Spec:sync check

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Overview

The synchronization check capability in GridLAB-D will be implemented to perform paralleling for two independent power grids. This could be used to parallel two separate power systems, or to reconnect a microgrid to the bulk power system. In the simulation, the frequency and voltage metrics are checked. When conditions are satisfied, the sync_check object sends a closure command to its parent switch_object.

Published properties (i.e., GLM inputs)

The mapping between the properties and variables is listed in Table 1. The sync_check object will inherit all standard Object_(directive) values as well. The variable definitions are presented in Table 2 of the next section "Variable definitions". Note that the properties listed here represent two different modes for calculating the from/to voltage conditions - MAG_DIFF and SEP_DIFF. Notes are included as to which property is used for which mode.

Table 1 - Mapping between Properties and Variables

Property	Mapped Variable	Data Type	Descriptions
armed	sc_flag	Boolean	Turns on/off the action functionality True - This object is functional False - This object is disabled
volt_compare_mode	volt_compare_mode	Enumeration	Determines which voltage difference calculation approach is used. This determines how the difference between from and to node voltages are compared. Two entries are supported: MAG_DIFF - consider only the magnitude of the overall complex difference between the from and to voltage. e.g., abs(F-T). This is the default mode. SEP_DIFF - consider the difference of voltage magnitude and difference of voltage angle separately. e.g., abs(|F|-|T|) and abs(angle(F)-angle(T)).
frequency_tolerance	eps_freq	Double	The user-specified tolerance in Hz for checking the frequency metric. Used by both volt_compare_mode types.
voltage_tolerance	eps_volt	Double	The user-specified tolerance in per unit for checking the voltage metric. Used only by the MAG_DIFF mode of volt_compare_mode.
voltage_magnitude_tolerance	eps_mag_volt	Double	The user-specified tolerance in per unit for the difference in voltage magnitudes for checking the voltage metric. Used only by the SEP_DIFF mode of volt_compare_mode.
voltage_angle_tolerance	eps_ang_volt	Double	The user-specified tolerance in degrees for the difference in voltage angles for checking the voltage metric. Used only by the SEP_DIFF mode of volt_compare_mode.
metrics_period	t_ud	Double	The user-defined period when both metrics are satisfied. Used by both volt_compare_mode types.
delta_trigger_mult	delta_trigger_mult	Double	Multiplier of the appropriate voltage/frequency tolerances to trigger deltamode. e.g., in MAG_DIFF mode, voltage_tolerance and frequency_tolerance will be multiplied by this value, and if current conditions are within that band (and sync_check is armed), deltamode will be triggered/maintained until either a close action occurs, or the band is exited.

Two sample sync_check objects defined in the glm file are shown as follows.

Using only the difference of the overall magnitude:

object sync_check
{
   name sc_m12;
   parent swt_01;
   armed true;
   volt_compare_mode MAG_DIFF;
   frequency_tolerance 0.6 Hz;
   voltage_tolerance 0.01; //Unit: pu
   metrics_period 5 s;
   delta_trigger_mult 2.0;
}

Using the difference of the voltage magnitudes and angles separately:

object sync_check
{
   name sc_m12;
   parent swt_01;
   armed true;
   volt_compare_mode SEP_DIFF;
   frequency_tolerance 0.6 Hz;
   voltage_magnitude_tolerance 0.01; //Unit: pu
   voltage_angle_tolerance 5 deg;
   metrics_period 5 s;
   delta_trigger_mult 2.0;
}

Variable definitions

Variables of the sync_check functionality are defined as follows:

Table 2 - Variable Definitions

Variable	Data Type	Unit	Definition
Flag
sc_flag	Boolean	N/A	Turns on/off the action functionality True - This object is functional False - This object is disabled
volt_compare_mode	Enumeration	N/A	Determines the algorithm to use for comparing the voltages of the from and to nodes of the switch_object MAG_DIFF - Use only the magnitude of the difference - e.g., abs(F-T) SEP_DIFF - Use the difference of the magnitude and angle separately - e.g., abs(|F|-|T|) and abs(angle(F)-angle(T))
swt_status	Enumeration	N/A	Status of the parent switch object. Valid states are: OPEN - Its parent switch object is open and no current can flow CLOSED - Its parent switch object is closed and conducting
Voltage
volt_m1	Complex	V	The voltage phasor measured at the 'from' node of its parent switch
volt_m2	Complex	V	The voltage phasor measured at the 'to' node of its parent switch
volt_norm	Double	V	Nominal voltage, which is set as the value pulled from the variable/attribute 'nominal_voltage' of either the 'from' or 'to' node object of its parent switch object
volt_diff	Double	V	The magnitude of the difference between volt_m1 and volt_m2 - used by MAG_DIFF
volt_diff_pu	Double	pu	volt_diff normalized against volt_norm - used by MAG_DIFF
volt_diff_mag	Double	V	The magnitude of the difference between the magnitude of volt_m1 and the magnitude of volt_m2 - used by SEP_DIFF
volt_diff_mag_pu	Double	pu	volt_diff_mag normalized against volt_norm - used by SEP_DIFF
volt_diff_ang	Double	deg	The magnitude of the difference between the angle of volt_m1 and the angle of volt_m2 - used by SEP_DIFF
Frequency
freq_m1	Double	Hz	The frequency measured at the 'from' node of its parent switch
freq_m2	Double	Hz	The frequency measured at the 'to' node of its parent switch
freq_diff	Double	Hz	The absolute value of the difference between freq_m1 and freq_m2
freq_diff_pu	Double	pu	freq_diff normalized against nominal_frequency (powerflow global)
Tolerance
eps_freq	Double	Hz	The user-specified tolerance for checking the frequency metric
eps_volt	Double	pu	The user-specified tolerance for checking the magnitude of the overall voltage difference. Used for the MAG_DIFF mode of operation
eps_mag_volt	Double	pu	The user-specified tolerance for checking the magnitude of the difference of voltage magnitudes. Used for the SEP_DIFF mode of operation
eps_angle_volt	Double	deg	The user-specified tolerance for checking the magnitude of the difference of voltage angles. Used for the SEP_DIFF mode of operation
t_ud	Double	sec	The user-defined period when both metrics are satisfied
delta_trigger_mult	Double	N/A	User-defined multiplier for the appropriate tolerances to trigger/maintain deltamode
Timer
t_sat	Double	sec	The total period (initialized as 0) during which both metrics have been satisfied continuously
dt_dm	Double	sec	Current deltamode timestep

Methodology

Quasi-Steady State Time Series (QSTS)

In QSTS mode, the sync_check object will check current voltage and frequency metrics against the appropriate tolerance values, but multiplied by the deltamode_trigger_mult variable. If they are within this expanded range and the sync_check is armed, deltamode will be triggered.

Deltamode

Paralleling will only occur when the grid-alignment conditions are met. While the voltage and frequency metrics are within the multiple of deltamode_trigger_mult and the appropriate tolerances, deltamode will continue to be requested (SM_DELTA). e.g., if in MAG_DIFF mode, the sync_check is armed, and the magnitude of the voltage difference is less than deltamode_trigger_mult * eps_volt, a request to stay in deltamode will be sent.

The pseudocode of the sync_check method is shown as follows. It is run in each step of the deltamode.

algorithm sync_check is
    input: freq_m1, freq_m2, volt_m1, volt_m2 
    output: Flag swt_cmd such that swt_cmd is true for sending the closure command, false for monitoring
    freq_diff ← abs(freq_m1 - freq_m2)
    
    if (volt_comare_mode == MAG_DIFF) do
         volt_diff ← abs(volt_m1 - volt_m2)
         volt_diff_pu ← volt_diff/volt_norm
         
         if (freq_diff_pu <= eps_freq) and (volt_diff_pu <= eps_volt) do
              t_sat ← t_sat + dt_dm
         else do
              t_sat ← 0
         
    else do
         volt_diff_mag ← abs(abs(volt_m1) - abs(volt_m2))
         volt_diff_mag_pu ← volt_diff_mag/volt_norm
         
         volt_diff_ang ← abs(angle(volt_m1) - angle(volt_m2))
         
         if (freq_diff_pu <= eps_freq) and (volt_diff_mag_pu <= eps_mag_volt) and (volt_diff_ang <= eps_ang_volt) do
              t_sat ← t_sat + dt_dm
         else do
              t_sat ← 0
         
    if (t_sat >= t_ud) do
         swt_cmd ← true
    else do
         swt_cmd ← false

    return swt_cmd

Validation

This subsection provides an outline on how the sync_check object will be tested to ensure its functionality. The current plan is to use two 4-node test systems interconnected through a sync_check object, which is open initially. The frequency and voltage values measured at the 'from' and 'to' nodes of the sync_check object are initialized in different values. The deviations must be larger than the user defined tolerances. The frequency and voltage of the 'from' node of its parent switch object are manipulated by a player towards the measurements of the 'to' node. Once the deviations are both within the tolerance longer than the user defined period, a 'closure' command should be sent to close its parent switch object. This sample use case will be included in the autotest for the sync_check object.

See also

• Requirements
• Implementation
• sync_ctrl
• IEEE Std C50.13^TM-2014