Grounding

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Aspects on grounding
April 30, 2004
by Frans Provoost, Sjef Cobben, Eindhoven University of Technology Jeroen van Waes, Holland Railconsult Maarten van Riet, Nuon Technical Projects and Consultancy Lex van Deursen, Eindhoven University of Technology

Modern electronic appliances receive signals from different sources like electricity, cable television, communication systems etc.. Their vulnerability for disturbances requires a well-designed grounding concept. In the ideal situation all grounding systems should be integrated in a so-called global earthing system. This grounding system must secure safety for both people and apparatus. In early days a reasonable earthing was obtained by metal objects like copper water tubes, gas pipes and sheets of PILC cables. Nowadays the water and gas tubes are made of plastic whilst power and telecommunication cables more and more contain plastic isolation. Local earthing of apparatus or total installations of a customer is not sufficient in many cases. In order to obtain a proper grounding the grounding systems of electricity suppliers and customers should be interconnected. This is only allowed when faults in the electricity systems will not result in dangerous situations for the customer. Possible dangerous situations do not only occur due to faults in the low voltage system, but also due to faults in high and medium voltage systems. The consequences of these faults have been studied and verified by measurements and field tests. This article describes the possible dangers and some countermeasures that can be taken. The first part contains (partly well known) information about grounding systems in electricity networks. The second part is dedicated to consequences of faults. The subjects in the article are related to typical power supply networks, which consist of underground cables for low and medium voltage and a combination of cable and overhead lines for high voltage.

1

An introduction to grounding aspects

Electrical energy is injected, transported and supplied at several voltage levels. Most systems make use of a so-called three-phase AC system. The three signals (for voltage or current) have a relative phase angle of 120 degrees. In a normal situation there is symmetry and the sum of the three signals is zero. This means

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that only three conductors are required. Symmetry however is not always kept especially during faults in the network (short circuits between phases and mostly in combination with earth). To prevent undesired situations neutral conductors and proper grounding systems are necessary. Various grounding methods are used for network protection and to prevent dangerous situations for people and apparatus. In the past, separate grounding principles were applied for each voltage system. The introduction of sensitive electronic devices and installations requires an integrated grounding philosophy. This integration even expands towards other networks entering at customers like telephone and cable television connections. This philosophy not only requires extra investments, but also a change in thinking for those who are responsible for operating and managing of the networks. In a low voltage grid the grounding systems of the various networks come together inside consumer electronic appliances like computers, video and television. A well-engineered grounding concept is needed to reduce disturbances and to keep the EMC requirements towards apparatus within an acceptable level. The choice of the low voltage grounding philosophy is of high importance. In the past the lead water tubes and the armours of PILC cables had a significant contribution towards electrical safety due to their good contact with the soil over a large distance. Nowadays the water tubes are replaced by plastic ones and PILC cables will be substituted by XLPE cables. This results in a nonneglectable influence on safety aspects and therefore the low voltage grounding philosophy has to be reconsidered. Domestic grounding systems must prevent users and apparatus from hazardous shocks during short circuits within the building. During such a disturbance the grounded apparatus will carry a voltage relative to normal ground. This voltage is dependent on the impedances of the network components and the grounding system. When a person touches a grounded apparatus during a disturbance his touch voltage will be a part of that voltage. This will result in a current through the body. In order to prevent victims this current must not last too long, so the fault has to be switched off in time. Switching of the fault is done by the protection device. In most households this is a fuse or a circuit breaker. To switch off in time the circuit resistance needs to be low. The circuit resistance is mainly determined by the impedance of the return path. Several grounding principles are applied nowadays (figure 1).

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MV LV

L1 L2 L3 N PE

A TT System

MV LV

L1 L2 L3 N PE

A TN-S System

MV LV

L1 L2 L3 PEN

A TN-C System

MV LV

PEN PE

L1 L2 L3 N PE

A TN-CS System

Figure 1: Various types of LV grounding systems

1.1

TT grounding system

In a TT (Terre Terre) grounding system, the earthing (PE) of the customer is separated from the earthing of the electricity supplier. The customer is responsible for the maintenance and proper functioning of his own grounding system. The return path consists of the grounding circuit at the customer, the grounding impedance of the MV/LV transformer and the resistance through ground between customer and transformer. The value of the circuit resistance must be low (for fuses Rc < 30/In ). In the past, the circuit resistance was kept within a safe region by low values of In , the metal water tubes and the PILC cables. Replacement of water tubes and cable insulation by plastics form a threat for safe values. In areas with a high ground resistance the safe value can no longer be obtained. Residual Current Devices (RCD) must be used then. But these devices (especially the 30 mA RCD) show failure rates between 5 and 10%. Larger customers can never have a safe value for their circuit resistance, due to the high values of nominal

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current. Customers with a TT grounding are not vulnerable for potential rises due to short circuits in the network of the supplier. Lightning strokes however can result in high voltage differences between both the active parts as between active parts and grounding systems, which can result in damage of equipment. Especially apparatus connected to various types of networks, like radio, television, video and IT equipment, is vulnerable for this kind of disturbances.

1.2

TN grounding system

In a TN (Terre Neutral) grounding system the electricity supplier provides the safety earth of the customer by means of a low resistant connection with the grounding of the MV/LV transformer. This system will therefore always guarantee sufficient low impedance for the return circuit. A distributed grounding system is obtained, which minimizes the risk of lack of return connection for the customers. Also safety at lightning strokes is increased strongly, due to considerable reduction of overvoltages between neutral and ground. Extra (small) electrodes in the low voltage network take care of a good distribution of externally induced (lightning) currents. The connection of customer earthing and network earthing will make the customer vulnerable for faults in the electricity network of the supplier. In a global earthing system this will not be limited to faults in the low voltage system, but also faults in systems with higher voltage levels. These types of faults have to be taken into account, the consequences should be studied and additional measures might be required. TN systems exist in several configurations. In a TN-S system the neutral and ground conductor are separated. From EMC standpoint of view this is the best solution both for 50 Hz and high frequencies especially with shielded cables, where the shield is used as grounding conductor. A minor disadvantage of a TN-S system is that it requires 5 conductors. Another disadvantage is that a break of the neutral results in undefined voltages in apparatus and that a break in the PE conductor leads to safety risks. In a TN-C system neutral and ground are combined into one (PEN) conductor. The advantage of this system is that it only requires 4 conductors instead of 5. This reduces cable costs and is therefore often applied in industrial networks. There are two main disadvantages in this system. First disadvantage is the safety and voltage problem that occurs when the PEN-conductor is interrupted as already described in the TN-S system. The second problem is the stray currents, which are a result of return currents (due to asymmetry but also multiples of third harmonics) that not only flow through the PEN conductor but also through metal parts connected to these conductors. These stray currents induce magnetic fields, which can influence the control of processes. Small magnetic fields of 1µT are able to distort the image on the screens of CRT tubes. Magnetic field over 100 T can be dangerous for human body.

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The TN-CS system is a combination of TN-S and TN-C. The power supply network is a TN-C system and the customer network is a TN-S system. In the Netherlands, for instance, most of the new build low voltage networks are based on this TN-CS system. To guaranty safety for the customers the TN-C network consists of a 4 conductor underground cable with copper sheath of sufficient equivalent cross section and plastic insulation. At all joints and at the customers the neutral and the sheath are interconnected. This ensures a safe current return path even if the neutral conductor is broken. The risk of simultaneous broken neutral and broken shield is neglectable. In this way the system combines the advantages of TN-S and TN-C. The paralleling of neutral and PE also reduces the return impedance for asymmetric currents and thus dips due to switching of large single-phase loads.

2
2.1

Dangerous situations due to faults
Faults in low voltage networks

A phase to ground (or neutral) fault in a low voltage TN network will give a voltage on the grounding system of the customers (figure 2). The value of the voltage is defined by the quotient of the return impedance and the total impedance. Due to the inductance of the transformer and the resistance of the cable the highest fault voltages will occur at the end of the cable. So the fastest fault clearance time is required for faults at the end of the cable. On the other hand these faults result in the lowest short circuit current and when fuses are used in the longest switch off time. This limits the longest applicable cable length. If no measures would be taken, the maximum cable length would be limited to 310 meters for a 150 mm2 Al low voltage cable. Cooperation and innovation between utilities and manufacturers of fuses resulted into the introduction of faster fuses (figure 3). Therefore the maximum length of 150 mm2 Al low voltage cable could be increased to 445 meters. This is sufficient for towns and cities. In rural areas the cable load is lower, resulting in lower nominal currents for the fuses, which can switch off faster. Nowadays, longer cable lengths may be applied.

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Low voltage fuses 250 A
10

1

Time (sec)

0,1

Normal Fast

0,01

0,001 1000

1500

2000 Current (Amps)

2500

3000

Figure 2: The development of faster LV fuses

L1 L2 L3 If If

Uf

Ut

Ze

Figure 3: Touch voltages in a LV network due to a fault in the LV network

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2.2

Faults in the medium voltage networks

When a (single or two phase) fault to earth in a medium voltage grid occurs, the return current partly flows through the shield of the MV cables and partly through the ground via the grounding electrodes of the MV transformerhouses. These currents through the grounding electrodes results in a potential rise of the grounding system on the low voltage side of the MV/LV transformer. This system is connected to the electrical ground of the LV customers (figure 4). To prevent casualties this voltage rise must be limited. The main parameters, which will influence this potential rise, are the size and duration of the return current, the grounding impedance of the transformer house and the distribution of the return current between cable sheath and ground. The size of the return current is depending on the type of fault and on grounding of the network. With two-phase to ground faults the largest part of the fault current returns through the faulted phase. Only a small part returns through cable sheath and grounding systems. Single phase to ground faults have a higher absolute value of return current through sheath and ground. In nongrounded MV networks the fault current is fully determined by the capacitance of the cables. Typical values are in the range between 200 and 600 amps. In grounded networks the fault current is depending on the impedance of the grounding transformer and the distance from the substation. Typical values are in the range between 700 and 2000 amps. The duration of the fault is depending on the type of fault, the fault current through the protection device and the protection scheme. Two-phase (to ground) faults give rather high fault currents of several kilo amps. These currents will be switched off very fast (less than 0.3 sec). Single phase to ground faults can occur either spontaneous or due to construction works. Spontaneous single phase to ground faults often have a repetitive character. Often they start due to some contamination in the insulation, especially in the oil of the joints. The event lasts just one to four periods after which isolation restores. This process can repeat several (sometimes more than 20) times. During these faults the voltage between the two non-faulted phases and the cable sheath in the whole network is raised towards coupled voltage. This stresses the isolation of these phases and will at the end result in a two or three phase to ground fault (figure 5). Digging up a cable results at first hand in a continuous single phase to ground fault. This fault remains until the fault current is switched off. When switching off takes too long time the second phase will be taken by the crane resulting in a two-phase fault. The so-called lasting ground faults will be switched off by the over current relays in the network. Typical switch off currents are between 0.9 and 1.5 seconds.

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L1 L2 L3

If

Ut Ie Ze

Figure 4: Touch voltages in a LV network due to a fault in a MV network

An inte rm itte nt s ingle phas e fault trans form s into a 2 phas e fault ( 8 Fe bruary 1999 )

10000 8000 6000 4000

45.49

33.84

34.51 35.55

36.96 37.75 I_A I_B

Current (A)

2000 0 -2000 -4000 -6000 -8000 -10000 Tim e ( s e conds afte r 16:17 )

I_C

Figure 5: An intermittent single phase to ground fault transforms into a two phase fault The distribution of the return current between earth and cable sheath can be divided into three parts. Close to the fault the current will flow into earth. Close to the substation the current will come out of the earth. In between there will be equilibrium. The current

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distribution in that part is totally defined by cable parameters and earth resistivity. Old PILC cables have steel armour, which forces a large part of the current inside the cable. Modern XLPE cables have good conducting shields of copper wire. Seen from the fault point this equilibrium will be reached after 1 to 2 kilometres. This value depends on grounding resistance of the transformer houses and the resistance of the cable sheath. The transition length close to the substation is somewhat lower due to the low grounding resistance of the substation. Insight of the different fault types, modelling, calculating and on line tests in MV networks resulted in the conclusion that safety aspects especially play a role in stub ends of the MV network, especially when XLPE cables are used. Here extra measures have to be taken.

2.3

Faults in high voltage networks

In a high to medium voltage substation the grounding systems of the high voltage, the medium voltage and the telecom network are interconnected. The return current due to a phase to ground fault in high voltage network will not only run over the earth return wires, but also through the substation grounding and also through the shields of the medium voltage and telecom cables. Especially cables, which are running in parallel with the faulted line will carry a significant amount of the current. These currents can result into local voltage rise at medium voltage houses in the neighbourhood and in overheating of signal cables. In most cases the fault is switched off within 5 periods so damage will be low. A new ’danger’ is introduced by GSM antennas in HV towers. The GSM antennas are fed by a low voltage cable. Often there are also other low voltage customers connected to that cable. The grounding of the communication apparatus, the low voltage supply and the HV tower have to be interconnected in order to prevent damages due to a lightning stroke at the tower. A lightning stroke however can also cause a flashover across the insulator. This will result into a single phase to ground fault with fault currents of more than 10 kA. A part of the fault current will flow through the tower towards the grounding premises. A significant part of this current will flow over the shield of the low voltage cable, giving earth potential rise and dangerous situations at the low voltage customers. This theory has been validated by means of a real time measurement. This is the dilemma for the earthing. On one hand the grounding systems have to be interconnected for preventing damages during a lightning stroke. On the other hand the grounding systems have to be separated in order to prevent damages at the low voltage customers due to backflashovers. For this problem the best solution is to feed the communication system from the low voltage by means of an isolation transformer. The grounding system on the primary side of this transformer is connected to the grounding system of the secondary installation by means of a surge arrester. During a lightning

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stroke, the arrester limits the voltage difference between the two grounding systems to a reasonable value. During the following ground fault, the arrester forms a proper insulation between the two grounding systems (Figure 6). The requirements for the arrester are quite high, because it still has to function after the lightning stroke, so it must not be damaged by the dissipated energy during this stroke. The concept is nowadays stated as a solution in guidelines for telecommunication like the ITU recommendation.
10kV/400V Below tower structure >30m Provider 1 Provider 2 Junction Box Plastic tube (>6m)

Low voltage supply of GSM providers in High Voltage towers

To junction box Plastic tube Sheath and neutral coupled

To installation provider

Grounding cabinet and HV tower interconnected

Connection of the isolation transformer. The surge arrester couples the grounding systems during a lightning stroke and separates the grounding systems during the short circuit that follows.

Figure 6: Solving the problems of GSM antenna’s in HV towers

3

Reflections

In a so-called global earthing system all grounding systems (not only of different voltage levels, but also of other systems like cable television and telephone) are interconnected. From the EMC point of view this is the most preferable configuration, on condition that safety can be guaranteed during all disturbances in the various systems and overheating of cable sheaths and interference with signalling voltages is limited below an acceptable level. This means that the level of occurring fault voltages must be under a specific level. Also the currents through the cores of signalling cables must be limited to prevent heating and unwished disturbances. The global earthing concept results in a safe functioning of electrical apparatus. When applying this concept all aspects of potential dangers must be known and solved. In technical and scientific literature calculations are shown, but they are hardly supported by convincing measurements. Often essential data necessary for good calculation is missing. Sometimes important EMC data like maximum allowable currents through shielding

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of signalling cables are unknown. Measurements will therefore give a necessary contribution towards theoretical assumptions. Full-scale experiments give a large insight, but have the disadvantage, that a part of the network has to be taken out of service. Dedicated experiments with injection currents having frequencies unequal but relatively close to the network frequency are therefore preferable. Global earthing is discussed worldwide. Due to difference in network structures one unique solution cannot be given. Further research is required.

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For further reading • Oirsouw, P.M. Van, Provoost, F., Safety: a very important factor in cost-optimal low-voltage distribution network design, 16th International Conference on Electricity Distribution (CIRED), Amsterdam, The Netherlands, 18-21 June; • J.B.M. van Waes, J.F.G. Cobben, F. Provoost, M. van Riet, A.P.J. van Deursen, P.C.T. van der Laan, Fault Voltages in LV networks during 1-phase MV shortcircuit, On our way to a total earthing concept, 15th International Conference on Electricity Distribution, Session 2: Power Quality and EMC, CIRED Nice99, 1-4 June 1999, pp. 115-120; • J.B.M. van Waes, F. Provoost, J. van der Merwe, Current Distribution in LV Networks during 1-phase MV Short-Circuit, IEEE Power Engineering Society 2000 Winter Meeting, Singapore, 23-27 January 2000; • J.B.M. van Waes, A.P.J. van Deursen, P.C.T. van der Laan, M.J.M. van Riet, F. Provoost, J.F.G. Cobben, Risks due to lightning strikes on high voltage towers with LV applications, 25th International Conference on Lightning Protection (ICLP 2000), Rhodos, Greece, 18-22 September 2000, Volume B, pp. 837-841; • J.B.M. van Waes, M.J.M. van Riet, A.P.J. van Deursen, F. Provoost, Safety aspects of GSM systems on high voltage towers, invited paper, 9th International Conference on Transmission & Distribution, Operation & Live-Line Maintenance (ESMO2000), Montral, Canada, 8-12 October 2000, pp.165-168; • J.B.M. van Waes, F. Provoost A.P.J. van Deursen, M.J.M. van Riet, J.F.G. Cobben, P.C.T. van der Laan Experimental Study on safety Aspects of GSM systems in HV Towers, 16th International Conference on Electricity Distribution, Session 2: Power Quality and EMC, CIRED Amsterdam 2001, paper 2-12; • J.B.M. van Waes, A.P.J. van Deursen, F. Provoost, M.J.M. van Riet Current distribution during HV and MV faults in LV systems, Workshop on EMC Measurement Techniques for Complex and Distributed Systems, Lille, France, 11-12 June 2001; • J.B.M. van Waes, A.P.J van Deursen, M.J.M. van Riet, F. Provoost, Safety aspects of GSM systems On High Voltage Towers: An Experimental analysis, IEEE Transactions on Power Delivery, Vol. 17, No. 2, April 2002, pp. 550-554; • J.B.M. van Waes, M.J.M. van Riet, Dr. A.P.J. van Deursen, F. Provoost, J.F.G. Cobben A systematic approach to improving grounding systems, invited contribution Transmission and Distribution World, Vol. 54, No. 6, June 2002, pp. 74 -77; • J.B.M. van Waes Measurement of the current distribution near a substation during a single phase to ground fault. Contribution to Question QI-2.1. Proceedings CIGRE 2002, Group 36 Power System Electromagnetic Compatibility; • J.B.M. van Waes, F. Provoost, M.J.M. van Riet, A.P.J. van Deursen, Measurement of the current distribution near a substation during a single phase to ground fault, 17th International Conference on Electricity Distribution, Session 2: Power Quality and EMC, CIRED Barcelona 2003, 12-15 May 2003; • J.B.M. van Waes, M.J.M. van Riet, A.P.J. van Deursen, F. Provoost, J.F.G. Cobben, Dutch experiences with GSM systems in HV Towers, invited paper, Telecommunications and the Electricity Industry Conference, Chester, England,19-20 June 2003; • ITU recommendation K57: Protection measures for radio base stations sited on power line towers, approved September 2003.

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