Fault in Asynchronous Machines

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CC II RR EE DD 17th International Conference on Electricity Distribution Barcelona, 12-15 May 2003

FEE_Bizjak_A1 Session 4 Paper No 37 - 1 -

INFLUENCE OF FAULTS IN DISTRIBUTION NETWORK ON BEHAVIOR OF ASYNCHRONOUS

GENERATORS IN SMALL HYDROELECTRIC POWER PLANTS

Grega BIZJAK

University of Ljubljana, Faculty of Electrical Engineering - Slovenia

[email protected] INTRODUCTION

The number of small hydroelectric power plants (SHPP) in

Slovenia has greatly increased over the last 20 years. In

some mountainous regions with a large number of streams

the production of electrical energy from SHPP now covers

up to 50% of power demand. In such small power plants

three phase asynchronous machines are normally used as

generators and only the largest among them use

synchronous generators. SHPP are usually connected

directly to the local low-voltage or middle-voltage network.

Electrical power networks in these wooded and

mountainous rural area normally consist of overhead

transmission lines and there are a large number of short circuit faults and line interruptions each year. Each fault

represent a disturbance to the generators.

Generators react to the disturbance in two ways. As the

voltage in the network is reduced, the generator can not get

rid of all its active power so it will start to increase speed.

In asynchronous machines this can lead to a very high

speed and unstable operation. On the other hand, the large

number of new generators in the network can increase the

short-circuit power beyond the limits allowed by the

installed equipment.

Digital simulation is an appropriate tool for the

investigation of such problems. Through simulation, it is possible to establish the behavior of the network and

machines prior to the occurrence of the critical conditions.

This paper presents the results of the simulation of

behavior of generators in SHPP in the Upper Savinja

Valley in Slovenia.

THE DISTRIBUTION NETWORK IN THE UPPER

SAVINJA VALLEY

Electrical distribution network of the Nazarje

substation

The distribution network in the Upper Savinja Valleyoperates on 20 kV voltage level and is supplied from the Nazarje substation. This substation is the only supply pointfor this network and has no transformation capabilities. The110/20 kV transformation is carried out at the Mozirjesubstation and both substations are connected by four 20kV overhead lines. The Upper Savinja Valley network operates as a radial network. It supplies the 20/0,4 kV

substations arranged along the main supply lines andconnected to them.

There are also some 50 small hydroelectric power stations(SHPP) in a network, more than 30 of which are normallyin operation. Most of them have one or two asynchronousgenerators, and there are also two synchronous generatorsin the network. The total average active power generation isaround 2000 kW, which represents 45% of the load demandin the valley.

A model of the observed network was made in a digitalsimulation program to enable the simulation of generator behavior during faults.

Network simulation model topology

In the computer simulation model of the Upper SavinjaValley network all the main elements are included. The 110kV network is represented by a slack generator with theshort-circuit power of 2210 MVA and the R/X ration of 0,1.

The model includes both 110/20 kV transformers installedin Mozirje substation. The three-winding 110/20 kVtransformers are modeled with a two-winding transformer

model in Yy0 connection. The tertiary winding was notmodeled. The neutral point of the 110 kV winding isisolated and the neutral point of the 20 kV winding isearthed through the 80 Ω resistor. In this way, the one- phase short-circuit currents are limited to the values around150 A.

The model also includes the complete 20 kV network of theUpper Savinja Valley area, together with all four 20 kVoverhead lines between the Nazarje and Mozirjesubstations. The 20 kV overhead lines and cables are allmodeled with a nominal PI-model and appropriate data.

All generators included in the network are modeled with a

Park model of an asynchronous or in two cases of asynchronous machine. As these generators are normally notequipped with voltage or speed regulators, the constantturbine torque and the constant excitation voltage of bothsynchronous generators were taken into account. Thegenerator models are connected to the 20 kV grid throughthe models of 20/0,4 kV transformers in localtransformation substations. These transformers are Yy0connected with an insulated neutral point on the 20 kV sideand an earthed neutral point on the 0,4 kV side.





The transformers in 20/0,4 kV substations (TSS), where nogenerators are connected, are not modeled. Instead, thelumped impedance is used to represent the power consumption in the area supplied by such substations. Thesimplified one-pole scheme of the modeled network isshown in Fig. 1.

Figure 1 - One-pole scheme of the observed network

Operation of the network

The configuration and the reference point of the network used in the model were taken from a “normal” day’soperation. The Upper Savinja Valley network is suppliedwith one 110/20 kV transformer in the Mozirje substation.This transformer is supplying one main bus bar in thissubstation which is connected with the Nazarje substations by two out of four overhead lines between the Mozirje and Nazarje substations. Each of this two overhead lines inoperation is supplying one of the two main bus bars in the Nazarje substation. The bus bars are not interconnected andthe total load of the area is divided between them into twounequal parts. The 20 kV network of Upper Savinja Valleyoperates as a radial network with all loops open.

The average yearly load is used to represent the load flowsituation in the network model. This load is distributedamong the lumped resistances in 20/0,4 kV substationsalong the 20 kV lines, which represent the load in notmodeled 0,4 kV network.

There are also 32 generators in operation, which are

included in the model. From these 32 generators, 30 areasynchronous generators and two are synchronousgenerators. All the generators are loaded with active power of up to 80% of their nominal power. The reactive power of asynchronous generators depends on the model parametersof each generator. The total power factor (cos ϕ) is around0.8. The total generation of active power is 2,104. MW.Generator models are not equipped with model of speedregulator and turbine. The constant torque on generator axisis used to represent the turbine instead.

DYNAMIC ANALYSIS OF THE NETWORK

With the help of a dynamic simulation program, theinfluence of the faults in the network on the behavior of thegenerators was calculated. A three-phase short-circuit faultwas simulated at different locations in the network and withreference to different residual voltages. The residualvoltage at the fault point is used to represent more distantfaults, such as those in 0,4 kV network which is not fullyincluded in the model of the network.

Influence of three-phase short-circuit faults on the

behavior of generators in SHPP

Three-phase short-circuit faults represent the most severefault in the network for the operating generators. When thefault occurs, the network voltage is reduced across thewhole network. The lowest voltage is at the short-circuit point and is equal to zero if the fault is a bold one (directconnection between the phases). The residual voltage ongenerator terminals depends on the distance between theshort-circuit point and the generator, on the impedance between the generator and the short-circuit point, and onthe current through this impedance. It could be near zero if the fault is at the generator terminals, or almost nominal if the fault is very far away.

If the residual voltage on the generator terminal is near thenominal one, the fault represents only an additional load tothe generator. In such a case, the asynchronous generator will slow down and the generator current will increase. Onthe other hand, if the residual voltage during the fault islower, the generator can not give away its active andreactive power. The surplus of the mechanical power over the electrical one will increase the speed of theasynchronous generator.





Increase in the speed of asynchronous generators

during three-phase short-circuit fault

During the three-phase short-circuit fault with low residual

voltage an asynchronous generator will increase in speed asit can not give away the electrical power. To find out howlarge this increase in speed can be at normal fault clearingtimes, a series of simulations of three-phase short circuitfaults were carried out at the terminals of a typicalasynchronous generator in a network (at TSS Sentjanz).Different residual voltages were considered (from 0% up to75%) as well as different fault clearing times (from 100 msto 400 ms). The relative increases in speed in differentcases are shown in Table 1. The normal speed (relative tothe synchronous speed) of this asynchronous generator is1.014

TABLE 1 - Increase in speed of asynchronous generators at different

residual voltages and fault durations on a 20 kV bus-bar in TSS Sentjanz

Residual voltage 0% 10% 20% 30% 50% 75%

Fault clearingtime

n/nsinh n/nsinh n/nsinh n/nsinh n/nsinh n/nsinh

100 ms 1,120 1,098 1,089 1,085 1,081 1,057

200 ms 1,356 1,299 1,254 1,198 1,103 1,041

300 ms 1,713 1,614 1,518 1,417 1,166 1,042

400 ms >2,00 >2,00 >2,00 1,724 1,285 1,045

As the results show, the most dangerous fault is one near the generator. With a normal fault clearing time of 400 msin a 20 kV network, faults with a residual voltage of lessthan 20% will result in a speed twice as fast as synchronousspeed. If the residual voltage is higher (a more distant fault)the speed increase is lower and at a 50% residual voltage,the speed increase is less than 30%. At an even higher residual voltage such as 75%, the speed increase is only afew percent of synchronous speed. The behavior of theasynchronous generator during a 300 ms fault with 0%residual voltage is shown in Fig. 2.

Some additional simulations were carried out to show theinfluence of the fault location (distance) on the residualvoltage on generator terminals. It was found that even acable with a length of 1.25 km contributes 20% to theresidual voltage. A residual voltage as large as 70% occursat a fault point which is approximately 4.5 km away fromthe generator. Thus the only really dangerous faults arethose that occur very close to the generator.

Influence of generators on the short-circuit current in a

network

The next step is represented by simulations of short-circuitfaults (one-pole and three-pole) in a network both with and

without the consideration of generators in SHPP. The aimof these simulations was to determine the influence of generators in SHPP on the amplitude of short-circuitcurrents in the network. Four different fault locations wereconsidered. At each location a bold short-circuit fault (one- phase or three-phase) was simulated. The fault was clearedafter 300 ms without any other change in the network.

The results show that generators have a very smallinfluence on a one-pole short circuit current. In all cases,the short-circuit current is larger when generators innetwork are involved, but the increase is less than 5%.More precise results are given in Table 2.

Figure 2 - Asynchronous generator values during a fault (300 ms,0% residual voltage) on a 20 kV bus-bar

In addition to the currents, voltages in phases without faults

were also calculated. It was found out that the difference involtage is even smaller than in the current. The influence of generators in a network on voltage during a one-phase faultis less than 3%.

Figure 3 – Voltages and currents in Nazarje substation during one- phase short-circuit fault at TSS Poljane (duration 300 ms)





The results show that the generators in a network, althoughthey supply up to 45% of the load, have a very smallinfluence on the one-phase short-circuit currents or onvoltage during one-phase short-circuit faults.

TABLE 2 - One-phase short-circuit currents in a network with andwithout the contribution of generators in SHPP

Fault location With SHPP Without SHPP

IL1 (A) IL2 (A) ∆ IL3 (A)

TSS Poljane 201,0 198,0 3,0

TSS Planina 178,0 173,2 4,8

TSS Igla bife 170,9 167,8 3,1

TSS Sestre Logar 155,2 149,1 6,1

TABLE 3 - Voltages at Nazarje bus-bar during one-phase short circuitfaults

Fault location With SHPP Without SHPP

UL2 (pu)

UL3 (pu)

Umax (pu)

UL2 (pu)

UL3 (pu)

Umax (pu)

∆ Umax

(pu)TSS Poljane 1,659 1,861 1,861 1,637 1,842 1,842 0,019

TSS Planina 1,584 1,779 1,779 1,534 1,749 1,749 0,049

TSS Igla bife 1,534 1,705 1,705 1,491 1,682 1,682 0,023

TSS Sestre Logar 1,469 1,609 1,609 1,419 1,579 1,579 0,030

The contribution of the generators to the three-phase shortcircuit currents is much larger. As can be seen from Table4, the difference between a fault current with or withoutconsideration of the generators can be over 25%. Theincrease in the fault current is larger at more distant pointsof the network than in faults near the main bus-bars.

TABLE 4 - Three-phase short-circuit fault currents in a network withand without the contribution from the generators in SHPP

Fault location With SHPP Without SHPP ∆ Imax

IL1(A) IL2(A) IL3(A) IL1(A) IL2(A) IL3(A) (A)

TSS Poljane 2137, 2393, 2274, 1901, 2089, 1978, 303,

TSS Planina 1314, 1502, 1444, 1116, 1180, 1124, 322,

TSS Igla bife 1004, 1111, 1092, 874, 917, 885, 194,

TSS Sestre Logar 654, 716, 710,9 576, 593, 580, 122,

Although the generators contribute a large part of the faultcurrent, the voltage in a network during a three-phase faultis nearly the same in both cases (with or without thegenerators in the network). The difference between them isless than 2%, as is shown in Table 5.

TABLE 5 - Voltages at Nazarje bus-bar during three-phase shortcircuit faults

Fault location With SHPP Without SHPP ∆ Umax

UL1 (pu)

UL2 (pu)

UL3 (pu)

UL1 (pu)

UL2 (pu)

UL3 (pu)

(PU)

TSS Poljane 0,672 0,700 0,686 0,659 0,688 0,677 0,012

TSS Planina 0,815 0,838 0,825 0,804 0,826 0,819 0,012

TSS Igla bife 0,866 0,883 0,869 0,854 0,870 0,861 0,013

TSS Sestre Logar 0,924 0,931 0,915 0,915 0,921 0,909 0,010

As well as the increase in fault current during three-phaseshort-circuit faults, a large increase in post-fault currentwas also found. The reason for this were asynchronousgenerators in the network. During a fault, the speed of thesegenerators increases, as described in the previous section.After the fault is cleared, the slip of these generators is verylarge (up to –1.0) so the current is sufficiently high and canreach 3-5 times nominal current.

CONCLUSIONS

Simulations of short-circuit faults in a network with a largenumber of small distributed generators provided somevaluable results.

The first problem we identified was that of instability andexcessive speed. If a fault occurs close to a generator and isnot cleared fast enough, the speed of asynchronousgenerator can reach as much as twice synchronous speed before the fault is disconnected. This indicates thatgenerators should be equipped with additional short circuitand under-voltage protection to prevent unwantedoperational states and damage.

On the other hand, generators have very little influence onconditions in the network except on three-phase short-circuit currents. The influence on one-phase short-circuitcurrents as well as on voltages during faults is practicallynegligible.

The results of these simulations were of practical value tothe electricity distribution company. After the results wereobtained, the generators’ protection devices were checked,as well as the circuit breaker capacities in the substations.In the period since most of the SHPP were put intooperation, no serious problems have occurred in the Upper Savinja Valley network, in spite of the large number of generators in the network.

REFERENCES

[1] Bizjak, G.; Žunko, P., 1999, Raziskava razmer v 20 kVomrežju zgornje Savinjske doline po vključitvi RPLjubno, Final report on project for Elektro Celje, Celje,

Slovenia

[2] Bizjak, G.; Povh, D.; Völcker, O.; Žunko, P., 1993,Calculation of Transient Phenomena, Proceedings"Athens Power Tech", NTUA - IEEE/PES, JointInternational Power Conference 1993, Greece

[3] Bizjak G.; Žvikart D., 1999, Vpliv malih hidroelektrarnna razmere v distribucijskem omrežju, 4th conferenceSLOKO-CIGRE 1999, Slovenija

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Fault in Asynchronous Machines