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Power System Simulation Challenges

Petros Aristidou, Frédéric Plumier, Christophe Geuzaine and Thierry Van Cutsem

Department of Electrical Engineering and Computer Science, University of Liège, Belgium

p.aristidou@ieee.org and f.plumier@ulg.ac.be

Introduction

Power system simulations are routinely used to check the response of electric power systems to large disturbances. Operation of non-expandable grids closer to their stability limits and unplanned generation patterns stemming from renewable energy sources require dynamic studies. Furthermore, under the pressure of electricity markets and with the support of active demand response, it is likely that system security will be more and more guaranteed by emergency controls responding to the disturbance.

Static

(computation of new

system equilibrium)

Dynamic

(computation of system

evolution through time)

Power System Simulations

+ Fast

- Neglects the dynamic

evolution and the effect of acting protections

between the pre and post disturbance point

- Time consuming

+ Considers the dynamic

evolution and the effect of acting protections

between the pre and post disturbance point

Dynamic simulations find application in:

Dynamic Security Assessment: an evaluation of the

abil-ity of a certain power system to withstand a predefined set of contingencies and to survive the transition to an acceptable steady-state condition.

Hardware-in-the-loop: the addition of a real component

(electronic control unit, real engine, etc.) in the simulation

Operator training: training control center operators through

the simulation of possible contingency scenarios

Simulation assisted design for planning, control and se-curity

and more. Parallel Computing Algorithmic Improvements

Fast power system dynamic simulations

Theoretic Background

In dynamic simulations, power system components are modeled by sets of nonlinear stiff hy-brid Differential-Algebraic Equations (DAEs) based on their physical properties and control schemes. A large interconnected power system may involve hundreds of thousands of such equations spanning very different time scales and undergoing many discrete transitions.

M M M M M Transmission Network

Distribution Network EMT Network

Interface

VT

Injectors

VDt

Figure 1: Decomposed Power System

The power system can be considered as a collection of components interfacing with each other through the Transmission Network (TN) as shown in the Figure. All the compo-nents connected to the TN that either produce or consume power in normal operating condi-tions (such as power plants, induction motors, other loads, etc.) are called injectors. Bigger components that include many components, like distribution networks, can be considered

as super-injectors. This leads to a natural

decomposition of the power system allowing us to apply Domain Decomposition Methods (DDMs) to increase simulation performance.

The

i

-th injector/super-injector can be described by a DAE system of the form:

Γ

i

x

˙

i

= Φ

i

(x

i

,

V )

where

x

i the model’s internal states,

i

)

``

=

(

0

if the

`

-th equation is algebraic

1

if the

`

-th equation is differential and

V

the

vector of TN voltages.

At the same time, under the fundamental-frequency approximation, the TN can be described by the linear algebraic equations:

0

= DV − I = g(x, V )

Domain Decomposition Methods

DDMs allow the solution of each injector/super-injector of the decomposed system in Fig. 1 sep-arately and concurrently. Several schemes differ mainly by the method of exchanging interface variables, that is the variables shared between the TN and the injectors/super-injectors.

Basic classification of DDM based on the interface exchange scheme:

1. Schwartz: interface variables are updated after computing a converged solution of each

decomposed sub-system

2. Schur-complement: a global reduced system is used to compute the interface variables

before performing a Newton iteration on each decomposed sub-system

DDMs yield acceleration of the simulation procedure in two ways:

Numerically: exploiting the localized nature of power systems to avoid many unnecessary

computations (factorizations, evaluations, solutions)

Computationally: exploiting the parallelization opportunities inherent to DDMs.

Large Interconnected Power System Example

Test-case based on a large-size power system representative of the Western European main transmission grid (UCTE area):

• 15226

buses and

21765

branches,

• 3483

synchronous machines represented in detail together with their excitation systems, volt-age regulators, power system stabilizers, speed governors and turbines,

• 7211

other models (equivalents of distribution systems, induction motors, impedance and dynamically modeled loads, etc.).

• 146239

DAE states in total

Physical energy flows 2008 *

UCTE region

Synchronous operation with UCTE region

Total: 334658 GWh UCTE: 285182 GWh

* Not to be confused with contractual electricity exchanges

Values in GWh DE DE PL PL SK SK HU HU CZ CZ CH CH NO SE AL GB DK_W MA DZ TN DK_East UA ME ME IE ES PT NL BE LU TR MD UA_W RO RO RS RS BGBG GR GR AT AT SI SI HR HR BA BA GR GR MK MK BY IT IT FR FR 10597 1315 4227 15 4564 1661 1758 1658 181 95 1189 4 628 1142 1289 152 806 7 2382 3095 288 10 2382 104 1434 91 3755 1171 177 766 2552 6685 233 7554 773 258 2378 2082 239 737 2664 2061 2221 5301 95 439 2683 1210 873 1367 7449 106 24162 400 12841 8787 3548 7940 6909 13858 2709 14997 5607 839 721 56 5334 1326 26 10569 868 12448 923 2650 835 829 1629 1517 7286 2036 18859 8121 3008 5578 96 2066 147 2507 543 778 1973 586 7180 4814 424 1195 2364 5302 4928 1140 14 720 4733 178 332 554 31 12 30 1 1 4 1 3156 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 1 2 4 6 8 10 12 14 16 18 20 22 24 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 # of cores Real-time=240 s Base Algorithm=663 s Elapsed Time (s) Speedup

The disturbance simulated on this system consists of a short circuit near a bus lasting

5

cycles

(

100

ms at

50

Hz), that is cleared by opening a double-circuit line. The system is simulated over

a period of

240

s with a time-step of 1 cycle (

20

ms) using a Schur-complement based DDM.

Extension to Hybrid Simulations

More detailed dynamic simulations demand the representation of power system components by their ElectroMagnetic Transient (EMT) models. Although more accurate, EMT simulations are extremely time consuming. To accelerate the simulation, a multi-rate technique (using a Schwartz based DDM) is used to combine fundamental-frequency simulation with EMT simu-lation. The objective of this hybrid approach is to obtain more accurate simulations than with the fundamental-frequency approximation, while saving computing time by applying the de-tailed model to a subsystem only (see Fig. 1). It also allows to remove some limitations of fundamental-frequency simulations, such as the difficulty of simulating unbalanced faults. The test system is the 74-bus, 102-branch, 20-machine Nordic32 model. The Figures show the re-sponse of the system to a disturbance when modeled in EMT (EMTP-RV) and hybrid (FF-EMT).

-15 -10 -5 0 5 10 0 0.5 1 1.5 2 2.5 ia (kA) t (s) EMTP-RV FF-EMT -15 -10 -5 0 5 -1 0 1 2 2.02 2.04 -1.5 -1 -0.5 0 0.5 1 1.5 0 0.05 0.1 0.15 0.2 va (pu) t (s) EMTP-RV FF-EMT

Conclusions

In the future, the rising need for simulating larger power system models, including active distri-bution networks, will further increase the computational burden of dynamic simulations. Apply-ing DDMs can improve the dynamic simulation performance and accuracy and allow the safe and economic operation of power systems closer to their stability limits.

Selected Publications

[1] Petros Aristidou, Davide Fabozzi, and Thierry Van Cutsem. Exploitation of localization for fast power system dynamic simulations. In Paper submitted for presentation at Power Tech 2013 conference.

[2] Petros Aristidou, Davide Fabozzi, and Thierry Van Cutsem. A Schur Complement Method for DAE Systems in Power System Dynamic Simulations. Domain Decomposition Methods in Science and Engineering XXI, 2013.

[3] Petros Aristidou and Thierry Van Cutsem. Dynamic simulations of combined transmission and distribution systems using decomposition and localization. In Paper submitted for pre-sentation at Power Tech 2013 conference.

[4] Frédéric Plumier, Christophe Geuzaine, and Thierry Van Cutsem. A multirate approach to combine electromagnetic transients and fundamental-frequency simulations. In Paper sub-mitted for presentation at IPST 2013 conference.

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