Paper id 316

1

Abstract— The paper discusses the results of the case studies

devoted to the impact of distributed generators connected to a distribution network on the transient stability of transmission network, with the critical fault clearing time applied as an performance index. The case studies are based on the two-machine scheme, employing also a model of the Baltic power system. The authors analyse a wide technological range of the distributed generators and a variety of the modelled operating conditions. The transient stability is estimated using EUROSTAG software for simulation of power systems.

Index Terms—Power System Dynamic Stability, Distributed

Generation, Distributed Energy Resources

I. INTRODUCTION he integration of distributed energy resources (DER) in the distribution networks presents new challenge

affecting their operating conditions [1]. To ensure reliable power supply if such integration takes place, stability issues are of primary importance. In particular, the angle stability problem is among those determining directions of new research [2-4].

The paper estimates the DER impact on the angle stability of a transmission network [5]. From the methodological point of view, the estimation procedure includes several steps: the determination of a power system’s configurations for which the angle stability is an operational constraint; the selection of the basic configuration for a distribution network with DER; the definition of the aspects important for a further development of the study scenarios, the selection of an appropriate performance index.

Manuscript received April 20, 2007. The research activity in this area is supported by European Union as part of

Integrated Project “The birth of European Distributed EnErgy Partnership that will help the large-scale implementation of distributed energy resources in Europe (EU-DEEP)”.

I. Svalova is with the Riga Technical university, Kronvalda bulv. l, Riga, LV-1010, Latvia (e-mail: svalova@eef.rtu.lv).

A. Sauhats is with the Riga Technical university, Kronvalda bulv. l, Riga, LV-1010, Latvia (e-mail: sauhatas@eef.rtu.lv).

A.Svalovs is with the Riga Technical university, Kronvalda bulv. l, Riga, LV-1010, Latvia (phone: +371-67089931, e-mail: svalovs@eef.rtu.lv).

II. METHODOLOGICAL STEPS

A. Main aspects of the study scenario development When studying a whole system with new DER-grids [6-7],

the most important aspects influencing the results are as follows. • The configuration of a distribution network must be

selected in such a manner that it suits to different types of the grid (cable/non-cable, urban, industrial and other) where addition of DER-units would be profitable.

• The actual DER generation technologies in use should be presented by appropriate dynamic models for synchronous generators (SG), induction generators (IG), as well as by a model describing the types of generation connected to the distribution network, including the converter interface-based generation type (CONV).

• The operation of the protection devices for DER units. • The volume of DER-based generation (Penetration Level)

varies from 0 up to 25-30% of the total generation in a system.

• New distribution grids including DER should be added to the model only in the case when they can influence the system’s dynamic response. The list of the nodes where DER-grids can be connected to the system is to be prepared preliminarily.

• A suitable range of the pre-fault operational conditions should be covered.

• Faults of appropriate type should be selected for each step of the Case study.

B. Description of the power system configuration For the case analysis given below two different

configurations of a power system containing high-voltage transmission and low-voltage distribution parts were chosen.

Case Study 1 includes the actual transmission system of the Baltic States linked by 330 kV lines and connected to power systems of Russia and Belarus (which have also 750 and 500 kV lines). This case is also denoted the Baltic Case. Figure 1 illustrates the structure of the grid under consideration.

The transmission system configured as shown in this figure is described in the Case Study 1 section of the paper.

Estimation of the Distributed Generation Impacts on the Angle Stability of the

Two-Machine Scheme Inesa Svalova, non-member, Antans Sauhats, Member, IEEE, and Andrejs Svalovs, non-member

T

1975978-1-4244-2190-9/07/$25.00 ©2007 IEEE PowerTech 2007

Paper id 316

2

Fig. 1. Configuration of the Case Study 1 that includes power systems of the Baltic States, Belarus and Russia.

Another case study – the Two-Machine Case – considers a 400÷800 MW generation connected to the system by a double 400 kV transmission line. A distribution network includes DER-based production in two zones (DER Zone 1 and DER Zone 2), each of them connected to one end of the line (see Figure 2).

AC Generator

Pnom=1000 MW AC System (infinitive bus)

DER Zone 1 DER Zone 2

Double-circuit 400 kV lines

P

Load

Fig. 2. The two-machine scheme with DER

The power normally flows from the generation side to the system. The second case is more general and its study results may have more wide meaning as compared with Case Study 1.

C. Description of section of the distribution network Possible growth in the DER-based production was

modelled both for the Baltic Case and the Two-Machine Case by adding a number of distribution network sections that have the identical basic configuration throughout the model.

The basic configuration of the study network includes a 10 kV network with four SG or IG units that are connected to nodes 57, 58, 78 and 79. The nominal power of each generator is 6 MW, the total consumption and production of the modelled grid being 21 MW and 24 MW, respectively.

A CONV-based distribution grid was modelled but without loads representing a “park” of generation.

A 110/10 kV transformer is located between nodes 1000 and 2000. The 110 kV grid has a link to the 330 kV grid in the model.

The scheme described above presents an actual distribution grid to be extended in the next several years and serves an industry area having active connection of new customers. The structure of the selected grid is presented in Figure 3.

Fig. 3. Diagram of the distribution network model

The next methodological step provides the selection of an appropriate performance index.

D. Performance index A very common indicator of transient stability of

synchronous generator is critical fault clearing time (CCT), which is defined as the maximum duration of the fault which will not lead to the loss of synchronism of one or more generators [8].

The following changes were analysed with increase in DER share: 1) in CCT for different scenarios; 2) in characteristics of the dynamic response of whole

system. Change in the CCT is calculated by the formula:

%,100*DERwithoutscenarioforCCT

DERwithscenarioforCCTCCTinChange = (1)

The chosen software package EUROSTAG is able to

calculate the CCT automatically for a given data set [9].

III. DYNAMIC MODELS OF DER UNITS AND SIMULATION TOOLS

For the simulation phase, EUROSTAG software package was chosen. It contains the necessary dynamic models, provides a possibility for developing the user-defined models, and has useful complementary tools for dynamic simulations.

The following dynamic models were chosen from EUROSTAG library [10].

A. Synchronous generator The modelling of SG is done according to Park’s classical

theory [11]. The models used the following assumptions: 1) Stator windings sinusoidally distributed along the air-gap

as far as the mutual effects with the rotor are concerned. 2) Stator slots cause no appreciable variation of the rotor

inductances with the rotor position. 3) Magnetic hysterisis and magnetic saturation effects

1976

Paper id 316

3

negligible. The simplified model of SG (M6 model) was chosen for the

studies. In this model the rotor is represented solely by the exciter winding in the direct axis and no winding along the q axis.

B. Induction generator For the induction generator models the following

assumptions are made: 1) Slot effects, saturation, hysterisis and eddy currents are

neglected. 2) Purely sinusoidal distribution of flux waves is assumed. 3) Rotor structure is considered symmetrical.

The simplified model of IG (M11 model) was chosen which neglects the rotor transients.

C. Converter interface-based unit model The model of variable-speed turbine with direct driven

synchronous generator was used. The model name is INTERDDG.

D. Modelling of unit protections DER-units as a rule have their own protection devices that

detect low- or over-voltage conditions and disconnect such units from the grid in the case of emergency. This kind of protection was activated as an appropriate automation model (A20).

During the study it was revealed that each DER-SG unit needs out-of-step protection. The model was developed and added to the EUROSTAG library.

The corresponding protection device operates like the known Wide Area Measurements System [12], which controls the actual difference in the rotor angles.

IV. CASE STUDY 1: IMPACT OF DER ON THE STABILITY OF BALTIC NETWORKS

A. Basic configuration of the power systems in the Baltic States

In this Case, the parallel operation of several power systems - those of Estonia, Latvia, Lithuania, Belarus and North-West Region of Russia – was modelled (see Figure 1).

The mentioned systems possess a number of specific features ensuring their stability [13].

From the stability point of view, the most dangerous for the power systems is a sudden loss of the 750 kV line that connects the Leningradskaja s/st and the Kalinin NPP.

The critical stability conditions were modelled by faults of the following types.

Fault 1. A three-phase short circuit with the subsequent disconnection of the 750 kV line Leningradskaja s/st and the Kalinin NPP. In the model, the pre-emergency power flow of 500 MW through this line was assumed. The emergency power transfer in the transmission system abruptly changes its direction and transits through the lines of the Baltic States

and Belarus to the centre of Russia. As this takes place, the generators of a high-capacity (approx. 3000 MW) power plant start to oscillate with high amplitude. This results in voltage and power swings in the Baltic network.

Fault 2. A three-phase short circuit at a 330 kV busbar of the Plavinas Hydro Power Plant (located in Latvia). This results in the loss of two 330 kV lines and four generators (90 MW each).

Three groups of scenarios were introduced in which faults of the considered types are acting.

B. Overview and Simulation Results for Group 1 The pre-emergency power flow on the monitored 750 kV

line: 500 MW; Fault 1 type. In the computations the CONV models have been employed, which are able to withstand rather large and long-lasting voltage deviations. The maximum increase in the DER share equals 20%.

For Group1 the simulation scenarios and results are shown in Table I.

As a result of Fault 1, all generating sets (large and small)

together with their protections response to the voltage and power flow oscillations.

A 330 kV transmission system under consideration possesses the stability margin that is big enough to accumulate new DER-units of the desired volume. This is the reason for the minimum change in CCT (the reduction only to 98%).

C. Overview and Simulation Results for Group 2 In the second group of Scenarios more active introduction

of IG-units (in comparison with SG) was implied. The studies were concentrated on the influence of IG that were introduced additionally to other types of generators on the dynamic performance of the system.

The CCT value for the situation without DG is close to 13 seconds. The system holds a big stability margin in relation to the fault considered. As a result of Fault 2, the swings of electric parameters (voltage and power flow) in the system are considerably smaller than in Group 1 of simulations.

Results of modelling show considerable changes in the dynamic characteristics of the system (see Table II). The CCT reduces down to 89% when the IG proportion is 75% of the

TABLE I SIMULATION SCENARIOS AND RESULTS FOR GROUP 1

Share of DER [%]

SG [%]

IG [%]

CONV [%]

CCT change [%]

0 - - - 100 10 50 40 99 20 40 40 98 10 25 40 35 98 10 50 40 100 20 40 40 98 20 25 40 35 100

1977

Paper id 316

4

DER production. The same total DER percentage with a more active participation of SG does not change the CCT value.

The introduction of DER into the Baltic networks is described in more detail in [14].

TABLE II SIMULATION RESULTS FOR GROUP 2

Share of DER [%] SG [%] IG [%] CCT change [%]

0 100 35 65 100 25 75 100 10 50 50 100 35 65 100 25 75 89 20 50 50 100 25 75 89 25 50 50 100

30 35 65 89

Further, the presented paper considers another study approach, which includes the two-machine case.

V. CASE STUDY 2: TWO-MACHINE CASE

A. Basic configuration of Two-Machine Case The dynamic behaviour of a two-machine transmission

network with DER (operated in the distribution network) connected to its ends (see Figure 2) are studied in more detail in a series of specially developed simulations using appropriate models of the network and generators.

The simulations are based on several scenarios considered below. They correspond to the variations in normal operating conditions followed by a three-phase short-circuit and the subsequent switching-off of faulted circuit of the double 400 kV line. The faults differ in their location and duration. The high-volume load of 1000+j850 MVA is located close to the System side.

B. Overview of Scenarios Three groups of Scenarios (the total 22) were considered:

1) a group including scenarios intended for testing the validity of the model of a two-machine scheme with DER in operation. In this group only SG (100%) are simulated (Scenarios 1, 2, 3, 4);

2) a group including scenarios studying differences in CCT by introducing combinations of different generator types (Scenarios 5-19);

3) a group including scenarios that reveal the influence of other factors, namely: changes in the system consumption, in the number of 110/10 kV transformers, in the voltage relay settings, etc. (Scenarios 20, 21, 22).

The overview of scenarios is illustrated by Table III, which gives the initial data for simulations taking into account pre-fault and fault conditions.

TABLE III INITIAL DATA FOR SIMULATIONS

Shares of DER-units [%] Sc.

No.

DER location

side

Gen. Output[MW]

DER parts

(x24MW) SG IG INV

Export from DER[MW]

Faultclose

to

1 Gen. 800 0 to 10 100 - - - Gen.2 Gen. 400 0 to 14 100 - - 300 Gen.3 Syst. 800 0 to 16 100 - - - Gen.4 Syst. 400 0 to 14 100 - - - Gen.5 Syst. 400 0 to 14 25 75 - - Gen.6 Syst. 400 0 to 16 35 65 - - Gen.7 Syst. 400 0 to 12 40 60 - - Gen.8 Syst. 400 0 to 12 50 50 Gen.9 Syst. 400 0 to 12 20 70 10 240 Gen.

25 75 - - 10 50 40 20 40 40

10A Syst. 400 4

25 40 35

24x INV-parts

Gen.

50 50 - 10 50 40 20 40 40

10B Syst. 400 8

25 35 40

24x INV-parts

Gen.

11 Syst. 400 0 to 14 100 - - - Syst.12 Syst. 400 0 to 14 25 75 - - Syst.13 Syst. 400 0 to 16 35 65 - - Syst.14 Syst. 400 0 to 12 40 60 - - Syst.

15A Syst. 400 4 to 12 20 70 10 240 Syst.10 50 40 20 40 40 15B Syst. 400 4 25 40 35

24x INV-parts

Syst.

16 Syst. 800 0 to 14 25 75 - - Syst.17 Syst. 800 0 to 16 35 65 - - Syst.18 Syst. 800 0 to 16 40 60 - - Syst.19 Syst. 800 4 to 12 20 70 10 240 Syst.20 Gen. 800 14 100 - - - Gen.21 Gen. 800 14 100 - - - Syst.22 Gen. 800 14 100 Syst.

VI. TWO-MACHINE CASE SIMULATION RESULTS The purpose of the two-machine case is to estimate the

influence of the distributed generation on the dynamic stability of a transmission network. The calculations have been performed by simulating the parallel operation of a transmission network represented by the two-machine scheme and distribution networks with distributed generation.

Simulation results based on specially developed Scenarios show that provision of the angle stability may be a problem for the distribution networks with DER synchronous generators.

A. Simulation Group 1 The mutual influence of transients in transmission and

distribution grids may have both positive and negative impact on the transmission stability (see Figure 4).

1978

Paper id 316

5

Change in CCT for Scenarios 1 - 4

-8 -6 -4 -2 0 2 4 6 8

10 12

0 5 10 15 20 25 30 35 40 45 50

DER Gen./Total Gen. (%)

Change in CCT (%) Scenario 1Scenario 2Scenario 3Scenario 41

2

3 4

Fig. 4. Change in CCT values for Scenarios 1-4

The results for Scenarios 1 and 2 indicate an increase in the CCT with the increasing DER share of up to 23%. The maximum CCT deviation from the base value (11%) corresponds to the maximum share of DER.

The reason for this is the additional power flow arising when DER-SG are switched off the network by their own protection. In the distribution network an active power deficiency occurs, which is covered owing to generator G1. In this case, at the instant of each SG-unit’s switching-off the power portion that up to this point was transferred through the high-voltage line to the system side is now redistributed from this line to the distribution network side. The angle of generator G1 with respect to the system slightly decreases when one of the SG-units is switched off but continues to increase because of the acceleration. As the number of DER units increases they are also completely disconnected by their protection, but in this case there is an increase in the additional load to G1, with respective increasing.

Connection of the distributed generation to another side of the two-machine scheme (close to the infinite bus) leads to a marked change – the decreases with the DG share increasing. The reason is the growth in active power, which is additionally transmitted through the high-voltage line every time when one of the DER-SG is switched off.

The change of 6% corresponds to 46% of DER in Scenario 4 (see Table III), and 32% of the total generation by DER in Scenario 3. From the dynamic point of view, this means that installation of DER-SG should be done carefully, with appropriate preliminary simulations. The results may entail some special actions in order to cope with the DER stability problem.

B. Simulation Group 2 In the Scenarios 5-9 up to 16 sections of a distribution

network were additionally connected. In this case the DER share varied from 0 up to 46%, with the SG sharing from 25 to 40%. Here, IG and CONV type generators were present in addition to synchronous ones.

The results of simulations indicate an increase in the CCT with an increased share of DG up to 49%. The maximum

CCT deviation from the base value making up 3% occurs at the maximum DG share (32%).

In Scenario 10, large-scale introduction of CONV (35-40%) has shown no influence on the CCT. The system’s dynamic characteristics coincide with those obtained in the presence of SG and IG only. Thus such introduction should be estimated as a stabilizing factor.

It should be noted that change in Fault types in Scenarios from 11 to 15 results in a CCT decrease in comparison with Scenarios 5-9. Thus, the connection of distributed generation to the infinite bus is a decreasing factor for the CCT value.

The results for Simulation Group 2 have shown that the DER-unit response is sensitive to the swings of generator G1. All DER-units go/turn out to be out-of-step and are switched off. Such sensitivity can be reduced through reduction in the amplitude of G1 fluctuations or modification of the distribution network parameters. The mentioned changes were realised in the third group of scenarios.

C. Simulation Group 3 In the third group of scenarios (Scenarios 20, 21, 22) the

following modifications of the network model were accepted: a) a change in the consumption in the system, b) a change in the number of transformers 110/10 kV

operating in parallel, c) a change in the maximum/minimum voltage relay settings

installed at DER-units, d) a change in the transmission network parameters, e) a change in the fault resistance.

Scenario 20 compares self-balancing and import operational modes of the distribution part. The total load in the distribution network is increasing by 200% from 336 MW up to 1008 MW. Output of 14 DER-sections in both variants is 336 MW.

Rotor angle variations in time are illustrated in Figure 5.

0 1 2 3 4 5 6 7 8 9 10

-50

0

50

100

150

200

250

1

2

Rotor angle (deg)

Time (s)

a 0 1 2 3 4 5 6 7 8 9 10

-50

0

50

100

150

200

250

1

2

Rotor angles (deg)

Time (s)

b Fig. 5. Rotor angle variations for self-balancing (a) and import (b) modes: 1 – Generator G1; 2 – DER-SG units

The generator G1 reduces its post-fault angle from appr.

60° down to appr. 15° that corresponds to 9% change in CCT. Comparing the specified operational modes, the noticeable increase in the consumption stabilises generators.

Scenario 21 compares behaviour of DER-SG and their

1979

Paper id 316

6

protections tripping of one circuit of the transmission lines without a short circuit and with a short circuit. DER-SG start asynchronous operation even responding to the circuit removing without a fault.

In Scenarios 1-21 one circuit of the transmission line was switched off while the second one remained in operation, converting the transmission part into weak connection. The resultant transients led to disconnection of almost all DER-SG units either by voltage or angle protection. Therefore the purpose of Scenario 22 was the determination of measures directed to improvement of the overall stability for the studied scheme.

An improvement in the transmission network was modelled by adding a third transmission circuit. However, this did not produce any noticeable effect. Tripping of one of the three circuits without a short circuit causes out-of-step condition for several DG SG.

The modification in fault resistance leads to non-operation of the minimum voltage relays and no loss of stability occurs even following a prolonged short circuit.

Generally, DER-units have higher probability of out-of-step condition occurrence than power plants connected to a transmission grid.

VII. CONCLUSION The mutual influence of transients in transmission and

distribution grids may have both a positive and a negative effect on the transmission stability, which depends on the combination of a number of the operational and fault parameters. They are as follows. 1) The quantity of operating DER connected to the

transmission grid that may be weak or strong. 2) The proportion of DER protection devices that acts

responding to the fault in the transmission part. 3) Tripping of DER-units may both increase and decrease

the power flow in the transmission, thus providing stabilisation or destabilisation of the angle positions.

4) The active distribution grids may operate in three modes from the power exchange point of view. These are: export, import, and self-balancing. Each of the modes may result in a completely different stability situation.

5) DER-SG units can lose synchronism following sudden and significant changes in the mutual rotor angles of major power plants.

Out-of-step protection should be applied to individual DER-SG units or to the connection transformers of a distribution grid.

VIII. REFERENCES

[1] New ERA for electricity in Europe. Distributed Generation: key issues, challenges and proposed solutions, European Commission, Directorate-General for Research, 2001.

[2] Definition and classification of power system stability. CIGRE/IEEE 38, 2003, 30 pp. Ref. No. 231.

[3] J.G. Slootweg, W.L. Kling, Impacts of distributed Generation on Power System Transient Stability, IEEE Power Engineering Society Summer Meeting, 2002 Volume 2, pp. 862 – 867.

[4] A.M. Azmy, I. Erlich, Impact of Distributed Generation on the Stability of Electrical Power Systems, IEEE Power Engineering Society General Meeting, 2005, pp. 1337 – 1344.

[5] M. Reza, P.H. Schavemaker, J.G. Slootweg, W.L. Kling, L. van der Sluis, Impacts of Distributed Generation Penetration Levels on Power Systems Transient Stability, IEEE Power Engineering Society General Meeting, 2004, pp. 2150 – 2155.

[6] M.K. Donnelly, J.E. Dagle, D.J. Trudnowski, G.J. Rogers, Impact of the Distributed Utility on Transmission System Stability, IEEE Transactions on Power Systems, Vol. 11, No. 2, May 1996, pp. 741-746.

[7] V.V. Thong, E. Vandenbrande, J. Soens, D. V. Dommelen, J. Driesen, R. Belmans, Influences of Large Penetration of Distributed Generation on N-1 Safety Operation, IEEE Power Engineering Society General Meeting, 2004, pp. 2177 – 2181.

[8] F.V. Edwards, G.J. Dudgeon, J.R. McDonald, and W.E. Leithead, Dynamics of Distribution Networks with Distribution Generation, IEEE Transaction on Power Systems, 2000, pp.1032-1037.

[9] Eurostag package, Tractebel-EDF, October 2004, Release 4.3. [10] Eurostag package, Standard Model Library, Tractebel-EDF,

October 2004, Release 4.3. [11] P.Kundur, Power System Stability and Control, McGraw-Hill,

Inc., 1993. [12] System Protection Schemes in Power Network, Task Force

38.02.19, Electra, Nr. 196, June 2001. [13] A. Sauhatas and A. Svalov, Statistical Optimisation of a

Complex of Local Devices for Prevention of Out-of-Step Conditions, in Proc. 14th Power System Computation Conf., Sevilla, June 2002.

[14] A. Sauhatas, I.Svalova and A. Svalov, Loss-of-synchronism in a system with increased share of DER, in Proc. 3rd International Symposium on Modern Electric Power Systems (MEPS06)., Wroclaw, September 2006.

1980