Capítulo 4 4 Diseño Metodológico
4.5 Instrumentos de Recolección de Resultados
At a system cold start (black start), no reference signals are available from the sensors since the system is not operating. At start-up, the system is forced to remain in Black Start for a pre-defined interval (0.150 sec) to allow for the system controllers to start and the PLLs to lock onto any available reference sources. The black start references signals are generated from an internal signal generator operating at the fixed frequency of 60 Hz and at a fixed voltage level at the nominal primary voltage. At initial system startup, the black start source will remain active
until the grid is available at which time the tracking algorithm will be engaged to eventually allow the microgrid to join the grid.
Table 5-1 Synchronization Sources
Sync Source Enumerated Type
Black Start 0
Grid 1
Track 2
Island 3 Open-Loop 4
Figure 5-5 Synchronization Reference Generation
5.3.2 Island
If no valid grid signal is present, the selected source will be ‘Island’ resulting in the references signal to be generated from a dq-PLL which uses the transformer primary voltage to generate the frequency reference. The voltage reference signal is set to a fixed magnitude (nominal primary voltage) at the desired frequency. The dq-PLL must be active and processing the primary voltage prior to the island being
formed in order for the PLL to generate a valid synchronization signal to be used by the voltage controller.
5.3.3 Grid
If the grid signal is present and valid, a dq-PLL extracts the grid references from the grid sensor which is located on the secondary side of the transformer on the grid side of the PCC contactor. The extracted values must be corrected for the 30° phase shift that has been introduced by the delta-wye connection of the transformer. The tracking voltage reference sets the reference magnitude to match the grid.
5.3.4 Open-Loop
The open-loop synchronization source used is the same as the black start source. The open-loop is only used when the operation of the system is under manual control by adjusting the modulation index to regulate the output to reach the desired secondary voltage (120 VRMS).
Table 5-2 Transformer Turn Ratios
Phase Turn Ratio
A 17.48 B 17.78 C 17.80
5.4 Frequency Tracking
One of the important control functions is to adjust the phase of the island system to match the phase of the grid. Once a valid grid frequency has been determined, the tracking function begins to slowly adjust the tracking reference waveform to match the grid phase. Once the tracking frequency matches the grid, the intertie relay will reclose allowing the microgrid to join the grid. The tracking function is disabled and the tracking waveform locks to the grid when the connection to the grid is established. If the grid is not valid the tracking function locks onto the island reference. The tracking operation is controlled by the Track Logic function which bases its decision making on the specified synchronization source.
The voltage matching function (shown in Figure 5-6) extracts the grid magnitudes for each phase and generates the corresponding voltage magnitudes for the primary references. The turn ratios from Table 5-2 are used to translate the magnitudes from the secondary (grid) to the primary side of the step-up transformer. The variation in the turns ration between phases is a result of differences in the windings during the fabrication of the transformer. Differences between phases can also be introduced due to component differences between the phases (inductors, capacitors, etc.).
Figure 5-6 Frequency Adjustment System
5.5 Tracking Technique
The frequency adjustment algorithm shown in Figure 5-6 calculates the difference between the grid frequency (ωtGrid) and the island frequency (ωtIsland). The initial
modified delta is fed into a simple proportional gain control loop. The result (u) is
combined with the grid frequency (feed forward term). The particular difficulty with this technique is the process used to generate the frequency difference which in turn is used to generate the new desired frequency (ωtAdjust). The cause of the
difficulty is that the frequency signals are ramp functions from zero to 2π at which point the signals are reset to zero.
Figure 5-7 Frequency Synchornization Cases
Two general conditions can exist: the island leads the grid (Cases 1, 3, 5 & 6) or the island lags the grid (Cases 2, 4, 7 & 8). The different cases are shown in Figure 5-7. The simple case when the grid and the island are in phase is when the difference is 0 rad/s (or 2π rad/s which is equivalent). The following is a high level description of the frequency synchronization cases as shown in Figure 5-7. Initially, the difference is calculated and used to determine which case is applicable.
raw delta = ωtGrid - ωtIsland
Case 1: Island Leads Grid, Island Resets to Zero
delta = -2π + ωtGrid - ωtIsland
Case 2: Island Lags Grid, Grid Resets to Zero
delta = 2π + ωtGrid - ωtIsland
Case 3: Island Leads Grid, Island above π, Grid bellow π
if (raw delta) > π/2
delta = -2π + ωtGrid - ωtIsland
endif
Case 4: Island Lags Grid, Island below π, Grid above π
if (raw delta) < -π/2
delta = 2π + ωtGrid - ωtIsland
endif
Case 5: Island Lags Grid, Island and Grid below π Case 6: Island Lags Grid, Island and Grid above π Case 7: Island Leads Grid, Island and Grid below π Case 8: Island Leads Grid, Island and Grid above π
if (raw delta > 0)
if (raw delta) > π/2
delta = -2π + ωtGrid - ωtIsland
if abs(delta) > π
delta = raw delta endif
endif else
if (raw delta) < -π/2
delta = 2π + ωtGrid - ωtIsland
if abs(delta) > π
delta = raw delta endif
endif endif
5.6 Island Validity
The island operation is considered valid if the primary transformer voltage and frequency are within the following limits:
6.24 ≤ / ≤ 7.62 \ (5.10)
58.8 ≤ + ≤ 61.2 O (5.11)