Synchronization Control

Background and Motivation

The synchronization control capability in GridLAB-D™ has been designed and implemented to coordinate the connection of distributed generation units to the grid. The sync_ctrl object in GridLAB-D™ enables automatic synchronization between two independent power grids by actively adjusting a generation unit's frequency and voltage setpoints to match grid conditions. The controlled generation unit can be either a diesel generator (diesel_dg object) or a grid-forming inverter (inverter_dyn object in GFM set-up). This functionality supports applications such as paralleling separate power systems or reconnecting microgrids to the bulk power system. During simulation, the sync_ctrl continuously monitors frequency and voltage magnitude differences measured by its associated sync_check object. When both frequency and voltage differences fall within specified tolerance bounds, the sync_check object becomes armed and ready for connection. Until these synchronization criteria are met, the sync_ctrl actively controls the generation unit through dual PI controllers to minimize the differences and achieve safe paralleling conditions.

Functionality

The connection, data, and work flow between the GridLAB-D™ objects connected to enable and simulate the synchronization control are shown in Figure 1.Synchronization control actions are enabled through the sync_ctrl object in conjunction with sync_check and switch objects in the powerflow module, and diesel_dg and inverter_dyn objects in the generators module.

Sync Ctrl Dataflow
Figure 1: Sync Ctrl Dataflow

Specifically, this controller:

  1. Controls grid synchronization - It works in conjunction with a sync_check object to monitor when conditions are appropriate for connecting a generation unit to the grid.

  2. Operates in two modes:

    • Mode A - Active controlling period when both frequency and voltage metrics are satisfied,
    • Mode B - Monitoring period when the controller keeps monitoring metrics but the switch is not closed.

  3. Uses dual PI controllers to adjust generator setpoints:

    • Frequency PI controller - Adjusts the frequency setpoint or power setpoint based on frequency differences,
    • Voltage magnitude PI controller - Adjusts the voltage setpoint based on voltage magnitude differences.

  4. Controls generation units, which include specifically for the current GridLAB-D™ implementation:

    • Diesel generators (diesel_dg),
    • Dynamic inverters (inverter_dyn).

  5. Monitors synchronization metrics:

    • Frequency tolerance (upper and lower bounds in Hz),
    • Voltage magnitude tolerance (in per unit).

  6. Adjusts setpoints gradually - The PI controllers incrementally adjust the controlled generation unit's frequency and voltage setpoints to bring them within acceptable tolerance ranges for safe synchronization.

Synchronization Control Parameters and Implementation Details (sync_ctrl)

Figure 2 details the flowchart for sync_ctrl behavior in transient mode.

Sync Ctrl Flowchart
Figure 2: Sync Ctrl Flowchart

Details on the variable types and units expected by GridLAB-D™ are given along with the code insider documentation in Table 1. The following subsections detail the key parameters that GridLAB-D™ implementation offers for modeling the sync_ctrl object. This specific object has been designed and implemented to be integrated and used for transient mode analysis, under the assumption the call to transient mode was triggered by either the device arming the sync_ctrl object, or by something elsewhere in the system making the adjustments for synchronization to occur. Otherwise, no explicit functions are performed by the sync_ctrl object in QSTS mode.

Control Flags

  • armed: Global/published flag that enables the synchronization control functionality
  • sct_volt_cv_arm_flag: Internal (hidden) flag to specifically enable voltage control output
  • sct_freq_cv_arm_flag: Internal (hidden) flag to enable frequency control output

Two Operating Modes

Once active, sync_ctrl runs in either one of the operating modes:

  • Mode A - Active Controlling Period during which the controller adjusts the voltage and frequency settings of the controlled generation unit actively. In this mode:

    • Both frequency and voltage PI controllers are running,
    • Actions last for duration controlling_period[s], the user-defined period in seconds when both voltage and frequency metrics are satisfied (default: 1 second).

  • Mode B - Monitoring Period during which the controller monitors the voltage magnitudes and frequency and counts a timer, determining when to switch to mode A if needed. In this mode:

    • Controllers keep monitoring but switch remains open,
    • Controllers wait for synchronization conditions to be satisfied,
    • Monitoring continues for duration monitoring_period[s], the user-defined period in seconds while both frequency and voltage metrics are not violated and the switch object is not closed (default: 10 seconds).

Reference Object

  • sync_check_object: Reference to the sync_check object in the powerflow module that provides measurement feedback,
  • controlled_generation_unit: Reference to the controlled generation unit, either diesel generator of dynamic inverter, which represents the actuator of the PI controllers.

Frequency Tolerance Bounds

  • frequency_tolerance_ub_hz: Upper frequency tolerance bound in Hz
  • frequency_tolerance_lb_hz: Lower frequency tolerance bound in Hz

Voltage Magnitude Tolerance

  • voltage_magnitude_tolerance_pu - Voltage magnitude tolerance in per unit (default: 0.01 pu = 1%)

PI Controller Parameters - Frequency/Voltage Magnitude

  • pi_freq_kp/pi_volt_mag_kp: Proportional gains (default: 1)
  • pi_freq_ki/pi_volt_mag_ki: Integral gains (default: 0.1)
  • pi_freq_ub_pu/pi_volt_mag_ub_pu: PI controller output (\(P_{\text{set}}\)/\(f_{\text{set}}\), and \(V_{\text{set}}\) respectively) upper bound in per unit (default: 1)
  • pi_freq_lb_pu/pi_volt_mag_lb_pu: PI controller output lower bound in per unit (default: 0)

Example

The following glm file sample defines a sync_ctrl object.

object sync_ctrl
  {
    name sct_f01f02;
    flags DELTAMODE;

    armed false; //starting as disarmed

    sync_check_object grid_resyncer; //the sync_check object linked to this sync_ctrl object

    controlled_generation_unit Diesel_1; // the controlled generation unit can be either a DG or INV
    // controlled_generation_unit Inverter_1;

    controlling_period 2; // control period when in mode A
    monitoring_period 15; // monitoring period when in mode B

    frequency_tolerance_ub_hz -0.7;
    frequency_tolerance_lb_hz -0.1;

    pi_freq_kp -2;
    pi_freq_ki -0.2;

    voltage_magnitude_tolerance_pu 0.02;
    pi_volt_mag_kp -2;
    pi_volt_mag_ki -0.2;

    pi_volt_mag_ub_pu 1.65;
    pi_volt_mag_lb_pu 0.35;

    pi_freq_ub_pu 1.0;
    pi_freq_lb_pu 0;
  }

Summary of Key Parameters

Table 1: Key Parameters and Variables of the GridLAB-D™ Synchronization Control (sync_ctrl) Object
Published Name Unit Type Description
armed bool Flag to arm the synchronization control functionality.
sync_check_object object The object reference/name of the sync_check object, which works with this sync_ctrl object.
controlled_generation_unit object The object reference/name of the controlled generation unit (i.e., a diesel_dg/inverter_dyn object), which serves as the actuator of the PI controllers of this sync_ctrl object.
frequency_tolerance_ub_hz Hz double The user-specified tolerance in Hz for checking the upper bound of the frequency metric.
frequency_tolerance_lb_hz Hz double The user-specified tolerance in Hz for checking the lower bound of the frequency metric.
voltage_magnitude_tolerance_pu pu double The user-specified tolerance in per unit for the difference in voltage magnitudes for checking the voltage metric.
controlling_period s double The user-defined period when both metrics are satisfied and this sync_ctrl object works in mode A.
monitoring_period s double The user-defined period when this sync_ctrl object keeps on monitoring in mode B, if both metrics are not violated and the switch object is not closed.
pi_freq_kp double The user-defined proportional gain constant of the PI controller for adjusting the frequency setting.
pi_freq_ki double The user-defined integral gain constant of the PI controller for adjusting the frequency setting.
pi_freq_ub_pu pu double The upper bound of the output (i.e., the control variable 'Pset'/'fset') of the PI controller that adjusts the frequency difference in per unit.
pi_freq_lb_pu pu double The lower bound of the output (i.e., the control variable 'Pset'/'fset') of the PI controller that adjusts the frequency difference in per unit.
pi_volt_mag_kp double The user-defined proportional gain constant of the PI controller for adjusting the voltage magnitude setting.
pi_volt_mag_ki double The user-defined integral gain constant of the PI controller for adjusting the voltage magnitude setting.
pi_volt_mag_ub_pu pu double The upper bound of the output (i.e., the control variable 'Vset') of the PI controller that adjusts the voltage magnitude difference in per unit.
pi_volt_mag_lb_pu pu double The lower bound of the output (i.e., the control variable 'Vset') of the PI controller that adjusts the voltage magnitude difference in per unit