The demand for a no-break power supply has increased significantly in the past few years as more and more industrial customers operate sensitive loads such as data centers, banks and telecommunications. The standard solution for these customers is the in-house installation of an uninterruptible power supply (UPS) system in combination with a diesel generator to overcome the longer supply interruptions. This solution introduces several disadvantages, including the need for complex control systems, the cost of repair and maintenance, the additional space required for the equipment and complying with emission requirements.

In areas with a high load and high density of customers such as business centers, a utility network often can be designed to offer an alternative source of power. Even though the reliability of supply in these areas is likely to be exceptional, the customer demand for redundant connections is on the rise. Faced with this problem, Elektrizitätswerk der Stadt Zürich (ewz) is developing an attractive solution with an independent secondary network that could be expanded into a smart network.

Standard Security of Supply

The Zürich distribution network is connected to the national grid by four 220/150-kV substations. A 150-kV subtransmission system supplies 15 150/22-kV substations. The majority of the 150/22-kV substations benefit from being connected to at least three circuits, each with sufficient capacity to supply the substation's maximum demand. This level of redundancy ensures the 150-kV network does not contribute to system outages. Instead, system fault outages occur largely because of substation equipment failures on the lower-voltage systems.

The 22-kV system is designed to operate with closed radial loops that have an average feeder length of 5 km (3.11 miles), each of which supplies eight to 10 22-kV low-voltage transformer substations. These, in turn, supply the 10 closed-loop low-voltage feeders that supply energy to an area with a radius of 250 m (820 ft).

To determine the reliability indices for customers supplied by 22 kV and 0.4 kV, ewz developed a tool based on a block diagram technique adapted to a simple network topology. Using the tool, different power supply solutions can be evaluated. The tool calculates overall outage rates, conveyed in times, based on the same parameters used for predefined components such as transformers, busbars, cables and UPS batteries.

The average interruption frequency for ewz customers supplied by 22 kV is 0.08 per annum, or about one interruption every 12.5 years with a duration of 45 minutes. For customers supplied by 0.4 kV, an interruption can be expected every six years with a mean duration of 90 minutes, or 0.168 per annum.

Second or Alternative Source of Supply

To increase system reliability, a second or alternative power supply from a utility network is a sound solution that can be relatively cost effective in areas with a high density of customers. This other supply is often achieved through connection to a transformer substation supplied by an alternative medium-voltage loop. However, the best solution is when the two loops are supplied by different medium-voltage/low-voltage substations.

For a medium-voltage supply, although the outage rate remains unchanged (0.08 per annum), the outage time is reduced to the period required to switch the load to the healthy second circuit. Depending on circumstances, this period varies from a few seconds to minutes. In these situations, rotating a mass energy-storage system rather than batteries is a good alternative for high-load, short-duration interruptions.

Provision of Additional Networks

The availability of a second or alternative power supply can be embedded in the main distribution network. This arrangement involves additional operational demands and administration costs because of the need to plan switching operations and prearrange power reserves. These costs are reduced when the redundant power supply is connected to a separate medium-voltage network

In 2007, ewz began a pilot project of planning such an independent secondary medium-voltage network in the city center of Zürich, where growing demand for a redundant power supply is expected. The network plan consisted of two circuits, each connected to adjacent substations. The transformer substations on the route of each circuit are ring-connected to the circuit. The plan also called for additional cables to be installed in existing empty cable ducts, so the secondary network could be established in a cost-effective way.

The network is supplied by a reserve transformer in the 150/22-kV substation and can automatically switch to a 150/22-kV transformer in an adjacent substation, which means the redundant circuit components are better utilized. The use of a 22-kV medium-voltage network is limited by the cross-sectional area of the 150-sq mm (0.23-sq inch) cable to 9 MVA for each circuit. Both circuits are supplied by the reserve transformer in substation 1, with the possibility to change the supply through automatic switching to substation 2. This may be necessary for maintenance reasons or in the event of a primary busbar or transformer fault in substation 1.

It is possible to allocate loads up to double the current-carrying capacity, namely 18 MVA to one circuit. If half of the customer load, or 9 MVA, is supplied through the standard grid substation 1 and the other half by substation 2, in the event an outage affects all the customers supplied by one substation, the reserve transformer in the healthy substation would supply the remaining 9 MVA. The probability of a simultaneous fault occurring in both circuits is so unlikely it can be neglected.

Application of the Additional Networks

To date, ewz has offered the potential for an improved level of supply reliability to 13 customers (six medium voltage and seven low voltage) with a total load of 9.9 MVA. These potential users would allow cost-efficient operation of the network, as the average cost for a secondary grid connection without a UPS is 200 euros/kVA (US$272/kVA).

If a UPS is required, then buffering is needed when connected to a secondary network. The use of an additional feeder is an alternative to emergency generation in the form of one or more diesel generators. If a customer who is also connected to the secondary network requires a UPS, then the customer is required to bridge the duration of the interruption caused by switching from one network to the other with batteries, flywheels or other means.

This network design practice does not eradicate the risk of a complete system blackout that could follow load shedding should the European Union for the Coordination of Electricity Transmission (UCTE) network suffer from low frequency.

Table 1 shows the reliability key parameters for a medium-voltage customer with four different supply arrangements: a standard connection; additionally using a UPS with a diesel generator; a secondary power supply connection; and a secondary network connection. The results are shown for two situations, without a risk of blackout and with a blackout risk. Additionally, for cases 2, 3 and 4, values are calculated with and without the buffering of a UPS (values shown in brackets in the table). As no statistical value can be assigned to the blackout frequency, the trend cannot be determined. Therefore, a blackout rate of once every 20 years with a mean duration of 2 hours has been assumed.

Table 1 shows that the blackout risk for a medium-voltage customer with a standard network connection is an interruption every 7.5 years (H = 0.13) with a mean duration of 72 minutes (T = 1.216 hour). For cases 2 and 3, the supply reliability is the same with or without a blackout risk, respectively. One interruption every 111 years (H = 0.009) for a period of 5 minutes (T = 0.080) is the case without a blackout risk, and one interruption every 21 years (H = 0.046) for a period of 2 hours (T = 2) with a blackout risk.

Integrating Backup Generation in the Secondary Network

To minimize the blackout risk for those customers who do not wish to invest in the purchase and operation of diesel generators, larger backup generators may be integrated into the secondary network. The first choice for the positioning of these backup generators is installation within the 150/22-kV substations. This outsourcing solution then effectively transfers the financial responsibility of distributed generation to the utility, and the customer then has a similar supply reliability (variant 4 in Table 1) as in variant 2.

Cost-benefit studies by ewz that considered the various alternative solutions available to medium-voltage customers to increase the supply reliability were based on a standard load of 1000 kVA for customers with and without self-contained power supply. For those situations where a secondary network is installed, the total cost is shared by the number of potential customers.

The average estimated cost of an interruption to customers in the commercial sector is 48,000 euros (US$65,300) per 1000-kVA load disconnected. Potential customers for secondary network connections include banks and insurance companies where a single interruption can cost some 100,000 euros (US$136, 500).

Table 2 shows the estimated capital investment costs and the estimated annual expenditure incurred by medium-voltage customers opting to increase the supply reliability or minimize the risk of blackouts for each of the four alternative solutions presented in Table 1.

When backup generation is not installed as in variants 2 and 3 in Table 1, investment costs for battery storage become substantially higher because of the need for a longer discharge time. For example, the customer requires time to shut down the computer systems. This overcompensates for the additional costs for the backup generator in the variant 4.

Distributed Generation and Embedded Storage Devices

The installation of a secondary network offers the opportunity to integrate distributed generation into that network instead of a standard network. This is advantageous because the power-quality problems are limited to the secondary network, operation of the standard network remains unchanged and distributed generation may serve as the backup power source in a blackout situation.

However, keeping distributed generation off the standard network has a potential drawback in that it creates additional system losses. As the main load is still connected to the standard network, the power flow from the distributed energy sources is transformed twice in the substations.

Uncontrollable energy in-feeds connected to the secondary network, namely combined heat and power and the use of nonlinear devices such as semiconductors, can be absorbed by storage devices. Included in the range of storage devices ewz is considering are batteries and supercapacitors, which are able to offer smoothing characteristics during peak load periods.

The specifications for storage systems differ from those used by UPS systems, as the batteries considered for integration into a secondary network would be subject to more onerous charging and discharging cycles that would adversely affect the reliability and operational life.

Improving Customer Service

Elektrizitätswerk der Stadt Zürich started researching an independent secondary network in the Zürich city center in 2007 and has developed several possible solutions to improve the supply reliability to key medium-voltage customers. The project has now reached the stage where the utility is able to improve customer service by giving its medium-voltage customers the option to select an alternative solution to achieve an uninterrupted power supply. The integration of embedded generation and storage devices remains subject to further research and technical analysis for the future.

Table 1. Reliability indices for different power supply connections for a medium-voltage customer.
Medium-voltage power supply connection Reliability indices Power supply reliability

Without blackout risk With blackout risk H = 0.05 events/annum T = 2 hours/annum
Case 0
Connection to standard network (without UPS)
H (events/annum) 0.080 0.130
T (hours) 0.727 1.216
P (hours/annum) 0.058 0.158
Case 1
Self-contained power supply with UPS
H (events/annum) 0.010 0.016
T (hours) 0.034 0.034
P (hours/annum) 0.0003 0.0005
Case 2
Second power supply to another substation
H (events/annum) 0.0090 [0.080*] 0.046 [0.130*]
T (hours) 0.0800 [0.080*] 2.000 [0.817*]
P (hours/annum) 0.0007 [0.006*] 0.092 [0.101*]
Case 3
Secondary network
H (events/annum) 0.0090 [0.080*] 0.046 [0.130*]
T (hours) 0.0800 [0.080*] 2.000[0.817*]
P (hours/annum) 0.0007 [0.006*] 0.092 [0.101*]
Case 4
Secondary network with backup generator
H (events/annum) 0.0170 [0.080*] 0.024 [0.130*]
T (hours) 0.0070 [0.067*] 0.056 [0.056*]
P (hours/annum) 0.0010 [0.005*] 0.001 [0.007*]
* Values in brackets are the power reliability indices for customer installations without UPS.
Table 2. Investment and annual costs for improving supply reliability.
Expenditure UPS Secondary power supply connection Secondary network without backup generation Connection to the secondary network
Investment cost, euros/kVA 450 650 450 450
Investment cost, US$/kVA 614 887 614 614
Annual revenue cost, euros/kVA 62 58 56 53
Annual revenue cost, US$/kVA 85 79 76 72

Juerg Dieter Bader (juerg.bader@ewz.ch), Ph.D., studied mathematics and physics at the Federal Institute of Technology Zürich and earned his doctorate in modeling atmospheric processes. Bader joined ewz in 1995 and now works in the network design department, where his responsibilities include developing future networks.

Hansruedi Luternauer (hansruedi.luternauer@ewz.ch), El. Eng. HTL, studied electrotechnology at the FH Muttenz and joined ewz in 1985, where he is now responsible for the network design department.

Lukas Kueng (lukas.kueng@ewz.ch), Ph.D., was awarded his doctorate in electric engineering at the Federal Institute of Technology Zürich for his work on developing an electric machine for hybrid vehicles. Kueng joined ewz in 2001 and is responsible for the distribution network.

Companies mentioned:

Elektrizitätswerk der Stadt Zürich www.ewz.ch

Union for the Coordination of Transmission of Electricity www.ucte.org