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The objective of the example model is to demonstrate the functionality of each of the PB protection blocks according to the IEEE 1547-2018 and IEEE 1547-2020 standards [1] [2]. The demonstrations are performed in closed loop considering the interaction of all the control layers in the converter. The procedures, operating points, and some of the control parameters presented in this section are based on the IEEE 1547.1-2020 standard [2]. The example model was developed to show all the features of the PB protection system library. Thus, the user can select the PB protection system scenario only by changing a simple setting in the Reference Generator region.
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To demonstrate all the functionalities of the PB protection system library, the voltage and frequency of the grid (symbolized by three AC sources at the right of the blue region) are varied according to the reference generator block. Also, the Rate of Change of Frequency (ROCOF) and the connection breaker can be controlled by the reference generator block. The reference signals vary depending on the selected scenario. By default, the quick example scenario is selected. However, the user can select other scenarios to assess a particular PB protection system functionality.
Throughout the tests, the model is run at a sampling period of 50e-6 s. The simulation parameters and control variables, which are loaded from the ‘SmartInverterLib_Protection_init’ file, are described as below:
Fnom=60; System System frequency (Hz):
Ts=50e-6; Simscape Power Systems sample time (s)
Srated = 5e3; Inverter Inverter nominal 3-phase power (VA)
Pnom = 5e3; Inverter Inverter nominal 3-phase active power (W)
Vnom = 208; Nominal inverter output line- Nominal inverter output line-to-line voltage(Vrms)
Vdcnom = 400; Nominal Nominal DC link voltage (V)
Ilim = Srated/(Vnom/sqrt(3))*sqrt(2); Inverter Inverter output current limit
L1 = 5e-3; L filter inductor L filter inductor (H)
R1= 0.1; L L filter resistor (ohm)
kpp = 2; Active power controller Active power controller proportional gain
kip = 60; Active Active power controller integral gain
kpq = 2; Reactive Reactive power controller proportional gain
kiq = 100; Reactive Reactive power controller proportional gain
kpI = 2; Current Current controller proportional gain
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- Frequency Protection
- Instantaneous value (FREQUENCY)
- ROCOF Rate of Change of Frequency (ROCOF)
- Voltage Protection (VOLTAGE)
- Bridge Protection
- AC side (BRIDGE AC)
- DC side (BRIDGE DC)
- Islanding Detection (ISLANDING)
- General Electric (GE) Frequency Method
- General Electric (GE) Voltage Method
- General Electric (GE) Voltage+Frequency Method
For each validation scenario, the Time-to-Trip time (TtT) is shown. For all scenarios, the primary control block is programmed in closed-loop control for both active and reactive power. To select each scenario, the scenario in the "Test Generator" must be modified.
Scenario 0 - Quick Example
The quick example scenario is configured to show three of the most important features of the PB protection system library: frequency protection, voltage protection, and islanding detection. For this test, the frequency limit was defined as Fo = 61.2 Hz with a Time-to-Trip (TtT) of 3 s, the voltage limit was defined as Vu = 0.5 p.u. with a Time-to-Trip (TtT) of 2 s, and the islanding detection method was set to General Electric (GE) Voltage+Frequency scheme.
Step id | Test Description | Testing Time | Cummulated TimeCumulated Time | Signals of interest |
|---|---|---|---|---|
0.1 | Adjust the power output to 1 p.u. | 3 | 0 | P, Freq, Vrms, Trip, VRTm, TtT |
0.2 | Ramp the grid frequency to the frequency point 0.99Fo | 2 | 3 | |
0.3 | Ramp the grid frequency to the frequency point 1.01Fo1 | 1 | 5 | |
0.4 | Measure the clearing time (time-to-trip) from the moment that the frequency achieves Fo | 4 | 6 | |
0.5 | Clear the fault and ramp the grid frequency to nominal values | 2 | 10 | |
0.6 | Step down the voltage to 1.01Vu | 1 | 12 | |
0.7 | Step down the voltage to 0.99Vu | 2 | 13 | |
0.8 | Clear the fault and step up the voltage to nominal values | 2 | 15 | |
0.9 | Adjust the power output to 0.6 p.u. | 1 | 17 | |
0.10 | Open the connection breaker and watch the trip signal | 2 | 18 |
The figure below shows the test results for the Quick Example. First, the frequency is increased above the limit, this starts the counting of the F_TtT signal. When the F_TtT signal reaches the Time-to-Trip (TtT) for frequency ( 3 s3s), the trip signal is activated. Then, the voltage is decreased below the limit. Notice that the VRTm Voltage Ride-Through (VRT) Mode signal is set to 2, which means that the power injection becomes zero and no trip signal is activated as part of the momentary cessation mode. Then, after 2 s, the trip signal is activated. Finally, the connection breaker is opened at t = 18 s. The islanding detection algorithm causes the voltage and frequency to move away from the nominal values and the trip signal is activated by the Rate of Change of Frequency (ROCOF) protection.
Scenario 1 - Frequency Protection
Frequency protection is divided into instantaneous value and Rate of Change of Frequency (ROCOF). For the instantaneous value, the frequency of the grid is varied below the specified limits and then, outside of those limits, Time-to-Trip (TtT) is measured. For the ROCOF, the ROCOF of the grid is varied under the limit and then, varied again outside of the limit to count the TtTto count the Time-to-Trip (TtT). Limits and times are defined as follows:
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Fu2 = 56.5;
Tuf1 = 3;
Tuf2 = 0.16;
Rate of Change of Frequency (ROCOF):
ROCOF_threshold = 3;
ROCOF_average_time_window = 0.1;
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In this scenario, the instantaneous value of the frequency is moved around the limits to evaluate Time-to-Trip (TtT). The scenario sequence is presented below:
Step id | Test Description | Testing Time | Signals of interest |
|---|---|---|---|
1.1 | Adjust the power output to Prated=5kW, and set the rate limit to 1Hz/s on the grid frequency. | 3 | P, Freq, Trip, TtT |
1.2 | Ramp the grid frequency to the frequency point 0.99FoN | 2 | |
1.3 | Ramp the grid frequency to the frequency point 1.01Fo1 for at least 1.5TofN | >1.5TofN | |
1.4 | Measure the clearing time (time-to-trip) from the moment that the frequency achieves Fo1 and re-initialize the inverter to nominal values | 1 | |
1.5 | Clear the fault | 2 | |
1.6 | Repeat steps 1.2 to 1.5 for N=2. | ||
1.7 | Repeat steps from 1.2 to 1.6 for under frequency (Fu, Tuf) limits. |
Simulation results are presented below. It is seen that the converter trips when expected according to the frequency variations. The Time-to-Trip (TtT) measurements show an adequate result as configured for each frequency trip region.
Scenario 1.2 - Rate of Change of Frequency (ROCOF)
In this scenario, the Rate of Change of Frequency (ROCOF) of the grid is varied around the limit value, and the Time-to-Trip (TtT) is evaluated. The scenario sequence for the ROCOF sequence for the Rate of Change of Frequency (ROCO) validation is presented below:
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For this scenario, the voltage amplitude of the grid is varied below the specified limits and then, outside of those limits, Time-to-Trip (TtT) is measured. Also, the voltage ride-through mode through (VRT) mode output signal is recorded. Limits and times are defined as follows:
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Step id | Test Description | Testing Time (s) | Signals of interest |
|---|---|---|---|
3.1 | Adjust the power output to Prated=5k. | 3 | P, Q, Vgrms, TtT |
3.2 | Step the grid voltage to the voltage point 0.99Vo2 | 1 | |
3.3 | Step the grid voltage to the voltage point 1.105Vo2 | 1 | |
3.4 | Measure the clearing time (time-to-trip) from the moment that the voltage achieves Vo1. Record the voltage ride-through (VRT) mode | 0 | |
3.5 | Clear the fault and re-initialize the inverter at nominal values | 2 | |
3.6 | Repeat steps 3.2 to 3.5 for N=2 for OV and N=2 and 3 for UV | - | |
3.7 | Step grid voltage to 1.01Vo2 and watch for momentary cessation | 2 | |
3.8 | Step grid voltage to 0.99 Vu1 and watch for momentary cessation | 0.5 |
Simulation results are presented in below. The voltage protection block generates a trip signal whenever the time defined for each voltage region passes. However, the voltage ride-through mode (VRTm) signal specifies the ride-through mode that the converter should assume during abnormal conditions. It can also be noticed that, when the voltage ride-through mode (VRTm) is equal to to 2 (momentary cessation), power injection is zero, but the trip flag is not activated until the TtT has Time-to-Trip (TtT) has passed. As it is appreciated from the simulation results, the TtT for Time-to-Trip (TtT) for each voltage region shows adequate results, and the trip signal and expected trip signal match with a slight difference caused by the RMS voltage estimation filter.
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The bridge protection validation is divided into AC side and DC side. For both scenarios, the active power reference is increased at t = 2.5 s and the trip signal is activated when the instantaneous current crosses the defined threshold and the action time delay has passed. The Time-to-Trip (TtT) is measured for each test. However, for the sake of simplicity, the Time-to-Trip (TtT) was measured using ScopeView for both scenarios. The parameters for the tests are defined as follows:
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Important note: Depending on the desired test scenario (AC test or DC test), the two two threshold values must be manually replaced in the mask as follows:
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The values with the "_nom" extension are used as regular threshold values that are used in all the scenarios presented in this helpfilehelp file. To run the AC or DC bridge protection test scenario, just replace the "_nom" extension by "_AC" or "_DC" in both fields.
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For this release of the Smart Inverter Library, the General Electric (GE) method for active islanding detection was implemented [3]. This method is suitable for three-phase power systems and has three different configurations: General Electric (GE) Frequency, General Electric (GE) Voltage, and General Electric (GE) Voltage+Frequency. All of them produce positive feedback in the voltage and/or frequency with the output current to generate abnormal conditions. These abnormal conditions (voltage and/or frequency) are detected by the voltage and/or frequency protection blocks of the library. The configuration of the Islanding Detection feature is implemented inside the Wave Reference block. Refer to its help file for more details.
For this scenario, the converter injects 0.6 p.u. of active power and, at t = 6 s, the grid-connection breaker is open to induce an unintentional islanding event. The figure below shows the simulation of an unintentional islanding event without using any islanding detection method. Although the RMS voltage is reduced, this deviation is not enough to trigger a trip signal from the voltage protection block. For cases like this, where the power sharing before islanding is similar to the local load demand after islanding, active detection algorithms such as the General Electric (GE) scheme are required.
Scenario 4.1 - General Electric (GE)-Frequency Scheme for Islanding Detection
The figure below shows the results using the General Electric (GE) Frequency scheme. The trip signal is activated once the breaker is open. However, it is noticed that it is activated by the voltage protection instead of the frequency protection block. It is also noticed that the frequency is having an unstable oscillatory behavior between the breaker opening time and the trip signal acknowledgment.
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The figure below shows the results using the General Electric (GE) Voltage scheme. The trip signal is activated once the breaker is open. It is also noticed that the voltage is having an unstable oscillatory behavior between the breaker opening time and the trip signal acknowledgment. The voltage protection detects the abnormal condition and sends the trip signal accordingly.
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