System Dynamics and EMT Simulation

DIgSILENT PowerFactory provides a basic simulation kernel, which, together with a comprehensive model library and a graphical, user-definable modeling system (DIgSILENT Simulation Language (DSL)), provides an extremely flexible and powerful platform for solving power system dynamic problems. Any combination of meshed 1-, 2-, and 3-phase AC and/or DC systems can be represented and solved simultaneously, from HV transmission systems, down to residential and industrial loads at the LV distribution levels. It includes transient analysis problems concerning short-, mid- and long-term dynamics, with adaptive step-sizes ranging from milliseconds to minutes.

DIgSILENT PowerFactory features integrated analysis of classical stability problems using the RMS simulation mode as well as electromagnetic transients via EMT simulation. In other words, PowerFactory incorporates solution techniques making additional EMT software obsolete. This approach has many advantages over the classical application of two separate software systems such as:

No duplication of data entry and case definitions
User defined models need to be setup only once
Absolutely compatible modeling features for RMS and EMT
Easy cross-check of RMS results via EMT simulations
No additional investments in other software and training

DIgSILENT PowerFactory is the leading software in power system dynamics. It’s modeling flexibility and precision, it’s numerical robustness and performance and especially it’s comprehensive model library is providing everything required to implement all kind of dynamic studies in the most efficient and economic way.

System Dynamics (Stability, RMS)

At DIgSILENT, it is a principle policy to provide the most accurate simulation models and up-to-date solution algorithms, to enable analysis of the complete range of power system dynamic phenomena. On basis on fast and reliable adaptive step-size algorithms, the simulation kernel features excellent precision when solving short-, mid- and long-term stability aspects.

Time frames for power system stability studies

The dynamic simulation tools available to the user in DIgSILENT PowerFactory incorporate the following features:

Calculation of initial conditions is carried out prior to the dynamic simulation, and is based on a solved load flow and other conditions such as power plant settings. Network representation and algorithm options are selected, with the following system representations available:
- positive sequence only - the classical RMS representation for simulation studies
- steady state a-b-c RMS representation, allowing any kind of asymmetrical fault;
Highly accurate and adaptive integration technique for solving the AC and DC network load flow and dynamic model equations. This is combined with a non-linear electromechanical model representation to enable a high degree of solution accuracy, algorithmic stability and time range validity;
Models for both solid and salient pole generators down to the sub-transient reactance and time constant level, with enhanced saturation effects, and Canay reactance incorporated;
A sophisticated model for asynchronous machines that includes primary and secondary leakage reactance saturation, rotor resistance slip-dependency, and an additional single/double cage asynchronous machine with parameter identification;
DC motor models, ASD systems (Adjustable Speed Drives), double-fed induction machine, PWM converter and other power electronic elements such as softstarter, inverter and rectifier;
General load models where load inertia, bus voltage and frequency dependence is represented. A special lumped load model to accurately represent feeders containing a high percentage of motors in the load. The capability of modeling motor stall effects is included, and was developed on the basis of comprehensive system tests.
The user can interrupt the simulation at any time, either manually or by scheduled interrupt time or automatically via interrupt conditions. When the simulation is interrupted, most DIgSILENT commands such as displaying or printing power flow results, checking the bus voltages, calculating eigenvalues or analyzing the controller status, etc. can be activated.
By activating predefined fault types, or by accessing and modifying DIgSILENT variables any type of fault can be realized. Typical faults are:
- line, transformer, feeder load and generator tripping
- starting/tripping of synchronous and asynchronous machines
- load shedding and shunt switching
- application and clearing of faults at substations or along lines
- change of controller set-points; controller failure
- synchronization of isolated areas
- injection of signals generated by a DSL device.
The user can interrupt the simulation at any time, either manually or by scheduled interrupt time or automatically via interrupt conditions. When the simulation is interrupted, most DIgSILENT commands such as displaying or printing power flow results, checking the bus voltages, calculating eigenvalues or analyzing the controller status, etc. can be activated.
Any DIgSILENT variable, or any quantity identified in the transmission network or dynamic models, may be selected for simulation observation or for later plotting within x/t or x/y diagrams or any other VI provided. In addition to these variables, the DSL algebraic expression interpreter and logical expression evaluator which can calculate any user defined quantity. Plotting files may be retained for re-plotting in comparison with subsequent runs
Simulation monitoring window log of all simulation event procedures, which allows a detailed analysis of manually entered or automatically initiated events.

Long-term Stability

In many cases stability calculations must be run for long periods thus taking effects of slower control systems such as boiler control, network exchange control or transformer tap-changer control into account. Other applications are varying loads or applications of wind power where the impact of wind speed fluctuations must be analyzed. In such cases, short-term and mid-term dynamics have already reached steady state but slower transients are still being observed.

Long-term stability simulations require therefore adaptive step-size algorithms that allow an automatic variation of step-sizes within the range of milliseconds and several minutes without any decrease of precision or even manipulation of transient behavior.

DIgSILENT PowerFactory features a highly accurate long-term simulation algorithm with precise, event-controlled integration step-size adoption.

RMS Simulation with a-b-c Phase Representation

The a-b-c phase, steady state component representation of the power system, allows the fundamental frequency analysis of any asymmetrical fault combination, including single and double phase line interruptions. This representation is valid for electromechanical transients and 1, 2, and 3-phase systems with or without asymmetrical pre-loading. This system representation mode fully avoids tedious hand-calculations of equivalent fault impedance. It also allows for accessing any a-b-c phase quantity for plotting or precise modeling purposes (e.g. protection devices).

EMT Simulation

DIgSILENT PowerFactory also provides an EMT simulation kernel for solving power system transient problems such as switching over-voltages, ferro-resonance effects or sub-synchronous resonance problems. In the EMT simulation environment, the network model and associated devices are not represented as constant steady state impedances, but by the differential equations that govern their behavior. For transmission and distribution network lines and cables, the transient PI models as well as distributed parameter models are provided. Additional standard built-in models include:

Transmission lines (according to tower layout), distribution network lines and cables
Passive RLC branches, filters and sources
2 and 3 winding transformers for 1, 2 or 3 phase systems, including saturation effects
VT, CT and PT models
Series capacitor, MOV’s and bypass switches
HVDC valve groups (6 and 12 pulse Graetz bridge configurations) and other FACTS devices such as SVC’s, UPFC’s and TCSC’s
Circuit breaker models

Special numerical integration methods have been implemented in DIgSILENT PowerFactory in order to avoid numerical oscillations caused by switching devices and other non-linear characteristics. The calculation of initial conditions is carried out prior to the EMT simulation, and is based on a solved load flow (symmetrical or asymmetrical).

The DIgSILENT Modeling Flexibility

DIgSILENT PowerFactory features an unmet flexibility for implementing user specific modeling needs. The basic flexibility level is realized via graphical object wiring diagrams – called Model Frames. They allow for a comfortable configuration of functional block relations using object signal connections. Any existing PowerFactory object can be plugged into such a "slot". Frames can be lumped and nested to any degree of complexity. Hundreds of objects such as power system equipment (e.g. busbars, generators, lines, transformers, motors), relays, relay components, CTs, VTs, measurement files, FFT devices, real time clock, RMS signal transducer, parameter identifiers, controllers, power plant control components, A/D converter, RPC links, result files or display objects are at the user’s disposal. In cases where additional functions are required, such functions can be build using the DSL language.

The most critical and decisive factor for producing reliable steady state and transient calculation results is the accuracy and completeness of the applied system model representation. Methods for solving this task especially for stability analysis purposes, range from the traditional way of using software which allows interfacing of user-defined models at the FORTRAN level - typically via connection lists (e.g. PSS/E)- to the block-oriented approach which is based on the provision of predefined basic block macros, connected at the case definition level (e.g. NETOMAC, NEPLAN). In addition, most modern commercially available general purpose simulation tools can only be used for flexible and not specific system representation (e.g. SIMULINK). In most cases the above mentioned approaches do not cover the special characteristics of electrical power systems adequately requiring iterative solution techniques to be able to determine the initial AC/DC load flows and to solve nonlinear grid characteristics during the simulation process.

The DIgSILENT Simulation Language – DSL

Although DIgSILENT PowerFactory contains a comprehensive model library and powerful built-in functions, there are many cases in which the user may want to implement additional control options and calculation functionality. For these reasons, the DIgSILENT Simulation Language (DSL) was developed.

DSL allows the creation of any kind of static or dynamic multi-input/multi-output model. Typical applications are:

Voltage controller and excitation systems
Power system stabilizers (PSS)
Primary and secondary controllers
Prime mover units
Motor driven machines
FACTS controllers
any type of protection device, such as: distance relays, undervoltage relays, over-current relays, load-shedding relays, unit trip devices, electronic motor starting devices (EMR).
any comprehensive calculation procedure required in the control environment
Supervisory control devices

To provide a flexible modeling and simulation tool that forms part of an integrated steady state analysis and stability program, a control system based simulation language was developed. The following main features of the DIgSILENT Simulation Language (DSL) are considered to be most relevant:

The DSL simulation tool falls into the category of Continuous System Simulation Languages (CSSL) including a complete mathematical description of (time-) continuous linear and nonlinear systems. The DSL is dedicated to common control and logic diagrams leading to a non-procedural language as the sequence of elements could be chosen arbitrarily. In sum, a DSL model is directly convertible into a graphical block diagram representation.
Provision of a flexible definition of macros, which can be: algebraic equations, basic control elements like PID, PTn or even complete physical subsystems such as HVDC valve groups or excitation systems. In addition, various intrinsic functions like "select, lim, limits, lapprox, picdrop", as wells as "interrupt procedures" are included.
Provision of various formal procedures for error detection and testing purposes, e.g. algebraic loop detection, reporting of unused and undefined variables and missing initial conditions.
Automatic calculation of initial conditions is supported – an important feature especially when complex, nonlinear equations must be solved iteratively.
DSL models are interfaced on all DIgSILENT level functions such as load flow, fault analysis, stability analysis, protection coordination and harmonic analysis, etc. Therefore multi-level modeling is given for the different steady state descriptions and transient time domains (short/mid-term, long-term and electromagnetic).
DSL models can be generated directly on the graphical level by drawing the "block diagram". Any "block" may contain another DSL model, a macro or any sequence of DSL syntax. The DSL-editor will then generate the DSL description automatically and will also provide direct model testing functions such as eigenvalue analysis or step-response tests.

DSL Implementation

The DSL is a semi-independent module that is appropriately linked to the program kernel via the graphical interface - Model Frames (FMs). A FM is drawn in form of a block diagram that defines the "wiring" of the different functions required. The blocks can be understood as "slots" which are used to "plug-in" the appropriate models. The definition of frames is completely flexible featuring e.g. the definition of relay frames, plant frames or any other functions.

Signal input and output channels: Any variable defined within the kernel (actually more than 5000) or a DSL model can be accessed in a read-and-write mode.
Interrupts: Conditions derived by DSL models can cause interrupts to be sent to the program kernel where they are then scheduled within the event queue.
Output and Monitoring: Conditions may trigger an output to be displayed on simulation monitor and stored in the simulation log file.

Advanced Features

DSL models feature the direct interaction with external processes such as DAQ interfaces, SIMULINK modules or other software systems.
Procedures written in C++ code can be directly linked via appropriate interface mechanisms.
The numerical integration of DSL models, interrupt scheduling and input-output signal processing is handled automatically by the program kernel. In addition, if the output of a DSL model is an electric current that is added to the appropriate total bus current - which is the case if a load or generator model is created - all Jacobean elements necessary for static analysis like load flow and for the iterative simulation procedures will be calculated automatically.

Interpreter versus C++ Code

DSL model definitions are included in the DSL model library using their native language. This method can be compared to procedures used in conventional programming (e.g C, Pascal or FORTRAN). The main difference however, is that DSL does not require any compiling or linking procedures as DSL works like an interpreter, building up an RPN list (Reverse Polish Notation) which is then processed automatically during runtime. Although DSL model interpretation is slightly slower than a compiled code in terms of execution times, the process of model development and testing is significantly faster than code compiling which requires linking and program reloading.

In order to cut down the DSL model execution times, an optimised DSL to C++ cross-compiler is also available, featuring the generation of dynamic link libraries (DLL) which is automatically loaded during program start-up, respective run time initialisation or directly re-loaded during program execution. With this option the user can implement self defined models on all calculation levels including network branch and bus elements with its maximum possible execution speed.

Parameter Identification

Built-in system identification and general optimization procedures provide an easy and accurate method to perform model parameter identification on the basis of system tests and field measurements. The PowerFactory identification tool is applicable for parameter estimation of multi-input multi-output (MIMO) systems, which are described by any type of nonlinear DSL model. The identification procedure itself is fully integrated in the graphical frame definition and block diagram level and also features parameter estimation of integrated models which forms part of a power system model such as loads or generators.

The provided optimization procedures are highly generic and can also be used for optimally tuning parameter such as PSS settings according to defined model response functions.

Eigenvalue Analysis

The DIgSILENT PowerFactory modal analysis tool features small signal analysis of a dynamic multi-machine system. System representation is identical to the time domain model. It covers all network components such as generators, motors, loads, SVS, FACTS, or any other component used for the system representation including also controllers and power plant models.

The calculation of eigenvalues and eigenvectors is an extremely powerful tool, e.g. for low-frequency oscillatory stability studies, PSS tuning, determination of interconnection options and basic parameter, and is a natural complement to the time domain simulation environment. It also allows for the computation of modal sensitivities with respect to generator or power plant controllers, load characteristics, reactive compensation or any other dynamically modeled equipment.

Eigenvalue analysis is with DIgSILENT PowerFactory performed in an easy, well-defined, and almost automatic procedure. The calculation steps are described as follows:

Based on a converged and adjusted power flow, the modal analysis starts with the calculation of the systems initial conditions; alternatively any interrupted status of a time domain simulation could be used as initial condition.
The system A-matrix is constructed automatically for the complete system (including generators, general loads, predefined system plant and controller models as well as DSL-devices). Therefore the overall system representation of the homogeneous form is:

dx/dt = A x
System and model linearization - incl. user defined models - is performed by iterative procedures. Limiting devices are disabled automatically. The representation of the network model is equivalent to the simulation model, allowing a direct comparison between time domain simulations and modal analysis results.
System order reduction is automatically performed for zero rows and columns. This may occur, if a time constant or a gain has been set to zero.
All eigenvalues are calculated and listed in their appropriate order. Based on the calculated eigenvalues and eigenvectors, the normalized participation matrix for the system oscillation is computed and the oscillation vectors for all modes are displayed graphically.

DIgSILENT PowerFactory can deal with multiple eigenvalues and eigenvectors which is an important feature when identical units are operated in parallel.

Depending on the program version, the resultant system A-matrix may have an order of up to 2500 and more - and hence may describe more than 250 machines modeled in detail and thus resulting in a correct damping pattern. If a system with a higher order is being analyzed, the application of the DIgSILENT selective eigenvalue analysis is recommended.