Appropriate Grounding System for Grid-connected

This article has been accepted for publication in abyfuture of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI Downloaded from Iran library: (www.libdl.ir) | Sponsored Tehranissue Business School (www.tbs.ir) 10.1109/TIA.2015.2422814, IEEE Transactions on Industry Applications

Appropriate Grounding System for Grid-connected Small-scale Synchronous Generators Moein Abedini

Mahdi Davarpanah

Majid Sanaye-Pasand

ECE School University of Tehran [email protected]

ECE School University of Tehran [email protected]

Senior Member, IEEE ECE School, University of Tehran [email protected]

 Abstract – Small scale synchronous generators (SSSGs) are widely utilized as distributed generators in power systems. A large fault current due to the SSSG phase to ground short circuit may result in excessive damages to the stator core and windings in a way that major repairs such as core disassembling and rewindings become necessary. Thus, the fault current should be properly restricted to prevent such damages and reduce the required capital and time for SSSG repairs. In this research work, a model to calculate the SSSG internal fault current is developed. Meanwhile, an appropriate index to quantify the damages to the faulty SSSG is presented. Afterwards, influencing parameters on reduction of the damages are investigated when either frequently used TT or TN grounding systems are applied for SSSG. Studies show the stator damages are mainly caused by the SSSG fault current rather than that of the system. Accordingly, to reduce the SSSG vulnerability to the stator earth fault, not only some applicable strategies are proposed for the both grounding systems, but also the favorable grounding system is determined. Index Terms — Generator internal fault, grounding system, small scale synchronous generator, stator earth fault.

T

I.

INTRODUCTION

WO types of grounding systems including protective and electrical groundings should be utilized for power system generators, especially small-scale synchronous generators (SSSGs). The generator frame should be directly connected to the ground as the protective grounding system to mitigate the touch voltage and prevent electric shock hazards due to a stator earth fault [1]. The stator neutral terminal should also be connected to the ground as the electrical grounding system to reduce overvoltage during phase-to-ground short circuits, supply single-phase loads and simplify detection of fault occurrence. The phase-to-ground fault current on a directly grounded system may be even higher than that of a three-phase fault [2]. Therefore, for large generators, neutral is usually connected to the ground through an impedance to decrease the phase-to-ground fault current [3-5]. Meanwhile, low voltage (LV) SSSGs are usually grounded by using few rods or just one ground well which results in a ground resistance of about 5 Ω [6]. Since the phase-to-ground fault current is properly limited under this condition, there is no need to utilize an additional impedance between the neutral terminal and the ground for LV SSSGs.

Different types of connections can be applied between the SSSG protective and electrical grounds among which TT and TN systems are the most practical approaches [7-8]. In the TT grounding system, the SSSG frame and corresponding neutral terminal are separately connected to the ground. However, bonding should be utilized to unify the both grounds for the TN system. Stator earth fault is one of the most prevalent short circuits in generators. A high magnitude ground fault current results in excessive amount of dissipated energy at the fault point which can damage the insulation between core laminations and electrically connect them together [910]. This results in a local overheating as well as an asymmetrical magnetic flux distribution which leads to mechanical parts vibrations [1]. In such a case, to repair the generator, it requires noticeable amount of capital and time to remove all the stator windings and replace the faulty core laminations. Whereas, low dissipated energy at the fault point results in much less damage to the laminations in a way that often no comprehensive winding or core disassembling is required. The dissipated energy amount depends on the fault current amplitude and its duration. Both SSSG and the system to which SSSG is connected contribute to the fault current. The fault current supplied by the system is cleared fast by operation of the generator circuit breaker (GCB). However, the generator keeps injecting some current into the stator earth fault point depending on the de-excitation system, the rotor circuit time-constant, and the generator inertia [11]. Although this decaying fault current usually has a low amplitude, the corresponding energy of the weakly damped generator fault current can be much more than that of the system fault current. The dissipated energy amount directly depends on the SSSG grounding system as well. In other words, the decaying fault current time constant and amplitude are related to (1) the de-excitation system, and (2) the utilized TT or TN grounding system. In this paper, a grid-connected LV SSSG is appropriately modeled to study the generator internal fault current for various fault locations. Using the developed model, the fault currents separately supplied by the grid and generator can be determined for both of the widely used TT and TN grounding systems. Meanwhile, the energy dissipated at the fault point is estimated based on the calculated currents and corresponding durations for both grounding systems.

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0093-9994 (c) 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.


(a )

(b )

(c )

(d )

Fig. 1. Partitioned faulty winding of phase A

In addition, influence of the ground resistance value on the dissipated energy is evaluated. Moreover, an appropriately designed crowbar system is developed as the SSSG deexcitation system that alleviates the dissipated energy for both TT and TN systems. Finally, the preferable grounding system is proposed for the grid-connected SSSGs. II.

Fig. 2. Various excitation systems including a simple de-excitation system

After interrupting the current passing through the AC side of the rectifier by the AC or DC field breaker, the rotor current requires a closed loop to be gradually diminished. To approach this objective, a freewheel diode is required when a DC field breaker is exploited, as shown in Fig. 2-a. Whereas, the rotating rectifier diodes can play the role of freewheel diode when an AC field breaker is applied, Fig. 2-b. Figs. 2-c and 2-d show a thyristor rectifier connected to the field winding through brushes and slip rings, which represents the shunt excitation system [2]. To interrupt the AC side rectifier current, either of the AC or DC breaker opening or thyristor extinguishing can be utilized. In addition, a freewheel diode is usually exploited to gradually damp out the SSSG rotor current. By utilizing one of these de-excitation systems, once the AC side rectifier current is interrupted, the field winding terminals are short circuited. This widely used strategy results in a decaying DC field current with the time constant of about 4 sec for SSSGs [15].

STUDIED SYSTEM MODELING

Synchronous Generator with Internal Fault To simulate the synchronous generator internal fault, the stator winding at which the phase-to-ground fault occurs should be divided into two parts, including exterior and interior windings [12], [13] and [14]. As illustrated in Fig. 1, the faulty phase winding is partitioned into a1 as the interior winding, which is terminated at the neutral (N), and a2 as the exterior winding, which is adjacent to the generator terminal (T). Furthermore, similar to many others research works, it is assumed that the generator windings are sinusoidally distributed in space and the system is magnetically linear. The generator model consists of electrical, magnetic, and mechanical equations of the generator and its associated prime-mover [12], [13], and [14]. A comprehensive generator model including the internal stator earth fault is developed and implemented for studies of this paper, which is not scrutinized here due to the number of pages restriction. A.

Power System Model Fig. 3 shows the modeled power system under study including the low voltage SSSG which is connected to an external grid comprising a voltage source and its Thevenin impedance. Parameters for the system and SSSG are tabulated in Appendix. A stator winding internal fault can be considered at different locations using the developed model. C.

Excitation and De-excitation Systems To be able to accurately model generator internal fault currents, the excitation system should be exactly modeled. After detection of an internal fault by the SSSG protection system, not only the stator current should be interrupted by opening the GCB, but the rotor excitation current should also be interrupted. However, the large rotor inductance results in a noticeable overvoltage if the rotor current is suddenly forced to zero. To avoid such an overvoltage, the rotor current should be gradually reduced. In doing so, one of the excitation systems of Fig. 2 can be exploited. The excitation systems illustrated in Figs. 2-a and 2-b can be utilized with a permanent magnet generator or an auxiliary winding regulation excitation principle. For such brushless excitation systems, the AC voltage output should be converted to the DC voltage by a three-phase rotating diode rectifier [2]. B.

III. COMMONLY USED GROUNDING SYSTEMS To more completely comprehend the problem under study, first the commonly used grounding systems are scrutinized. Grounding systems are classified based on various strategies to connect the neutral terminal and metallic frames of electrical devices, comprising the SSSG, transformer, electrical loads, and other equipment [7]. Fig. 4 illustrates TT and TN grounding systems, which are widely used for LV networks, e.g., with 400-V rated voltage. In the TT system, the frame and neutral terminal are distantly connected to the ground (LG) which is shown in Fig. 4-a. The generator neutral terminal and its frame are connected to the same ground for the TN system, as illustrated in Fig. 4-b. 2



RS T N

N

Fig. 3. Schematic diagram of the power system under study

Fig. 5. Implementing the TT system for SSSG and the TN system for distribution panel

ibody

RS T N

ibody N SSSG

LV distribution panel

LG a) TT grounding system

b) TN grounding system

Foundation and armatures connected to grounding rods

Fig. 4. Grounding systems for a low voltage SSSG

Fig. 6. Implementing the TN system for both SSSG and distribution panel

Electrical and protective ground resistances of the TT system, namely Rg1 and Rg2 are about 5 Ω. In such a condition, the phase-to-ground fault current is limited to about 20 A, which cannot activate some conventional protection devices of distribution systems, e.g., fuses. To guarantee fault detection using the less sensitive protection devices of LV distribution systems, the TN system is commonly applied. In addition, the TN system can practically be implemented with less capital and effort than those of the TT system. Accordingly, the TN system has become more common in many countries [16], [17]. The SSSG grounding system can be implemented regardless of the distribution network grounding system. For example, LV distribution panels and metallic frames of other equipment can be grounded based on the TN system while the TT system is used for SSSG. To do this, the SSSG stator frame and metallic part of the connected prime-mover can be grounded at a distant location with respect to the SSSG electrical grounding system, as illustrated in Fig. 5. On the contrary, the TN system can be used for both the SSSG and other equipment of the LV distribution system. For such a case, SSSG and other equipment can physically be installed on a same foundation, which results in an electrical bonding for the protective and the electrical grounds, as shown in Fig. 6. In other words, such a system should essentially be utilized where both of the following conditions are fulfilled: - The SSSG and other equipment of the distribution system, e.g., the distribution transformer and LV panels, are located on a same foundation and their metallic frames are connected to the armatures of the foundation. - The TN system is exploited for the distribution system equipment.

IV. SHORT CIRCUIT CURRENT FOR SSSG INTERNAL FAULTS TT Grounding System SSSG utilizing the TT grounding system is physically installed according to Fig. 5 and is electrically modeled by Fig. 4-a. The electrical ground resistance Rg1 and protective ground resistance Rg2 are both considered equal to 2 Ω in this case study 6 . Fig. 7 shows SSSG fault currents for a phase-to-ground short circuit at 50% of the stator winding, where ia1 and ia2 are the currents of the partitioned phase windings, respectively for the SSSG neutral and terminal sides. ibody is also the current passing through the SSSG stator core. As shown in Fig. 7, although both ia1 and ia2 exceed 1.5 pu, ibody is limited to 0.02 pu (i.e., 36 A) by the ground resistances of Rg1 and Rg2. This low amplitude current can damage the insulation between core laminations only when the fault current continues for a long time, as discussed in the next section. A.

TN Grounding System SSSG utilizing the TN grounding system is physically installed according to Fig. 6 and is electrically modeled by Fig. 4-b. When a stator earth fault occurs, the short circuit current supplied by SSSG passes only through a closed loop comprising of the frame and the faulty winding. Therefore, the SSSG short circuit current is not influenced by the Rg ground resistance. It should be noted that a large Rg can only reduce the fault current supplied by the upstream system. However, the system fault current contribution to the dissipated energy at the fault point is usually much less than that of the SSSG. B.

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This article has been accepted for publication in abyfuture of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI Downloaded from Iran library: (www.libdl.ir) | Sponsored Tehranissue Business School (www.tbs.ir)

i (pu)

2 1 0 -1 -2 0.03 2 1 0 -1 -2 0.03 0.04 0.02 0 -0.02 -0.04 0.03

a1

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i (pu)

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a2

(pu) body

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i (pu)

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i (pu)

10.1109/TIA.2015.2422814, IEEE Transactions on Industry Applications

Time (sec)

i

0.07

0.08

0.09

0.1

0.04

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0.06

0.07

0.08

0.09

0.1

0.04

0.05

0.06

0.07

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0.09

0.1

Time (sec)

(3)

where k is equal to 1.15 for the above mentioned condition, while it is considered in the range of 1 to 2 by other researchers [20], [21]. The k and 25 L factors are respectively considered equal to 1.5 and 1 in this study. Accordingly, the following index is introduced in this paper to quantify the SSSG damages caused by an internal fault

E D   i (t )1.5 dt

(4)

Furthermore, to prevent damages to the stator core and winding, the dissipated energy should be restricted to 30 kJ, as confirmed by experiments [22], [23]. Both the SSSG and system to which SSSG is connected contribute to the internal fault current. The system fault current is quickly interrupted by opening the PCC circuit breaker, e.g., five cycles after fault occurrence. However, the generator continues injecting the fault current depending on the rotor time-constant as discussed in Fig. 2. The longer time-constant, the larger dissipated energy and thus the more stator damages. Although the weakly damped generator fault current usually has low amplitude, the resultant ED due to the SSSG fault current can be much more than that of the system fault current. In addition, the dissipated energy amount directly depends on the SSSG grounding system. Fig. 9 shows the dissipated energy for the both grounding systems while a phase-to-ground fault is located at 75% of phase-A stator winding. For this study, the electrical and protective ground resistances are considered equal to 0.1 Ω, as the worst case. In addition, the GCB and the field breaker are simultaneously tripped 100 ms after the fault occurrence. In this condition, the dissipated energy at the fault clearing instant is calculated as 70 and 10 kJ for the TN and the TT systems, respectively. Afterwards, the dissipated energy ascends with a noticeable rate in a way that 5 sec after the fault instant, the calculated energy is respectively equal to 390 and 26 kJ for the TN and TT systems. Based on the obtained results for this case, the SSSG contribution to the dissipated energy is 95% and 82% of the total energy for the

SSSG DAMAGE ANALYSIS

where the fault current and fault resistance are respectively denoted by i and Rf, and ED represents the dissipated energy. It should be noted that the fault resistance mainly comprises of the arc resistance, which is intrinsically variable and is dependent on the fault current. For instance, the following equation is proposed by [19] to represent the dependency of fault resistance on the fault current for a generator stator earth fault

L

0.06

E D  25 L   i k dt

SSSG damages due to stator ground fault is proportional to the dissipated energy at the fault point. For the studied short period of time, the SSSG with an internal fault is usually considered as an adiabatic system and thus the energy can be calculated by (1) E D   R f  i (t )2 dt

0.85

0.05

where L represents the arc length. Combining (1) and (2) results in

Therefore, the ground resistance has insignificant effect on the fault energy. Meanwhile, the TN system stator earth fault current is considerably more than that of the TT system. Thus, protective equipment of the SSSG and the point of common coupling (PCC) to the system more rapidly respond to clear the fault. Fig. 8 shows the fault currents for a phase-to-ground short circuit at 50% of the stator winding where Rg is considered 2 Ω. In such a case, ibody current peak is about 5.5 pu which is extremely higher than that for the TT system. Consequently, the SSSG which is grounded based on the TN system experiences much more damages as compared with that of the TT system. To more quickly detect the fault and also to reduce the touch voltage, it was suggested by some researchers to use the TN system for microgrids [16-17]. On the contrary, the SSSG internal fault causes much more damages to the core laminations and the stator winding for this system. To determine a proper grounding system for a SSSG, the SSSG damages for both of the studied grounding systems should be evaluated. To approach this objective, the dissipated energy at the fault point is introduced as an appropriate index which is discussed in the following section.

R f  25

0.04

Fig. 8. Fault currents of partitioned phase windings and SSSG core for the TN grounding system

Fig. 7. Fault currents of partitioned phase windings and SSSG core for the TT grounding system

V.

5 3 1 -1 -3 -5 0.03 3 2 1 0 -1 -2 -3 0.03 6 4 2 0 -2 -4 -6 0.03

(2) 4



10 TN System TT System

8

(pu)

300

6

body

200

i

Dissipated energy (kJ)

400

4

Rg=0.1 

100

Rg=5 

2 0 0

1

2

3

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5

Time (sec)

6

7

8

9

0 0

10

Fig. 9. Dissipated energy due to a stator internal fault for the TN and TT systems

Rg=10  20

40

60

Fault location (%)

80

100

Fig. 10. Current passing through SSSG core versus fault locations and ground resistance in TN system

TN and the TT systems, respectively. Consequently, based on the performed analyses, it is concluded that the stator damages are caused mainly by the SSSG fault current rather than the system fault current.



VI. PROPOSED APPROACHES TO MITIGATE STATOR DAMAGES



As previously discussed, reduction of the SSSG fault current can noticeably decrease the core and winding damages at the fault point. This section investigates strategies to reduce the SSSG fault current amplitude and duration for both of the grounding systems.

Fig. 11. Crowbar system to quickly damp out the rotor current

To prevent unfavorable ohmic loss, the resistor should be connected in series with a thyristor or diode, which keeps the resistor out of service during the normal operation. This method is already used for large generators. It is proposed to apply a similar approach for SSSGs as well. Fig. 11 shows a modified de-excitation system in which instead of a single freewheel diode (Fig. 2-a), combination of a resistor (Rc) and a diode is utilized, which is called “crowbar system”. The crowbar causes the excitation current to be highly damped. Moreover, the generator current injected to the fault point is significantly decreased. Based on various simulations it is revealed that although increment of the resistance enhances the fault current damping rate, it results in some overvoltage across the rotor winding terminal as well. Furthermore, the increment of damping rate for crowbar resistances of more than 15 Ω is negligible. Thus, the proposed crowbar resistance of 15 Ω is considered for the SSSG under study. Fig. 12 shows the ibody fault current under the following assumptions: - Phase-to-ground fault at 75% of the stator winding - TN grounding system for the studied SSSG - GCB and field breaker interruption 100 ms after the fault instant As illustrated in Fig. 12-a, a relatively large fault current passes through the fault point even after opening the GCB and field breaker. In addition, the fault current is damped slowly and it continues to flow until about 12 sec after the fault occurrence. However, after the GCB interruption, not only the fault current amplitude is noticeably decreased by using the crowbar, but also SSSG supplies the fault current only for less than 600 ms, as illustrated in Fig. 12-b.

TN Grounding System Some SSSGs under operation exploit the TN grounding system. This grounding system provides such a high stator earth fault current that the fault can simply be detected by the conventional protective relays. On the other hand, the high current amplitude leads to excessive stator core damages and should be damped out as fast as possible. This may be accomplished using a high ground resistance or a properly designed de-excitation system which are discussed as follows. 1) Increasing ground resistance Fig. 10 illustrates the influence of the ground resistance and fault location on the fault current amplitude. Various simulations studies using the developed model are carried out. Obtained results illustrate that when the location of the internal fault approaches near to the phase terminal, the current passing from the SSSG core ibody increases more. Furthermore, for the TN system, the ground resistance is bypassed in the fault loop and thus the SSSG fault current is high and weakly damped. Consequently, increasing the ground resistance does not provide a reasonable solution to mitigate the SSSG damages for the internal fault occurrence. 2) Designing proper de-excitation system After the GCB interruption, the internal fault current amplitude only depends on the generator field current, which is weakly damped due to the high ratio of the field winding inductance to its resistance. Fast reduction of the field current can be provided using an appropriate resistor, which should be placed in series with the field winding after opening the field breaker. A.

5



Dissipated energy (kJ)

500

ibody (pu)

10 5 0 -5 -10 0 10 5 0 -5 -10 0

400

1

2

3

(a)

4

5

300

Without crowbar and tGCB =100 ms Rc= 15  and tGCB = 100 ms Rc = 15  and tGCB= 70 ms Rc = 15  and tGCB = 50 ms

ibody (pu)

200 100

0.2

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0.6

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Time (sec) (b)

10

20

30

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50

60

70

Fault location (%)

80

90

100

Fig. 13. Influence of proper crowbar system and GCB interruption time on the dissipated energy

Fig. 12. Current passing through the SSSG core (a) without crowbar system, and (b) with crowbar system 2

Enhancement of the excitation current damping speed using the crowbar effectively decreases damages to the stator core at the fault point. Under this condition, core damages mainly depend on the GCB interruption. Fig. 13 illustrates the effect of the crowbar system and also the GCB interruption time (tGCB) on the dissipated energy during internal faults for various phase-A to ground fault locations. More quick GCB operation, more reduction of the thermal core damages. As shown in Fig. 13, for a phase-to-ground fault at 50%, 270 kJ energy is dissipated at the fault point without using a crowbar. Whereas, the energy can be limited to about 50 kJ using a 15 Ω crowbar. Although the dissipated energy is significantly reduced, it is still more than the acceptable threshold. Our studies show that the energy exceeds the threshold for faults beyond 35% of the stator winding. Thus, even using a proper de-excitation system, the core damages cannot be prevented in the TN system. Accordingly, although some SSSGs which have been designed and installed by reputable international companies operate under the TN system (even without crowbar), utilizing such a grounding system is not preferable for SSSGs to avoid winding and core damages caused by an internal fault.

ibody (pu)

1.5

Rg1=Rg2=0.1  Rg1=Rg2=1  Rg1=Rg2=2 

1

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0 0

10

20

30

40

50

60

Fault location (%)

70

80

90

100

Fig. 14. Current passing through SSSG core versus fault locations, Rg1 and Rg2 for TT system TABLE I EFFECT OF GROUND RESISTANCE ON THE DISSIPATED ENERGY FOR TT SYSTEM

Rg2(  )

Rg1(  )

TT Grounding System 1) Increasing ground resistance Fig. 14 illustrates the effect of electrical and protective ground resistances on the fault current amplitude. On the contrary to the TN system, increasing the ground resistances considerably reduces the fault current passing through the stator core ibody in the TT system. As shown in this figure, the maximum current amplitude is less than 8% of the SSSG rated current, when the both ground resistances are equal to 2 Ω. It should be noted that few rods, one ground well or their combinations are usually used to provide grounding systems in the LV networks, which often results in a grounding resistance larger than 2 Ω. Computed dissipated energy for various Rg1 and Rg2 values is given in Table I, which is less than the acceptable threshold if Rg1+Rg2 is more than 0.22 Ω. Such a condition is usually easily fulfilled in LV systems.

Dissipated energy (kJ) 1 0.2 0.5

2

0.1

32.01

17.02

6.04

2.34

0.2

21.78

12.19

5.04

2.09

3.25

1.58

0.75 0.61 0.51

0.5

B.

0.1

12.95

7.01

1

9.73

4.62

2.07

1.15

2

8.18

3.42

1.32

0.74

0.93 0.87

It should be noted that the calculated values given in Table I are obtained for GCB interruption of 100 ms. For the most probable condition of Rg1 and Rg2 equal to or greater than 2 Ω, the GCB interruption time should be less than 25 sec to limit the dissipated energy to 30 kJ. This condition is also easily satisfied by the protection systems. Accordingly, unlike the TN system, application of SSSG under the TT grounding system can well attenuate the core and winding damages caused by stator earth faults. 2) Designing proper de-excitation system Utilizing a proper de-excitation system e.g., the crowbar system shown in Fig. 11 can decrease the amplitude and time-constant of the decaying fault current and the corresponding dissipated energy. As an example, for Rg1 and Rg2 equal to 2 Ω, the dissipated energy at the fault point is just 0.51 kJ when the PCC breaker opens after 100 ms, as 6



shown in Table I. In such a condition, de-excitation system described in subsection VI.A.2 limits the energy to 0.19 kJ. This energy is much less than the permissible threshold and prevents the core and winding damages. As the energy can well be restricted using the TT grounding system, unlike the TN system, utilizing the crowbar system is not essential for the TT system.

N SSSG

LG

B

A

VII.

DISCUSSION

Some proposals are provided in this section to more sensitize the SSSG protection system. In addition, some practical aspects to apply the TT grounding system are scrutinized.

Surface potentials (pu)

Fig. 15. Locating SSSG protective and electrical grounding rods in a TT system

Proposing a Sensitive Relay for Earth Fault Detection Although application of larger Rg1 and Rg2 resistances decrease the core and winding damages, it desensitizes the conventional SSSG phase overcurrent (OC) relays to recognize stator earth faults for the TT system. To enhance both SSSG protection system sensitivity and coverage, utilizing an earth OC relay is proposed which measures the current passing through the SSSG protective ground system, i.e, Rg2. For the system under study with Rg1=Rg2=2 Ω, the maximum short circuit current flowing through Rg2 is 0.08 pu (140 A). Thus, to increase the relay sensitivity, a current transformer with the turn ratio of 50/5 A is used. The pickup current of the proposed earth OC relay can be set equal to 0.1 times of the CT rated current, i.e., 5 A at the CT primary side. This sensitive relay can well cover about 95% of the stator winding against internal phase-to-ground faults. It should be noted that the proposed relay is very sensitive and provides almost complete coverage over the stator winding length. Moreover, since only negligible stray capacitive currents pass through the protective ground of the healthy SSSG, it practically remains stable against system transients or external faults. In some applications, the SSSG frame or the corresponding prime mover and piping systems may have multiple connections to the ground. To apply the proposed method, these parts should be connected to a single grounding point. A.

1

0.5

0 0

5

10

15

20

25 30 LG (m)

35

40

45

50

Fig. 16. Surface potential distribution of GPR along AB line

to the protective ground at distant LG. Along an arbitrary AB line, the surface potential distribution is calculated using a FEM-based software and the obtained result is illustrated in Fig. 16. As shown in Fig. 16, GPR reduction to about 5% can be achieved if the electrical grounding point is located at least 36 meters far from the protective grounding point in the system under study. This requirement can practically be fulfilled by connecting the SSSG neutral terminal to the distant grounding point using an insulated cable. VIII.

CONCLUSION

Proper selection of a grounding system for small scale synchronous generators was scrutinized in this paper to mitigate damages to the SSSG due to the stator phase-toground faults. To approach such an objective, the generator internal fault is modeled and the fault current is separately determined for the two commonly used TN and TT grounding systems. Investigations on the TN system, which is applied for some distribution systems, reveal that such a large current passes through the stator earth fault point that it usually leads to considerable damages to the SSSG core and windings. Although such damages can be alleviated by using a proper de-excitation system, the dissipated energy at the fault point is still more than the permissible threshold for many cases. On the contrary, the stator fault current is noticeably less than the threshold where the TT system is applied. The damages can even be reduced further by using a proper de-excitation system. It is concluded that the TT grounding system can be used as a viable and appropriate approach for distribution system SSSGs. In addition, a novel earth overcurrent relay is proposed for the TT system to enhance both the protection system sensitivity and coverage. Meanwhile, some practical aspects to apply the TT grounding system are discussed.

Implementing TT Grounding System To be able to implement the TT grounding system successfully, some practical aspects should be considered. In this section, these points are discussed using an actual example. Magnetic coupling between ground rods in the TT system should be as low as possible to avoid the effect of ground potential rise (GPR) [24]. To fulfill this requirement, the electrical ground should be sufficiently far from the protective ground. As an actual example, a 1.25 MVA, 400 V SSSG is considered to be installed on a 3.5*4.5 m2 foundation. As illustrated in Fig. 15, the foundation armatures are grounded at its four associated corners by 3-meter rods. The SSSG frame is connected to this ground, and the neutral terminal is remotely grounded with respect B.

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[18] R. Kamel, A. Chaouachi, K. Nagasaka, “Design and testing of three earthing systems for microgrid protection during the islanding mode,” Smart Grid and Renewable Energy, no. 1, pp. 132-137, Aug. 2011. [19] L. E. Fisher, “Resistance of low voltage arcs,” IEEE Trans. on Industry Applications, vol. 6, pp. 607–616, Nov./Dec. 1970. [20] L. J. Powell, “The impact of system grounding practices on generator fault damage,” IEEE Trans. on Industry Applications, vol. 34, no. 5, pp. 923–927, Sep./Oct. 1998. [21] H. Stainback, “Predicting damage from 277v single phase to ground arcing faults,” IEEE Trans. on Industry Applications, vol. 13, no.4, pp. 307-314, Aug. 1977. [22] R. Mcfadden, “Grounding of generators connected to industrial plant distribution buses,” IEEE Trans. on Industry Applications, vol. 17, no. 6, Nov.1981. [23] R.R. Conrad and D. Dalasta, “A new ground fault protection system for electrical distribution circuits,” IEEE Trans. on Industry Applications, vol. 3, no.3, May 1967. [24] IEEE Guide for Safety in AC Substation Grounding, IEEE Std. 80, 2000.

APPENDIX Table II PARAMETERS OF SSSG AND POWER SYSTEM [15] Generator capacity 1.25 MVA Rated voltage 400 V Ra 0.0026 pu Xd 3.92 pu Xq 2.35 pu X’d 0.194 pu X”d 0.165 pu X”q 0.173 pu T’d0 3.634 sec T’d 0.180 sec T”d 0.018 sec Ta 0.027 sec Xl 0.10 pu RTh 0.05 pu LTh 0.2 pu

REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12]

[13]

[14] [15] [16]

[17]

Moein Abedini received the B.Sc. and M.Sc. degrees in electrical engineering from the University of Tehran, Tehran, Iran, in 2011 and 2013, respectively, where he is currently pursuing the Ph.D. degree in electrical engineering. His research interests include power system protection problems and stability studies.

IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems, IEEE Std.142-1991, Dec. 1991. D. Reimert, “Protective Relaying for Power Generation Systems,” Taylor & Francis, 2006. IEEE Guide for Generator Ground Protection, IEEE Std. C37.101, 1993. IEEE Guide for AC Generator Protection, IEEE Std. C37.102, 1995. D. Shipp and F. Angelini, “Characteristics of different power systems neutral grounding techniques: fact & fiction,” IEEE Industry Technical Conference, Seattle, USA, pp. 107–116, June 1990. Grounding Connection in Distribution Networks, Iran Std. 32, Dec. 1994. Code of Practice for Earthing, British Standards Institute, second edition, 1998. C. Preve, “Protection of Electrical Networks,” ISTE Ltd, London, 2006. D. Braun, G. S. Koeppl, “Intermittent line-to-ground faults in generator stator windings and consequences on neutral grounding,” IEEE Trans. on Power Delivery, vol. 25, no. 2, April 2010. M. N. Rajk, “Ground-fault protection of unit-connected generators,”AIEE Trans. on Power Apparatus and System, pp. 1082– 1094, Oct. 1958. “Grounding and ground fault protection of multiple generator installation on medium-voltage industrial and commercial power systems—Parts 1–4: Protection Methods,” Working Group Report IEEE Trans. on Industry Applications, vol. 40, no. 1, pp. 11–32, Jan./Feb. 2004. P. Reichmeider, C. Gross, D. Querrey, D. Novosel, S. Salon, “Internal faults in synchronous machines. I. The machine model,” IEEE Trans. on Energy Conversion, vol. 15, no. 4, pp. 376–379, Dec. 2000. A. Megahed, O. Malik, “Synchronous generator internal fault computation and experimental verification,” IEE Proceedings on Generation, Transmission and Distribution, vol. 145, no. 5, pp. 604– 610, Sep. 1998. A. Megahed and O. P. Malik, “Simulation of internal faults in synchronous generators,” IEEE Trans. on Energy Conversion, vol. 14, no. 4, pp. 1306–1311, Aug. 2002. Leroy-Somer company, catalogue of electrical and mechanical data of synchronous generators, available at: http://www.leroysomer.com/ documentation_pdf. N. Jayawarna, N. Jenkins, M. Barnes, M. Lorentzou, S. Papthanassiou, and N. Hatziagyriou, “Safety analysis of a microgrid,” Future Power System Conference, Amsterdam, Netherlands, Nov. 2005. R. Kamel, A. Chaouachi, K. Nagasaka, “Comparison the performances of three earthing systems for microgrid protection during the grid connected mode,” Smart Grid and Renewable Energy, no. 2, pp. 206-215, Aug. 2011.

Mahdi Davarpanah received the M.Sc. and Ph.D. degrees in electrical engineering from The University of Tehran, Iran in 2005 and 2013, respectively. Currently, he is an Assistant Professor with the School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran. His research interests include Power System Protection, Control, and Transients.

Majid Sanaye-Pasand (M’98–SM’05) received the B.Sc. degree in electrical engineering from The University of Tehran, Tehran, Iran, and the M.Sc. and Ph.D. degrees from The University of Calgary, Calgary, AB, Canada, in 1994 and 1998, respectively. Currently, he is a Professor with the School of Electrical and Computer Engineering, University of Tehran, Tehran, Iran. His research interests include Power System Protection, Control, and Transients. He is also an editor of IEEE Transactions on Power Delivery.

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Appropriate Grounding System for Grid-connected

Appropriate Grounding System for Grid-connected

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