Single Earthed Neutral and Multi Earthed Neutral.


Single Earthed Neutral and Multi Earthed Neutral:

  •  In Distribution System Three Phase load is unbalance and non linear so The Neutral plays an important role in Distribution system.
  • Generally, distribution networks are operated in an unbalanced configuration and also service to consumers. This causes current flowing through neutral conductor and voltage dropping on neutral wire. The unbalance load and excessive current in neutral wire is one of the issues in three phase four-wire distribution systems that causes voltage drop through neutral wire and makes tribulations for costumers. The existence of Neutral earth Voltage makes unbalance in three phase voltages for three phase customers and reduction of phase to neutral voltage for single phase customers.
  • MULTI-GROUNDED three-phase four-wire service is widely adopted in modern power distribution systems due to having lower installation costs and higher sensitivity of fault protection than three-phase three-wire service. The neutrals play an important role in power quality and safety problems.
  • The multi grounded neutral system is the predominant electrical distribution system used in the United States.
  • It allow an uncontrolled amount of electric current to flow over the earth unrestrained, posing the potential of harm to the public and to animals causing electric shocks and is presumed responsible for undetected electrocutions.
  • The protective grounding used in low voltage,600-volt and below, applications will be described and used to explain the hazards involved with the present day multi grounded neutral distribution System, used in the United States. This will allow the reader to see the parallels between the safe low voltage distribution system and the dangerous medium voltage multi grounded neutral distribution system.
  • The reasons for the development of the three phase, four-wire, multi-grounded systems involve a combination of safety and economic considerations. The three-phase, four-wire multi-grounded design has been successfully used for many years and is well documented in the standards including the National Electrical Code (NEC).
  • It is Crucial decisions to adopt Multi Grounded Neutral System “save money” by the adoption of the multi grounded neutral electrical distribution system in the cost of the public’s safety.

 Multi Grounded Neutral System (MEN):

          

  • Fig shows the multi-grounded neutral systems commonly used by the electric utilities in North America. The neutral grounding reactor is used by some utilities to reduce the available ground fault current while at the same time still maintaining an effectively grounded system.
  • The multiple earthed neutral (MEN) system of earthing is one in which the low voltage neutral conductor is used as the low resistance return path for fault currents and where its potential rise is kept low by having it connected to earth at a number of locations along its length. The neutral conductor is connected to earth at the distribution transformer, at each consumer’s installation and at specified poles or underground pillars. The resistance between the neutral conductor of the distribution system and the earth must not exceed 10 ohms at any location.
  • NEC Article 250 Part X Grounding of Systems and Circuits 1 kV and Over (High Voltage)
  • (A) Multiple Grounding: The neutral of a solidly grounded neutral system shall be permitted to be grounded at more than one point.
  • (B) Multi-grounded Neutral Conductor: Ground each transformer, Ground at 400 m intervals or less, Ground shielded cables where exposed to personnel contact.

Single Grounded Neutral:

                    

  • Fig Show Single Grounded Neutral Which is different from Multi Grounded System .Figure shows the neutral also connected to earth, but the neutral conductor is extended along with the phase conductors. The configuration shown in figure allows electrical loads, transformers to be placed between any of the three phase conductors, phase-to-phase and/or phase-to-neutral.
  • This connection, phase to neutral will force electric current to flow over the neutral back to the transformer. So far, this electrical connection is acceptable, as long as the neutral is insulated or treated as being potentially energized, but modifications will be made in the future that will negate safety for the public and animals.
  • The ground connection would typically be located in the distribution substation. This may appear insignificant, but the differences are significant

 Advantages of Multiple Grounded Neutral Systems:

(1)  Optimize the Size of Surge Arrestor:

  • Surge arresters are applied to a power system based on the line-to-ground voltage under normal condition and abnormal conditions. Under ground-fault conditions, the line-to-ground voltage can increase up to 1.73 per unit on the two, un faulted.
  • Application of surge arresters on a power system is dependent on the effectiveness of the system grounding. The over voltage condition that can occur during a ground fault can be minimized by keeping the zero sequence impedance low. Therefore, optimization in sizing the surge arresters on the system is dependent on the system grounding.
  • An effectively grounded power system allows the use of a lower rated surge arrester. The lower rated surge arrester provides better surge protection at a lower cost. An effectively grounded system can only be accomplished using a properly sized, multi-grounded system neutral.
  • With Single Grounded Neutral System require the use of full line-to-line voltage rated arresters. This increases the cost of the surge arresters while at the same time reduces the protection provided by the surge arrester. In addition, if the fourth wire neutral is not multi grounded, it would be good practice to place surge arresters at appropriate locations on that conductor.

(2)  The zero sequence impedance is lower for a multi grounded system than the single point grounded neutral system.

(3)  Freezing and arctic conditions have an adverse impact on the zero sequence impedance. A multi-grounded system neutral will still lower the zero sequence impedance over a single point ground. In fact, without the multi-grounded system, it is more probable that insufficient fault current will flow to properly operate the ground fault protection.

(4)  Cost of Equipment for the multi-grounded system is lower.

(5)  Safety Concerns on Cable Shields.

  • Medium voltage and high voltage cables typically have cable shields (NEC requirement above 5 kV) that need to be grounded. There are several reasons for this shield:
    • To confine electric fields within the cable
    • To obtain uniform radial distribution of the electric field
    • To protect against induced voltages
    • To reduce the hazard of shockIf the shield is not grounded, the shock hazard can be increased. With the shield grounded at one point, induced voltage on the shield can be significant and create a shock hazard. Therefore, it is common practice to apply multiple grounds on the shield to keep the voltage limited to 25 volts.
    • This practice of multi grounding cable shields includes the grounding of concentric neutrals on power cables thereby extending the need for multi grounding of neutrals on the power system.

 Disadvantages of Multiple Neutral Grounding:

 (1)  Less Electrical Safety in Public and Private Property.

  •  With a multi grounded neutral distribution system it is necessary to have an electrical connection to earth at least 4 times per mile to keep the voltage on the multi grounded neutral from exceeding approximately 25 volts making it safe for the linemen should they come into contact with the neutral and the earth.
  • As per NESC Rule 096 C in the section with the multi grounded neutral conductor connected to earth at least 4 times per mile and at each transformer and lightning arrester there are now multiple paths over and through the earth that the hazardous electric current can flow over continuously, uncontrolled.
  • The path that this current flow takes through the earth cannot be determined. We cannot put an isotope on each electron and trace its path as it flows uncontrolled through the earth. It is irresponsible to permit stray uncontrolled electric current to flow into and over private property.
  • The National Electrical Code (NEC) requires the neutral in the service disconnect and over current panel board to be connected to the earth also. Now the secondary neutral is connected to earth a second time. A parallel connection of the neutral to earth now exists permitting hazardous electric current to flow continuously uncontrolled over the earth.

(2)  Earth Fault Protection Relay setting is complicated.

 Advantages of Single Grounded Neutral System:

 (1)  More Reliable and Safe System.

(2)  Protection Relay Setting is more easy in Single Grounded Neutral:

  •  Protective relays need to sense abnormal conditions, especially those involving a ground fault. The single point grounded system, with or without a neutral conductor, current flowing into the ground should be considered abnormal (excluding normal charging current). For sensing of ground faults are:

  • A current transformer in the location where the neutral is grounded can be used to sense the ground fault (zero sequence) current.
  • A zero sequence CT enclosing the three phase and neutral conductors.

  • Four CT residue circuit (Three CT residual with neutral CT cancellation).
  • Protecting against ground faults on a multi-grounded neutral system is more difficult than the single point grounded system since both neutral and ground fault currents must be considered.
  • Neutral current and likewise ground fault current can flow in both the neutral and the ground. So, We have must calculate both current as  the amount of neutral current which may flow in the circuit, and the ground fault setting must be above this neutral current. This is self explanatory from Fig.

(3)  Sensing of Ground Fault current :

  • While the sensing of the ground fault current in the single point grounded system is less complex than the multi grounded system, the amount of ground fault current on the single-point grounded system may be greatly limited due to the fact that all ground fault current must return through the earth. This is especially true where the earth resistivity is high, the soil is frozen or the soil is extremely dry.

Reference:

John P. Nelson Fellow, IEEE ANSI/IEEE Std 142-1991

Westinghouse Electric Corporation, Electrical Transmission and Distribution Reference Book NFPA 70.

Jeffery Leib, Train-Car Crashes on the Rise,Denver Post Newspaper, November 7, 2002

R.T. Beck and Luke Yu, Design Considerations for Grounding Systems.

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Type of Electrical Power Distribution systems


Type of Electrical Power Distribution Systems:

  • Electrical power is distribution either three wires or Four wires (3 wire for phases and 1 wire for Neutral). Voltage between Phase to Phase Called Line Voltage and Voltage between Phase and Neutral is Called Phase Voltage.
  • This Forth wire may or may not be distributed in Distribution System and Same way this neutral may or may not be earthed
  • Depending of this neutral condition (Earthed-not Earthed-access-not access) there are various type of earthing System.
  • The neutral may be directly connected to earth or connected through a resistor or a reactor. This system is called directly earthed or Earthed System.
  • When a connection has not been made between the neutral point and earth, we say that the neutral is unearthed.
  • In a network, the earthing system plays a very important role. When an insulation fault occurs or a phase is accidentally earthed, the values taken by the fault currents, the touch voltages and over voltages are closely linked to the type of neutral earthing connection.
  • A directly earthed neutral strongly limits over voltages but it causes very high fault currents, here as an unearthed neutral limits fault currents to very low values but encourages the occurrence of high over voltages.
  • In any installation, service continuity in the event of an insulation fault is also directly related to the earthing system. An unearthed neutral permits service continuity during an insulation fault. Contrary to this, a directly earthed neutral, or low impedance-earthed neutral, causes tripping as soon as the first insulation fault occurs.
  • The choice of earthing system in both low voltage and medium voltage networks depends on the type of installation as well as the type of network. It is also influenced by the type of loads and service continuity required.
  • The Main objectives of an earthing system are Provide an alternative path for the fault current to flow so that it will not endanger the user, Ensure that all exposed conductive parts do not reach a dangerous potential, Maintain the voltage at any part of an electrical system at a known value and prevent over current or excessive voltage on the appliances or equipment.
  • Different earthing systems are capable of carrying different amounts of over current. Since the amount of over current produced in different types of installation differs from each other, required type of earthing will also differ according to the type of installation. so in order to ensure that the installation goes with the existing earthing system or else to do any modification accordingly, we need to have a proper idea of the present earthing system. It would enhance the safety as well as the reliability
  • As per IEC 60364-3 There are three types of systems:

(1)  Unearthed System:

  • IT System.

(2)  Earthed System:

  • TT
  • TN (TN-S, TN-C, TN-C-S).
  • The first letter defines the neutral point in relation to earth:
  1. T = directly earthed neutral (from the French word Terre)
  2. I =unearthed or high impedance-earthed neutral (e.g. 2,000 Ω)
  • The second letter defines the exposed conductive parts of the electrical installation in relation to earth:
  1. T =directly earthed exposed conductive parts
  2. N =exposed conductive parts directly connected to the neutral conductor

 Unearthed System:

 (1) IT system unearthed (High Impedance earthed neutral)

  • First Letter I= the neutral is unearthed at Transformer or Generator side.
  • Second Letter T= Frame parts of the loads are interconnected and earthed at Load Side
  •  It is compulsory to install an over voltage limiter between the MV/LV transformer neutral point and earth.
  • If the neutral is not accessible, the overvoltage limiter is installed between a phase and earth.
  • It runs off external over voltages, transmitted by the transformer, to the earth and protects the low voltage network from a voltage increase due to flashover between the transformer’s medium voltage and low voltage windings.

Advantages:

  1. System providing the best service continuity during use.
  2. When an insulation fault occurs, the short-circuit current is very low.
  3. Higher operational safety only a capacitive current flows, which is caused by the system leakage capacitance if an earth fault occurs.
  4. Better accident prevention the fault current is limited by the body impedance, earthing resistance and the high impedance of the earth fault loop.

Disadvantages:

  1. Requires presence of maintenance personnel to monitor and locate the first fault during use.
  2. Requires a good level of network insulation (High leakage current must be supplied by insulating transformers).
  3. Overvoltage limiters must be installed.
  4. Requires all the installation’s exposed conductive parts to be Same Voltage level. If this is not possible RCDs must be installed.
  5. Locating faults is difficult in widespread networks.
  6. When an insulation fault with reference to the earth occurs, the voltage of the two healthy phases in relation to the earth take on the value of the phase-to-phase voltage So when Select Size of equipments it is need to higher insulation level of the Equipments.
  7. The risk of high internal over voltages making it advisable to reinforce the equipment insulation.
  8. The compulsory insulation monitoring, with visual and audible indication of the first fault if tripping is not triggered until the second fault occurs.
  9. Protection against direct and indirect contact is not guaranteed.
  10. 10. Short-circuit and earth fault currents may cause fires and destroy parts of the plant.

 Earthed System:

 (1) TT system directly earthed neutral

  • First letter T=the neutral is directly earthed.
  • Second letter T= the exposed conductive parts of the loads are interconnected and earthed.
  • The transformer neutral is earthed;
  • The frames of the electrical loads are also connected to an earth connection

System characteristics:

  1. High earth fault loop impedance
  2. Low earth fault current
  3. Utility company need not to provide earth for consumer

Advantages:

  1. save earth wires
  2. The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it.
  3. TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing
  4. Does not have the risk of a broken neutral.
  5. The simplest system to design, implement, monitor and use.
  6. Easily find location of faults.
  7. Upon occurrence of an insulation fault, the short-circuit current is small.
  8. Reduces the risk of over voltages occurring.
  9. Authorizes the use of equipment with a normal phase to earth insulating level.

Disadvantages:

  1. High demand of E/F relays.
  2. Individual earth system needs higher investment.
  3. Higher touch voltage.
  4. Induce Potential gradient.
  5. Switching upon occurrence of the first insulation fault.
  6. Use of an RCD on each outgoing feeder to obtain total selectivity.
  7. Special measures must be taken for the loads or parts of the installation causing high leakage currents during normal operation in order to avoid spurious tripping (feed the loads by insulating transformers or use high threshold RCDs, compatible with the exposed conductive part earth resistance).
  8. Very high fault currents leading to maximum damage and disturbance in telecommunication networks.
  9. The risk for personnel is high while the fault lasts; the touch voltages which develop being high.
  10. Requires the use of differential protection devices so that the fault clearance time is not long. These systems are costly.

(2)TN System: Neutral-connected exposed conductive part

  • First Letter T = the neutral is directly earthed at Transformer.
  • Second Letter N=the Frames of Electrical loads are connected to the neutral Conductor.
  • There are two types of TN systems, depending on whether the neutral conductor and Earth conductor are combined or not:

(a)TN-C:

  • In TNC System (the third letter C=combined Neutral and Earth Conductor), the neutral and Earth conductors are combined in a single conductor and earthed at source end.
  •  This Combined Neutral-Earth wire is than distributed to Load side.
  • In This System Earthing connections must be evenly placed along the length of the Neutral-(Earth) conductor to avoid potential rises in the exposed conductive parts at Load Side if a fault occurs.
  • This system must not be used for copper cross-sections of less than 10 mm² and aluminum cross-sections of less than 16 mm², as well as downstream of a TNS system (As per IEC 60364-5).

System Characteristics:

  1. Low earth fault loop impedance.
  2. High earth fault current.
  3. More than one earth fault loops.

Advantages:

  1. No earth wire required; allow of multi-point earth,
  2. Better earthing continuity.
  3. Neutral never have float voltage.
  4.  Impedance of earth fault loop could be predicted.
  5. The TNC system may be less costly upon installation (elimination of one switchgear pole and one conductor).

Disadvantages:

  1. If not multi-point earthed, and the neutral earth broken, the exposed metallic part may have float voltage.
  2. High earth fault level,
  3.  intervene the operation of earth fault protective device.
  4. current operated type device is not appropriated, voltage detected type could be employed.
  5. Third and multiples of third harmonics circulate in the protective conductor (TNC system).
  6. The fire risk is higher and, moreover, it cannot be used in places presenting a fire risk (TNC system).

(b)TN-S:

  • In TN-S system (the third letter S=Separate Neutral and Earth Conductor) neutral of the source of energy is connected with earth at one point only, generally near to the Source. The neutral and Earth conductors are separately distributed to load.
  • In This System Earthing connections must be evenly placed along the length of the Neutral-(Earth) conductor to avoid potential rises in the exposed conductive parts at Load Side if a fault occurs.
  • This system must not be used upstream of a TNC system.

System characteristic:

  1. Low earth fault loop impedance
  2. High earth fault current

Advantages:

  1. Use of over current protective devices to ensure protection against indirect contact.
  2. Earth fault protection device operates faster.
  3. Allow multi point earth, better earthing continuity; minimize the use of earth fault relay because of low earth fault loop impedance.

Disadvantages:

  1. Switching on occurrence of the first insulation fault.
  2. The TNC system involves the use of fixed and rigid trunkings
  3. Requires earthing connections to be evenly placed in the installation so that the protective conductor remains at the same potential as the earth.
  4. A tripping check on occurrence of the insulation fault should be carried out, if possible, when the network is being designed using calculations, and must be performed during commissioning using measurements; this check is the only guarantee that the system operates both on commissioning and during operation, as well as after any kind of work on the network (modification, extension).
  5. Passage of the protective conductor in the same trunkings as the live conductors of the corresponding circuits.
  6. high earth fault level  under earth fault condition,
  7. low power factor (high inductance of long cable)
  8. Requires extra equal potential bonding.
  9. On occurrence of an insulation fault, the short-circuit current is high and may cause damage to equipment or electromagnetic disturbance.

(c)TN-C-S System:

  • The Neutral and Earth wires are combined within the supply cable.
  • Typically this will be a concentric cable, with the live as the central core, and a ring of wires around this for the combined neutral and earth.
  • At the property, the Neutral and Earth are separated, with the earth terminal usually being on the side of the cutout. Inside the cutout, the live and neutral are linked.
  • Throughout the supply network, the combined earth/neutral conductor is connected to the ground in multiple places, either buried underground or at the poles for overhead supplies.
  • This multiple earthing is why a TNCS supply is often called PME (Protective Multiple Earthing).

Advantages:

  • Cost for core cable is cheaper than a 3 core
  • . As the outer sheath is usually plastic, there are no problems with corrosion.

Disadvantage:

  • When the combined earth/neutral conductor is broken. This results in a voltage appearing on the exposed metalwork in the customer’s property, which can be a shock risk.
  • This happens as the earth and neutral are connected in the cutout, and there is no direct connection to the ground other than in the supply network.
  • In the event of a fault, the current flowing in the customer’s earthing conductors can be much greater that for a TNS system.
  • It is also possible to get unusual circulating earth currents between properties, particularly where some properties have metal water pipes and others have plastic

Reference:

  • Protection of Electrical Network-Christophe Prévé.

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