8      E-TRAN PSCAD Library

All E-TRAN components ask for data in a Per Unit system that is consistent with PSS/E (regardless of whether the model is used in a single phase or three phase application).  The standard data entry system is as follows:

-        Rated Voltages are Line to Line, kV, RMS

-        Voltage Initial Conditions in Per Unit

-        Power Flow is three phase MW and MVAR

-        MVA Rating is 3 phase

-        Frequency in Hz

-        RXB is entered in PU on 100 MVA base (note RXB for sources/machines are entered on the MVA base).

-        Angles in Degrees

Each E-TRAN model (and most PSCAD models) has a NAME field.  This field is used by the Substitution Library to determine the from bus, to bus and circuit number (see Chapter 6, Substitution Library for more information).

The E-TRAN load model is shown to the right.

The E-TRAN load model supports the following load types:

-   Constant PQ

-   Constant Current

-   Constant Impedance

-   Constant RLC

The model interfaces to a variable RLC branch, and the RLC parameters are switched (once a cycle) to match the given load characteristic. 

The load model measures the +ve sequence fundamental frequency voltage (see Measurements in Chapter 8.9) and then varies the RLC load to match the selected characteristic.  The measured voltage has a minimum limit of 0.8 PU and a maximum limit of 1.25 PU, which determines the state of the load during faults and overvoltages. 

The text shown on the model (100 +j25 above) is the power flow at the rated voltage set point (see more on this below).

The dialog below appears when you edit the parameters of this model:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.

The PQ load values are entered for the rated voltage (1pu) operating point.  The actual load PQ flow will vary according to the loadflow and the load type selected.

The Initial Load Voltage parameter is the load voltage from a solved loadflow (read from the bus data of the PSS/E .raw file).  This is used to set the initial RLC values of the load.  The initial load RLC values are held for 0.2 seconds to give the EMTDC simulation time to reach steady state.

2. The E-TRAN load model will represent the selected load type as a variable RLC branch whose RLC parameters are switched (once a cycle) to match the given load characteristic. 

3. The Hold Signal parameter (this can be a variable) can be used to hold the RLC values at their current levels:

-        If a “1” is entered, the dynamic load model will change the RLC values once a cycle to match the selected load characteristic.

-        If a “0” is entered, the load model will hold its RLC values at the previous level (or at the initial condition based on the Initial Load Voltage).

-        If a variable name is entered (such as “Hold”) then the user can create a signal which dynamically changes from 0 to 1 etc…  This is useful if you want to prevent the RLC values from changing during a fault.

4. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

5. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).  Note the load parameters always ask for 3 phase PQ and the line to line voltage, even if the device is operating in a single phase circuit (see Chapter 1.18.2).

The next page of the data entry (Outputs) lets the user measure the actual PQ flow into the device as well as the instantaneous current.  To monitor these quantities:

-        Enter a unique name at the desired parameter (such as “Pmeas_Load1”)

-        Go back to your PSCAD circuit and place a “datalabel” (with the same name as above)

-        Connect the datalabel to a “channel” component

-        Right click on the channel component to plot this signal

Click “OK” to save your changes and exit to PSCAD or click “Cancel” to exit without saving your changes.

The E-TRAN generator/source model is shown to the right.

This model is a fixed frequency voltage source (either an infinite bus or a source behind impedance).  This model does not represent the changing frequency or exciter characteristics of a machine or generator (use the standard PSCAD Master Library models for this).

This model has a convenient option, which makes it easy to initialize.  The user can enter the terminal voltage (magnitude and phase) and the PQ flow into the source.  This model can then internally computes the “E behind Z” (note the standard PSCAD Master Library sources always require entry of the “E behind Z” directly and you must perform the complex arithmetic yourself).

The dialog below appears when you edit the parameters of this model:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.

2. The Generator Impedance Type can be set to:

- Infinite Bus, in which case the source voltage is directly fed into the EMTDC solution and will directly appear as the measured voltage on the bus.  This is equivalent to a source impedance of zero.

- Series RL, in which case the source voltage source will contain an impedance, with an associated voltage drop.

Note:  Data for this generator model is often translated from generator data of a loadflow program.  The ZR, ZX ZSORCE parameters are often not required nor used in the loadflow solution, but they do play a critical role in short circuit and transient studies.  If the ZR, ZX generator data is not available, then you should consider modifying your loadflow program data files before translating the circuit using E-TRAN.  See Chapter 9.1 for more information.

3. If a source impedance is entered in #2 above, then the user can enter the voltage at the bus terminals (magnitude and phase) and the PQ flow into the source, and the model will internally compute the “E behind Z” so as to give the desired terminal conditions based on the loadflow.  Alternatively the user can directly enter the “E behind Z” magnitude and phase.

4. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

5. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).  Note the load parameters always ask for 3 phase PQ and the line to line voltage, even if the device is operating in a single phase circuit (see Chapter 8.2).

The next page of the data entry (Initial Conditions and Outputs) is where the initial source voltage (or terminal voltage) and initial loadflow PQ flow are entered, and where you can measure the instantaneous current out of the source.  To monitor the source current:

-        Enter a unique name at the desired parameter (such as “Imeas_Gen1”)

-        Go back to your PSCAD circuit and place a “datalabel” (with the same name as above)

-        Connect the datalabel to a “channel” component

-        Right click on the channel component to plot this signal

Click “OK” to save your changes and exit to PSCAD or click “Cancel” to exit without saving your changes.

The E-TRAN PI section component is used for very short lines (in V3 and V4).

When you have a longer transmission line (whose travel time is longer than one time step), then you can use the Bergeron traveling wave line model (consult the PSCAD  help system).

The E-TRAN PI Section Component.

The PSCAD V3 Bergeron Line Model (using the Line Constants Interface)

The PSCAD V4 Bergeron Line Model (using the Single Line Diagram Line Constants Interface).

The E-TRAN PI section component dialog is shown below:

1. Refer to Chapter 8.2) for an explanation on the system of units for data entry.

The RXB data entry should already be “long line corrected” and is the same data as required in the PSS/E .raw data format.

2. The line length is used only for display, since the RXB values are entered in PU for the entire length of line.

3. The branch can be made to be Ideal, in which case there is a direct short between the “from” and “to” busses.  If the absolute value of the PSS/E reactance is less than 0.0001 PU, PSS/E internally treats the branch as an ideal branch (to improve convergence properties).  E-TRAN also will make the device Ideal if the absolute value of the PSS/E .raw branch reactance is less than 0.0001 PU.

4. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

5. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).  Note the load parameters always ask for 3 phase PQ and the line to line voltage, even if the device is operating in a single phase circuit (see Chapter 8.2).

Single Phase PI sections are represented as a series RL impedance (from the sending end to the receiving end) as well as capacitors to ground at each end of the line.

Three phase transmission lines are represented with a three phase mutually coupled transformer.  This method gives the correct +ve and zero sequence impedances of the line, yet does not require a separate “zero sequence network” and associated neutral connections at each end of the line.  The “separate zero sequence network” method is an old way of representing pi lines on analog simulators (as it can be realized with physical connections) and should be avoided.

The next page of the data entry (Shunt Admittances) allows you to enter shunt admittances at either end of the line.  These are represented by constant RLC branches to ground (as per the PSS/E .raw format standard).  When the transmission line is disabled, the shunts at each end of the line are also disabled.

Click “OK” to save your changes and exit to PSCAD or click “Cancel” to exit without saving your changes.

The E-TRAN transformer and phase shifter component is shown to the right.

The transformer and phase shifter are modeled as a mutually coupled branch element in PSCAD/EMTDC.  The three phase transformer is comprised of three single phase units (more detailed models are possible in the PSCAD Master Library).

This component assumes a wye-wye grounded configuration, which matches the assumptions in loadflow programs.  Consult Chapter 6 for more information on how to substitute more detailed transformer models.

The dialog below appears when you edit the parameters of this model:

1. Refer to Chapter 8.2) for an explanation on the system of units for data entry.

2. The question “Use Ideal Model” allows the user to represent the linear magnetizing reactance in the circuit or to assume it is 0.0.  The PSS/E .raw format assumes an Ideal Model (with magnetizing current = 0.0).  If you are planning to represent saturation (next parameter page) then you should use the Ideal Model to avoid representing the linear magnetizing branch twice (once in the circuit and a second time in the saturation model).

3. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

4. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).

Three phase transformers are represented with a three singe phase mutually coupled transformers.  This representation is valid if the actual transformers are built from single phase units, and will be reasonable accurate for 5 limb core transformers.  If the transformer is a 3 limb core transformer, then the UMEC transformer model in the PSCAD Master Library should be used (this model requires more detailed data, such as the core geometry).

5. Shunt Admittances can be entered at each end of the transformer to be consistent with the PSS/E .raw format.  These are represented by constant RLC branches to ground.  When the device is disabled, the shunts at each end are also disabled.

The second page of transformer/phase shifter parameter data is shown below:

1. A Tap Changer Winding can be specified.

2. This device can also be a phase shifter (the model graphics will change accordingly).

3. Enter the tap setting in PU.  The Tap Setting can be a constant or a variable.  If you have a 230/115 kV transformer (where winding 1 is 230 kV), and you enter a tap of 0.9 PU on winding 2, this is effectively a 230/(115*0.9) kV transformer.

4. Enter the phase shift angle in Degrees.  The phase shift is achieved with a near-quadrature voltage source connected in series with the transformer and shunt current injections.  The voltage and current sources are dynamically calculated to give the correct phase shift angle based on the measured bus conditions and frequency.

5. The Impedance Correction Factor is used as a multiplier to the series leakage reactance.

E-TRAN computes the correction factor based on the tap (or phase angle) and the appropriate Impedance Correction Table # that is available in the PSS/E .raw input file.  If the tap setting or phase angle is changed in PSCAD (i.e. not in the PSS/E .raw file and via E-TRAN), then you will have to manually consult the Impedance Correction Table, derive the Impedance Correction Factor, and re-enter it in this component.

Currently the Impedance Correction Table # is not used, but is transferred from the PSS/E .raw file for convenience.  Future versions of PSCAD may allow the Impedance Correction Table to be entered directly into the Tap Changer subroutine, in which case the Impedance Correction Factor can be automatically updated to give the effect of the leakage reactance changing as a function of a phase shift or tap change.

The third page of transformer/phase shifter parameter data is shown below:

1. Saturation is represented in the PSCAD transformer models by use of the TSAT21 subroutine (consult the PSCAD User’s Manual for more information on the theory of saturation).

The last page of the data entry (Monitoring of Currents and Fluxes) lets the user measure the instantaneous winding currents, magnetizing current and flux.  To monitor these quantities:

-        Enter a unique name at the desired parameter (such as “Imeas_T1_W1”)

-        Go back to your PSCAD circuit and place a “datalabel” (with the same name as above)

-        Connect the datalabel to a “channel” component

-        Right click on the channel component to plot this signal

If you are representing a 3 phase transformer (either 3 electrical connections in PSCAD V3 or a SLD model in V4) then the name you enter will create an array of 3 real signals (one for each phase).  When you create a datalabel connected to a channel, use the () syntax to extract phase quantity (i.e. use Imeas_T1_W1(2) to extract the measured current for phase B of the appropriate winding).

Click “OK” to save your changes and exit to PSCAD or click “Cancel” to exit without saving your changes.

The E-TRAN Switched Shunt model is shown at the right.

The model interfaces to the electrical network as a constant RLC branch.  The model does not represent the switching actions of adding/removing various stages of the switched shunt nor does it represent a reactive power controller.

The initial PQ values are taken from the loadflow file based on the number of device stages currently in operation at the loadflow set point.

The text shown on the model (100 +j25 above) is the power flow at the rated voltage set point.

.

The dialog below appears when you edit the parameters of this model:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.

2. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

3. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).  Note the parameters always ask for 3 phase PQ and the line to line voltage, even if the device is operating in a single phase circuit (see Chapter 8.2).

The next page of the data entry (Outputs) lets the user measure the actual PQ flow into the device as well as the instantaneous current.  To monitor these quantities:

-        Enter a unique name at the desired parameter (such as “Pmeas_SwSh1”)

-        Go back to your PSCAD circuit and place a “datalabel” (with the same name as above)

-        Connect the datalabel to a “channel” component

-        Right click on the channel component to plot this signal

Click “OK” to save your changes and exit to PSCAD or click “Cancel” to exit without saving your changes.

The E-TRAN DC Link model is shown at the right.

This model is a simple version of a DC link based on the steady state phasor equations of a DC link (also what is used in loadflow programs), which is:

Where:

-        VdcR   = DC Voltage across 1 six pulse group (kV)

-        EvR     = Converter Transformer Secondary (i.e. Valve Side) Rated Voltage (kV, L-L, RMS).

-        aR       = Converter Firing Angle

-        XcR     = Converter Transformer Leakage Reactance (converted to ohms on the valve side)

-        IdcR    = DC Current (kA)

The Rated DC Voltage should be divided by the total number of six pulse valve groups before this equation is applied.

A more detailed representation of a DC link is possible in PSCAD/EMTDC by using commutation-based models (such as the G6P200 valve group model in the PSCAD Master Library) or by constructing the valve group from individual thyristors. 

The commutation models will be more accurate, but require more expertise in the design and implementation of the firing controls etc…  If you are primarily interested in getting the correct fundamental frequency power flow and control response (or if you do not have any more detailed information available), then the E-TRAN DC Link model is suitable.

This simple DC link model interfaces to the ac electrical network as a voltage source at each end of the DC link.  The model internally computes and outputs the DC Voltage and DC Current (the current is computed via an integration of the voltage across the DC line using the DC Resistance and the total DC inductance (Line inductance plus the Smoothing Reactors at each end)).

The model has internal DC controls (described below) but can accept firing angles from external DC controls.

The dialog below appears when you edit the parameters of this model:

1. The Rated DC Voltage should be entered for all six pulse groups (i.e. it is the DC Line Rated Voltage).  For example:

-        If this is a 500 kV monopolar DC link with 2 six pulse valve groups (one star and one delta), then enter 500 kV (and enter 2 for the Number of Valve Groups in the next parameter section).

-        If this is a +/- 500 kV bipolar DC link with 2 six pulse valve groups in each pole, then enter 1000 kV (and enter 4 for the Number of Valve Groups in the next parameter section).

-        A twelve pulse valve group should be considered as 2 six pulse valve groups (one star and one delta).

The DC Line Resistance and Inductance should be entered as the equivalent value for the normal mode of operation.  If you have a bipolar DC Link, enter the DC Resistance and Inductance of a single pole.

2. Run Control: Enter 1 to always run the DC link (0 to Block).  Typically this is a variable name (such as “RunDC”), which lets you construct a 0,1 signal with any standard PSCAD component (perhaps the manual switch).  You should then connect a datalabel (to the output of the component) with the same variable name entered.

3. The Direction of Power can be entered as an integer (i.e. 1 or –1), or it can be a variable name.  A value of 1 indicates a +ve DC Power Flow from Converter 1 to Converter 2 (the converter numbers 1 and 2 are shown on the model).

4. The ratings can be specified at the Rectifier or the Inverter.  The DC Voltage can change (due to the line drop due to the DC resistance) so the ratings must be specified at one end (usually the Rectifier).

5. Use Fast Initialization is a feature to force the DC link to reach steady state sooner.  It will calculate the firing angles at each converter in order and initialize the DC link controls. 

If the Fast Initialization feature is not used, then each converter will have its controls initialized to the “Initial Firing Angle After Deblock” value (on the Rectifier or Inverter Control Parameters page), usually 90 degrees.  The normal control gains and time constants will then be used to reach an operating point (in much the same way as a recovery from a fault or block).

6. Use Internal or External Controls – This DC link model has an internal built-in control system (see the Rectifier or Inverter Control Parameters page) which can be used, or the user can model the controls with their own models and then pass the firing angle to this model.

A third option (Infinite Bus at Each AC Terminal) will result in an ideal voltage source (i.e. 0.0 resistance) at each converter.  The voltage source magnitude and phase are taken directly from the loadflow file and entered in the “Converter 1(2) Data” parameter page.

This feature is useful if a DC link does not start up properly and you would like to debug the case to see if the problem is due to the DC control system or if the problem is elsewhere in the network.

7. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (PSCAD V4 only) or a Single Phase circuit (V3 or V4).

8. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

9. Input Alpha in Degrees or Radians – This parameter is only used if you select the “Use External Controls” option above.  Many of the standard PSCAD Master Library HVDC Control system models output radians.

The next 2 parameter pages (for Converter 1 and 2) appear as follows:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.

2. The Number of Series Valve Groups parameter should be the total number of six pulse valve groups.  For example:

-        If this is a 500 kV monopolar DC link with 2 six pulse valve group (one star and one delta six pulse group), then enter 2 for the Number of Valve Groups (and 500 kV for the Rated DC Voltage in the previous parameter section).

-        If this is a +/- 500 kV bipolar DC link with 2 six pulse valve group in each pole, then enter 4 for the Number of Valve Groups (and enter 1000 for the Rated DC Voltage in the previous parameter section).

-        A twelve pulse valve group should be considered as 2 six pulse valve groups (one star and one delta).

3. The Transformer Leakage Reactance is entered in PU on the Transformer Base MVA (which is a three phase MVA rating).  These parameters are used to convert the leakage reactance in PU into Henries.

4. The Tap Setting (pu) is based on the Valve Side Voltage / Primary Side Voltage.  A Tap Setting greater than 1.0 will result in a larger voltage applied on the DC side of the valve group.

5. The DC Voltage and DC Current Transducer Time Constants are used in a simple measurement of the DC quantities.  The transducer response is implemented as a simple first order lag (real pole).

6. The Smoothing Reactor value should be entered at each end (enter 0.0 if this scheme does not have a smoothing reactor).

7. The Initial AC Bus Volts and Angle are used in the Fast Initialization feature and when each converter is represented as an Infinite Bus (see the previous parameter page).

The Master Power Controller and VDCL Control Parameters are:

1. The Master Controls can either control the DC Power or the DC Current.

2. The Master Controller Set-Point can be either a power order (pu) or a current order set point (pu), depending on the parameter above.

3. The Scheduled DC Voltage is used by Fast Initialization feature (main parameter page) and is also used as the Set-Point for the DC Voltage Control Mode (see the Main Control circuit on the following pages).

4. Time Constant for DC Volts Measurement in Master Controls is used to smooth or low-pass filter the measured DC Voltage before being applied to the “P by U” (Power divided by Voltage) so as to avoid oscillations in the DC Current Order output of the Master Controls.

5. The Voltage Dependant Current Limit control block diagram are shown on the next page.

The Rectifier I-Order Increase parameter is used to transiently create a higher current order at the Rectifier than at the Inverter (this is achieved by effectively using a different VDCL characteristic at the Rectifier).  This will build up the DC Voltage faster during recovery from faults (since both the Rectifier and Inverter can be in current control mode).  In the internal controls, this effect is implemented as a dynamic current margin.

Not shown on the VDCL VI diagram is the parameter  “VDCL Maximum +ve Ramp Rate (pu/sec)”.  The output of the VDCL (the current order) is rate-limited in the +ve direction to prevent fast set-point changes in the DC link (which can often result in commutation failures and oscillations).

The internal Master Power Controls are based on the following diagram:

A Voltage Dependant Current Limit (VDCL) is used to lower the DC Current Order when the DC Voltage is low.  The block diagram for the internal VDCL Controls is based on the following VI diagram:

The Rectifier and Inverter have a nearly identical control circuit as shown below:

The Control Parameters (for the Inverter) are entered as follows:

1. The PI controller has a proportional gain and an integral time constant which must be entered.  This model also accepts variables (instead of constants) which allows the user the ability to make the PI gains non-linear (perhaps changing with the mode of operation).  The input to the PI controller is PU (for DC voltage and current) and radians (for gamma control).  The output of the PI controller is radians (this will be scaled later to Degrees for output).

2. The minimum and maximum firing angles are entered here (either as constants or variables).  These are used internally in the PI controller (to prevent integrator wind-up) as well as applied to the final output of the controller.  A good trick to initialize the PI controller is to set the min and max limits to the same value (this allows the user to perform ramps etc for deblocking and fault recovery).

3. The Nominal Extinction Angle (degrees) should be entered here (either as a constant or as a variable).  This is the Gamma Order of the gamma control loop.  Note the Gamma Margin will be added to the ordered value.

4. The Current, Voltage and Gamma Margins are entered next (either as constants or variables).  One of these should be 0.0 in order to set the steady state operating point.  The margins ensure that the different modes of operation do not conflict and “fight” each other.

5. The Initial Firing Angle After Deblock should be entered (in degrees, either as a constant or as a variable) to define the initialization of the control system after a blocking action.  At the beginning of a run, this variable is used if the “Fast Initialization” feature is turned off.

6. The Relative Gains of each of the three control loops is used to allow the user to enter different gains and time constants for each loop.  It is used as a multiplier in the errors of each control mode before it is used in the PI controller.  If 0.0 is entered, then this disables that control mode.

The Relative Gains allow each control mode to have a different gain, but does not allow the ratio of the Proportional Gain to the Integral Time Constant to be modified.  If this is required, then you should enter variable names for the PI gain and time constant, and then modify these gains depending on which control mode is currently in use.  It is recommended to use a realpole or lowpass filter to smooth changes in the proportional gain (otherwise a step in the output firing angle will result).

The DC Controls (shown earlier) effectively provide for DC Voltage, DC Current and Extinction Angle (Gamma) control modes of operation.  This results in a VI diagram as shown below:

The control mode used at any instant is determined by a Select Min (at the Inverter) and a Select Max (at the Rectifier).  The output of the selection component is the controller error, which is finally fed to the Proportional Integral controller (PI).

The Rectifier control circuit utilizes DC Voltage and DC Current control modes, whereas the Inverter adds a third control mode (Gamma Control).

The mode of operation (current control, voltage control or gamma control) is determined by the intersection of the Rectifier and Inverter control characteristics (in steady state this is determined by the margins).

The Current Error Control will generate the “notch” characteristic (as shown above) to smooth the transition between DC Voltage and DC Current control modes. It operates in the current margin region and reduces the ordered DC Voltage by the same PU value as the current error (i.e. the slope is 1.0).

The VDCL (see the previous section) control circuit will prevent the converter operating in a low voltage high current condition (it reduces the current order when the DC Voltage is too low).

The next Parameter Page (for commutation failure modeling at the Inverter) is shown as follows:

1. The Inverter can often suffer from a Commutation Failure (or Com-Fail) whenever there is an AC fault, large transient increase in DC Current or fast phase shift in the AC voltages.  The Com-Fail occurs when 2 thyristors in the same phase or a valve group (i.e. Valve 1 and 4 for example) conduct at the same time.

Com-Fails are best represented by Commutation/Switching based models (such as the G6P200 valve group model in the PSCAD Master Library).  In this DC link model, the instantaneous valve voltages are not available, so the true Com-Fail criteria cannot be reproduced.  However, it is possible to approximate the Com-Fail based on the measurement of AC Voltage or based on the measured Gamma Extinction Angles.

In this phasor based DC link model, the com-fail will result in the Inverter DC Voltage being set to 0.0 and the AC Currents being injected into the Inverter bus being ramped to 0.0.  The 0.0 DC voltage will cause the DC Current to quickly increase (as it is the integral of VR – VI) until the Rectifier DC Current controls increase the firing angle to reduce the current back to its set-point (the set-point will no longer be 1.0 since the VDCL will have operated).

2.   If the second option is selected (Com-Fail Occurs When Inverter AC Voltage is Low), then the AC Voltage Level can be entered.  A Com-Fail will occur whenever the measured AC bus voltage is lower then the set-point, and the Com-Fail will clear once the AC Voltage rises above the entered level.

This representation of Com-Fail is sometimes used in transient stability simulations.

3. If the third option is selected (Com-Fail Occurs When Inverter Extinction Angle is Low), then the measured Extinction Angle (not the Nominal Extinction Angle Order!) will be compared to the entered level. 

This representation is based on the turn-off characteristics of a thyristor…  When a thyristor turns off (at a current zero), the voltage across the thyristor must stay reverse biased for some period of time (known as the Minimum Extinction Time) or else the thyristor will re-fire.

Note in this phase based DC link model, the measured Gamma is calculated based on the DC link equations, whereas in the real system (or commutation models) the measured Gamma is based on a direct measurement of the time period in which the thyristor remains reverse-biased.

If this option is used, the Com-Fail will be held for a fixed duration (which the user enters, typically one cycle).

All AC measurements in E-TRAN are designed to extract +ve sequence, fundamental frequency quantities (corresponding to the output of loadflow/stability programs).  This is challenging in a transients program in which the instantaneous calculation algorithm can produce distorted waveforms (due to harmonics etc…), DC offsets, changing frequencies etc… (these challenges are identical to those faced in the real system).

The component appears in PSCAD as shown to the right:

The E-TRAN measurements are performed using a recursive Discrete Fourier Transform (DFT) technique. The method developed:

-        Eliminates all the effects of characteristic harmonics

-        Outputs the RMS magnitude and phase angle of the fundamental frequency

-        Reacts to transients over 1 period of the fundamental

-        Uses minimum storage and uses a very efficient algorithm

Additional notch filtering is performed to eliminate non-characteristic harmonic distortions and DC offsets (the magnitude and gain effects introduced by the filtering functions are corrected for both magnitude and phase angle including frequency changes).  The model also tracks changes in the fundamental frequency (as occurs when used with machine/generator models) and uses the measured frequency to update the filtering magnitude and phase correction terms...

The DFT algorithm has an inherent response delay of one cycle (the time period over which the Fourier Transformer is performed), so the corresponding voltage, P and Q measurements will have a corresponding delayed response time.

The PSCAD Master Library contains other measurement models (based on other techniques such as RMS etc…) which can be used as required.

The dialog below appears when you edit the parameters of the Measurement model:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.

2.   This model can measure +ve sequence, fundamental frequency voltages as well as PQ flow (which requires voltage and current inputs).

3. Outputs can be in kV, kA, MW and MVAR or in Per Unit.

5. The “Circuit Type” can be selected so the component can measure Three Phase quantities (requires entry of array dimension 3 quantities), or Single Phase quantities.

The Series Capacitor model in PSCAD outputs a fixed capacitance value into the PSCAD circuit.

The component appears in PSCAD as shown to the right:

Note in PSS/E .raw format, a –ve X can also be entered in branches which are between busses of a different bus voltage or as part of an equivalent circuit for a 3 winding transformer.

When the bus voltages at each end of the branch are different (and the branch has not been identified as a transformer), E-TRAN will convert the branch to a transformer (not a series capacitor).

There is not enough information in the PSS/E .raw file format to distinguish between –ve reactance used as part of a 3 winding transformer equivalent circuit and –ve reactance for series capacitors.

For this reason, users should check the E-TRAN log file and also check all instances of Series Capacitors in the translated circuit to ensure the representation has been interpreted correctly. 

The frequency response of a –ve inductance and a capacitance are very different even though they share the same fundamental frequency impedance!

The dialog below appears when you edit the parameters of this model:

1. Refer to Chapter 8.2 for an explanation on the standard system of units for data entry.  Note the series X should be entered as –ve for capacitor values (as per the PSS/E format).

2. The branch can be made to be Ideal, in which case there is a direct short between the “from” and “to” busses.  E-TRAN will make the device Ideal if the absolute value of the PSS/E .raw branch reactance is less than 0.0001 PU.

3. If you disable this device, then the component will appear as “greyed out” and the circuit simulation will ignore this component.  The initial status of the device (i.e. enabled or disabled) is directly obtained from the status of this device in the PSS/E .raw file.

4. The “Circuit Type” can be selected so the component can be used in a Three Phase circuit (with 3 separate electrical connections), a Three Phase SLD (V4 only) or a Single Phase circuit (V3 or V4).

The next page of the data entry lets the user enter shunt admittances (similar to the line model) to adhere to the PSS/E branch data format.

The Common Get and Set models in PSCAD allow up to 1000 real quantities and 1000 integer quantities to be directly transferred between pages or sub-pages in a PSCAD circuit.  They are designed to be “stacked” so multiple variables can be entered/retrieved at one time.

This component is used to control the E-TRAN load model.  By default the load model is initialized to the values determined by the loadflow, and released after 10 seconds.

The Common Set component appears in PSCAD as shown to the right:

The Common Get component appears in PSCAD as shown to the right:

The normal way of transferring control signals from one page to another in PSCAD is to use the Import and Export features to transfer the signals from the current page to a higher/lower level page.  This requires modifying the page component definition and can be very time consuming (particularly if you have multiple levels of page components). 

This component transfers the information between pages using a Common Block.  This method has the advantage that it can transfer a variable from any page directly to another page, but it does not control the order in which the Set and Get operations are performed. For this reason, it is suggested that this model only be used for constants or for variables which do not depend critically on the order of the Get/Set operation.

The normal Import/Export method is more difficult to use, but it ensures that the Set operation is always performed before the Get in the same time step…

The dialog below appears when you edit the parameters of this model:

1. The parameter name is used only on the graphical display in PSCAD.  You can enter a single space if you do not want a label to appear.

2. Select either an Integer, Real or Boolean (logical true or false) variable type.

3. Enter the Variable Index, which is the array element to transfer.  This is what separates one real number from another, as they will read/write to a unique element of an array in the Common block.

The Initialization component should be placed on a sub-page of a model in the Substitution Library (see Chapter 6 for more information on the Substitution Library).

The Initialization component appears in PSCAD as shown to the right:

If E-TRAN encounters this component in a sub-page in a Substitution Library, it will automatically replace its data with the initial terminal voltage, phase angle, real power and reactive power (calculated based on the loadflow solution).  This information can be used to initialize any device contained on the sub-page.

If the device in the Substitution Library is a shunt device (such as a load or generator…), then this component should only appear once on the sub-page.  If it is a series device (transformer, dc link…) then 2 copies (one for the left bus and one for the right bus) should be used.  See the next section for more information.

E-TRAN also uses this component to determine if a 30 degree phase shift should be added or removed from the loadflow phase angle…  The total calculated angle (the loadflow angle with or without the 30 degree shift added/subtracted) will also be transferred to the synchronous machine component for its initialization.  See the next section for more information.

The dialog below appears when you edit the parameters of this model:

1. The Branch type should be set according to the type of device.  Branch to Ground should be used for generators, loads, switched shunt devices (anything that has a single bus connection).  For Series Devices, this initialization component should be copied and used twice on the sub-page, once for the left side bus, and once for the right side bus.  Left is defined based on the name of the page (such as E_123_789_C1, in which case the left bus would be 123, the right bus would be 789 and this is for circuit “C1”).    See Chapter 6 for more information on how to use page components in the Substitution Library.

2. Select the phase shift angle of this bus.  Loadflow data is quite often entered ignoring the 30 degree shift resulting from delta windings of transformers.  If the 30 degree shift is entered in the transformer branch data in the loadflow (or if this is for a wye-wye transformer), then this parameter should be left as “No phase shift”.  If the 30 degree shift is not entered in the loadflow and there is a wye-delta transformer, then you should select either –30 or +30 degrees.

The phase shift angle (-30 or +30) will be added to the phase angle of the bus voltage and output from this component.  This information is also read in by E-TRAN and transferred to the synchronous machine initial phase angle.

3. This data will be automatically updated by E-TRAN (based on the loadflow solution) when it translates a case.