Global earthing supplies the safe grounding of low-, medium-and even high-voltage (LV, MV, HV) networks. Providing integrated protection against lightning and switching events, a well-designed single grounding system is invaluable for networks connected to customers' premises.
NUON is one of the major power utilities in the Netherlands supplying gas, water and electricity to 5 million customers in five provinces, including the capital city of Amsterdam. To investigate the consequences of global earthing in the regions the transmission and distribution utility serves, NUON entered into a research contract with Eindhoven University of Technology (TU/e). The contract offered cooperative benefits to both parties. NUON gained access to modern, unconventional measuring techniques developed at TU/e, and TU/e gained access to NUON's network to perform measurements at all system voltage levels. Working together, the utility and TU/e analyzed system problems and found solutions with firm backup from theoretical investigations conducted on LV, MV and HV systems.
Most common LV systems are TN or TT systems. The first ‘T’ represents a solidly grounded system of the LV neutral at the MV/LV transformer. The second letter (T or N) represents the connection of the exposed conductive parts to the same grounded point of the LV system.
A TN-C system has a neutral combined with the protective earth (PE) conductor.
A TN-S system has a separate PE conductor.
A TN-CS system has a combined neutral/earth conductor, between the transformer and the building, that is separated at the entrance and inside the building.
A TT system has a separate electrode not connected to the neutral for each customer.
In the Netherlands, all networks below 50-kV are comprised of underground cables. The older MV and LV cables have lead sheaths that provide low impedance grounding via good contact with the often-damp soil. The new or replacement MV and LV cross-linked polyethylene (XLPE) cables that utilities currently install have a copper sheath inside and an outer insulating polyethylene jacket that effectively increase the grounding impedance.
Historically, TT and TN systems existed on the LV side. In a TT system, each customer has an individual grounding arrangement; therefore, faults on LV and MV systems may cause voltages on the neutral, but this rise in kilovolts is normally not accessible nor does it pose a hazard to the customer.
NUON now favors the TN-CS system on the LV side of the transformer where the various grounding conductors at the transformer and at customer's terminals interconnect or bond together. The distributed grounding reduces the probability of a customer not having a safe ground and insures improved lightning safety at lower overall costs. However, faults on the electrical network may migrate into the LV grid grounding, causing unacceptably high touch voltages on LV customer's installations.
As a fault on the LV network may cause touch voltages at customer premises, fast switch-off or isolation is required. The critical position of a fault on the LV network is at the remote end of the cable. Therefore, to keep the touch voltages within an acceptable range, the practical safe length of a 150 mm2 Al cored, Al sheathed cable is 310 m (1020 ft). NUON increased this range to lower the cost of the distribution network by following consideration of these options:
Providing additional connections or bonding between the phase and neutral conductors of the LV cables used for distribution to decrease the return impedance.
Installing a separate PE conductor in parallel with the cable.
Using a special fast fuse, (improved inverse time/current characteristic) with good selectivity that offers the same result. NUON can install lengths up to 445 m (1460 ft) without impairing safe operation.
In the past problems occurred in buildings with a TN-C system as materials within the building's construction carried part of the return current and induced magnetic fields (B).
There are two main reasons to reduce magnetic field strength:
To comply with recommended health standards, the magnetic fields in the human body should be <100 microT (for the general population and during a 24-hour day) according to the International Commission on Non-Ionizing Radiation Protection guideline.
Magnetic fields near computer monitors should be <1microT to avoid distortion on a monitor screen.
The power-frequency currents over signal cables may introduce extra interference on non-power cables.
Even with a TN-S system, problems can occur, for example, because of the position of single phase cables. Shielding may help solve the problems. Load currents may range up to 1kA with no zero sequence current. A television set situated 50 cm (20 inches) above the ceiling of the basement was disturbed and required a 10- to 20-factor reduction of the magnetic-field strength. Shielding with two stacked aluminum plates proved an efficient solution.
Transfer MV Faults to LV Network
When using a TN system, give attention to phase to ground faults on the MV network. In floating networks, the magnitude of fault current at a single phase to ground short circuit is governed by the characteristics of the cables installed, particularly the total cable capacitance and the layout of the network. Some of the fault current flows into the local grounding system at the MV/LV transformer, resulting in touch voltages at the premises of connected LV customers. Considering the amplitude and duration of the fault current, NUON distinguishes different MV networks based on the value of the phase to ground current:
I Fault<330 A Floating network, maximum fault duration 8 hours
330<I Fault<485 A Extra capacitors increase fault current above 485 A, resulting in fast switch off
I Fault>485 A Fault currents above 485 A have a fault clearance time <1 sec.
At 10kV/400V distribution substations, NUON grounds all sheaths of MV/LV cables. During a MV phase to ground fault, the major part of the current returns via the sheath of the faulted cable. A fraction of the fault current (Ie) flows into the soil via the local grounding structure (Re), and part of the fault current may return via other systems (telecom, cable television and gas pipes).
XLPE cable has a copper sheath with a smaller series resistance compared to lead-sheathed cables that have the advantage of providing a good contact with the soil, the steel armour reducing the ground return current. For a theoretical cable model, the designer should know the exact material parameters (for example, mr for steel-armored cables). However, in practice, power-utility cables are laid in the same trench as other cables and other unknown parameters make an accurate calculation of the current distribution difficult.
To verify the theoretical modeling, experiments were conducted in 1998 when several phase to ground faults were made on the 10-kV network. The current distribution and relevant voltages in the LV network were measured for several grounding configurations. In the original experiment, fault voltages up to 60V per 100 A fault current were observed at two LV customer installations. For currents less than 300 A, this situation may last up to 8 hours. Extra electrodes in the LV network reduce the touch voltages significant; for new rural networks, extra electrodes having a value of 5 ohms (max.) are placed at the end of each LV cable.
GSM Systems on HV Towers
The recent growth of cellular phone base stations has increased the demand for an elevated location for the base stations. A lightning stroke may induce flashover of the insulators and a subsequent phase to ground fault. The power-frequency current distributes over the lightning-protection conductors and the soil below the line via the tower grounding electrodes. A fraction of the fault current flows into the LV cable towards the distribution transformer feeding the GSM. This current also may cause dangerous touch voltages and overstress the insulation of electric apparatus on the premises of other customers supplied by this distribution transformer. Most sites where GSM systems are installed in this way are located in rural areas, with no extended grounding grid.
To study the effects at power-frequency current distribution in the event of short circuit, TU/e and NUON conducted a field test on the 150-kV double-circuit transmission line between Doetinchem (DTC) and Ulft. The figure above shows an overview of the test circuit with the GSM base station mounted on Tower 33 at 8.3 km (5.2 miles) from DTC and 2 km (1.24 miles) from Ulft. The sky wires are grounded at all towers and at both substations. The 10/0.4-kV distribution transformer located 300 m (985 ft) from Tower 33 also supplied several other customers.
One of the 150-kV systems was de-energized with one phase short circuited to ground at Tower 33. NUON injected a small current from a 10-kV network in DTC and measured the voltage between the neutral and the local ground at locations LV1 and LV2. Because this LV network had a TT configuration, these voltages are related to the touch voltages that occur if applying a TN grounding arrangement. The measured values between the neutral and telecom grounds when scaled up to the full short-circuit current would have exceeded the insulation requirements for telecom equipment, such as a modem.
TU/e simultaneously measured the current distribution at DTC, Ulft and Tower 33 with the measured current distribution. The currents and voltages were normalized to a 100-A injection current. Note that 36% of the current flows to ground at Tower 33 and more than 50% of this current flowed from the tower footing to the LV cable connected to the GSM base station; the measured voltage levels were all of the same magnitude. The field test results were compared with a theoretical simulation of the DTC Ulft short circuit using EMTP.
Utilities seek a solution that provides safety for every situation irrespective of the level of short-current and tower-footing resistances. Consider the following options:
Select location for GSM based on maximum fault current.
Install LV cable in metal tube.
Add electrodes along the LV cable route or at nearby towers.
Petersen coils may limit the fault current (option A), but as the majority of HV networks in the Netherlands comprise mainly underground cable, this grounding principle is no longer applicable. A combination of options (B) and (C) could provide reliable protection, but in the event of a low-source impedance at the tower base, (because of the HV network and phase conductors), it is difficult to change the current distribution. Since local parameters such as the soil resistivity and the presence of other buried cables are important, each site needs individual consideration. However, NUON considers this unpractical and uneconomical as GSM base stations can be sited at any location. Therefore, NUON has selected option (D), the installation of a LV/LV transformer to isolate the HV and the MV/LV networks. In the Netherlands, the large number of GSM base stations already installed and still planned, the dense population, and the serious consequences of a flashover justify these extensive safety measures.
Each cellular phone provider installs a separate transformer to avoid interruption during maintenance. NUON also installs an isolation transformer below the tower structure. A LV cable with an insulated jacket connects the junction box to the isolation transformer that is protected by a surge arrester. An additional grounding electrode, maximum length of 5 m (16 ft) is placed at the junction box for the lightning current that passes through the arrester. The LV cable extends to at least 30 m (98 ft) to the nearest distribution cabinet to avoid “bypassing” the fault current through the soil.
The cabinet is connected to the tower grounding structure, special attention being given to the incoming LV cable. As flashover between the cable and the cabinet would nullify this complete safety concept, as an additional precaution against insulation breakdown, the LV cable is protected by a plastic insulating tube for a distance of 6 m (20 ft) from the tower.
The close cooperation between NUON and TU/e examined many aspects of system earthing and produced a satisfactory range of cost-effective solutions designed to enhance the safety and protection to all utility customers. In particular, the installation produced to safeguard GSM base stations now provides NUON the opportunity to benefit from the revenue that can be generated by permitting cellular phone providers to site base stations on transmission towers. Furthermore, the combined use of existing towers helps to preserve visual amenities.
J.B.M. van Waes received the MS degree from the Eindhoven University of Technology in 1996. As a researcher at TU/e, he was responsible for various projects related to grounding for industry. As a Ph.D. student in a cooperation, he continued studying faults on LV, MV and HV grid systems using a theoretical and experimental approach.
A.P.J. van Deursen received the BS and Ph.D. degrees from the Nijmegen Catholic University, The Netherlands, in 1965 and 1976, respectively. Since 1986, he has been a lecturer on electromagnetic compatibility at TU/e.
M.J.M. van Riet received the MSEE degree from the TU/e in March 1979 and joined PGEM, now NUON. During his professional career with NUON he has managed large projects including SF6 150kV/10kV substations, 50-kV and 150-kV cable projects. van Riet is currently head of NUON's Research and Development Department.
F. Provoost received the MSEE degree from the University of Eindhoven in 1982 and joined ASEA in Ludvika Sweden. In 1986, he joined PGEM, now NUON, investigating HV and LV network systems, power quality and grounding.
J.F.G. (Sjef) Cobben received the BSEE degree in 1979 and joined PGEM, now NUON. His professional career appointments include work on projects and studies on MV and LV network systems, power quality, grounding and requirements for LV installations. He is a member of several national and international normalization committees involved with requirements for LV installations.