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Loss-of-Synchronism Detection Automatics for the Networks with Distributed Generation

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

Abstract--Loss-of synchronism processes in the distribution

networks with distributed energy resources are considered. They are influenced by new conditions that were not actual for the transmission system. To eliminate the loss-of-synchronism, special detection automatics is used. The application of such an automatics in low-voltage networks with distributed generation is specific. To meet the changed requirements, new algorithm of the automatic device is introduced. It is based on assumptions of voltage range and impedance angle. Equivalent non-synchronous emfs are modeled The method was tested by simulation of the loss-of-synchronism condition that takes place in the distribution networks.

Index Terms--Power System, Distribution Network, Distributed Generation, Power System Stability, Loss–of-synchronism Protection, Power System Simulation.

I. INTRODUCTION he electric power industry has been undergoing fundamental changes. The transmission and distribution

sectors are facing particularly significant challenges as they are forced to cope with wholesale and retail energy markets, re-regulation, and the need to keep up with infrastructure investments amidst the uncertainty in business models. Hopefully, developments in the electric power distribution technology and the marketplace may hold the keys as to solving some of the problems faced by the distribution system. The distributed energy resources (DER) are predicted [1, 2, 3] to play an increasing role in the future electric power systems. This requires a fresh analysis of the distribution landscape in order to identify the key challenges responding to the public interest.

A great number of units in a distribution system exert a significant impact on the power system’s stability. The loss-of -synchronism problem for the generators in the transmission network is studied well enough [4, 5]. The devices for detection and elimination of the mentioned process are widely spread in the networks which are at an increased risk of losing the synchronism [6].

In this paper the possibility of application of the considered

A. Sauhats is with Faculty of Power and Electrical Engineering, Riga Technical University, 1 Kronvalda boulv., Riga, LV-1010, Latvia (e-mail: sauhatas@eef.rtu.lv)

A. Svalovs is with Faculty of Power and Electrical Engineering, Riga Technical University, 1 Kronvalda boulv., Riga, LV-1010, Latvia, Latvia (e-mail: svalovs@eef.rtu.lv).

I. Svalova is with Institute of Electrical Engineering, Riga Technical University, Riga, Latvia (e-mail: inesa_svalova@yahoo.com)

devices is examined with the purpose of providing a stable operation of networks with distributed generation (NDG). Considered are the dynamic stability and the loss-of-synchronism. The application of a loss-of-synchronism detection algorithm in NDG is discussed.

II. SPECIFIC FEATURES OF THE DISTRIBUTION NETWORKS FROM THE STABILITY POINT OF VIEW

The loss-of-stability processes in the NDG are influenced by the following new conditions:

• a number of new generation technologies are available, which are profitable for DER operating in the same AC distribution network with synchronous generator (SG);

• operational parameters of relay protection and automatics in NDGs differ from those of transmission networks and should be taken into account;

• SGs are low-inertial if connected to the distribution network;

Various technologies used for electricity generation [7, 8]: • rotating machines coupled to synchronous AC

generators (steam turbines, gas turbines, diesel engines, spark ignition engines, large water turbines);

• rotating machines coupled to induction generators (small water turbines, fixed speed wind turbines, variable speed wind turbines - doubly fed (wound rotor) induction generators);

• AC current sources coupled via electronic inverter systems (variable speed wind turbines with synchronous or induction generators, wave and tidal devices);

• DC current sources coupled via electronic inverter systems (fuel cells, photo-voltaic, some wind turbines).

The electrical characteristics of synchronous generators, induction generators and electronic inverters are quite different, particularly with respect to continued operation under fault conditions that may lead to loss of synchronism.

Table 1 shows the conditions of continued operation in the case of a short-circuit in the AC grid.

TABLE 1.

OPERATION OF GENERATORS UNDER FAULT CONDITIONS

Synchronous generators

Induction generators

Electronic inverters

Electronic inverters - new requirements

Operate if current values are not

dangerous/critical

Operate if voltage values

are not low

Switch-off if U<90%Urat or at

high currents

Able to operate under definite

fault conditions

T

2

When a close short circuit appears, SGs and the devices

with the electronic converters possessing "ride-through capability" [9, 10] can continue their operation. DER of other types are switched-off by appropriate protection. Automatic partial disconnection of DER can bring heavier conditions as to keeping stable operation, with the generating power deficiency arising, power flow changing and the network condition worsening. The generators and the majority of customers supplied by the network, do not accept this regime.

Operation of relay protection and automatics in the case of a fault in the network can also lead to the loss of stability. The cause of this lies in the traditional arrangement of the relay protection, whose cost in distribution networks is relatively low. The response times of such protection are considerably higher than those in the transmission networks. Furthermore, the relay operation is simultaneously tripping several closely located DERs, and for those staying connected the probability to lose the synchronism increases.

For low-inertial SGs the loss of stability proceeds more rapidly [11] and they require faster solutions for its prevention.

The transient processes in NDGs are illustrated in the following section, taking into account all the above mentioned conditions using a simulation example.

III. ANALYSIS OF LOSS-OF-SYNCHRONISM PROCESS IN NDG

The dynamic behaviour of NDG and its dependence on the factors mentioned above 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 the five scenarios considered below. They correspond to the variations in normal operating conditions followed by two three-phase short-circuits. These faults differ in their location and duration.

The selected scheme presents the planned extension of a part of the existing Latvian 10 kV distribution network. Synchronous generators and generators coupled via electronic inverters (EI) and their groups were introduced as predicted development of DER penetration level.

The selected values of rated power are: 6 MW for SGs, 6 MW for an individual EI, and 20 MW for a group of EIs.

The fault duration (3-phase s.-c.) varies when estimating the Critical Clearing Time (CCT) [4, 5].

Below, the developed scenarios are given. Scenario 1. Four SGs are connected at nodes 57, 58, 78

and 79, covering 100% of local load in this part of the grid (Figure 1). 3-phase short-circuit at line between nodes 55 and 57 (circuit 1) causes relay protection operation for the faulted line and the SG at node 57. Considering modern relay protection, the fault duration in this scenario is 0.3 s, which is greater than the CCT. All the four SGs that remained connected to the grid have lost synchronism. Figure 2 shows the change in the generator rotor angles during the process.

Fig. 1. Diagram of the distribution network model for Scenario 1

0

20

40

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80

100

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160

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-0.1 0 0.1 0.2 0.3 0.4 0.5Time (sec)

Rot

or a

ngle

(deg

)

Mach.78 Mach.58 and 79

Fig. 2. Scenario 1 – Change of rotor angles

Scenario 2. Here, the fault duration is smaller (0.2 s) on

the line 56-58 (circuit 1). Stability violation is found only for one SG connected at node 78 (Figure 3). In this Scenario a part of SG set keep synchronism, with the short-circuit duration smaller than the time of relay protection operation.

Scenario 2 was chosen for further analysis of SG stability in the presence of EI.

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360

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

Time [sec]

Rotor angle [deg] Mach. 78

Mach. 57

Mach. 79Mach. 58

Switched-offat 0.2 sec

Fig. 3. Scenario 2 – Change of rotor angles

Scenarios 3 and 4. The SG at node 79 was replaced by a EI with the rated power of 6 and 20 MW, respectively. No changes in the fault duration and location (Figures 4 and 5).

3

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360

-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

Time [sec]

Rotor angle [deg] Mach. 78

Mach. 57

Mach. 58Switched-off

at 0.2 sec

At node 79 generator with electronic inverter 6 MW(not synchronous generator)

Fig. 4. Scenario 3 – Change of rotor angles

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-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

Time [sec]

Rotor angle [deg]

Mach. 78

Mach. 57

Mach. 58Switched-off

at 0.2 sec

At node 79 generator(-s) with electronic inverter 20 MW(not synchronous generator)

Fig. 5. Scenario 4 – Change of rotor angles

Scenario 5. Two EIs are connected at nodes 54 and 79. As

Figure 6 shows, the SG at node 79 is removed. Figure 7 illustrates the dynamic behaviour of the remaining SGs.

INV INV

Fig. 6. Diagram of the distribution network model for Scenario 5

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0

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-0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2

Time [sec]

Rotor angle [deg]

Mach. 78

Mach. 57

Mach. 58Switched-off

at 0.2 sec

At nodes 54 and 79 generators with electronic inverter 6 MW each

Fig. 7. Scenario 5 – Change of rotor angles

The data needed for the simulations and the results

obtained in stability estimation are collected in Table 2. The model of EI protection system simulates tripping of the

generation source in the response to the low voltage condition at the connection node.

TABLE 2. OVERVIEW OF SCENARIOS AND RESULTS

Scenario

1 2 3 4 5 Pre-fault conditions

Generation [Generators at node] SG 57, 58, 78, 79 SG 57, 58, 78, 79 SG 57, 58, 78

EI 79 SG 57, 58, 78

EI 79 SG 57,58,78 EI 79,

54 Covering of internal load from DERs

100% (75% SGs, 25% EIs)

100% (75% SGs, 25% EIs)

100% (75% SGs, 25% EIs)

160% (47% SGs, 53% EIs)

125% (60% SGs, 40% EIs)

Fault conditions (short circuit) Location: Line and circuit 55-57 (1) 56-58 (1) 56-58 (1) 56-58 (1) 56-58 (1) Duration [sec] 0.3 0.2 0.2 0.2 0.2 Switched off elements

Line 55-57(1) SG 57

Line 56-58(1) SG 58

Line 56-58(1) SG 58, EI 79

Line 56-58(1) SG 58, EI 79

Line 56-58(1) SG 58, EI 79, 54

Post-fault conditions SG staying connected to the grid [node] 58, 78, 79 57, 78, 79 57, 78 57, 78 57, 78

Loss of synchronism [SGs at nodes] 58, 78 and 79 78 78 57 and 78 57

Figure with results of the transient 3 4 5 6 7

4

Considering the simulation results for Scenarios 2 and 3, we can conclude that removal of an SG with its replacement by a EI of the same power does not influence the outcome of the transient process (Figures 3 and 4).

In Scenario 4, stability conditions are worse than in Scenario 3 – for a greater number of SGs a stability violation was revealed.

In Scenario 5, dynamic behaviour of SGs (at nodes 57 and 79) differs from that in Scenario 3.

Resume: the SGs connected to the distribution networks with operation of the existing relay protection are exposed to angle instability. The presence of EI influences this process. For the study network the level of EI participation in the generation up to 25% and higher results in heavier stability conditions.

To eliminate the loss-of-synchronism, special detection automatics is widely used. It protects networks and generators from dangerous damages. The use of such automatics in a NDG is specific as considered in the following sections together with proposed development of its algorithm.

IV. AUTOMATICS FOR LOSS-OF-SYNCHRONISM DETECTION The devices for loss-of-synchronism detection and

elimination (out-of-step and first swing protection) are widely used in the networks having the increased risk of the loss of synchronism [6].

A. Overview The loss-of-synchronism protection systems can be

organised in several ways presented in Table 3. The first and the second types of system could be realised

with quite low expenses. However, their main limitation is the setting-based operation that can incorrectly match the an actual transient process.

The third type provides a real-time view of the actual conditions but requires additional investments. Its reliable operation depends on the availability of communication channels.

Considering the loss-of-synchronism detection automatics in NDGs the following question should be answered – are there special requirements to the operation of such automatics in an NDG?

B. Requirements to the automatics installed in NDG The use of the loss-of-synchronism detection automatics in

the NDG involves the following aspects:

• cost of such type of automatics should be reduced as compared with the transmission grids;

• in NDGs the local automatics should be used as more cheap and reliable;

• operation algorithms should provide correct and effective functioning in the presence of different types of generation (SG, EI, others). Participation of each type may vary from zero up to 100%. This requirement is not actual for transmission systems;

• the setting selection process should be simplified as much as possible due to the fact that the personal of a distribution network can be not experienced enough in dynamic simulations. An alternative approach is to use automatics algorithms that do not require settings at all.

A local system that operates without pre-selected settings and meets the mentioned requirements can be realised as shown in the next section

C. The Proposed Algorithm The algorithms of devices for elimination and liquidation

of unstable processes in the transmission networks are well developed and verified in practice [13-16]. They are based on monitoring the parameters of the transient: an angle that equals or corresponds to the angle between equivalent non-synchronous emfs, rate of change of the apparent impedance and others. For example, in the Baltic States (Lithuania, Latvia and Estonia) the local automatic devices for prevention of the angular instability in the transmission grid have gained wide acceptance [17].

The mentioned automatic devices control voltage U and current I at the node A of the two-machine equivalent scheme of the power system (Figure 8). The circuit demonstrates the parallel operation of two generators, G1 and G2, the load represented by impedance ZM; the network’s elements responsible for the energy transmission are represented by impedances Z1Σ, Z2Σ.

G1 G2

M

ZM

Z1Σ Z2ΣA N

U

I

E1 E2

Fig. 8. The two-machine equivalent circuit

POSSIBLE ORGANISATIONS OF T

Organisation of Out-of-Step Protection System Measurements

1. Local device 2. Coordinated set of local devices

Local measurements of voland current

3. Wide-area (or global) system Synchronised measuremenvoltage phasors collected f

several locations

TABLE 3. HE OUT-OF-STEP PROTECTION SYSTEMS

Decision Selection of separation place

Local trip tage Based on limits and settings that present equivalents of a

whole system Settings provide coordination of

local trips ts of rom

Based on system- wide view of the process and its

Local trip

actual dynamics (sometimes remote trip)

5

The computations (modelling) are based on the hypothesis that the results of preliminary analysis can give the values of resistances Z1Σ and Z2Σ between node A and nodes M and N (see Figure 8). Then

Σ⋅+= 11 ZIUE

Σ⋅−= 22 ZIUE } (1)

and the angle δ between equivalent non-synchronous emfs

⎟⎠⎞⎜

⎝⎛=

2

1arg EEδ .

In this connection we will consider calculation of the equivalent non-synchronous emf vectors, E1 and E2, based on the following two hypotheses. 1. There are known moduli of emf, E1 and E2 (in power

systems the voltages at node points under normal conditions vary within relatively narrow limits) [18, 19].

2. There are known angles of impedances Z1Σ and Z2Σ (in a network these angles vary within relatively narrow limits). Thus we can write:

( ) ( )( ) ( ) ⎪⎭

⎪⎬⎫

⋅⋅−⋅⋅⋅−

⋅⋅+⋅⋅⋅+∗

∗

IpZUIpZU = E

IpZUIpZU = E

ΣΣ2

Σ

11Σ11Σ2

1Σ

22222

(2)

where 1ΣE and ΣE 2 are known (given as the settings)

moduli of the simulated voltages, and 1p and 2p are the

operators of turning by the angles 1ϕ and 2ϕ , i.e.

and , and

1j1 ep ϕ=

2j2 ep ϕ= 1ϕ =arg(Z1Σ), 2ϕ =arg(Z2Σ).

After transformation (1) the following equations for determination of the moduli of impedances Z1Σ and Z2Σ can be obtained:

( )( ) ⎪⎭

⎪⎬⎫

=+⋅⋅+⋅⋅⋅−⋅

=+⋅⋅+⋅⋅⋅+⋅∗∗∗

∗∗∗

22Σ

2222Σ

222Σ

21Σ

2111Σ

221Σ

EUpIUpIUZ IZ

EUpIUpIUZ IZ(3)

Second-order equations (2) contain only real coefficients and can be solved for 1ΣZ and ΣZ 2 . Only a positive root of

either equation is retained, while a negative one is discarded. The impedances Z1Σ and Z2Σ obtained, the emfs E1Σ and E2Σ

can be estimated according to to equation (1). After that we can calculate angle δ thus realising the corresponding algorithm for loss-of-synchronism identification.

In this paper we will consider the feasibility of this algorithm (further in the text Algorithm) under the conditions of the NDG operation.

D. Testing of Algorithm The proposed Algorithm was tested in Scenarios 2-5. An

appropriate software package for simulation of dynamic transient processes in the power system was used [12].

We have modelled the operation of devices installed in different places: at each SG and at the transformer connecting the 10 kV distribution network with a 110 kV grid. The graphic representations of the results are given in Figures 9-

12. The calculated value of the angle between the

asynchronous emfs fully corresponds to a real transient process: the modelled angle reaches 180° at the time of non-synchronous swing, and the character of the changes in this angle corresponds to the actual changes in the rotor angles. This is true both for the devices installed at SGs and for those installed at the 110/10 kV transformer.

This means that the loss-of-synchronism by all or several SGs can be determinate with the help of the proposed Algorithm. The problem of the device location might be actual for NDGs, however its consideration is beyond the scope of the present paper.

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-0.2 0 0.2 0.4 0.6 0.8 1

Time [sec]

Simulated angle [deg]Installed at Mach. 78 Installed at transform.

Fig. 9. Algorithm results for Scenario 2

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240

-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Time [sec]

Simulated angle [deg] Installed at Mach. 78Installed at transform.

Fig. 10. Algorithm results for Scenario 3

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-0.2 0.0 0.2 0.4 0.6 0.8 1.0

Time [sec]

Simulated angle [deg]

Installed at Mach. 78

Installed at transform.

Installed at Mach. 57

Fig. 11. Algorithm results for Scenario 4

6

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240

-0.2 0 0.2 0.4 0.6 0.8 1

Time [sec]

Simulated angle [deg]

Installed at transform.

Installed at Mach. 57

Fig. 12. Algorithm results for Scenario 5

The results obtained allow for a positive estimation of

Algorithm’s characteristics in the process of control over the difference of angles between the vectors of asynchronous emfs in the distribution network.

V. CONCLUSIONS 1. The real risk of loss-of-synchronism in NDG has been

considered and estimated. 2. Simulation results have confirmed that operating

conditions of an NDG may lead to a loss-of-synchronism within a system.

3. Analysis of the angle between two simulated emfs ensures the timely detection of loss-of-synchronism in NDGs.

4. The results of Algorithm’s testing have confirmed that it can be used for providing a reliable power supply for customers.

VI. ACKNOWLEDGMENT 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)”.

VII. REFERENCES [1] Integration of Renewable Energy Sources and distribution Generation in

energy Supply Systems, European Commission, Directorate-General for Research, 2001.

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

[3] O. Linkevics, M. Balodis, M. Bockarjova, A. Sauhats, V.Chuvychin, I.Svalova, “Will distributed energy resources make a revolution in the power sector of Baltic States?”, in Proc. EPE-PEMC 2004 Conf., Riga, September 2004.

[4] P.M. Anderson, A.A. Fouad, Power System Control and Stability, The Iowa State University Press, Ames, USA, 1977.

[5] P. Kundur, Power System Stability and Control, McGraw-Hill, Inc.,1994.

[6] Advancements in Microprocessor Based Protection and Communication, IEEE Tutorial Course, IEEE Operations Center, 1997, 127 pp.

[7] Technical guide to the connection of generation to the distribution network, DTI, 2003.

[8] J.G. Slootweg and W.L. Kling, “Is the Answer Blowing in the Wind”, IEEE power & energy magazine, november/december 2003, pp 26-33.

[9] M.Pöller and S.Achilles, “Aggregated Wind Park Models for Analyzing Power System Dynamics”, [Online]. Available: http://www.digsilent.de/Consulting/Publications/T

[10] E.On Netz “Netzanschlussregeln für Hoch- und Höchstspannung, Stand August 2003”, E.On Netz GmbH, Bayreuth, 2003.

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

[12] V. Ivanovs, V. Rimarevs, J. Gerhards and A. Mahnitko, “Calculations of Steady and transient states in the complex power systems according to the program MUSTANG” in Proc. 5th International Conference Control of Power & Heating Systems, Zlin, May 2002, pp. 6.

[13] A.G.Phadke and J.S.Thorp, Computer Relayinng for power systems, Research studies press Ltd, England, 1990.

[14] Y. Ohura, M. Suzuki, K. Yanagihashi, M. Yamaura, K. Omata, T. Nakamura, S. Mitamura, and H. Watanabe, "A Predictive Out-of-Step Protection System Based on Observation of the Phase Difference Between Substations", IEEE Transactions on Power Delivery, Vol. 5, No. 4, pp.1695-1704, Nov. 1990.

[15] J. van Eyssen, “Introducing a new application philosophy for out-of-step protection”, Developments in Power System Protection, 25-27th March 1997, Conference Publication No. 434, IEE, 1997.

[16] V. Centeno, J. De La Ree, A. G. Phadke, G. Michel, J.Murphy and R.Burnett, "Adaptive Out-of-Step Relaying using Phasor Measurement Technique", IEEE Computer Application in Power, pp. 12-17, October 1993.

[17] A. Sauhatas, K. Brinkis and A. Svalov, "Automatic Devices for Prevention and Elimination of Asynchronous Operation in Power Systems", in Proc. 13th Power System Computation Conf., Trondheim, July 1999.

[18] 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.

[19] R. Pelissier, Energetical systems, Dunod, Paris, 1975.

VIII. BIOGRAPHIES Antans Sauhats received Dipl.Eng., Cand.Techn.Sc. and Dr.hab.sc.eng. degree from the Riga Technical University (former Riga Polytechnical Institute) in 1970, 1976 and 1991 respectively. Since 1991 he is Professor at Electric Power Systems. Since 1996 he is the Director of the Power Engineering Institute of the Riga Technical University.

Inesa Svalova received the Mag. Sc. ing. degree in chemistry from Riga Technical University in 1992 and Mag. Sc. ing. degree in electrical engineering in from Riga Technical University 2003. Now she is Dr.Sc. student.

Andrejs Svalovs received the Dr. Sc. ing. degree in electrical engineering from Riga Technical University in 1998. His areas of interest include power system automatics, relay protection of power systems, microprocessor devices, loss-of-synchronism protection and statistical approach to power systems simulation.