Study of Effectiveness of Under-excitation Limiter in Dynamic Modeling of Diesel Generators

Saeed Mohajeryami, Zia Salami, Iman Naziri Moghaddam Energy Production and Infrastructure (EPIC)

Electrical and Computer Engineering, University of North Carolina at Charlotte Charlotte, NC 28223, USA

Email: smohajer, zsalami, inazirim @uncc.edu

Abstract— The Under Excitation Limiter (UEL) has been introduced in the excitation systems mostly to prevent the loss of synchronism of generators and also prevent exceeding machine under-excited capability limit which otherwise leads to overheating of the stator of the machine and also challenges different part of generator protection and control system. In this paper, its effect on the modeling of Diesel Generator’s excitation system during island operation (i.e. local network) is studied. The simulation results performed in the Matlab/Simulink using IEEE type ST2A excitation model suggests that in the local network with mainly inductive loads the UEL is ineffective and it can be neglected in the modeling.

Keywords: Diesel Generators, Excitation System, Under Excitation Limiter (UEL), IEEE type ST2A excitation system, Electrical Distribution System (EDS), Island Operation, Local Network, Modeling

I. INTRODUCTION

As part of regulations and standards process within power generation stations, the plant owners need to model and study their Electrical Distribution System (EDS), Static and/or dynamic modeling, to reflect the behavior of the system under extreme loading condition, different modes of operations, and disturbances (e.g. fault). Diesel generators is one of the major equipment which needs to be dynamically modeled and analyzed, especially in nuclear power plants, due to their critical function as a backup source to operate power plant in safe condition.

One of the most important modifications introduced by IEEE in excitation system models is introducing Under Excitation Limiter (UEL). It forced IEEE to change most of its previously proposed models [1] to accommodate this change. IEEE used designation of “A” in front of excitation models type in 1992 version [2] and later in 2005 version [3] representing this new change.

The UEL, also known as the Minimum-excitation limiters (MEL), are intended to prevent the loss of synchronism of generators and also prevent exceeding machine under-excited capability limit. In 1940s, when dynamic stability was the major problem, they were proposed to avoid ill-operation during the steady state stability limit. But later, industry found some other applications for it like preventing situations leading to overheating in the stator end region of the synchronous machine [4]. These situations lay mostly in the

outside of the UEL region in machine capability curve. After introduction of loss-of-excitation relays, the UEL became useful in preventing its false operation in the UEL region as well [3].

UEL limits the imaginary part of the stator current which is due to the injected reactive power from the grid into the machine. This paper will address how impact of UEL is negligible during the transient stability analysis when there are no large capacitive loads in the local network. Many papers have identified how UEL works during large disturbances [4]-[8].

In [9], [10] some UEL models for stability analysis were introduced and some recommended tips for implementation have been discussed. Moreover, other types of limiters including Instantaneous and Inverse Time over Excitation Limiters, and Volts per Hertz Limiters have been reviewed in [10].

In this paper, one of the comprehensive and popular models of excitation system models (IEEE type ST2A) has been chosen to test the effect of UEL under different loading conditions. The Matlab/Simulink has been chosen as simulation software to realize a typical plant local loads network with presence of diesel generator.

In section II, the modeling has been discussed in detail. The chosen excitation system model along with UEL model and the test network have been described and their parameters have been given in Annex I and II. Section III is allotted to presentation of results with relevant discussion. Conclusion has been made in section IV.

II. MODELLING

A. Excitation System Model ST2A is a model of static excitation system which utilizes

both terminal current and voltage to comprise the power source. It uses the uncontrolled rectifier to make a DC source for static field [2]-[3].

ST2A has been chosen for this study mostly because there is only one difference from its previously proposed model, ST2. This difference is addition of UEL. Furthermore, this model has three inputs which make it more complicated and sensitive to changes in the model and system. The model has

978-1-4799-4881-9/14/$31.00 ©2014 IEEE

Figure 4. Test Case in Simulink

been shown in Figure 1. The parameters have been taken from IEEE sample data which enlisted in Annex I.

Figure 1. IEEE type ST2A excitation system

B. Under Excitation Limiter (UEL) IEEE introduced two different UEL models to cover wide

range of UEL applications. Type UEL1 is circular characteristics and Type UEL2 is single or multiple segment straight line characteristics. Type UEL1 senses terminal voltage and current as its input while Type UEL2 uses directly active and reactive power to compare them with the under excitation limits [2],[3],[9]. Since Type UEL2 uses active and reactive power, it has been used in this study. The model has been shown in Figure 2. and its parameters have been taken from sample data provided by IEEE and it has been presented in Annex II. The UEL region in capability curve has been shown in Figure 11. in Annex II.

Figure 2. IEEE UEL Type2 model

Figure 3. Test Case system single line diagram

C. Test case The small scaled system chosen for this study represents a plant modeled by 1 MW resistive load, 2250 HP, 2.4 kV asynchronous motor load (ASM) and a synchronous generator/diesel generator unit (SM) rated 3.125 MVA, 2.4 kV. The plant is fed from a 25 kV distribution network through 6 MVA, 25/2.4, Yg/D transformer [12]. This test system is one of the standard example systems introduced in the Simulink which has been tailored to accommodate the goals of this study. The SLD of the network has been shown in Figure 3. The original network consists of the inductive network and with closing the CB of the capacitive load, 1.2 Mvar, the network becomes mainly capacitive.

Initially the diesel generator is in standby and is delivering no active power and the plant is fed by distribution network and the SM functions as condenser to keep the bus voltage at 1.0 p.u.

A solid three phase to ground fault occurs at t=1.0 s on the distribution network which makes the circuit breaker (CB) to operate at t=1.1s. The islanding follows by sudden increase of diesel generator loading. During the islanding, the excitation system and speed governor react to transient changes to keep the system stable. The model for this system, in the Simulink, is shown in Figure 4.

III. TEST RESULTS For observing the effect of UEL in the system, two

scenarios have been introduced. The first one is the effect of UEL on the excitation system during islanding period of the local network when the loading is inductive and the second one is the same situation in the presence of capacitive load (when CB of capacitor is closed). In the second scenario, 1.2 Mvar capacitive load has been added to the local network. These two scenarios reflect the two distinguished loading situations which the former is the normal loading situation of local networks in the power plants and the latter one is the extreme situation which has been studied to cover all different loading situations.

A. Inductive Loads Figure 5. shows the terminal voltage and field voltage of

the diesel generator respectively. As it’s been shown in these figures, the UEL doesn’t have any effect on the system. Figure 6. shows the active and reactive power of diesel generator, as expected, before islanding, the diesel generator is in standby (the active power is zero) and it acts like condenser (the reactive power is around 0.25 p.u.).

Figure 5. a. Terminal Voltage b. Field Voltage both for Inductive Loading

During fault and islanding, the diesel generator supplies the local loads. As it is shown in Figure 6. , the reactive power doesn’t go beyond the UEL region and it’s positive during the simulation. This answers why UEL is inactive during the simulation and it doesn’t affect the terminal voltage and field voltage. Figure 7. shows (a) the signal of error voltage (the first summation point in the figure 1), (b) the output signal of UEL block, and (c) High value (HV) gate which compares the signals of (a) and (b) and let the highest value to go through. As it is shown in Figure 7. , with comparing figure 7(a), 7(b) and 7(c), it’s obvious that 7(c) is almost 7(a) and the UEL doesn’t have any impact on the output of HV gate.

Figure 6. a. Active Power b. Reactive Power both for inductive loading

Figure 7. a. Error Voltage b. UEL Block Output Signal c. High Value Gate Signal

B. Capacitive Loads Figure 8. presents the terminal voltage and field voltage of

the diesel generator respectively. With comparing the behavior of voltage with and without presence of UEL block, obviously it has been shown that they got affected drastically. The UEL block, outside the UEL region tries to change the field voltage to keep the reactive power in the UEL region. Figure 9. presents the active and reactive powers of diesel generator and the reactive power is kept within the UEL Region. During the islanding, the capacitive loads inject reactive power to the system. Figure 10. shows (a) the signal of error voltage, (b) the output signal of UEL block in the excitation system model and (c) HV gate. As it is shown, in this situation, the UEL is active and its signal is dominant most of the time and it affects the output of the excitation system.

Figure 8. a. Terminal Voltage b. Field Voltage both for Capacitive Loading

Figure 9. a. Active Power b. Reactive Power both for Capacitive loading

Figure 10. a. Error Voltage b. UEL Block Output Signal c. High Value Gate Signal

IV. CONCLUSION

It is been discussed that there are two different, and complex, IEEE UEL models to cover wide range of applications for synchronous generators.

Since diesel generators are typically designed to supply partial in-house plant loads during emergency condition which are mainly inductive loads (e.g. motors); the necessity of having UEL in the excitation system model of diesel generator is been questioned.

Two scenarios have been introduced in this study to examine the impact of inductive and capacitive net loading on the UEL. As it is shown in figures 5-7, the UEL block is ineffective during the islanding which diesel generator will supply power to the local network that are largely inductive. On the other hand, figures 8-10 show that in capacitive net loading which is impractical, the UEL cannot be neglected and its presence should be considered in the modeling.

Therefore, it can be concluded that for diesel generator excitation system modeling that are in island operation, the UEL block can be neglected without compromising the accuracy of modeling when the system loading is mainly inductive.

ANNEX I. SAMPLE DATA FOR A TYPE ST2A EXCITATION SYSTEM [3]

ANNEX II. SAMPLE DATA FOR A TYPE UEL2 UNDER EXCITATION LIMITER

MODEL [3]

Figure 11. UEL region in Capability Curve

REFERENCES

[1] Report, I.C., "Excitation System Models for Power System Stability Studies," Power Apparatus and Systems, IEEE Transactions on, vol.PAS-100, no.2, pp.494,509, Feb. 1981 [2] IEEE Recommended Practice for Excitation System Models for Power System Stability Studies," IEEE Std 421.5-1992 [3] IEEE Recommended Practice for Excitation System Models for Power System Stability Studies," IEEE Std 421.5-2005 [4] Ribeiro, J. R., "Minimum excitation limiter effects on generator response to system disturbances," Energy Conversion, IEEE Transactions on, vol.6, no.1, pp.29,38, Mar 1991[5] de Oliveira, S.E.M.; Groetaers Dos Santos, M., "Impact of under-

excitation limit control on power system dynamic performance," Power Systems, IEEE Transactions on, vol.10, no.4, pp.1863,1869, Nov. 1995

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[7] Jia, X. M.; Choi, S.S., "Design of Volts per Hertz limiter with consideration of the under-excitation limiter control actions," Energy Conversion, IEEE Transactions on, vol.16, no.2, pp.140,147, Jun 2001

[8] Patel, S. et al "Performance of Generator protection during major system disturbances," Power Delivery, IEEE Transactions on, vol.19, no.4, pp.1650,1662, Oct. 2004

[9] Hurley, J.D., "Underexcitation limiter models for power system stability studies," Power Engineering Society General Meeting, pp.980,984 Vol. 1, 12-16 June 2005

[10] Mummert, C.R., "Excitation system limiter models for use in system stability studies," Power Engineering Society 1999 Winter Meeting,pp.187,192 vol.1, 31 Jan-4 Feb 1999

[11] Berube, G.R.; Hajagos, L.M.; Beaulieu, R.E., "A utility perspective on under-excitation limiters," Energy Conversion, IEEE Transactions on,vol.10, no.3, pp.532,537, Sep 1995

[12] MATLAB SimPowerSystems software (version 7.12., R2011a), The MathWorks, Inc., Natick, USA, 2011.

Saeed Mohajeryami received the B.Sc and M.Sc from Ferdowsi University and University of Tehran, Iran, respectively in 2008 and 2010. He joined as a PhD student and research assistant in University of North Carolina at Charlotte, US in 2012. His research interest includes mostly power system real time

dynamic modeling, design and analysis, power system equipment protection, grid and interconnection study, and also system identification in power system.

Dr. Zia Salami is Associate Professor of Electrical and Computer Engineering in UNC Charlotte, graduated in 1998 with Ph.D. degree in Electrical Engineering, Power Systems and Control. His prior experience before joining the university includes 13 years of domestic and international industry experience in the nuclear

power and energy market working with AREVA Inc. He has served in several leadership roles as an advisory engineer, technical consultant, project manager, and coordinator. Dr. Salami is a Senior Expert in electric power system network and his research and focus area are mainly in electric power system generation and distribution system real time dynamic modeling, design and analysis, power system equipment protection, Grid and interconnection study.

Iman Naziri Moghaddam received the B.Sc from IKIU (Imam Khomeini International University) and the MSc from Amir Kabir University of Technology (Tehran Polytechnic), Tehran Iran, in 2007 and 2010, respectively. He is currently a PhD student and research assistant in University of North Carolina at Charlotte,

USA. His main research interests are: Modeling and system identification, fault detection, and application of control theories in power system.