NUON, A REGULATED ELECTRICITY DISTRIBUTION COMPANY IN THE NETHERLANDS, faces a continual challenge to reduce system operation and maintenance costs, while increasing network availability and reliability. Driven by both technological and economic factors, NUON and the other electricity distribution companies in the Netherlands are forced to examine in more detail the design of new or refurbished low-voltage power distribution networks.

The technological factors are:

• Grounding system trends from Terra-Terra (TT) to Terra-Neutral(TN)

• Dispersed generating systems that can cause voltage problems.

The economic factors are:

• Decreasing engineering margins resulting in reduced network investment costs

• A low-investment network should not increase operating costs, introduce poor voltage conditions or create unsafe situations for the connected customers.

THE CHANGE IN GROUNDING PRACTICE

One technological factor is that many distribution companies are changing their policy from a TT-grounded system (where the installation is grounded at the client's location) to a TN-grounded distribution system (where the electricity company provides the grounding as a service through the network). The electricity companies formerly used the metallic water pipes to ground both the low-voltage terminals and customer home electric systems.

Now that water companies use plastic instead of metal pipes, this grounding method is becoming obsolete. One solution is to apply grounding electrodes for each customer instead of grounding at the water system. An alternative method would be for the distribution company to provide the grounding together with the electric system. In the latter case, the electricity company is responsible for proper grounding and making calculations to ensure safety at all times.

A further technological factor that drives the need for accurate calculation is the growth of dispersed generating systems in the low-voltage system, such as combined heat and power as well as solar energy. These systems introduce a reverse current path, which affects network operation and safety. In addition to the lower voltage limits, developers need to consider the upper voltage limits (see how in Fig. 1 the curves B, showing “with dispersed generation,” contrast with the curves A, showing the conventional “without dispersed generation”). Dispersed generation is now connected to the medium-voltage system, affecting voltage levels; therefore, these daily varying voltages also are taken into account.

One important economic and political factor is the burden caused by the liberalization of the electricity market. The conventional engineering method was based on the use of specified engineering margins, incorporated in well-known rules of thumb. Decreasing these margins results in reduced network investment costs, but existing operating costs, voltage quality and customer safety standards must be maintained. Therefore, the development of new networks becomes increasingly difficult.

In the Netherlands, almost 100% of the low-voltage distribution systems consist of underground cables. However, the methods of operating the distribution systems are historically based and vary greatly with networks operated radially and in meshed configurations. Meshed networks are operated as open or closed rings and a low-voltage network comprises a variety of cables and methods of network grounding. However, it is common to consider long-term practice, so that subsequent network adjustments are reduced to a minimum during the cable's lifetime.

SOFTWARE SOLUTION

Deregulation in the Netherlands led to new nationwide constraints imposed by the regulator and new international standards. The new demands led to the development and implementation of computer software that is based on a new integral engineering method. The first steps toward the software development were made with software company Phase to Phase in 1996. Gaia, the new computer program, comprises two major parts: the calculation of cost-optimal cable diameters regarding the technical constraints and the correction of the network for contact safety considerations.

The aim of Gaia's optimization function is to develop a low-voltage distribution network with minimal lifetime costs. The procedure is based on a combined integer and real approach, as well as on the optimization for economic operational management. In other words, the cheapest solution with respect to investment and electrical losses is sought for the estimated lifetime of the cable. A large cross-section cable requires a higher investment but has fewer losses and vice versa (Fig. 2).

Ultimately, the method selects the right cable with respect to load and voltage quality. The main variables in the process are cable diameters and transformer tap positions. The technical constraints are minimum and maximum voltage, maximum cable current and voltage fluctuations. The result strongly depends on the financial parameters, network design, electrical demand, growth rate and dispersed generation.

By adopting grounding according to the TT system, the customer's safety is almost totally determined by the impedance of the customer's own grounding provisions. However, with a TN system, the customer's grounding system is connected to the supplier's grounding system. The neutral and ground in the supplier system are coupled wherever possible.

When a phase-to-ground fault in the network cable occurs, the neutral and ground lines will obtain a voltage with respect to the “far-off ground.” In this case, a person's body experiences a voltage through his or her grounding conductor (Fig. 3). To avoid serious harm to the person, this voltage must be low enough or the fault must be switched off in time.

The time it takes to isolate the system depends on the magnitude of the person's contact voltage and current. The connection between the current passing through the body and the allowable time is stipulated in the IEC 479-1 standard. The voltage depends on the medium-/low-voltage transformer and the impedances of the low-voltage cable and the return path. The return path is determined by cable neutral and sheath, as well as by additional grounding electrodes and ground resistance.

In this context, the length of the network cable has a major influence on a person's safety; a longer cable extends the cutoff time and, therefore, the length of exposure. For this reason, extra attention is given to selecting the right network protection devices, because the actual cutoff time depends on the fault current magnitude and the fuse characteristic.

For a fast cutoff, the total circuit impedance has to be small; this directly limits the maximum cable length. For a low-fault voltage, the phase/return impedance ratio has to be large. If the network configuration does not meet these contact-voltage requirements, additional measures can be taken, such as increasing the cable diameter for an impedance reduction and, therefore, a quicker cutoff time; or adding grounding electrodes or connections for a neutral and ground network impedance reduction.

Determining contact safety during short circuits on the low-voltage network with a TN grounding system is complicated. In the method of calculation, the operating resistances and impedances play a role as well as the electromagnetic mutual coupling between the cable conductors.

The voltage at the “touch” point is calculated using normal longitudinal impedances and mutual impedances for all conductive elements: three phases, neutral and sheath. For the calculation of a single-phase fault in the low-voltage system, the mutual impedances may not be neglected. Therefore, a complete five-conductive-element system should be necessary, thus yielding a 25-element matrix for a cable. The cable impedances are calculated and stored in a cable database, so that the user is not confronted with these theoretical parameters.

Additionally, two specific technical functions are incorporated into the method for corrections in special cases, such as a voltage dip due to a single-phase motor start and simplified asymmetry calculations.

USABILITY, RESULTS AND VALIDATION

The computer program must have a graphical mouse-controlled user interface that enables the low-voltage network designer to calculate his or her design in a fast and practical way. Therefore, expertise must be built-in as rules and built-in in the components database. The users must be supported by means of a “Wizard,” which enables the quick construction of the electrotechnical design. The program must be able to contain company standard values for the calculations and must have flexibility for user-specific visualization and presentation.

Gaia provides all the user-friendly requirements, and all of the calculations can be accessed by a graphical user interface. The program has a fast, simple one-line editor that can distinguish between the phase, neutral and grounding networks in layers. The user is supported in the construction and alteration of the network by component databases that contain all the technical data of the components. Consideration is given to the most important technical preconditions, such as the loading capacity of the cables and transformers, voltage limits, motor start and the contact safety in the event of short circuits in the network. The interface and the calculation software run in Windows on a normal PC. The user can modify all variables, but a lock is built-in for specific companywide standards, such as voltage requirements, financial data and parameters for the safety calculation.

When it comes to human safety, a new network model and new cable data must definitely be validated for correctness by means of measurements in practice. The fault voltages and fault currents were measured by creating short circuits in an existing low-voltage network. The available results of field experiments show that the differences in contact voltages compared with the values determined using Gaia were less than 10%, with the results from Gaia always on the safe side.

GAIA IN PRACTICE

The use of Gaia has led to interesting results when compared with traditional design methods, with some companies saving considerably on new distribution networks. Furthermore, the computer program has effectively improved the operation of existing networks, resulting in reduced revenue costs.

The introduction of grounding by means of a TN system as a service to customers initially reduced the length of the network compared to the TT system where the voltage level is the only constraint. In TN systems, the choice of the network fuse protection needs more discipline, as the network cable length influences the short circuit cutoff time. The need to satisfy this requirement led to the development of faster standard switching devices designed to fit into the normal switchboard. These devices have a standard characteristic for normal overcurrents, but have very fast operating times in the event of excessive overcurrents due to network faults (Fig. 4).

In conclusion, the use of TN grounding systems no longer imposes major constraints in the length of the network. However, users must exercise care in selecting the correct length of the customer connection or service cable, as this is still an important factor in the safety calculations. When introducing TN systems in existing networks, problems can occur when modeling older cables as it is not apparent whether sheaths at joint positions are connected or not. Also, problems may occur when trying to model noninsulated copper conductors in the ground and older mass-impregnated cables with their sheaths in direct contact with the soil.

The technical network requirements must satisfy the expectations of both power consumers (standard household loads) and dispersed generation (photo-voltaic systems and combined heat and power), and medium-voltage system fluctuations must also be taken into account. The requirements verification for upper and lower limits to voltage is determined in a single calculation, which simplifies the projection in modern network planning.

The presence of a clear and open structure of the database concept ensures that a conversion from and toward the geographical information systems (GIS) is easy. This means that, in future systems, lines can be drawn in the GIS, the network will be calculated in Gaia and the results, such as cable types to be used, will be stored in the GIS. Using more accurate information from the network may result in improved results. With the current model, the calculations are made at only two load points only: maximum load with minimum generation, and minimum load with maximum generation.

Future use of daily load curves and daily generation curves for different loads and different forms of local generation, available on an hourly basis, should lead to improved accuracy.

The design of networks for lower voltages requires a considerable volume of reliable information. However, this information is not always available, so users of the resulting design should always use experience and be aware of possible inaccuracies in the designed network. Therefore, most of the electricity utilities in the Netherlands have taken considerable care to train their staff in the basics of the program, to ensure the designer and project working staff respect the results of the calculation. This implies that the network has to be laid out exactly as it was calculated. The model has to take into account changes to the network, for example the connection of new customers, and changes in the power demands of existing customers that result in the need to redesign the existing network. By adopting this procedure, the existing planning, construction and operation disciplines are maintained.

SUBSTANTIAL SAVINGS

Distribution companies have now been using Gaia for some four years now, and compared with previous design methods, this development has resulted in a reported 10% savings on cable investment costs. This computer program proves particularly valuable when required to demonstrate the economic and technical merit of this low-voltage distribution network design tool to utility management or the industry regulator.

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Frans Provoost earned his master's degree in electrical engineering from the Technical University of Eindhoven in 1982, and he joined ASEA (now ABB) in Ludvika, Sweden, where he worked on main circuit design for high-voltage dc projects. In 1986, Provoost joined NUON, one of the largest energy companies in the Netherlands, where his experience has been used on 150-kV grid planning and reactive power compensation projects. More recently, his responsibilities have included power quality, software development for network analysis, fault location methods for medium-voltage networks and feasibility studies for several HVDC connections to the Netherlands. Currently, Provoost is studying at Eindhoven University of Technology for his Ph.D. on intelligent networks.

Frans.Provoost@nuon.com