An investigation of Network Splitting for FaultLevel Reduction

Xueguang Wu, Joseph Mutale, Nick Jenkins and Goran Strbac

January 2003

Tyndall Centre for Climate Change Research Working Paper 25

Integrating Renewables and CHP into the UK Electricity System Tyndall Centre for Climate Change Research Project TC/IT 1.30

An Investigation of Network Splitting for Fault Level Reduction

(Revision 1)

Xueguang Wu, Joseph Mutale, Nick Jenkins, Goran Strbac

The Manchester Centre for Electrical Energy (MCEE), UMIST, UK

Email contacts: w.xueguang@umist.ac.uk j.mutale@umist.ac.uk

n.jenkins@umist.ac.uk g.strbac@umist.ac.uk

September 2003

1

Executive Summary The UK government has set targets that by the year 2010, 10% of the electrical energy consumed in the UK will be provided from renewable sources and 10GWe capacity of CHP plants will be installed by the same date. Achievement of these very ambitious targets will most likely entail the connection of a very large number of distributed generation to existing distribution networks. Connection of distributed generation on the scale that is envisaged will require practical solutions to a number of identified technical, commercial and regulatory challenges. One of the technical challenges is fault level management as connection of distributed generation often results in increased fault levels beyond the capacity of existing switchgear, especially in urban areas. In this report, results of investigations carried out to assess the effectiveness of the options available for fault level management as well as, where necessary, their impact on voltage profiles and system stability are presented and discussed. Five main methods for fault level reduction namely current limiting reactor, Is-limiter, superconducting fault current limiter, solid-state fault current limiter and network splitting have been reviewed and discussed. Network splitting was found to have the greatest potential for fault level reduction in the short term as it is relatively inexpensive and furthermore it has high reliability and flexibility. Modelling and simulations for various network-splitting scenarios performed using PASCAD/EMTDC confirm that fault level can be reduced significantly by network splitting. Furthermore, the quality of power supply to the customer and the transient stability of the DG can be improved by using fast-closing switchgear or a static transfer switch in the bus-section. The analysis shows that the fault current is sensitive to closure of the normally open point (NOP) on the ring effectively connecting the cable network in parallel with the bus-section breaker or the reactor.

Contents 1. Introduction..............................................................................................................2 2. Methods of fault level reduction .............................................................................3

2.1 Current limiting reactor ....................................................................................3 2.2 Is-limiter ..............................................................................................................5 2.3 Superconducting fault current limiter..............................................................7 2.4 Solid-state fault current limiter.......................................................................11 2.5 Network splitting ..............................................................................................11

3. Modelling and simulations ....................................................................................13 3.1 Modelling...........................................................................................................13 3.2 Simulations ........................................................................................................14

3.2.1 Fault current reduction .................................................................................16 3.2.2 Voltage drop.................................................................................................18 3.2.3 Transient stability.........................................................................................22

4. Conclusions .............................................................................................................23 5. References ...............................................................................................................24

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1. Introduction Climate change is the most pressing environmental challenge faced by the international community. There is increasing evidence that the main cause of this global problem is the emission of greenhouse gases (GHGs) from human activity. In response to the threat posed by climate change, world leaders signed the United Nations Framework Convention on Climate Change (UNFCCC) at the Rio Earth Summit in 1992 [1]. Under the Convention, 36 industrialised countries made voluntary commitments to reduce their GHG emissions from 1990 levels by the year 2000. Developing countries, whose per capita emissions have historically been far below those of industrial countries, agreed to establish inventories of GHG emissions, though not to specific targets. In 1997 the Kyoto Protocol to the UNFCCC was adopted. It binds industrialised countries that sign it to achieve quantified GHG reduction targets. Overall, these are designed to reduce industrialised country emissions by at least 5% from their 1990 levels by the commitment period 2008 to 2012 [2]. The targets for individual countries range from an 8% cut for the European Union to a 10% increase for Iceland. At present, there is some way to go before the Kyoto Protocol becomes legally binding. It is required that at least 55 countries ratify the Protocol, including industrialised countries (known as Annex I countries) accounting for at least 55% of industrialised GHG emissions. The latest information shows that 97 countries have ratified, but these only cover 37% of Annex I country emissions. Since the USA has decided not to ratify the Protocol, it will only enter into force if Russia signs up. The European Union has ratified the Protocol, and its Member States have agreed national targets. The UK share of the EU ‘burden’ has been translated into a 12.5% GHG emissions reduction target by 2008-2012. As part of its strategy to meet its 12.5% target, the UK Government has set itself two further targets to encourage the deployment of low carbon distributed energy sources. The first one is that by the year 2010, 10% of the electricity consumed in the UK should come from renewable sources and the second is that there should be 10GWe of Combined Heat and Power (CHP) installed by the same date [3]. The Government’s Performance and Innovation Unit (PIU) has suggested a more ambitious target of 20% electricity from renewable sources by 2020 [4]. The forthcoming Energy White Paper, due in early 2003, will confirm whether or not this further target will be adopted. Achieving these targets will require connection of a large amount of distributed generation (DG) into existing distribution networks. The 2010 targets alone will require the connection of up to 14GW of new distributed generation to the UK’s electricity distribution networks [5]. This report is concerned with cost effective management of fault levels as one of the many technical challenges that must be resolved in order to connect a significant amount of DG to existing distribution networks. The fault level management problem is particularly acute in large conurbations where fault levels are already close to the design limits of switchgear. The connection of DG in these areas requires some action to be taken to ensure that fault levels remain within the design limits of existing plant. This is an important safety issue which if unresolved would severely limit the amount of DG that can be connected.

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In this report, results of investigations carried out to assess the effectiveness of the options available for fault level management as well as, where necessary, their impact on voltage profiles and system stability are presented and discussed. The report begins by reviewing the methods that can be used to reduce fault levels. It then focuses on network splitting, modelling the impact of this technique on fault currents, voltage profiles and stability as well as investigating the issues that must be considered and required safeguards in applying network splitting. 2. Methods of fault level reduction Fault level is a measure of network robustness. A high fault level is a good indicator of the strength of the system suggesting close proximity to generating stations or a highly interconnected system. A high fault level implies low impedance between source and load and hence is associated with good system voltage profiles and low magnitudes of voltage dips when they occur. It also has a beneficial influence on the speed of operation of protective devices under fault conditions. Therefore on the whole a high fault level is not a bad thing. However, these benefits come at a price as high fault levels require switchgear and other equipment with high rupturing capacities, which is expensive. A balance must therefore be struck between the benefits of high fault level and the cost of necessary switchgear and other plant. In general short circuit breaking capacities of switchgear are standardised. In the event that the connection of DG causes the fault level to rise above the existing switchgear rating, it becomes necessary to find ways to reduce the fault level as a cheaper alternative to replacement of the switchgear, which in most cases is a costly solution. There are several methods that can be used to reduce fault levels in power systems. Some of the notable ones are listed below:

• Current limiting reactor

• Is-limiter

• Superconducting fault current limiter

• Solid-state fault current limiter

• Network splitting

The basic principles underlying each of the above methods as well as their typical application are discussed briefly in the following sections. 2.1 Current limiting reactor A current limiting reactor (CLR) is a series reactor connected into the circuit for limiting fault current [6, 7]. Although the CLR introduces impedance into the circuit degrading the voltage profile during normal operation, it can be a cost-effective solution obviating the need for upgrading switchgear in the system due to increase in the fault level. Some typical applications of the CLR are shown in Figure 1.

4

CLR

DG

CLR

CLR

CLR

DG

CLR

DG

CLR

Applications of the CLR for limiting fault level are varied. One of the more attractive applications is often the bus section application; see Figure 2(a). For this arrangement, the CLR is placed between two busbars to connect them together. This arrangement will be analysed in more detail later in this report.

The main disadvantage of the CLR is a high reactor impedance existing in normal operation. As indicated earlier, this could degrade the voltage supplied to the customers, and also influence the effective operation of tap-changing transformers.

Figure 1 Typical applications of the CLR

(a) Bus section

(c) Outgoing feeder

(d) Incoming feeder

(f) Generator and transformer connection

(e) Generator auxiliary connection

(b) Generator connection

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2.2 Is-limiter The Is-limiter shown in Figure 2 is a fault current limiter developed and supplied by ABB Calor Emag [8, 9].

The Is-limiter consists of two parallel conductors: a main conductor and a parallel fuse. Under normal operation, the load current flows through the main conductor. During a fault, a tripping device disconnects the main conductor, transferring the fault current to the parallel fuse with a high breaking capacity, which limits the fault current during the first rise (in less than 1 ms) [9]. The main applications of the Is-limiter are shown in Figure 3.

Figure 2 Diagram of the Is-limiter (Source(s): www.abb.de/calor)

Measuring and tripping device

Current transformer

Pulse transformer

Main conductor and interrupter

Parallel fuse

(a) Schematic of the Is-limiter

(b) Load current path uninfluenced (c) Fault current switched to fuse

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The Is-limiter is often used to couple two systems or parts of systems (as shown in Figure 3(a)), whose short-circuit withstand capability would not be sufficient if connected in parallel via a circuit breaker. The Is-limiter separates the system electrically into two parts before the fault current can endanger the system components.

Figure 3(b) shows the Is-limiter protecting a public supply system from the short circuit current supplied by the DG.

Figure 3(c) shows the Is-limiter installed in the connection between the public supply and the DG. The Is-limiter can interrupt the fault current supplied by the DG with a current-direction comparison.

Figure 3(d) shows the Is-limiter connected with a reactor in parallel. This arrangement can avoid the copper losses, large voltage fluctuations and the electromagnetic fields caused by the reactor.

(a) Is-limiter coupling two parallel systems (b) Is-limiter in the generator circuit

(c) Is-limiter coupling a generator to the public network with current-direction comparison

(d) Is-limiter and reactor connected in parallel

Figure 3 Main applications of the Is-limiter (Source(s): www.abb.de/calor)

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2.3 Superconducting fault current limiter Superconductor-based fault current limiters offer an alternative solution to controlling fault levels on the network [10, 11, 12]. A superconducting fault current limiter (SFCL), unlike reactors or high-impedance transformers, will limit fault current without adding impedance to the circuit during normal operation.

Most SFCLs are based on the “superconducting and normal” (SN) transition property. Superconductors are the only materials that change their resistance automatically from zero to a high value when a certain ‘critical current’ is surpassed. Early superconducting fault current limiters were too expensive for wide application in electrical utilities, since they were based on superconducting materials, which can only operate under extremely low temperatures (-269°C). With the discovery of high temperature superconductors (HTSs) fifteen years ago, the cooling problem has been greatly reduced. These new materials can be operated at much higher temperatures (-196°C) and can be cooled simply by using liquid nitrogen.

Figure 4 shows the typical resistivity (Ωm) of (a) Bi-2212 (Bi2Sr2CaCu2Ox) bulk superconductor and (b) YBCO (YBa2Cu3Ox) coated superconductor as a function of current density (A/m2) and temperature (K) [13].

In the event of a fault, the SFCL develops a high resistance limiting the fault current. With the SFCL, the utility can be provided with a low-impedance, stiff system with a low fault level, as shown in Figure 5.

Figure 4 Typical resistivity characteristics of Bi-2212 and YBCO superconductors [13]

(a) Bi-2212 bulk superconductor (b) YBCO coated superconductor

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SFCL

40MVAZt=5%

Z=0

Normal

11 kV

I_FL=2.1kA

SFCL

40MVAZt=5%

Z=20%

Faulted

11 kV

I_SC=8.4kA

In Figure 5, a large, low-impedance transformer (Zt=5%) is used to feed a busbar. The SFCL is installed between the transformer and the busbar for limiting the fault current. Normally, the SFCL does not affect the circuit, and the full-load current I_FL is 2.1kA. During a fault, the SFCL develops an impedance of 0.2 per unit (Z=20%), and the fault current I_SC is reduced to 8.4kA. Without the SFCL, the fault current would be 42kA.

There are two major types of the SFCL namely resistive and magnetic-shielded inductive [14]. In the resistive type of SFCL, the superconductor, using the YBCO thin film, makes use of the property of changing from a superconducting state to a resistive state and produces a resistance when an overcurrent flows. A diagram of the resistive SFCL is shown in Figure 6. The fault current pushes the superconductor into a resistive state directly and a resistance appears in the circuit. The advantage of the resistive SFCL is that the superconductor absorbs the energy of the fault current directly.

SFCL

In the magnetic-shielded inductive type of SFCL, the superconductor, using Bi-2212 bulk or YBCO thick film cylinder, makes use of the SN transition for its operation. The transition is triggered when magnetically induced screening currents exceed the critical current of the superconductor. A prototype magnetic-shielded inductive SFCL, which was developed by the Central Research Institute of the Electric Power Industry (CRIEPI) in 1995 [14], is shown in Figure 7.

Figure 5 Fault current control with a SFCL

Figure 6 Resistive SFCL

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Ideally, this type of SFCL has the characteristics of a transformer with a primary winding and a ‘shorted’ secondary winding [15], as shown in Figure 8. The secondary winding is the HTS cylinder whose function under normal conditions is to shield the flux produced by the primary winding from entering the iron core. The primary winding is connected directly to the circuit. If the secondary winding is driven beyond the critical current of the superconductor, it reverts to a resistive state. Hence, the flux coming from the primary winding enters the iron core. The inductance and hence impedance of the primary winding rapidly increase. Thus the fault current in the circuit is limited by the inductance of the SFCL.

The SFCL can be applied in a number of areas in distribution or transmission systems. Some typical applications are shown in Figures 9.

Figure 8 Typical circuit diagram of the magnetic-shielded inductive SCFCL

Figure 7 Schematic diagram of the CRIEPI inductive SFCL (Ichikawa and Okazaki 1995)

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SFCL

SFCL

SFCL

Figure 9(a) shows the SFCL in the main transformer circuit. The entire downstream busbar is protected by the SFCL. A large, low-impedance transformer can be used in this arrangement.

Figure 9(b) shows the SFCL in a feeder circuit. Individual feeder equipment, that is difficult to replace, such as underground cables or distribution switchgears, can be protected by the SFCL.

Figure 9(c) shows the SFCL connecting two busbars. The busbars are only separated by the SFCL during a fault.

In 1996, the first prototype high-temperature-superconductor SFCL with a rating of 1MVA at 10.5kV was installed in Switzerland [16].

On March 7, 2001, ABB demonstrated the world’s most powerful SFCL with a rated power of 6.4MVA at Baden, Switzerland [17]. However these devices are still very

(a) SFCL in the transformer circuit (b) SFCL in the feeder circuit

(c) SFCL in the bus-section position

Figure 9 Typical applications of the SFCL

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costly and on present projections, an economical SFCL will not be available for approximately another 5 to 10 years. 2.4 Solid-state fault current limiter Figure 10 shows the basic configuration of a solid-state fault current limiter (SSFCL) [18, 19, 20, 21].

Z1 SW1

C1

ZnO

L1

Z2BPS

GTO based switch

The SSFCL mainly consists of the capacitor bank C1, reactor L1 and GTO (gate-turn-off) or fast-closing switch SW1. It operates as follows: normally, the capacitor C1 and the reactor L1 combine to give a ‘zero’ impedance. Thus, the ‘zero’ impedance is presented within the circuit. When a fault occurs, SW1 bypasses the capacitor C1 at a high speed within 3ms [21], and the reactor L1 is immediately inserted into the network working as the fault current limiter.

The low impedance Z1 limits the inrush current through SW1. The over voltage protection device ZnO (zinc-oxide arrester), and the bypass switch BPS, which backs up the switch SW1 are also connected in parallel to the capacitors C1. The low impedance Z2 restrains the inrush current when BPS closes.

However, the SSFCL is still not widely used in practice due to its high cost, low reliability, and complicated auxiliary system. 2.5 Network splitting In practice, a much more common and less expensive approach to fault level reduction is network splitting, as shown in Figure 11.

Figure 10 Configuration of the SSFCL

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DG

Infinite system

Load2Load1

12.5MVA, 33kV/11kVZ=10%

33kV

11kV11kV

5MVA 2.0=′′dx

Rated Interruptcapacity250MVA

Rated interruptcapacity250MVA

SB12

SB01

LB1 LB2

SB11

SB00

12.5MVA, 33kV/11kVZ=10%

EB1

F3

F1

EB0

Bus1 Bus2

The network splitting scheme shown in Figure 11 uses the bus-section circuit breaker EB1 between Bus1 and Bus2. By splitting the network in this way, the impedance between the 33kV and 11kV systems increases from 5% to 10%, reducing the fault current coming from the public supply (33kV) significantly.

However, this scheme may decrease the flexibility of the DG. When busbar Bus2 needs maintenance, the DG has to be disconnected from the network, and therefore an alternative network splitting arrangement that avoids this problem is shown in Figure 12.

Figure 11 Network splitting

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DG

Infinite system

Load2Load1

12.5MVA, 33kV/11kVZ=10%

33kV

11kV11kV

5MVA 2.0=′′dx

Rated Interruptcapacity250MVA

Rated interruptcapacity250MVA

SB12

SB01

LB1 LB2

SB11

SB00

12.5MVA, 33kV/11kVZ=10%

EB1 EB2

F3

F1EB0

Bus1 Bus2

Figure 12 shows that one more circuit breaker EB2 is needed, but the flexibility and safety of the network as well as of the DG are improved. Under normal operation, the circuit breaker EB1 is open separating the 11kV busbar into two parts, Bus1 and Bus2. For a fault F1, LB2 can safely interrupt fault current within its rated capacity. For a fault F3, the DG and the Load2 can be switched to Bus1 by closing EB1 after fault clearance. When busbar Bus2 needs maintenance, the DG can be switched to Bus1 by closing EB1.

A more detailed study of the impact of network splitting, either directly through bus-section or by installing a reactor between the two sections of the busbars, is presented in the next section. The main issues of concern are voltage dips experienced by load customers and stability of the DG. 3. Modelling and simulations In this section, modelling and simulations for various network-splitting scenarios are presented and discussed. The simulation results were performed using PASCAD/EMTDC. The primary aim of this work is to study the efficacy of network splitting in fault current reduction by use of bus-section circuit breaker and reactor. A secondary but equally important objective is to study the impact of these network splitting options on voltage profiles and generator stability. 3.1 Modelling Network: The following system parameters were used in the simulations: Network primary voltage is 33kV, 50Hz, with an infinite short circuit power.

- Main transformers are each 12.5 MVA, 33/11 kV, reactance 10%.

Figure 12 An alternative network splitting method

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- Load1 and Load2 are each 5MVA and power factor 0.85. - The feeder impedance between LB2 and Load2 is 0.1+j0.5 Ω.

Generator: The following typical generator (synchronous machine) parameters were used to represent the distributed generator [22]:

- Rated capacity: 4.51 [MVA], - Rated voltage: 11 [kV], - Frequency: 50 [Hz], - Inertia constant: 1.05 [sec.], - Armature resistance (Ra): 0.01 [p.u.], - Potier reactance (Xp): 0.03 [p.u.], - D-axis synchronous reactance (Xd): 2.95 [p.u.], - D-axis transient reactance (Xd’): 0.25 [p.u.], - D-axis transient time (Tdo’): 5.50 [sec.], - D-axis sub-transient reactance (Xd”): 0.17 [p.u.], - D-axis sub-transient time (Tdo”): 0.05 [sec.], - Q-axis synchronous reactance (Xq): 1.35 [p.u.], - Q-axis sub-transient reactance (Xq”): 0.31 [p.u.], - Q-axis sub-transient time (Tqo”): 0.27 [sec.].

3.2 Simulations Three network models were used to investigate network splitting as an effective means of fault current reduction. The first network is the base case (case a), as it represents the normal network configuration. This case is shown in Figure 13 where the 11kV busbars Bus1 and Bus2 are shorted. In the second case (case b), the network is split by installing a reactor between Bus1 and Bus2 (see Figure 14). The last case (case c) is shown in Figure 15 depicting a split 11kV bar with a bus-section circuit breaker that is operated normally open.

DG

Infinite system

Load2

Load1

12.5MVA, 33kV/11kVZ=10%

33kV

11kV11kV

5MVA 2.0=′′dx

Rated Interruptcapacity250MVA

Rated interruptcapacity250MVA

SB12

SB01

LB1 LB2

SB11

SB00

12.5MVA, 33kV/11kVZ=10%

F3

F1

EB0

Bus1 Bus2

F2

Figure 13 Case a -11kV busbar connected solidly

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DG

Infinite system

Load2

Load1

12.5MVA, 33kV/11kVZ=10%

33kV

11kV11kV

5MVA 2.0=′′dx

Rated Interruptcapacity250MVA

Rated interruptcapacity250MVA

SB12

SB01

LB1 LB2

SB11

SB00

12.5MVA, 33kV/11kVZ=10%

F3

F1

EB0

Bus1 Bus2

F2

Reactor20%

DG

Infinite system

Load2

Load1

12.5MVA, 33kV/11kVZ=10%

33kV

11kV11kV

5MVA 2.0=′′dx

Rated Interruptcapacity250MVA

Rated interruptcapacity250MVA

SB12

SB01

LB1 LB2

SB11

SB00

12.5MVA, 33kV/11kVZ=10%

EB1 EB2

F3

F1EB0

Bus1 Bus2

F2

Figure 14 Case b - 20% reactor (0.2 per unit based on 12.5MVA) connecting the two 11kV busbars

Figure 15 Case c: Two 11kV busbars are split by bus-section circuit breaker EB1

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For each of the above cases the following studies were performed for various fault locations as appropriate: - Fault current reduction - Voltage drop - Transient stability The results of these studies are presented and discussed below. 3.2.1 Fault current reduction Based on the three cases set out above, a three-phase short circuit fault was applied at F1 (see Figures 13, 14 and 15) at 2.0 seconds and lasted 150ms. The fault currents flowing through LB2 are shown in Figure 16.

Figure 16 shows that the peak values of fault current have been reduced from 25kA to 17kA by using network splitting via a bus-section circuit breaker and 20kA using a reactor. If the busbar is separated (e.g. by a reactor), the fault current will be increased by connection of cable network through closing the normal opening point (NOP) (see Figure 17).

Figure 16 Fault currents flowing through the LB2

Case a

Case b

Case c

17

RC XC

Cable network

11kV

11kV

12.5MVA, 33/11kV,10%

12.5MVA, 33/11kV,10%

33kV, infinite system

NOP

Is_rms

F1

12.5MVA, 20%

Reactor

close 5MVA, 20%

DG

For a fault F1 in Figure 17, the fault currents and short circuit levels (SCLs) were calculated by considering typical cable circuit lengths, as shown in Table 1. Table 1 Fault current and SCL for typical cable circuit lengths

Magnitude of Impedance Fault current and SCL 11kV cable circuit length

(km) Cable (p.u)

Reactor (p.u)

Is_rms (kA)

SCL (MVA)

Is_peak (kA)

SCL_peak (MVA)

0 0.000 0.200 14.4 274.5 24.5 466.7 1 0.021 0.200 13.4 255.5 22.8 434.3 3 0.063 0.200 12.3 234.5 20.9 398.7 5 0.105 0.200 11.8 225.0 20.1 382.5 10 0.210 0.200 11.1 211.7 18.9 359.8 20 0.420 0.200 10.7 204.1 18.2 346.9 30 0.630 0.200 10.4 198.3 17.7 337.2 50 1.050 0.200 10.3 196.4 17.5 333.9 M M M M M M M ∞ ∞ 0.200 10.0 190.7 17.0 324.2

Assuming that: (1) The cable conductor is 185mm2, resistance 0.195Ω/km, and reactance 0.080Ω/km [23]. (2) All per unit values are based on 12.5MVA. (3) The first peak of fault current is: Is_peak = 1.7 × Is_rms [23]. (4) ∞ is an infinite value denoting an open circuit. Table 1 shows the impacts of cable lengths on the possibilitiy of limiting fault current using a reactor in the bus-section. The first peaks of fault currents increase from 17.0kA to 24.5kA, when the cable lengths decrease from infinite to zero. The SCLs at the 11kV busbar are increased from 190.7MVA to 274.5MVA. Similarly, if the busbar is split by a bus-section circuit breaker, the fault current will also be increased by connection of the cable network through closing of the NOP (see Figure 18).

Figure 17 Impact of the cable network on the reactor

18

RC XC

Cable network

11kV11kV

12.5MVA, 33/11kV,10%

12.5MVA, 33/11kV,10%

33kV, infinite system

NOP

Is_rms

F1

open

close 5MVA, 20%

DG

For a fault F1 (see Figure 18), the fault currents and SCLs can be calculated by considering typical cable circuit lengths. The results are shown in Table 2. Table 2 Fault current and SCL for typical cable circuit lengths

Magnitude of Impedance Fault current and SCL 11kV cable circuit length

(km) Cable (p.u)

Breaker (p.u)

Is_rms (kA)

SCL (MVA)

Is_peak (kA)

SCL_peak (MVA)

0 0.000 ∞ 14.4 274.5 24.5 466.7 1 0.021 ∞ 13.3 253.6 22.6 431.1 3 0.063 ∞ 11.9 226.9 20.2 385.8 5 0.105 ∞ 11.1 211.7 18.9 359.8 10 0.210 ∞ 10.0 190.7 17.0 324.2 20 0.420 ∞ 9.1 173.6 15.5 295.1 30 0.630 ∞ 8.8 167.9 15.0 285.4 50 1.050 ∞ 8.4 160.2 14.3 272.4 M M M M M M M ∞ ∞ ∞ 7.8 148.8 13.3 253.0

With reference to Table 2, it is noted that the first peaks of fault currents increase from 13.3kA to 24.5kA (almost double), when the cable lengths decrease from infinite to zero. The SCLs at the 11kV busbar increase from 148.8MVA to 274.5MVA. From the above analysis, it clear that, the effectiveness of reactors and network splitting in fault current reduction will be limited by connection of a cable network in parallel, particularly a short cable length. 3.2.2 Voltage drop Voltage drops cause concern when the voltage decreases below the normal rated value. A voltage drop occurring during a fault is often referred to as a sag or a dip.

Figure 18 Impact of cable network on the network splitting

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When a three-phase short circuit fault occurs at 2 seconds at F1 in Figures 13, 14 and 15, the 11kV busbar will experience short-term voltage drops, as shown in Figure 19. The fault F1 is cleared by opening LB2 after 150ms.

Figure 19 shows little effective difference of the voltage responses of 11kV busbar after clearance of the fault in three cases a, b and c. Similarly, for a fault occurring at 2 seconds at F2 in Figures 13, 14 and 15, the 11kV busbar will also experience voltage drops, as shown in Figure 20. The fault F2 is again removed by opening LB2 after 150ms.

Figure 20 11kV voltages at Bus2 for a fault at F2

Case c

Case b

Case a

Case c

Case b

Case a

Figure 19 11kV voltages at Bus2 for a fault at fault F1

20

Figure 20 shows the difference of the voltage drops during the fault due to feeder impedance between switch LB2 and the faulty point. The voltage drops are about 50%, 41% and 36% for case a, b and c respectively. The voltages at the 11kV busbar are almost the same after fault clearance. Additionally, for a fault occurring at 2 seconds at F3 in Figures 13, 14 and 15, the 11kV busbar will also experience voltage drops. Fault F3 is cleared by opening SB01 and SB12 after 150ms. Then, the DG and Load2 are switched from Bus2 to Bus1 by closing EB1 in 50ms after clearing the fault.

Figure 21 shows the differences of the recoveries of the 11kV busbar voltage in the three cases. In general, the voltage recovery is better when the network is split by a bus section circuit breaker than when a reactor is connected across the two sections of the 11kV busbar. Different types of electrical equipment will have different tolerances to voltage drop. The CBEMA (Computer Business Equipment Manufactures Association) curve has been widely quoted as providing some voltage quality guidance for other types of equipment [24]. This curve has recently been revised and is now known as the ITI (Information Technology Industry Council) CBEMA curve [25], shown in Figure 22.

Case c

Case b

Case a

Figure 21 11kV voltages at Bus2 for a fault at F3

21

Published by: Information Technology Industry Council (ITI) 1250 Eye Street NW Suite 200 Washington DC 20005 202-737-8888 http://www.itic.org

The ITI (CBEMA) curve was developed to be used as a guideline for manufacturers in designing power suppliers for use with sensitive electronic equipment. The vertical axis of the graph is the percent of rated voltage applied to a circuit. The horizontal axis is the time which the voltage is applied. The graph is divided into three regions: no interruption in function region, prohibited region and no damage region. Normally, equipment is expected to operate within the “no interruption in function” region. Equipment can be damaged when voltage spikes are severe enough to enter the prohibited region. In contrast, if the voltage drops are below the lower limit and the times which the voltages are continuously applied are exceeded, then the equipment can enter the no damage region. In this area, the normal function of the equipment is not to be expected, but no damage to the equipment should result.

Figure 22 ITI (CBEMA) curve

22

Given the simplifying assumption of the ITI (CBEMA) curve there is little effective difference between the voltage drops in this study. The fault duration times were assumed to be 150ms in the case a, case b and case c. According to the ITI (CBEMA) curve, the voltage drops and duration times in Figures 19, 20 and 21 will all be located in the “no damage” region. 3.2.3 Transient stability The fault level can be reduced by network splitting significantly. But, the DG may be subjected to a transient stability problem when a fault occurs on the system, particularly on a 33kV transformer feeder (see fault F3 in Figure 15). For transient stability investigation of the DG, a three-phase short circuit fault was applied at 2 seconds at F3 and lasted 150ms. The closing times of the EB1 were assumed to be 60ms and 50ms, respectively. The rotational speed of the DG is shown in Figure 23 (PSCAD/EMTDC simulation).

Curve I in Figure 23 shows transient instability of the DG after EB1 closing in 60ms. The DG can be stable if the closing speed of EB1 is fast enough, as shown the curve II in Figure 23. A static transfer switch connected in parallel with EB1 is shown in Figure 24. It can allow a fast transfer of the DG from a one busbar to the other within a quarter of a cycle [26, 27].

Figure 23 The rotational speeds of the DG I, EB1 closing in 60ms; II, EB1 closing in 50ms;

III, a static transfer switch connected in parallel with EB1

I

II III

23

Static transfer switch

EB1

Curve III in Figure 23 shows a large improvement in the transient stability of the DG through the use of a static transfer switch. In addition, the quality of power supply to the customer can be improved by the swift operation of the static transfer switch as it decreases the duration of the voltage drop. Thus, the sensitive equipment can be kept in the “no interruption” region of the ITI (CBEMA) curve. 4. Conclusions This report has reviewed the basic principles underlying the methods that can be used to reduce fault levels as well as their typical application. Five main methods namely current limiting reactor, Is-limiter, superconducting fault current limiter, solid-state fault current limiter and network splitting have been discussed.

The current limiting reactor is an effective means of fault current reduction in the network. However, it suffers from the disadvantage that it could degrade the voltage supply to the customer due to the voltage drop across it in normal operation. The Is-limiter is not presently used widely on the UK public distribution systems. The superconducting fault current limiter (SFCL) is an effective method to limit fault current. It is however still is too expensive to be applied in practice, at least not in the next 5 to 10 years. The solid-state fault current limiter (SSFCL) is also an efficient fault current limiting device. However, the SSFCL is not widely used due to its high capital cost and complex auxiliary system.

Network splitting was found to have greatest potential for fault level reduction in the short term as it is relatively inexpensive and furthermore it has high reliability and flexibility. This method was therefore analysed in greater detail to determine its effectiveness as well as the impact on power quality and transient stability. Modelling and simulations for various network-splitting scenarios were performed using PASCAD/EMTDC. These scenarios confirm the expected result that the fault level can be reduced significantly by network splitting via a bus-section circuit breaker. Furthermore, the quality of power supply to the customer and the transient stability of the DG can be improved by using fast-closing switchgear or a static transfer switch in the bus-section. However, analysis shows that the fault current is sensitive to closure of the normally open point (NOP) on the ring effectively connecting the cable network in parallel with the bus-section breaker or the reactor. Care should therefore be taken in cases where the cable impedance is low (short cable length) to ensure that the network is operated in a radial configuration rather than as a ring.

Given the simplifying assumptions of the ITI (CBEMA) curve there is little effective difference between the voltage drops in this study.

Figure 24 A static transfer switch connecting in parallel with EB1

24

5. References 1. UN, Earth Summit, United Nations Conference on Environment and Development

(UNCED), Rio de Janeiro, June 3-14, 1992. http://www.un.org/geninfo/bp/enviro.html 2. UNFCC, Text of the Kyoto Protocol, http://unfccc.int 3. ETSU/DTI, New and Renewable Energy: Prospects in the UK for the 21st

Century: Supporting Analysis, March 1999. 4. PIU, The Energy Review, February 14, 2002. 5. X Wu, N Jenkins and G Strbac, Impact of Integrating Renewables and CHP into

the UK Transmission Network (Tyndall Centre Project Report, UMIST), April 2002.

6. Alstom, Air Core Reactor, www.tde.alstom.com 7. ETSU/DTI, Likely Changes to Network Design as a Result of Significant

Embedded Generation, EA Technology Ltd, 2001. 8. ABB, Is-limiter, ABB Calor Emag, 2002, http://www.abb.de/calor 9. Hartung K.H., Is-limiter – The Solution for High Short-circuit Current

Applications, ABB Calor Emag, 2002, http://www.abb.de/calor 10. ABB, HTS Fault Current Limiter, www.abb.com 11. Siemens, Superconductive Current Limiter, www.siemens.com 12. TEPCO, Development of Fault Current Limiters, http://www.tepco.co.jp/rd/power/dtyodend/fcl/fcl-e.html 13. Passi J., et al, Superconducting Power Link for Power Transmission and Fault

Current Limitation, Physica, C 354 (2001), pp1-4. 14. WTEC, Power Application of Superconductivity in Japan and Germany,

September 1997, http://www.itri.loyola.edu/scpa/toc.htm 15. Meggs C., Dolman G., Mumford F. J., et al, HTS Thick Film Components for

Fault Current Limiter Applications, http://www.fmg.bham.ac.uk/papers/superconductors/FCL%20Components/htsthick.htm 16. ABB, The Worlds First Superconducting Device for Commercail Use by an

Electrical Utility, November 21, 1996, http://www.abb.com 17. ABB, The World’s Most Powerful Superconducting Fault Current Limiter, March

7, 2001, http://www.abb.com 18. Veda T, et al, Solid-state Current Limiter for Power Distribution system, IEEE

Trans PWRD 1993, Vol. 8, No 1, pp1796-1801. 19. Sugimoto S., et al, Principle and Characteristics of a FCL with Series

Compensation, IEEE Trans PWRD 1996, Vol. 11, No 2, pp842-847. 20. Sugimoto S., et al, Fault Current Limiting System for 500-kV Power System,

Electrical engineering in Japan, Vol. 127, No. 1, 1999. 21. Zou J., Chen J., Dong E., Study of Fast-closing Switch Based Fault Current

Limiter With Series Compensation, Electrical Power and energy System, 2002 (Article in Press).

22. Engineering Technical Report No.113, Notes for guidance for the protection of private generating sets up to 5MW for operation in parallel with electricity suppliers’ distribution system, 1995.

23. Bungay E.W.G. and McAllister D., Electrical Cables Handbook (second edition), BSP Professional Books 1990.

24. ANSI/IEEE, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, Std 141-1986, pg. 86.

25. ITIC, ITI (CBEMA) Curve Application Note, www.itic.org/technical/iticurv.pdf

25

26. Sannino A., Bollen M., Mitigation of Voltage Sags and Short Interruptions through Distribution System Design, www.elkraft.chalmers.se/publikationer

27. Inverpower Controls Limited, Medium Voltage Static Transfer Switch, http://www.inverpower.com/products/sts/sts.html

The inter-disciplinary Tyndall Centre for Climate Change Research undertakes integrated research into the long-term consequences of climate change for society and into the development of sustainable responses that governments, business-leaders and decision-makers can evaluate and implement. Achieving these objectives brings together UK climate scientists, social scientists, engineers and economists in a unique collaborative research effort.

Research at the Tyndall Centre is organised into four research themes that collectively contribute to all aspects of the climate change issue: Integrating Frameworks; Decarbonising Modern Societies; Adapting to Climate Change; and Sustaining the Coastal Zone. All thematic fields address a clear problem posed to society by climate change, and will generate results to guide the strategic development of climate change mitigation and adaptation policies at local, national and global scales.

The Tyndall Centre is named after the 19th century UK scientist John Tyndall, who was the first to prove the Earth’s natural greenhouse effect and suggested that slight changes in atmospheric composition could bring about climate variations. In addition, he was committed to improving the quality of science education and knowledge.

The Tyndall Centre is a partnership of the following institutions: University of East Anglia UMIST Southampton Oceanography Centre University of Southampton University of Cambridge Centre for Ecology and Hydrology SPRU – Science and Technology Policy Research (University of Sussex) Institute for Transport Studies (University of Leeds) Complex Systems Management Centre (Cranfield University) Energy Research Unit (CLRC Rutherford Appleton Laboratory)

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Recent Working Papers Tyndall Working Papers are available online at http://www.tyndall.ac.uk/publications/working_papers/working_papers.shtml Mitchell, T. and Hulme, M. (2000). A Country-by-Country Analysis of Past and Future Warming Rates, Tyndall Centre Working Paper 1.

Hulme, M. (2001). Integrated Assessment Models, Tyndall Centre Working Paper 2.

Berkhout, F, Hertin, J. and Jordan, A. J. (2001). Socio-economic futures in climate change impact assessment: using scenarios as 'learning machines', Tyndall Centre Working Paper 3.

Barker, T. and Ekins, P. (2001). How High are the Costs of Kyoto for the US Economy?, Tyndall Centre Working Paper 4.

Barnett, J. (2001). The issue of 'Adverse Effects and the Impacts of Response Measures' in the UNFCCC, Tyndall Centre Working Paper 5.

Goodess, C.M., Hulme, M. and Osborn, T. (2001). The identification and evaluation of suitable scenario development methods for the estimation of future probabilities of extreme weather events, Tyndall Centre Working Paper 6.

Barnett, J. (2001). Security and Climate Change, Tyndall Centre Working Paper 7.

Adger, W. N. (2001). Social Capital and Climate Change, Tyndall Centre Working Paper 8.

Barnett, J. and Adger, W. N. (2001). Climate Dangers and Atoll Countries, Tyndall Centre Working Paper 9.

Gough, C., Taylor, I. and Shackley, S. (2001). Burying Carbon under the Sea: An Initial Exploration of Public Opinions, Tyndall Centre Working Paper 10.

Barker, T. (2001). Representing the Integrated Assessment of Climate Change, Adaptation and Mitigation, Tyndall Centre Working Paper 11.

Dessai, S., (2001). The climate regime from The Hague to Marrakech: Saving or sinking the Kyoto Protocol?, Tyndall Centre Working Paper 12.

Dewick, P., Green K., Miozzo, M., (2002). Technological Change, Industry Structure and the Environment, Tyndall Centre Working Paper 13.

Shackley, S. and Gough, C., (2002). The Use of Integrated Assessment: An Institutional Analysis Perspective, Tyndall Centre Working Paper 14.

Köhler, J.H., (2002). Long run technical change in an energy-environment-economy (E3) model for an IA system: A model of Kondratiev waves, Tyndall Centre Working Paper 15.

Adger, W.N., Huq, S., Brown, K., Conway, D. and Hulme, M. (2002). Adaptation to climate change: Setting the Agenda for Development Policy and Research, Tyndall Centre Working Paper 16.

Dutton, G., (2002). Hydrogen Energy Technology, Tyndall Centre Working Paper 17.

Watson, J. (2002). The development of large technical systems: implications for hydrogen, Tyndall Centre Working Paper 18.

Pridmore, A. and Bristow, A., (2002). The role of hydrogen in powering road transport, Tyndall Centre Working Paper 19.

Turnpenny, J. (2002). Reviewing organisational use of scenarios: Case study - evaluating UK energy policy options, Tyndall Centre Working Paper 20.

Watson, W. J. (2002). Renewables and CHP Deployment in the UK to 2020, Tyndall Centre Working Paper 21.

Watson, W.J., Hertin, J., Randall, T., Gough, C. (2002). Renewable Energy and Combined Heat and Power Resources in the UK, Tyndall Centre Working Paper 22.

Paavola, J. and Adger, W.N. (2002). Justice and adaptation to climate change, Tyndall Centre Working Paper 23.

Xueguang Wu, Jenkins, N. and Strbac, G. (2002). Impact of Integrating Renewables and CHP into the UK Transmission Network, Tyndall Centre Working Paper 24

Xueguang Wu, Mutale, J., Jenkins, N. and Strbac, G. (2003). An investigation of Network Splitting for Fault Level Reduction, Tyndall Centre Working Paper 25