Yuriy Kazachkov Principal Consultant yuriy.kazachkov@siemens.com	Ping-Kwan Keung Consultant ping-kwan.keung@siemens.com	Jay Senthil Senior Staff Software Engineer jayapalan.senthil@siemens.com

Development of dynamic stability models in PSS®E to simulate wind farms as elements of power systems started with vendor-specific models. So far about 25 vendor-specific models have been developed.

Many of these models have been used successfully for years to study dynamic response of the systems with interconnected wind farms. However, since these models contain proprietary information, use of most of these models requires manufacturer’s authorization, which in turn makes the process of incorporating vendor-specific models into dynamic setups of utilities and power pools difficult.

In view of this, the idea of publicly-available generic wind models that can be parametrically adjusted to any practical implementation was well received by the community of transmission planners.

While attempting to create generic wind models, it became clear very early on that there was no way that one generic wind model could represent the various types of wind technologies. Based on the analysis of the current wind turbine market, the suggestion has been made to develop four following basic topologies of generic models based on the type of the generator and their interface with the grid:

Type 1 – conventional directly connected induction generator
Type 2 – wound rotor induction generator with variable rotor resistance
Type 3 – doubly-fed induction generator
Type 4 – full converter interface

Generic models of all four types will be included in the next point release of PSS®E version 31 as user-written models. These will eventually be made PSS®E standard models (i.e., supplied as part of the PSS®E library of models) in version 32, which is due in the Spring of 2009.

A brief description of generic models is provided below. These models are intended for transmission planning studies, which focus on grid disturbances, not the wind disturbances. This implies that wind speed is assumed to be constant during the dynamic simulation.

In power flow all generic models are represented as wind machines (a new category of machines in PSS®E-31), with reactive power set consistently with the type of wind turbine. All generic models are initialized directly from the power flow; no auxiliary subroutines, like IPLAN programs, are needed to either set up the power flow or create the dynamic input data file.

Each generic model includes two or more modules responsible for simulating specific controls or equipment. Some of these modules are shared by several generic models.

WT12T – wind turbine module
The block diagram of WT12T is shown in Figure 1. Input signals for this module are “aerodynamic” torque and electrical torque, which in turn are outputs of the pseudo-governor module WT12A and the generator/converter module respectively.

Input parameters for the WT12T module are the total inertia of the drive train H, which includes the rotor blades, the shaft, and the machine together with the gear box, the inertia of first two being a portion Htfrac of H, a mechanical damping of the shaft system, and the first frequency of mechanical oscillations Freq1. The outputs of the WT12T module are rotor speed deviations on the blade and machine sides, the rotor twist angle, and the machine rotor angle deviation. By setting Htfrac=0 the two-mass shaft system is converted into a conventional single-mass system where inertias of all drive train elements are combined.

Figure 1 - Block Diagram of WT12T Module

WT12A – Pseudo-Governor Module
The block diagram of the pseudo-governor model is shown in Figure 2. The key objectives in developing this model were to:

This module was designed and developed after thorough investigation of aerodynamic characteristics and pitch control of several vendor-specific wind turbines. The model uses the following two inputs: the rotor speed deviation, and the real power at the machine terminals. The filtered output is the “aerodynamic” torque on the rotor blade side.

Figure 2 - Block Diagram of WT12A Module

The WT1 Generic Model
The connectivity diagram of the WT1 wind generic model is shown in Figure 3.

Figure 3 - Connectivity Diagram of WT1 Model

The generator model WT1G is based on the standard PSS®E model of the induction generator CIMTR3. This model takes into account the rotor flux dynamics and can be used for single cage or double cage machines. At initialization this model calculates the actual reactive power consumption of the machine Qact at given terminal voltage and MW-dispatch. It places a “hidden” shunt on the machine terminal bus, with the size equal to a difference between Qgen from the power flow and Qact.

The WT2 Generic Model
The connectivity diagram of the WT2 wind generic model is shown in Figure 4.

Figure 4 - Connectivity Diagram of WT2 Model

The generator model WT2G is based on the standard PSS®E model of the induction generator CIMTR3. At initialization, the model determines the portion of the available external rotor resistance that would have to be added to achieve the steady-state operating point. During the simulation the value of the external rotor resistance comes from the electrical control module WT2E (Figure 5). This module uses the machine rotor speed and electrical power as inputs and calculates the portion of the available rotor external resistance to be added to the internal rotor resistance.

Figure 5 - Block Diagram of WT2E Module

The WT3 Generic Model
The connectivity diagram of the WT3 wind generic model is shown in Figure 6.

Figure 6 - Connectivity Diagram of WT3 Model

This model uses a simplified approach to aerodynamic torque calculation, which has been developed from the performance analysis of vendor-specific models. This calculation is done by a combination of the pitch control and wind turbine modules. The latter also includes provision for the two-mass shaft simulation, similar to the WT12T module described above.

The converter control module has two paths responsible for the machine torque control and reactive power/voltage control. The reactive control suggests a selection from various control options, namely:

• Remote bus voltage control
• Power factor control
• Reactive Power control

Outputs of these paths are active current and magnetizing voltage commands.

The generator/converter module transforms these slightly filtered commands into current injection of the doubly-fed induction machine to the grid. It has a provision for including a phase-locked loop to synchronize the generator rotor currents with the stator.

The WT4 Generic Model
The connectivity diagram of the WT4 wind generic model is shown in Figure 7.

Figure 7 - Connectivity Diagram of WT4 Model

The converter control module (Figure 8) includes reactive and active power controls. The former is similar to one used for the WT3 model. The active power control is based on the idea that we do not need to simulate a machine at all. Independent of the way the active power control is implemented and which criteria it uses, this control is responsible for keeping the power balance between the machine and the grid injection.

Figure 8 - Block Diagram of WT4E Electrical Control Module

Validation of Generic Wind Models
For all four generic models, validation tests have been performed using vendor-specific models as benchmarks: Mitsubishi MWT-1000 and Vestas V82 for WT1 models, Vestas V80 for WT2 models, GE 1.5/3.6 MW for WT3 models, and GE 2.5 MW and Siemens 2.3 MW for WT4 models. The criteria for comparison were Pelec, Qelec, and Vterm. Validation tests confirmed that generic models can be adjusted parametrically to benchmark models and the leverage for this adjustment is flexible enough.

Voltage and Frequency Protection
Standard PSS®E models FRQDCA and VTGDCA can be used to simulate the characteristics of respective protection systems or what is sometimes called the Low Voltage Ride Through characteristics.

Summary
The next point release of PSS®E version 31 will include a number of user–written generic wind models based on four types of generator-grid interfaces. These will be made PSS®E standard models in version 32. This article provides a brief description of four types of PSS®E generic wind models.

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