1. Siemens Wind Turbine Weak Grid Option Modeling in PSSE WECC library Hongtao Ma, Siemens Wind Power Joergen Nielsen, Siemens Wind Power
Good morning, everybody, I am hongtao, engineer from Siemens wind. Today I am going to make a presentation of “Siemens WP IR modeling”
Restricted © Siemens AG 2016

2. Background
Wind power penetration increased
More weak grid interconnected projects
Weak grid projects experienced voltage oscillations and turbine tripped
Weak grid option(WGO) was applied in projects to help stabilizing turbine/POI
ISO/Utility demand WECC model but WECC library model don’t have this option
First, I will introduce some background: more and more wind plant is integrated to the grid and ISO concerns the whole grid frequency stability. ISO prefer/required wind plant providing the same support as synchronous generator with IR function. Not only providing IR function on the real wind project to, ISO/Utility also asked for the IR model for them to do interconnection study. Some wind vendor may have already develop the IR model, but some ISO only accept WECC library model for wind project.
So the objective is to develop the accurate IR model in PSSE library to accommodate different vendors’ IR function feature. I am here to describe the ISO’s general requirement on IR and how Siemens wind plant realize IR function.

3. Weak Grid Definition What is a weak grid?
Short Circuit ratio (SCR) refers to 3-phase short circuit level MVA divided by the aggregate turbine MW at point of interconnection (POI)
An SCR above 4 is recommended to ensure steady-state stability in VDN Transmission code 2007 (Germany)
SCR≤ 4 is considered a ”weak grid”
Siemens wind turbine can be tuned to work for SCR=1.5
Now, I am going to the details of wind farm IR function and controls. We are concerning two main Scenarios on IR control. First, grid f drop and IR triggered, wind farm is controlled to output the boosted power. After the required time of boosted power, f is still low and not restored. Then wind farm will have to stop IR and restore the normal operation. The second Scenario is, while wind farm is under IR control, grid frequency is restored, wind farm need stop IR and restore to the normal operation.
In short, even wind farm is under IR control, grid frequency need to be monitored. It is not a fixed time of IR after being triggered.

4. Wind Project Challenge for Weak Grid Connection
Main concerns:
Low frequency voltage/power oscillations from small signal disturbance
FRT capability—voltage/power stably restored after fault is cleared
HVDC or Series capacitor connected wind plants has difficulty to restore stable operation after fault is cleared
Now, I am going to the details of wind farm IR function and controls. We are concerning two main Scenarios on IR control. First, grid f drop and IR triggered, wind farm is controlled to output the boosted power. After the required time of boosted power, f is still low and not restored. Then wind farm will have to stop IR and restore the normal operation. The second Scenario is, while wind farm is under IR control, grid frequency is restored, wind farm need stop IR and restore to the normal operation.
In short, even wind farm is under IR control, grid frequency need to be monitored. It is not a fixed time of IR after being triggered.

5. WGO Control Strategy Active current Id clamp
2. Active current ramp rate is limited
3. Id is hold for 1s to give the headroom of reactive power/current support to the grid voltage.

6. Wind Plant Performance without WGO(SCR ≈1.7)
Scenario 1. Standard wind turbine control
3 phase fault is applied at 1s.
After fault is cleared, the turbine ramp up the output active power
The turbine output reactive power dropped quickly
The grid voltage lose wind plant support and get another dip
Wind turbine in FRT mode again, which gain more reactive power/voltage support
FRT in-out is repeated to cause the oscillation
Here I will described Scenario 1 and point out those main point, which need to be modelled.
At the beginning, as grid frequency drops below the threshold,

7. Wind Plant Performance with WGO(SCR ≈1.7)
Scenario 2. Wind turbine response with WGO applied
After fault is cleared, the active power is not ramped to the rated power immediately
The turbine output reactive power to support POI voltage
Turbine restore stability and ride through the fault

8. Discussion: How to model WGO in WECC
Control is on turbine level and applied in projects
Siemens has WGO in PSSE user defined model
How can WECC implement WGO in the library model?