Учебники / 0841558_16EA1_federico_milano_power_system_modelling_and_scripting

20.2.7Direct-Drive Synchronous Generator

The model of the direct-drive synchronous generator is assumed steady-state, as the stator and rotor flux dynamics are fast with respect to grid dynamics. Furthermore, the converter decouples the generator from the grid. As a result of these assumptions, one has the DAE system described below. Table 20.8 summarizes and defines all parameters required by wind turbine with directdrive synchronous generator.

Table 20.8 Direct-drive synchronous generator parameters

Network Interface

The active and reactive powers injected into the grid depend on the grid side current ic,d + jic,q and voltage vc,d + jvc,q of the converter:

where the converter voltages are functions of the grid voltage magnitude and phase, as follows:

Machine Electro-Magnetic Equations

Assuming a permanent magnet synchronous generator, machine equations are

where the permanent field flux ψp represents the rotor circuit. The active and reactive power produced by the generator are as follows:

Turbine and Machine Mechanical Equations

The generator motion equation is modeled as a single shaft, i.e., (20.27), as it is assumed that the converter controls are able to filter shaft dynamics. For the same reason, no tower shadow e ect is considered in this model. In (20.27), the electrical torque is:

Substituting (20.50) in (20.51) leads to:

Mechanical equations are completed by the mechanical torque τt equation (20.25) and the turbine model (20.16) and (20.17)-(20.18) or (20.19)-(20.20) and the pitch angle control (20.21).

VSC Regulators

The back-to-back VSC that connect the generator to the grid allow controlling four quantities. Controllable quantities are the converter currents is,d, is,q , ic,d and ic,q. Typical controlled quantities are the active power injected into the grid, the grid side voltage, the dc voltage of the capacitor in the dc back-to-back connection, and the reactive power on the generator side. Several combinations have been proposed [3, 4, 119, 351]. A possible control scheme is as follows.

The currents on the generator side control the rotor speed and the generator reactive power:

where pw(ωm) is the power-speed characteristic (20.22) or (20.23) and qs0 is the reactive power determined at the initialization step.

The currents on the grid side control the active power and bus voltage:

The first of the previous equations indirectly models the dynamic of the dc system of the back-to-back connection. The steady-state error ps − pc is a model of VSC and capacitor losses. A pure integrator can be used for modelling a loss-less back-to-back VSC:

If the dynamics of the VSC dc connection are fast, one can simply impose:

Limits

All currents in (20.53) and (20.54) undergo anti-windup limiters. The limits have to be carefully calculated to avoid overloading. On the dc side, one has:

where imax is the VSC maximum current. Since (20.57) contains two variables, additional conditions are required to define imaxc,d and imaxc,q . For example:

c,q

imaxc,d = (imax)2 − i2c,q

Thus, one of the limits, e.g., imaxc,d is a function of the other current. Then, for the lower limits:

Similar expressions can be defined for the limits of generator stator currents

is,d and is,q .

Part IV

Spare Material and Concluding

Remarks

-Fixed position, fixed order. This is the oldest method of storing data and date back to the paper cards that were used in the seventies for loading and saving the information used by computers. Any data has is starting and ending columns and is assigned a maximum number of digits. Also the order in which data are listed is fixed. For example, in the IEEE CDF the bus card comes before the branch card, and the voltage magnitude must occupy columns 28 to 33 of the bus card [350]. The formats created by WSCC-EPRI [90], Eurostag [92], UCTE [318], and ENEL (INPTC1) [87] are other examples of this kind of format.

-Free position, fixed order. The position of the data is free, however, the order in which the data is listed in the file is fixed. For example, in the PSS/E format, the load data must follow the bus data, and the generator data must follow the load data [245]. In the bus data, the bus name must follow the bus identification number, etc. However, there is no restriction on the token position, and data can be separated by commas or by spaces. Other examples of this kind of formats are those of GE-PSLF [88] and FlowDemo.net [83].

-Free position, free order. The position and the order of the data are free. Typically, an ad-hoc parser is needed to read this kind of data files, which makes quite di cult to create an import/export utility. Some examples of such formats are the formats used by Simpow [302], DigSilent [75], PowerWorld [246] and InterPSS [359]. A special example of this format are also all data files written in Matlab (see for example, PSAT [194], PST [59], and Matpower [363]).

In this case, the data file is parsed by the Matlab interpreter, which makes extremely flexible the information that can be stored in the file, but also hard to export to other platforms if not using the Matlab interpreter.

-Mark-up languages Mark-up languages allows the maximum freedom for organizing data. The idea of mark-up languages is simple but powerful: each data is introduced by a pre-defined syntax which allows clearly identifying the data itself. Thus the position and the order of the data is not relevant. For example a widely used mark-up language is XML [342]. Another example is the Resource Description Framework (RDF) that is used for CIM data bases.

21.1.2Kind of Supported Data

This feature a ects the contents of the data file. Clearly, the more data kinds the format can support, the more complete and general the format is. A format that pretends to be application independent should provide as many kind of data as possible. For example, typical data kinds are as follows.

-Static Data. (e.g., power flow data).

-Dynamic Data. (e.g., synchronous machine and regulator parameters).

-Market Data. (e.g., generator and load bids).

-Short Circuit Analysis Data. (e.g., negative and zero sequence of generators and transformers).

-Graphical Data. (e.g., network scheme, geographical information system, etc.).

-Other Data. (e.g., FACTS data, user defined component data, etc.).

21.1.3Number of Files

Having multiple files for defining a network can be an useful feature if working with several scenarios for the same network. For example, one can use a single power flow data and work on several dynamic scenarios. This feature allows reducing the amount of data stored on the computer. However, nowadays, this is a not so critical issue taking into account that a 15000 bus system takes about 10 MB of disk space while modern hard disks contain hundreds of GB. Furthermore, the “modification command” described in the next subsection is a better option than the multiple-file feature for multi-scenario studies. The following items describes some examples.

-Single file. Most of the data formats requires a single file for defining the whole network. This is typical of most formats.

-Multiple fixed number of files. Some formats uses di erent files for different information. For example the power flow data is in some case separated from the dynamic data (e.g., CYME [68] and Eurostag [92] formats).

-Any number of files. The Simpow format [302] provides the possibility of including any number of files. Nested file inclusion is also allowed. This feature is useful in case of large networks, where the amount of data is cumbersome. The user can be interested in modifying only a small part of the network and it is thus easier to work on a small file that is then included in the main data file.

21.1.4Default Values, Prototypes and Data Manipulation

Accepting default data is both a feature of the data format and of the application that reads the data format. The application that reads the data file must assign a known default value to all parameters that are not specifically defined in the file. Thus, it is necessary that the documentation of the data format clearly specifies default values.

Similar to default value definition is data prototyping, which consists in referring certain device data to a common device prototype. Any device instance inherits the data of the prototypes. This technique is similar to creating

a class (prototype) and then instantiating several times that class.1 The advantage is that a change in the prototype data automatically update all the data referring to that prototype.

Finally, being able to modify data “on the fly” is an useful feature for scripting applications. For example, if one wants to solve the power flow for di erent load levels, it could be convenient to have a compact command included in the data file that modifies the load powers without the need of rewriting all load data.

The following items describes some examples.

-Default Values. Most formats, especially fixed position formats, does not support default values and force writing all data of a component, even if those data are not known or could be easily deduced by default.

-Device Prototyping. This feature is provided by the DigSilent format [75]. In case of big distribution networks where most of the transformers and protections are the same, data prototyping can save space. The Simpow format also provides a sort of device prototyping feature in the definition of synchronous machine regulators [302]. Synchronous machine data include the code of a certain AVRs and/or turbine governor prototype. This technique allows reducing the data file size if most of AVRs and turbine governors have same data.

-Modification Command. The Simpow format provides the powerful alter command for modifying the base case data. Matlab-based formats implicitly include modification commands, since any Matlab function and matrix manipulation can be included in the data file.

21.2Canonical Model

From the previous section, it is clear that if one wants to use di erent software tools, some data import/export utility is needed. Software companies such as PowerWorld and most open-source tools, such as UWPFLOW, InterPSS and PSAT, develop and provide adapters to import data. However, the direction is mainly one way, i.e., importing data in di erent formats into simulation applications, but generally not the other way round, i.e., exporting data to di erent data formats. This makes very di cult (if not impossible) to exchange power system simulation study case in a robust and reliable way.

Another strong restriction to the di usion and creation of data format adapters is the fact that the documentation of most commercial data formats is not freely available and, in some cases, is intentionally not complete.

Even if one can access the full and detailed documentation of any formats in use in the power system community, there is another important issue that

1The di erence between “classes” and “prototypes” is that a class defines a abstract type that in general does not contains any specific data, whereas a prototype is a template that defines data.