The city of Nanning, located in China's Guangxi Province, has an aging medium-voltage (MV) distribution network that is no longer able to meet the increasing demand for energy with reliability standards 21st century customers expect.
The traditional 10-kV network, designed on a radial-feeder basis, is no longer cost effective or efficient. Therefore, to realize the growth in the gross domestic product (GDP), in accordance with State Council forecasts, a major program of distribution network redesign and construction is now an urgent task for the Nanning Electric Power Supply Bureau (NEPSB).
Following the reform and opening of Nanning City, NEPSB produced a development plan for the 110-kV system. In the past few years, investment in system capacity and equipment standardization safety and reliability standards have reached a satisfactory level.
Typically, the city's 110/10-kV substations are supplied via two 110-kV lines equipped with 110-kV gas-insulated substations (GIS), and two 40-MVA 110/10-kV transformers that supply 20 outgoing 10-kV feeders. The redesign and upgrading of the 10-kV network forms the second phase of this investment program.
Distribution Network Problems
In the past, the design and development of Nanning's MV distribution network was limited by the available power capacity. This network now exhibits several major problems that Nanning must address to fulfill the expectations of the State Council, NESPB and customers. The 10-kV network, which comprises 1500 km (932 miles) of overhead line and 300 km (186 miles) of underground cable, now faces the following problems:
The capacity of the network is inadequate for the customer base with too many 10-kV radial-distribution lines. Customers with supply agreements for 2 MVA often are supplied via a single-circuit line with no alternative source of power in the event of a network fault. Also, the design of the high-voltage (HV) system and HV substations are such that in the event of a substation fault or circuit outage for maintenance, many customers experience long-duration power outages. Similarly, the design of the MV network results in outages for operation and maintenance work.
Many 10-kV feeders supply a single customer with a demand of 100 kVA. This form of supply is not economical, the customer has an unreliable source of supply and there are more 10-kV feeders to control from the city center 110-kV substations.
MV customers often install circuit breakers on their premises, thus the distribution substations supplying the customer increase in cost and complexity because the 10-kV feeders require secondary protection. The additional equipment is similar to that installed in a 110-kV substation that can increase the cost of a 10-kV substation by 10 million Yuan (US$1.2 million). Furthermore, there is more equipment to maintain, more prearranged outages and no improvement in supply reliability.
In the urban area, the MV overhead lines limit the power transfer, are a hazard to citizen safety and have an adverse visual impact on the environment.
NEPSB has a finite limit on the capital available to restructure the MV network, and the decision-making process became more complex as the large power customers constructed sections of the network. However, the utility acknowledged the need to establish a long-term development plan to improve the reliability, efficiency and operational safety of this network, while simultaneously increasing the capacity of the network to satisfy future load growth.
10-kV Network Development — Alternative Designs
The radial-feeder MV network is unacceptable to supply major cities. Internationally, ring main supply systems are commonly used in substations equipped with circuit-supply ring main units that comprise load-break switches, fused switches and circuit breakers. NEPSB now installs cross-linked polyethylene (XLPE)-insulated power cables that have a high reliability and a low fault rate. As the length of the XLPE feeders installed is short, standard ring main units (with load-break isolators and fuse switches for protection of the local 10-kV low-voltage (LV) transformer isolation and protection) are in use on NEPSB's 10-kV network. Circuit breakers and secondary protection systems are not used, thus improving the network with a modest increase in equipment costs.
The structure of networks based on the use of ring main units varies and each major city adopts a different approach. As part of the development program studies, NEPSB examined the design of Hong Kong's MV network. The network supplying this large city has three different design features that are dependent on the characteristics and type of loads supplied:
Single-ring circuit-supply network with an open point where the underground cable connects to the overhead line network.
Complete ring supply for the underground cable network operated with a midsection open point.
Multi-ring circuits on the underground cable system that operate closed with the benefit of unit protection to isolate a faulty section in the event of a fault.
Nanning considered the application of these three design alternatives for its urban power network.
In the single-ring circuit-supply network with an open point for overhead lines, several of the substations are equipped with automatic control features, and the in-line overhead tie switch or line isolator provides the normal open point for the ring.
The 10-kV substations are a combination of teed substations for smaller customers and standard ring main units for the larger capacity substations. The two power sources supplying the ring come from different bus bars located in one or two 110-kV substations. The units equipped with automation facilities and the source substation protection isolate and disconnect the faulty network section, rapidly restoring supply to the majority of consumers. This supply method has been adapted for suburban areas.
In the open-ring principle, all distribution substations equipped with standard ring main units are ring-connected to the 10-kV cable networks, which are supplied from two 110-kV substations. This underground cable design requires the thermal capacity of each section to supply the total load on the ring under all operating conditions in accordance with the n-1 principle.
This method of supply offers high reliability, as under normal operating conditions, the cables are not loaded beyond 50% thermal rating. However, the cost of installing underground cables is high; therefore, this method of supply is uneconomical and inappropriate for Nanning City.
Multi-ring or interconnected underground cable networks have open points that, in the event of a network fault, offer several alternative sources of supply. On these networks, the use of cable capacity is increased together with the system reliability as prearranged outages for plant maintenance can be managed without interrupting supplies to customers.
Interconnected networks prevalent in areas of high load density result in cable utilization values of 67% to 83%, thereby optimizing the return on the network investment. The interconnected network satisfies the n-1 principle for the 10-kV network and also for the 10-kV bus bars in 110-kV substations. NEPSB assess the improved reliability of the XLPE cables at 99.99%. Also, the utility equips each ring main unit with an earth-fault current-passage indicator, simplifying fault location and customer restoration.
The high-reliability interconnected 10-kV networks offer is not only suitable but more customers demand it. Thus, NEPSB decided to apply these design principles in the upgrading of Nanning's 10-kV network.
The design study team recommended the adoption of “TT” or “TTT” connections to different 110-kV substations for customers whose industrial or business processes cannot accept supply interruptions greater than 30 seconds. In practice, the customer is expected to install standby generation or a UPS system. While it adds to the overall cost of the supply connection, it is still more economical and reliable than the single customer-specific circuits installed in the past that are now only considered in exceptional cases.
NESPB's 10-kV Network Development Plan
NESPB's 110-kV substations accommodate two 110/10-kV transformers operating with a split 110-kV linked via a bus coupler. Similarly, the 10-kV bus bars are designed in two sections linked via a bus section circuit breaker. The first stage of the development plan was to identify positions on the existing 10-kV network that are required to establish switching stations to provide the desired level of interconnection between the 110/10-kV substations. Modern ring main units are compact, so it is not difficult to find switching sites for switching stations in city areas.
The number of distribution stations looped into the 10-kV cable depends on the thermal capacity of the cable and the customer's power demand. In general, the rating of each outgoing 10-kV feeder is limited to 8 MVA the continuous rating of a 240 mm2 cable. Distribution substations with capacities in excess of 630 kVA will be looped into the feeder and controlled by ring main units; smaller commercial customers are supplied from public distribution substations.
Strict operational control procedures are required to obtain the benefits from the interconnected 10-kV network, and NEPSB operates all load-break switches to minimize the risk of customer maloperations. In general, ring main units are not fitted with remote-control devices and, in view of the large number currently in service, the cost to provide these facilities would be prohibitive for the marginal improvement in overall system reliability.
NEPSB's new policy increases the level of interconnection on the 10-kV network and takes advantage of new technologies to improve reliability. It gives attention specifically aimed at the underground cable networks where the location of faults can be time consuming, compared to that for overhead line circuits. NEPSB estimates the three-year program to upgrade the 10-kV network will cost about 200 million Yuan (US$24 million).
As a result of the 10-kV interconnection, Nanning is considering redesign opportunities for the 110-kV system, together with improvements in the operational features of 110/10-kV substations, such as removing the 110-kV bus coupler. Over the long term, reducing the number of 10-kV circuit breakers installed will reduce the site area required for a 110/10-kV substation. In the future, NEPSB should find it easier to find sites for additional substations in Nanning City's densely populated areas.
Yan Jiajin graduated from Shanghai Jiaotong University in 1984 and joined the Nanning Electric Power Supply Bureau where he is now a senior engineer in the Design Department.