Bidirectional AC-DC Converter (Generic)

The Bidirectional AC-DC Converter (Generic) component, shown in Table 1, is a Schematic Editor library block from the Distributed Energy Resources section, of the Microgrid library. In Schematic Editor, it is presented as a BVSC (Generic) component, which is short for Bidirectional Voltage Source Converter. It is capable of operating in isochronous, droop, and grid following mode.

Table 1. Bidirectional AC-DC Converter (Generic) component in the Schematic Editor core library
component	component dialog window	component parameters
BVSC (Generic)		Property tabs: 1 - General 2 - DC Bus 3 - Grid codes 4 - Conv. Extras 5 - Grid Extras

Bidirectional AC-DC Converter (Generic) block diagram

The component consists of two main parts: a high level control subsystem and a low level control subsystem with a power stage and necessary measurements included.

High level control contains:

PQ Control block - Contains regulators for active and reactive power control. The PQ Control block receives power references and measured powers after the transformer (grid side) and calculates reference output currents. These current references are fed to the low level control logic. This control block is active in the grid following mode of operation.
Ramping elements - Limit the ROC (rate of change) of the active power reference, reactive power reference, voltage reference, and frequency reference. The rate of change for each function is configured through a corresponding input signal.
Frequency droop functionality - Calculates frequency reference signal when droop control mode is activated. This control unit is described in the documentation for the Frequency droop unit in DER generic components.
Voltage droop functionality - Calculates voltage references signal when droop control mode is activated. This control unit is described in Voltage droop unit in DER generic components.

Low level control subsystem main parts:

Currents and Voltage/Frequency control block - Contains regulators for inner current and voltage control loops. Input signals to the Currents Control block are reference currents and measured currents before the transformer (converter side). The input signal to the Voltage Control block is the reference voltage and measured voltage before the transformer (converter side).
Fault state machine - Checks all measurements, signals the fault state, and sends an alarm message. It also immediately opens the MCB (main circuit breaker) on any detected fault. A list of implemented faults is shown in Table 6.

Bidirectional AC-DC Converter (Generic) power stage

The AC power stage is shown in Figure 2. The main elements include the average model of the inverter, the input L filter, the transformer, and the MCB that connects the plant to the grid. The transformer can be excluded from the power stage if it is not necessary.

Figure 2. Bidirectional AC-DC Converter (Generic) AC power stage

The DC power stage is shown in Figure 3. The main elements of the DC power stage are the DC link and controlled current source.

Figure 3. Bidirectional AC-DC Converter (Generic) DC power stage

All power stage elements are parametrized from the component dialog box.

Bidirectional AC-DC Converter (Generic) inputs and outputs

This component has one user interface input, and one user interface output. The user interface input and user interface output are each arrays of 40 elements. Table 2 describes the input signals and their order, while Table 3 describes the output signals and their order.

All these signals are also defined in the corresponding Bidirectional AC-DC Converter (Generic) UI subsystem with built-in Probe components (for output signals) and SCADA Input components (for input signals), as shown in Table 4. These signals were split and joined by a properly configured DER (Generic) Output Split and DER (Generic) Control Join components. The Bidirectional AC-DC Converter (Generic) UI subsystem is located in the same place in the Schematic Editor library as the Bidirectional AC-DC Converter (Generic) component, and it can be unlinked from the library and adopted to the specific needs. Unit [pu] is associated to nominal values, which are either inserted as general tab parameters or derived from them.

Table 2. Bidirectional AC-DC Converter (Generic) user interface inputs
Number	Input	Description	Signal range	Default value
0	Enable	A digital input that enables the converter. The converter is enabled when the input value is high.	0 or 1	0
1	Converter mode	An analog input that sets the operation mode of the converter. Available modes and their respective codes are: grid following mode (code 0) droop (code 1) isochronous (code 2)	0 or 1 or 2	0
2	Reset alarms	A digital input that resets alarms if the converter is in a fault	0 or 1	0
3	Pref	An analog input that is the active power reference. This reference is active in grid following and droop mode. [pu]		0
4	Qref	An analog input that sets the reactive power reference. This reference is active in grid following and droop mode. [pu]		0
5	Frequency droop offset	An analog input that sets the offset for frequency in frequency droop control mode. [pu]		0
6	Frequency droop coeff	An analog input that sets the frequency droop coefficient for the frequency droop control mode [%]	[0, 100]	2
7	Voltage droop coeff	An analog input that sets the voltage droop coefficient for the voltage droop control mode [%]	[0, 100]	4
8	Vrms_ref	An analog input that sets the voltage reference. This reference is active in isochronous mode of operation. [pu]	> 0	1
9	Fref	An analog input that sets the frequency reference. This reference is active in the isochronous mode of operation. [pu]	> 0	1
10	Pref rate of change	An analog input that sets the rate of change of the active power reference ramping. [pu/s]	>= 0.001	0.1
11	Qref rate of change	An analog input that sets the rate of change of the reactive power reference ramping. [pu/s]	>= 0.001	0.1
12	Vrms_ref rate of change	An analog input that sets the rate of change of the voltage reference ramping. [pu/s]	>= 0.001	0.1
13	Fref rate of change	An analog input that sets the rate of change of the frequency reference ramping. [pu/s]	>= 0.0001	0.02
14	V alarm upper limit*	Upper limit for the Grid voltage out of range error [pu]	> 1	1.5
15	V alarm lower limit*	Lower limit for the Grid voltage out of range error [pu]	[0, 1)	0.5
16	F alarm upper limit*	Upper limit for the Grid frequency out of range error [pu]	> 1	1.5
17	F alarm lower limit*	Lower limit for the Grid frequency out of range error [pu]	[0, 1)	0.5
18	S alarm upper limit*	Upper limit for the Over power protection [pu]	> 1	1.5
19	I alarm upper limit*	Upper limit for the Over current protection [pu]	> 1	1.5
20	LVRT enable	Digital input that enables the LVRT unit. The LVRT unit is enabled when the input value is high.	0 or 1	0
21	LVRT Q contribution	An analog input that specifies additional reactive power contribution per percent of voltage drop in the case of a LVRT event. [%]	>= 0	2
22	LVRT P set	An analog input that defines the active power reference in the case of a LVRT event. [pu]	>= 0	1
23	VoltVAr enable	Digital input that enables the VoltVAr unit. The VoltVAr unit is enabled when the input value is high.	0 or 1	0
24	HzWatt enable	Digital input that enables the HzWatt unit. The HzWatt unit is enabled when the input value is high.	0 or 1	0
25	VoltWatt enable	Digital input that enables the VoltWatt unit. The VoltWatt unit is enabled when the input value is high.	0 or 1	0
26	Voltage droop offset	An analog input that sets the offset for voltage in voltage droop control mode. [pu]		0
27-39	reserved_ins	Inputs that are not used.		0

* If the values for inputs 14 to 19 are all zeroes, default values will be used.

Table 3. Bidirectional AC-DC Converter (Generic) user interface outputs
Number	Output	Description
0	Enable_fb	A digital output describing the converter enable/disable state. The converter is enabled if the output is high.
1	Converter_mode_fb	An analog output describing which control mode the converter operates in.
2	Pref_fb_kW	An analog output with the instantaneous value of the applied power reference in the converter. [kW]
3	Qref_fb_kVAr	An analog output with the instantaneous value of the applied reactive power reference in the converter. [kVAr]
4	Pnom_kW	An analog output with the value of nominal active power of the converter. [kW]
5	Qnom_kVAr	An analog output with the value of nominal reactive power of the converter. [kVAr]
6	Snom_kVA	An analog output with the value of nominal apparent power of the converter. [kVA]
7	Vnom_LL_V	An analog output with the value of nominal converter's voltage. [V]
8	Fnom_Hz	An analog output with the value of nominal converter's frequency. [Hz]
9	Vrms_ref_fb_V	An analog output with feedback information about the RMS value of the reference voltage. [V]
10	Fref_fb_Hz	An analog output with feedback information about the reference frequency value. [Hz]
11	Vgrid_rms_meas_kV	An analog output reporting the RMS value of the grid voltage, measured at the grid side of the transformer. [kV]
12	Fmeas_Hz	An analog output reporting the value of the grid voltage frequency. [Hz]
13	Pmeas_kW	An analog output reporting the instantaneous value of the three-phase active power output of the converter. [kW]
14	Pa_meas_kW	An analog output reporting the instantaneous value of the phase A active power output of the converter. [kW]
15	Pb_meas_kW	An analog output reporting the instantaneous value of the phase B active power output of the converter. [kW]
16	Pc_meas_kW	An analog output reporting the instantaneous value of the phase C active power output of the converter. [kW]
17	Qmeas_kVAr	An analog output reporting the instantaneous value of the three-phase reactive power output of the converter. [kVAr]
18	Qa_meas_kVAr	An analog output reporting the instantaneous value of the phase A reactive power output of the converter. [kVAr]
19	Qb_meas_kVAr	An analog output reporting the instantaneous value of the phase B reactive power output of the converter. [kVAr]
20	Qc_meas_kVAr	An analog output reporting the instantaneous value of the phase C reactive power output of the converter. [kVAr]
21	Smeas_kVA	An analog output reporting the instantaneous value of the three-phase apparent power output of the converter. [kVA]
22	PFmeas	An analog output reporting the value of the converter output power factor.
23	Vconv_rms_meas_V	An analog output reporting the RMS value of the converter voltage, measured at the converter side of the transformer. [V]
24	Vdc_V	An analog output reporting the instantaneous value of DC voltage at the output of the converter. [V]
25	Idc_A	An analog output reporting the instantaneous value of DC current at the output of the converter. [A]
26	MCB_status	A digital output reporting the information about the state of the main circuit breaker (contactor). The contactor is closed if the value is high.
27	state	An analog output reporting the code of the state of the converter.
28	alarm_msg	An analog output reporting the value of the alarm message. In order for the converter to return to normal operation after the alarm has been triggered, all alarms must be reset.
29	Lvrt_active_status	A digital output that signals the status of the LVRT unit. If high, the LVRT unit is active and contributing active and reactive power to the grid, otherwise it is not active.
30-39	reserved_outs	Outputs that are not used.

Table 4. Bidirectional AC-DC Converter (Generic) UI subsystem
Bidirectional AC-DC Converter (Generic) UI	Bidirectional AC-DC Converter (Generic) UI internals

Bidirectional AC-DC Converter (Generic) operational states

Table 5. Main state machine states
Code	Description
1	Starting up state – Represents the state of the converter between the moment signal activation is triggered and the moment when the converter starts operating, e.g. system checking and synchronization time.
2	Running state - Represents the operational state of the converter.
3	Disabled state - Reports if the converter is not enabled/operational.
4	Fault state – Represents the state of the converter if it encounters a fault. It is necessary to reset the alarms once the converter enters this state.

Table 6. Faults/Alarm messages
Code	Description
0	None of the alarms have been triggered
1	Over current protection. Input phase currents are greater than the maximum specified value.
2	Grid voltage out of range. Grid voltage is outside the specified range. This protection is active in grid following mode. If the voltage restores, the fault will be automatically reset.
3	Grid frequency out of range. Grid frequency is outside the specified range. This protection is active in grid following mode. If the frequency restores, the fault will be automatically reset.
4	Over power protection. Measured apparent power is greater than the maximum allowed apparent power.

Component dialogue box and parameters

The Bidirectional AC-DC Converter (Generic) component dialogue box consists of five tabs for specifying parameters of the component. Unit [pu] is associated to nominal values, which are inserted as general tab parameters or derived from them.

Tab: "1 - General"

In this component tab, general parameters of the Bidirectional AC-DC Converter and the grid can be specified.

Figure 4. Bidirectional AC-DC Converter (Generic) "1 - General" parameters

Table 7. Bidirectional AC-DC Converter (Generic) "1- General" parameters description
Parameter	Code name	Description
Nominal active power	Pnom	Bidirectional AC-DC Converter nominal active power. [W]
Nominal apparent power	Snom	Bidirectional AC-DC Converter nominal apparent power. [VA]
Nominal converter line voltage	Vnom_LL	Bidirectional AC-DC Converter nominal line voltage (transformer's primary side). [V]
Nominal frequency	fnom	Bidirectional AC-DC Converter Nominal frequency. [Hz]
Include transformer	include_transformer	Include the transformer in the Bidirectional AC-DC Converter power stage model.
Nominal grid line voltage	Vnom_sec_LL	Nominal line voltage of the grid (transformer's secondary side). [V]

Tab: "2 - DC Bus"

In this component tab, parameters that are related to the DC Bus part of the converter can be specified.

Figure 5. Bidirectional AC-DC Converter (Generic) "2 - DC Bus" parameters.

Table 8. Bidirectional AC-DC Converter (Generic) "2 - DC Bus" parameters description
Parameter	Code name	Description
Nominal DC voltage	nom_DC_voltage	Nominal DC voltage. [V]
Shunt resistance	Rshunt_DC	Shunt resistanc in DC Bus. [Ω]
Capacitance	Cdc	Capacitance in DC Bus. [F]

Tab: "3 - Grid codes"

In this component tab, different Grid code parameters can be specified. Currently, LVRT (Low Voltage Ride Through), VoltVAr, HzWatt, and VoltWatt functionalities are supported.

Figure 6. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" LVRT parameters

Table 9. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" LVRT parameters
Parameter	Code name	Description
LVRT requirement	lvrt_req	Automatically parametrizes voltage and time points according to the chosen requirements. If the Custom option is selected, voltage and current points become available for editing.
Voltage points	vs_lvrt	LVRT curve voltage points. [pu]
Time points	ts_lvrt	LVRT curve time points. [s]
Preview LVRT curve	preview	Displays the created LVRT curve.

Implementation details for the LVRT unit are given on the LVRT in generic DER components page.

Figure 7. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" VoltVAr parameters

Table 10. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" VoltVAr parameters
Parameter	Code name	Description
VoltVAr requirement	voltvar_req	Allows for defining voltage and reactive power points. Currently, only the Custom option is supported.
Voltage points	vs_voltvar	VoltVAr curve voltage points.
Reactive power points	qs_voltvar	VoltVAr curve reactive power points.
Preview VoltVAr curve	preview_voltvar	Displays the created VoltVAr curve.

If LVRT and VoltVAr functionalities are both enabled, then the VoltVAr curve has priority over the LVRT reactive power contribution setting (Q_contribution).

The VoltVAr curve is linearly extrapolated for voltage values below and above the minimum and maximum values. Also, the curve is limited by a maximum value of reactive power which can be produced or stored. This limit is calculated as $Q_{m a x} = (1 + o p c_p u) * Q_{n o m}$ , where opc_pu is the overpower capability.

Figure 8. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" HzWatt parameters

Table 11. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" HzWatt parameters
Parameter	Code name	Description
HzWatt requirement	hzwatt_req	Allows for defining frequency and active power points. Currently, only the Custom option is supported.
Frequency points	fs_hzwatt	HzWatt curve frequency points.
Active power points	ps_hzwatt	HzWatt curve active power points.
Preview HzWatt curve	preview_hzwatt	Displays the created HzWatt curve.

If LVRT and HzWatt functionalities are both enabled, then the HzWatt curve has priority over the LVRT active power contribution setting.

If the frequency value is below the minimum value or above the maximum value, then the Grid frequency out of range error will appear. The HzWatt curve is limited by a maximum value of active power which can be produced or stored. This limit is calculated as $P_{m a x} = (1 + o p c_p u) * P_{n o m}$ , where opc_pu is the overpower capability.

Figure 9. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" VoltWatt parameters

Table 12. Bidirectional AC-DC Converter (Generic) "3 - Grid codes" VoltWatt parameters
Parameter	Code name	Description
VoltWatt requirement	voltwatt_req	Allows for defining voltage and active power points. Currently, only the Custom option is supported.
Voltage points	vs_voltwatt	VoltWatt curve voltage points.
Active power points	ps_voltwatt	VoltWatt curve active power points.
Preview VoltWatt curve	preview_voltwatt	Displays the created VoltWatt curve.

If LVRT and VoltWatt functionalities are both enabled, then the VoltWatt curve has priority over the LVRT active power contribution setting. But, if HzWatt and VoltWatt functionalities are both enabled, then the HzWatt curve has priority over the VoltWatt curve.

The VoltWatt curve is linearly extrapolated for voltage values below and above the minimum and maximum values. Also, the curve is limited by a maximum value of active power which can be produced or stored. This limit is calculated as $P_{m a x} = (1 + o p c_p u) * P_{n o m}$ , where opc_pu is the overpower capability.

Tab: "4 - Conv. Extras"

In this component tab, additional parameters that are related to the converter can be specified.

Figure 10. Bidirectional AC-DC Converter (Generic) "4 - Conv. Extras" parameters

Table 13. Bidirectional AC-DC Converter (Generic) "4 - Conv. Extras" parameters description
Parameter	Code name	Description
Input filter impedance	wL_pu	Input filter impedance. Input filter is a simple L type filter. See Figure 2. [pu]
Transformer impedance	Zt_n	Determines the transformer impedance. The position of this transformer is shown in Figure 2. [pu]
DC link voltage margin	dc_link_margin	Voltage margin over the minimal allowed DC link voltage. This value restricts the maximum voltage that a converter can generate. [pu]
Overpower capability	opc_pu	Determines the overpower capability of the converter. [pu]
Shunt resistance	Rshunt_pu	The value of the shunt resistances that are used for the measurement of phase voltages at the converter input. The position of shunts is shown in Figure 2. [pu]
Faster execution rate	Tfast	The Faster execution rate at which part of the inner signal processing of the component will be executed. Should be approximately 5 to 10 times faster than the Slower execution rate. [s]
Slower execution rate	Tslow	The Slower execution rate, at which part of the inner signal processing of the component will be executed. This execution rate is inherited by the connected UI subsystem. Should be approximately 5 to 10 times slower than the Faster execution rate. [s]

Tab: "5 - Grid Extras"

In this component tab, additional parameters that are related to the grid can be specified.

Figure 11. Bidirectional AC-DC Converter (Generic) "5 - Grid Extras" parameters

Table 14. Bidirectional AC-DC Converter (Generic) "5 - Grid Extras" parameters description
Parameter	Code name	Description
Grid Short Circuit power	Sg_sc_pu	Short circuit apparent power of the grid that the converter is connected to. This parameter doesn't influence the power stage of the model, only the control part. Stable operation of the converter can be achieved by tuning this parameter. [pu]
Grid inductive power factor	PFg	Power factor of the grid that the converter is connected to. This power factor occurs when the grid is short-circuited. This parameter doesn't influence the power stage of the model, only the control part. Stable operation of the converter can be achieved by adjusting this parameter. [pu]

Example

Overall behavior and control methodologies can be better understood with the use of the given example:

Model name: bidirectional_AC-DC_converter_gen.tse

SCADA interface: bidirectional_AC-DC_converter_gen.cus

Path: /examples/models/microgrid/bidirectional_AC-DC_converter/bidirectional_AC-DC_converter (generic)