Monitoring Method for Off-grid Capacity of Direct Drive Wind Farm

A direct drive wind farm (DDWF) is composed of multitudinous permanent magnet synchronous generators (PMSG), with a small single-unit capacity. When the power grid mal-functions, some units in the DDWF may enter a fault traversal state, and some units may be disconnected from the grid. At this time, the DDWF exhibits a decrease in active output. Due to the large number of units in DDWFs, it is difficult to directly monitor each off-grid unit, resulting in the inability of the power grid to timely and accurately determine the load shedding amount. This article proposes a monitoring method for the disconnection capacity of DDWFs, which can calculate the disconnection capacity of DDWFs after a fault occurs by only monitoring the electric quantity at the connection point of the DDWF. And a DDWF grid connection model is built using PSCAD simulation software. The reliability of the monitoring is authenticated through simulation using a single-phase grounding short circuit fault as an example.


2.PMSG and Its Control Model
The PMSG is directly grid-connected through the converter.The operating features of the converter determine its external characteristics.A simplified grid connection diagram of PMSG is shown in Figure 1.The PMSG grid-side converter transmits the electric energy to the grid.The converter only plays the role of power conversion and transmission.Based on the equally active power on both sides of the converter, it can be obtained: In Figure 2, the control diagram of the PMSG grid-side converter is shown.After setting the voltage orientation, the AC side current of the grid side converter is decomposed into d and q axis currents.Among them, the active power is controlled by the d-axis current.The voltage outer loop is used to provide a reference value for the d-axis current, which is used to sustain an invariant DC bus voltage in the converter.According to the needs of the power grid, the reference value * q i can be modified to adjust reactive power output.

3.LVRT Control Strategy for PMSG
If the voltage at the grid-connected point of the DDWF is all within the voltage contour line or above in Figure 3, the wind turbine unit should ensure continuous operation without disconnection.Otherwise, the wind turbine unit is allowed to switch off.The dynamic reactive current output by the DDWF needs to meet the non-equality:   1.5 0.9 0.2 0.9 According to the standard of PMSG grid connection, the output reactive current of PMSG is proportional to the voltage drop.It shall meet the requirements of the equation:
In Figure 2, the reference value of active current is determined by the voltage outer loop.
The output current of the converter has amplitude limitation.Under the premise of reactive current priority output, the active current should meet the following equation.
lim it i -Converter current limiting.By synthesizing the above equations, the control equations for PMSG can be obtained.
1.5 0.9 0.2 0.9 The following equation can be obtained by discretization of Equation (8).

4.Monitoring Method for Off-grid Capacity
When the power grid malfunctions, the PMSG may experience the following three states: normal operation, LVRT, and disconnection from the grid.According to the LVRT specification of the DDWF, the reactive current output by the PMSGs in different states is inconsistent.Assuming a DDWF has p PMSGs.The single unit capacity is q MW.And the proportion of units operating normally during faults is x, the proportion of low voltage crossing units is y , and the proportion of off-grid units is z .
According to Kirchhoff's current law, the currents at the grid point of the DDWF are the sum of the current output by each unit.
By monitoring the electric quantity of the grid-connected point, the proportion of normal operation and low voltage crossing units can be calculated.The current of the grid-connected point is output by both normal operation and LVRT units.The proportion of normal operating units is obtained by removing the part of the active current that is output by the low voltage crossing unit, and then dividing it by the active current value d i  that should be output by all units during normal operation.Similarly, we shall remove the part of the reactive current that is output by the normal operating unit, and then divide it with the reactive current value q i  that should be output by the LVRT operation of all units.The result is the proportion of the normal operating units.According to equation (8), it can be concluded that Substituting Equation ( 14) into Equations ( 12) and ( 13), obtain After calculating the proportion of normal operation and LVRT units, the proportion of off-grid units can be obtained according to Equation (10).The off-grid capacity of the DDWF can also be calculated, which is p q z   MW.

5.Simulation Analysis
For the sake of verifying the feasibility and effectiveness of the off-grid capacity monitoring method, this paper simulates a DDWF in the PSCAD/EMTDC platform.The DDWF experienced a single-phase ground fault at 0.3 seconds.In Table 1, the specific system parameters are shown.

Conclusions
Wind power has drawbacks like volatility and randomness.Because of the continuous improvement of the wind power scale, the stability and safety of power systems face huge challenges.The defense against the disconnection from the grid of new energy has been included in the third-level security and stability standards of the power system.A capacity monitoring method for DDWF off-grid units after a malfunction occurs is proposed in this paper.By only monitoring the electric quantity at the grid-connected point of the DDWF, the off-grid capacity of the DDWF after the fault occurs is calculated.There is no need to monitor each PMSG within the DDWF, thereby reducing construction costs.

CP-
Output active power of PMSG; dc u -DC voltage of PMSG; C -Voltage stabilizing capacitance; d U and d i -D-axis voltage and active current at the grid-connected point of DDWF; r U -Voltage loss of converter.

Figure 2 .
Figure 2. The Control Diagram of the PMSG Grid-side Converter

d u and q 2 IK
u -Output voltage reference values of the grid-side converter; and -PI regulator capacity ratio and integral gain of active and reactive power control link; * d i and * q i -Current reference value; d i and q i -Current Actual value; d e and q e -Disturbance of voltage.
of the grid-connected point; _ d l i -Active current output by LVRT operation units; _ q n i -Reactive current output by the normal operation units.

Figure 4 .
Figure 4. Simulation Diagram of Grid-connected System for DDWFAs shown in Figure4, the DDWF has three PMSGs with the same capacity.After the fault occurred, the three PMSGs were placed in normal operation, LVRT, and disconnection from the grid by changing the control command.So, the off-grid capacity of the DDWF accounts for one-third of the total capacity, which means 1 2 1 3 z z   .As shown in Figure 5, by adjusting the transition resistance value, the degree of voltage drop at the grid-connected point of the DDWF is changed.The active current and reactive currents output by the PMSGs are shown in Figure.6 and Figure.7 when the fault phase voltage drops to 0 pu or 0.5 pu.Based on the data in Figure.6 and Figure.7, the parameters inTable 2 can be calculated We shall substitute all known parameters and variables into Equation (15), and the calculation results are 1 0.321 z  and 2 0.352 z  .There is a certain error with the set value.But the error is within a reasonable range and can be ignored.Hence one can see that the offline capacity monitoring method is accurate and feasible.Table 2. Measured and Calculated Values of Active and Reactive Currents (kA).

Figure 7 .
Figure 7. Current Curve of PMSG when Fault-phase Voltage at the Grid-connected Point is 0.5 pu.

Table 2 .
Table 2 can be calculated We shall substitute all known parameters and variables into Equation (15), and the calculation results are 1 0.321 z  and 2 0.352 z  .There is a certain error with the set value.But the error is within a reasonable range and can be ignored.Hence one can see that the offline capacity monitoring method is accurate and feasible.Measured and Calculated Values of Active and Reactive Currents (kA).
Current Curve of PMSG when Fault-phase Voltage at the Grid-connected Point is 0 pu.