stabilizes at a value of 0.40 p.u. The reactive power changes for a short time, but less than 2%. It levels off again within approx. 100 ms.
A symmetric short circuit on the busbar "MV" lasting 150 ms and a resulting residual voltage of 0% on "MV" was simulated. At voltage dip, the reactive current fed in increases to 100% after approx. 20 ms. Because the voltage falls below "umin_PLL", the phase measurement is blocked, which is also reported in the output window. After the error has been eliminated, phase measurement is restarted.
A symmetric short circuit on the busbar "MV" lasting 250 ms and a resulting residual voltage of approx. 30% on "LV" was simulated. The reactive power before the error was 200 kVar inductive (underexcited) and th e reactive current before the error approx. 29%. The reactive current increases to approx. 92% during the error, which fits to the expected reactive current. A manual calculation results in -29% + 2*(90%-30%) = 91%. Because the voltage falls below "umin_PLL", the phase measurement is blocked.
A symmetric short circuit on the busbar "MV" lasting 1000 ms and a resulting residual voltage of approx. 77% on "LV" was simulated. The reactive power before the error was 150 kVar capacitive (overexcited) and the reactive current before the error approx. 22%. The reactive current increases to approx. 51% during the error, which fits to the expected reactive current. A manual calculation results in 22% + 2*(90%-77%) = 48%. The ph ase measurement is not blocked in this case.
An asymmetric short circuit on the busbar "MV" lasting 150 ms and a resulting positive-sequence voltage of approx. 53% on "LV" was simulated. The r eactive power before the error was 0 kVar. The reactive current
R e v. D
15
increases to approx. 40% during the error, which fits to the expected reactive current. A manual calculation results in 0% + 2*(90%-53%) = 74%, but the current is limited to 40%. The phase measurement is not blocked in this case.
An asymmetric short circuit on the busbar "MV" lasting 250 ms and a resulting positive-sequence voltage of approx. 65% on "LV" was simulated. The r eactive power before the error was 200 kVar inductive (underexcited) and the reactive current before the error approx. -29%. The reactive current increases to approx. 18% during the error, which fits to the expected reactive current. A manual calculation results in -29% + 2*(90%-65%) = 21%. The phase measurement is not blocked in this case.
An asymmetric short circuit on the busbar "MV" lasting 1000 ms and a resulting positive-sequence voltage of approx. 88% on "LV" was simulated. The r eactive power before the error was 0 kVar. The reactive current increases to approx. 4% during the error, which fits to the expected reactive current. A manual calculation results in 0% + 2*(90%-88%) = 4%. The phase measurement is not blocked in thi s case.
In this simulation, the prescribed reactive power, i.e., the reactive power target value, was changed from 0% to 50%, to 50% and finally to 20%. The simulation result shows that the reactive power follows the target value, but that the reactive power is limited between -40% and 40%. With active power of 400 kW, these limits correspond to the power diagram in Figure 2.
In this simulation, the prescribed active power, i.e., the active power target value, was cha nged from 1 to 0.9 (capacitive), to 1, to 0.9 (inductive) and finally to 0.7 (capacitive). The simulation result shows that the active power follows the target value, but that the reactive power is limited to 40% after the last event, which corresponds to the limit in the power diagram in Figure 2.
The mains frequency was reduced in this simulation. The inertia constant of mains is set temporarily to a small value, and a load is connected that results in the frequency decreasing. The inverter control reacts to the decreasing frequency starting from 50.2 Hz, and the active po wer is reduced.
The mains voltage was increased in this simulation, which resulted in switching off the inverter.
R e v. D
16
The model can be used for simulating normal operation and mains errors (symmetric and asymmetric voltage dips and power reduction at increased mains frequency). In normal operation, the model either sets the required reactive power (signal "Q_user" of Q Control) or it sets the reactive power based on the required power factor (signal "Pfactor_user" of Q Control) depending on which controller mode is set. An external system controller (not included) can be connected with the inverter model and consequently change "Q_user" or "Pfactor_user".
1.
Import the project "Schneider_Conext_XC_680kVA_vx.x_enc" into PowerFactory v14.1.6 (or a newer version). The letters x.x display the newest version of the model.
2.
Copy the Library\ Schneider_Conext_XC folder and insert it into the library directory of your own project.
3.
Move the "Schneider_Conext_XC_680kVA" template into the template folder of your project.
4.
Move the object "ConexctXC_680kVA" into the Operating data library\power diagrams folder.
5.
Enter a new inverter model into your project by using the template ("General templates" button).
6.
Edit the Curr_Control model, and change the K factor in line with your needs.
7.
Edit the "Q Control" model, and select (after consultation with the manufacturer) either r eactive power default or power factor default.
8.
Edit the "Solar inverter" model, and enter the operating point on the load flow page (e.g., active power and reactive power).
9.
Calculate the load flow with consideration of the reactive power limits, and ensure that there are not any warnings or error messages s in the output window.
10. Calculate the initial conditions using the options "Simulation methods: RMS", mains model "Symmetric" or "Asymmetric" (depending on which error is to be calculated) and a simulation increment of 1 ms (an increment of 0.1 ms can be used if there are convergence problems). 11. As customary,define the variables as well as the events that are to be recorded. 12. Carry out the simulation, and define the diagrams to display the results.
R e v. D
17
[1] Schneider electric, information concerning inverter control "Questionnaire FGW TR4_rev_B.docx" [2] Schneider Electric, manufacturer certificate for specific data of the photovoltaic inverter, “Manufacturer's Declaration BDEW XC680 rev B.pdf” [3] E-mail exchange with Schneiderelectric incl. MATLAB models, June-August 2012
R e v. D
18
