ENEL Distribution in Italy, in cooperation with CESI (Milan, Italy), has developed a new standardized design for environmentally friendly low-cost high-/medium-voltage (HV/MV) distribution substations.

The goal includes minimizing the visual impact, electromagnetic field and audible noise emission of HV/MV substations and MV networks. ENEL also expects to reduce the capital and operation and maintenance (O&M) costs by limiting network redundancy and reducing energy losses. The design improvements will comply with the 1999 Electricity and Gas Authority service standard, increasing service quality.

Operationally, it is not easy to supply all MV feeders from adjacent HV/MV substations due to the length of time required to locate and manually close the load disconnectors on long feeders. In addition, adjacent feeders might not have sufficient thermal capacity to supply the total load or maintain voltage at an acceptable level. Previous design practices may adversely affect consumers who are exposed to voltage dips and supply interruptions.

ENEL's fault statistics confirm that 90% of all supply interruptions can be attributed to the MV network and MV/LV substations. Hence, in 1999, ENEL Distribution started an MV network automation project to install remote control facilities to disconnectors at 50,000 MV/LV distribution substations (25% of the total number of substations installed).

New Design Philosophy for MV Networks

New HV/MV substations are installed for load growth and to improve service quality. In practice, this reduces the length and loading of MV circuits and provides emergency supply, in most cases, from adjacent HV/MV stations.

Accordingly, ENEL is moving away from the old model where conventional HV/MV substations consisted of redundant transformers with large ratings and limited reserve from the network. Many long MV lines were supplied by HV/MV substations unequipped with remotely controlled sectionalizing points. The new model provides a large number of smaller HV/MV substations with one transformer (16-MVA, 25-MVA or 40-MVA rating) and limited transformer redundancy. Also, fewer shorter lines radiate from each new HV/MV substation, each provided with, on average, five remotely controlled load disconnectors along each feeder linking two substations.

While the new model increases the loading capability and reliability of the existing MV network, it requires more infeeds from the HV network. In practice, the expansion of the HV network will be moderate for the following reasons:

  • Distribution HV networks in Italy are extensive and meshed — about 45,000 km (28,000 miles) of 132- and 150-kV lines. Therefore, most of the new HV/MV small-capacity substations can be located underneath an existing line within the line right of way.

  • Where short lateral 132- and 150-kV lines are required to supply the new HV/MV substations, the length will not exceed 10 to 15 km (6.2 to 9.3 miles). Where appropriate, “light” HV lines (smaller conductors) are used to lessen the environmental impact.

  • Simplified prefabricated HV/MV substations can be better located (siting and routing).

General Schemes for HV/MV Substations

Figure 1 shows the single-line diagrams (showing only circuit-breakers “CBs” and HV disconnectors “DSs”) for HV/MV substations presently employed in Italy. Diagram A, or “H” scheme, is extensively applied in the HV/MV substations equipped with two transformers with the line-in, line-out and loop connection. The redundancy of transformer capacity and HV line capacity in closed-loop operation assures continuity of supply of the MV network in case of single-transformer or single-line outage.

Diagram B is the most suitable basic scheme for ENEL's new simplified HV/MV substations. Using a CB on the transformer's HV side facilitates switching and protection of the transformer. A forced or planned outage of the line terminated only with a DS causes the temporary outage of the transformer and MV feeders. This event will be rare since the HV distribution lines are relatively short, and high-speed, single-pole reclosures are expected to avoid transformer-supply interruption in most line faults. The HV part of a Diagram B substation is a prefabricated modulus. Adding a second equal modulus extends the substation, as shown in Diagram C.

It is thought that five to six simplified Diagram B substations can be inserted in a HV line terminating at two supply substations. The iteration feasibility of Scheme B along a line stems from the fact that a line or transformer fault normally should cause the outage of no more than one HV/MV substation. Diagram D also is used where one simplified substation is tapped from a line terminating at two supply substations. Contrary to Scheme B, Scheme D is not to be iterated along a line in order to avoid the simultaneous outage of more than one HV/MV substation.

Use of New Equipment Types

HV Switchgear

An important distinguishing feature of substations is the type of insulation and enclosure of components. New equipment using hybrid (SF6 and air) insulation is preferred due to the following advantages:

  • Compact design
  • Fast and reliable installation at site
  • No live parts within the reach of service personnel
  • Easy replacement in case of failure
  • Reduced need for civil works (a building is usually not required).

Stimulated by ENEL's requirements for this new type of substation, some major manufacturers have designed switching equipment that is simple, requires minimal maintenance and can be effectively integrated into the HV sub-transmission networks.

Figure 3 shows the circuit schematic of the “Y” HV hybrid-insulation switchgear, specifically called “Y2” as it includes two CBs and examples installed in 170-kV substations. Compared with single-line Diagram B in Fig. 1, the only difference is the omission of the DS on the bus bar side of line CB, because in the case of failure or major overhaul, the affected single-phase modulus is replaced with a spare unit. All the components are combined in a compact SF6-insulated, completely pre-assembled enclosure. Single-phase, SF6-insulated, non-compartmentalized enclosures are acceptable, because in event of an internal failure, one complete phase is replaced with a spare unit, and the faulty unit is returned for repair. The three sets of 170-kV bushings can be polymeric, forming the interface with the incoming and outgoing line and transformer. CBs, DSs, current transformers, voltage transformers and grounding switches are combined in a compact SF6-preassembled enclosure. Conventional air-insulated surge arresters are connected to the transformer HV terminals.

The Y switchgear includes remotely controlled motor operated DSs with built-in earthing switches and an interlocking system assures reliable operation. This switchgear is particularly suitable when underground cables supply the HV/MV substation. In this case, the HV air-SF6 bushings are replaced by SF6/cable and SF6/transformer interfaces. The Y2 hybrid switchgear is also installed when retrofitting existing substations.

MV Switchgear

A new design (ENEL patent) of air-insulated compact MV switchgear, offering a 68% reduction in volume compared to conventional air-insulated switchgear, is applied. Therefore, the complete MV switchboard can be factory-assembled and housed in a container, 2.75 m (9 ft) high, 2.5 m (8.2 ft) wide, with a maximum length of 12 m (39 ft).

HV/MV Power Transformer

A new design of a simplified power transformer is now available. The changes in design features compared with the traditional transformers are as follows:

Rated Power
Traditional Design (MVA) New Design (MVA)
ONAN ONAF ONAN ONAF
16 20 13 16
25 31.5 20 25
40 50 32 40
63 78 50 63

The new design has the same power losses, the electromagnetic design of the active parts is unchanged and the cooling system capacity is reduced.

Rated Secondary Voltage
Traditional Design (kV) New Design (KV)
10.4 10.8
15.6 16.2
20.8 21.6

The secondary voltage values for the new transformer has an improved voltage regulation range changing from a ±12 - 1.5% step to a ±8 - 1.5% on-load, thereby reducing the probability of the transformer over-excitation — an improvement that affords a substantial reduction of the acoustic noise in service.

MV Winding Neutral Connection

The MV winding of the new transformer is designed for neutral grounding via a Petersen coil. The values specified for the neutral current ground connection (1 min rating) are:

Rated Power (MVA) Neutral current (A)
16 300
25 450
40-63 600

The neutral connection conductor has the same cross-section as the phase conductor.

Neutral Insulation Level

Based on the specific over-voltages experienced on ENEL's distribution network resulting from single phase-to-ground faults and the surge arresters protection levels, a value of 123 kV is specified as the neutral rated insulation level compared with the present level of 145-170 kV. This value is also specified for the transformer on-load tap changer.

On-Load Tap Changer

Reducing the number of tap-changer steps leads to a voltage regulation by “substitution” instead of “course and fine,” reducing the number of required regulation windings from two to one. A unified tap changer can then be specified for all power ratings.

Bushings

The unified value of 170 kV is specified as the transformer bushings rated voltage (275-kV PFWL and 650-kV LIWL). The unified flange dimension eases bushing substitution for oil-air, oil-oil or oil-SF6 bushings.

Scheme for User Connections

Several schemes can be implemented using the simplified HV/MV substation with a single-power transformer. The Y HV switchgear consists of a combination of one (Y1) or two (Y2) CBs, and one or two remotely controlled disconnectors, respectively, according to the needs of network operability and the MV load location (Fig. 4).

For substations equipped with two power transformers, there are several options available to implement a typical H scheme. Each transformer can be fed via a single HV line or two separate HV lines (Fig. 5).

Expected Benefits and Developments

The use of HV/MV simplified substations installed between two existing substations will gradually lead to the distribution system evolving into a modular configuration as follows: ENEL plans to substantially increase the number of new simplified HV/MV stations equipped with a single transformer. Within five years, it is anticipated that 20% of the HV/MV substations in commission will be single-transformer substations. The length of existing MV circuits will be reduced by 40% to 50%, and they will double in number, reducing the voltage drop and losses on the MV network.

The majority of MV distribution networks in Italy operate with an ungrounded neutral. As the maximum ground fault current is reduced in proportion to line length, the probability of the self-extinction of phase-to-ground transient faults in overhead lines will increase.

The new design philosophy will increase the peak load utilization of HV/MV transformer capacity from 50% to 75% at times of peak load. Although the overall length of the MV network will remain almost unchanged, the effective power-transfer capacity will be doubled.

The proliferation of MV lines in close proximity to HV/MV stations will diminish as the new HV/MV substations will supply a maximum of 10 MV circuits. The use of remotely controlled load disconnectors will substantially reduce voltage dips and the frequency and duration of consumer interruptions.

Already, ENEL has installed five simplified substations, and it plans to install another 50 within a year — a few of which are underground installations.

Vincenzo Colloca has a degree in electrical engineering from Polytechnic of Turin. At ENEL Distribution S.p.A., he was the manager responsible of the unification of HV, MV and LV electric power components.

Enrico Colombo has a degree in physics from the University of Milan and is with CESI. He managed the project “Technological Innovation of the Components for Electrical Networks” and was responsible for the group “Switching Equipment and Surge Arresters” at ENEL S.p.A.'s Electrical Research Centre.

Network Configuration and Operational Model

HV subtransmission networks are operated at 132 kV (Northern Italy) or 150 kV (Central-Southern Italy), while the majority of the MV networks operate at 20 kV. Until recently, most of the HV/MV distribution stations were equipped with two HV/MV transformers rated at 2 × 25 MVA or 2 × 40 MVA ONAN (2 × 31.5 MVA or 2 × 50 MVA ONAF) and, in some cases, with 2 × 16 MVA, 2 × 63 MVA or 2 × 100 MVA transformers.

The non-metro substations were designed and built with outdoor HV switchyards (air-insulated), and MV switchgear in buildings. The HV SF6-insulated stations (GIS), and sometimes 20-kV SF6-insulated switchgear with fixed CBs, have been used for the metro substations.

MV networks are designed with radial feeders originating at one HV/MV substation and terminating at an adjacent HV/MV substation, each feeder supplying several MV/LV transformer substations. Network sectionalizing is provided at intermediate MV/LV substations. In the event of a fault in a MV line or unavailability of a HV/MV station, the sectionalizing points are relocated and the entire feeder is supplied from one substation. The installation of two HV/MV transformers per substation provides 100% redundancy in normal operating conditions, based on the requirement for rapid supply restoration to all the MV lines.