IEEE Power & Energy Magazine - November/December 2016 - 30

Wind Farm PCC Voltage (p.u.)

that grid-connected wind farms
should have low-voltage ride1.2
Voltage Outline
1.1
through (LVRT) capability, and
1
those without this capability shall
0.9
make corrective improvements
0.8
to add it. To define the require0.7
WT Must Not Trip
0.6
ments on the LVRT capability
0.5
of grid-connected wind farms,
0.4
China Electric Power Research
WT May Trip from the Grid
0.3
Institute (CEPRI) took the lead
0.2
0.1
in revising Technical Rule for
0
Connecting Wind Farm to Power
-1
0
0.625 1
2
3
4
System (GB/T 19963-2005) by
Time (s)
inserting explicit requirements
on the LVRT curve, active power
figure 6. LVRT requirements for wind farms.
recovery, and reactive support
capability. The rule has been in
force
since
30
June
2012.
Figure
6 shows the LVRT require✔ Some wind turbines were not equipped with FRT capability and even absorbed reactive power after their ments for wind farms.
According to the new standard, wind turbines shall be
passive crowbar acts during the fault. This degrades
able to keep operating for 625 ms when the voltage of the
system voltage even more.
✔ Wind farm reactive-power compensators cannot PCC drops to 20% of the nominal voltage. Wind turbines
react in time. After a number of wind turbines dis- should stay connected to the power grid and remain operatconnect from the grid due to low voltage, static var ing when the PCC voltage returns to 90% of the nominal
compensators (SVCs) and static var generators (SVGs) voltage in 2 s. Furthermore, for every wind power cluster
fail to respond correctly, and control strategies do with a total installed capacity over 1 GW, when voltage
not meet the system requirements or their response drops due to a three-phase short circuit fault on the power
time is incorrect. The grid voltage climbs due to the system, each wind farm should have the following dynamic
huge reactive-power surplus, which then causes other reactive power support capability during the LVRT process:
groups of wind turbines to disconnect from the grid
✔ When the PCC voltage is within 20-90% of the nomibecause of overvoltage protection.
nal voltage, the wind farms should be able to inject
reactive current to support voltage recovery. From the
✔ Reactive power compensators are allocated inefmoment the PCC voltage drops, dynamic reactive curfectively in wind farms. The reactive power rating
rent control should respond in 75 ms, and the duration
(MVAr) of single capacitors is too high, causing the
should not be fewer than 550 ms.
switching of such compensators to lead to unacceptable voltage fluctuation.
✔ Dynamic reactive current injected from a wind farm
This was the situation in China around 2011.
to the power system needs to increase at least linearly with the depth of the voltage depression. The exact requirement is given by the following formula:
Grid Code Formulation and
I T $ 1.5 # (0.9 - U T ) I N, for (0.2 # U T # 0.9), in which
Measurement and Test
To prevent the types of wind turbine trip-off incidents
U T is the PCC voltage of wind farm in per unit and
observed in 2011, specialized remediation work was carried
I N is the rated current of wind farm, respectively.
out by State Grid Corporation of China (SGCC) and power
High-voltage ride-through (HVRT) capability requiregeneration companies. Remediation measures included
ments are specified in Technical Regulation for Wind Farm
Reactive Power Configuration and Voltage Control, an
✔ requiring FRT capability of wind turbines
✔ confirming the response of reactive power compensators energy industrial standard formulated by CEPRI. The basic
requirements on HVRT are to tolerate 120% voltage for up
✔ verifying protective relay settings.
Those actions, and others, have achieved observable improve- to 200 ms and 115% voltage for up to 10 s.
ment in the FRT capability of wind farms across the country.

Formulation of New Grid Codes
To avoid the recurrence of large-scale wind turbine trip-off
and secure the reliable operation of wind farms and the power
grid, the State Electricity Regulator Commission (SERC) and
NEA promulgated a series of regulations, clearly requiring
30

ieee power & energy magazine

Field Testing Wind Turbines
Requiring that wind turbines have LVRT capability is
important for grid security. But this requirement is technically challenging for the wind turbine manufacturers. Therefore, it is important to test the actual LVRT performance of
operating wind turbines.
november/december 2016



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2016

IEEE Power & Energy Magazine - November/December 2016 - Cover1
IEEE Power & Energy Magazine - November/December 2016 - Cover2
IEEE Power & Energy Magazine - November/December 2016 - 1
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IEEE Power & Energy Magazine - November/December 2016 - Cover3
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