A NEW TRANSMISSION LINE TO INTERCONNECT THE POWER SYSTEMS OF ZAMBIA AND TANZANIA has been under consideration since 1995, when the first feasibility study was undertaken by the technical teams of the national utilities of Zambia (ZESCO), Tanzania (TANESCO) and South Africa (ESKOM). Although the study was firmed up in 1998, recent studies on another transmission line interconnecting the power systems of Tanzania and Kenya have given fresh impetus to the Zambia — Tanzania interconnector project. This development creates a real prospect that would allow Tanzania and Kenya to participate in the Southern African power market.

The future connection to Kenya has prompted the need to update the Zambia-Tanzania interconnector study, including looking at necessary reinforcement of the Tanzanian high-voltage (HV) network to support power flows to Kenya. This latest study on the long-planned interconnection, which was started in June 2003, will add another milestone to the structure of the Southern African Power Pool (SAPP).

Given the size of this project and the amount of money needed to be raised for it — more than US$1 billion — the study is being carried out in two phases. Phase 1 (the subject of this article) is purely technical and economic, and Phase 2 focuses on the financing of the project and its implementation, necessarily spread over several years in both Zambia and Tanzania. Given the objectives of the Office for the Promotion of Private Power Investment (OPPPI), at least part of the project will be open to private financing. Obviously, reaching a financial agreement for such a large project will take some time. Notwithstanding the time factor, it is highly desirable to update and optimize the technical side of the project.

THE SOUTHERN AFRICAN POWER POOL

SAPP was established in 1995 when representatives of the power utilities of Botswana, Mozambique, Angola, South Africa, Malawi, Swaziland, Namibia, Zaire (now the Democratic Republic of Congo), Tanzania and Zimbabwe signed the Inter-Utility Memorandum of Understanding. The purpose of SAPP is to provide reliable and economic electric power supply to customers of each of the member countries, and to that effect, it has introduced a framework to achieve coordination of system planning and operation throughout all member countries (Fig. 1). Among the expected benefits is a reduction in overall generating capacity, a reduction in fuel costs, an improved use of hydropower, and a reliable and stable power supply.

So far, the limitations of the transmission networks between the various countries and within the SAPP countries themselves have constituted a constraint on energy trading among SAPP countries. Therefore, the Zambia — Tanzania interconnector is a significant move toward the declared goal of SAPP, which is to enhance energy development and trading.

REGIONAL RESOURCES

In Zambia, the bulk of the generation is from hydropower (1608 MW), generated by three main power stations, namely Kafue Gorge (900 MW), Kariba North Bank (600 MW) and Victoria Falls (108 MW). These three plants are sufficient to support the power demand of Zambia and have some spare capacity available for export. Recently, the decision has been made to rehabilitate these plants and upgrade the generating capacity of the units. There is also the possibility to increase by two the number of units installed in the Kariba North plant.

In addition to these capacity extensions in progress, there are plans in Zambia to build two hydropower plants on the Kafue River to be located at Itezhi-Tezhi dam (120 MW) and Kafue Gorge Lower (600 MW). Following the completion of all these projects, Zambia will clearly have surplus power available for export for several years.

On the wider regional level, the most significant hydropower potential is located in the neighboring Democratic Republic of Congo (DRC). The largest hydropower station in DCR is Inga Dam (1774 MW of installed capacity) near Kinshasa on the lower Congo River. When fully developed, the Inga site will have an estimated total installed capacity of about 39,000 MW, with the total hydropower potential at this location being some 100,000 MW.

The peak demand in Zambia is expected to grow from 1300 MW in 2004 to 2245 MW in 2025. The table gives an idea of the order of magnitude of the power available for export from Zambia. It reveals the power surplus/deficit situation in Zambia up to 2025. The table also shows that a minimum of 308 MW and a maximum power of 841 MW will be available during the off-peak (70% peak load) periods in 2007. Imports from DRC combined with the local surplus will result in a considerable amount of available power for exports.

SELECTION OF TRANSMISSION ALTERNATIVES

The updated regional power study and Zambia's power surplus/deficit situation led to the selection of 400 MW as the new power-transfer level for the interconnector. Based on this level of power to be transmitted and the length of the interconnector, the next step was to determine the optimal voltage of the line. Five transmission alternatives were considered:

• 220 kV
• 330 kV
• 400 kV
• High-voltage direct current (HVDC)
• Back-to-back HVDC.

Interconnection at 220 kV and 330 kV were considered first, as these two voltages are used for the existing transmission systems in Tanzania (220 kV) and Zambia (330 kV). Due to the long distances involved, it was also considered logical to study the interconnection at 400 kV as well. This was the voltage used for the South Africa-Botswana-Zimbabwe and South Africa-Namibia interconnections. Whenever power has to be transmitted over a distance of 600 km or more, HVDC technology is a potential candidate. Therefore, schemes employing HVDC and HVDC back-to-back technologies were also evaluated.

Before starting power-system simulations, a preliminary screening process was used to evaluate and reject any unsuitable alternatives. Because the route length of the proposed Zambia-Tanzania interconnector is approximately 700 km (435 miles), this distance was considered too long for a 220-kV transmission line needed to carry an ultimate power of about 400 MW. The power losses on this long 220-kV circuit would be high and voltage regulation difficult; hence, this alternative was not evaluated.

Similarly, the back-to-back HVDC alternative did not present any clear benefit over the other alternatives. Back-to-back HVDC schemes are used when there is a problem of the interconnection of two systems due to large-phase angle differences, but with the Zambia-Tanzania interconnection this is not a problem. The back-to-back scheme would be more expensive than the remaining alternatives; thus, this alternative was not examined in detail either.

Therefore, the power flow studies were limited to the remaining three transmission alternatives to check technical feasibility. The final alternatives were then cost-estimated using preliminary cost estimates of 25%. As expected, the HVDC alternative was the most capital-intensive scheme and the 330-kV scheme the most economic. System losses were lowest for the 400-kV transmission scheme. Using a 12% discount rate, loss-load factor of 0.8 and a cost of purchase of energy of US$0.025/kWh in Zambia (a figure seriously contemplated for this project), the present-worth analysis revealed that a 330-kV transmission line would be the most cost-effective.

330-KV TRANSMISSION LINE STUDIES

The selected 330-kV scheme was simulated for the years 2007, 2012, 2017 and 2025. A 330-kV double-circuit transmission line was proposed from the Pensulo Substation in Zambia to the Mbeya Substation in Tanzania. This line would be routed through the Kasama Substation in Zambia to supply the remote loads in the northeastern parts of Zambia and to provide much-needed voltage support to the region. For the 200-MW power transfer stage in 2007, only one circuit would be strung on the double-circuit tower. By 2012, when the power to be transferred is expected to increase to 400 MW, a second circuit would be installed.

Available firm capacity in Zambia.

Year	Generation Capacity (MW)	Total Export (MW)	Zambia Peak Demand (MW)	Zambia Spin. Res. Cir. (MW)	For Tan Exp (MW)	For Tan Exp PD=70% (MW)	Firm Cap* For Tan Exp @PD=100% (MW)	Firm Cap for Tan Exp @PD=70% (MW)
2007	1932	290	1372	180	90	501	-104	308
2008	2052	290	1428	180	154	582	-51	377
2009	2052	290	1465	180	117	556	-88	351
2010	2652	290	1537	180	645	1106	338	812
2011	2652	290	1578	180	603	1077	338	812
2012	2652	290	1622	180	560	1047	295	781
2013	2652	290	1667	180	515	1015	249	750
2014	2652	290	1693	180	489	997	223	731
2015	2652	290	1697	180	485	994	220	729
2016	2652	290	1703	180	479	990	214	725
2017	2652	290	1748	180	434	958	168	693
2018	2652	290	1806	180	376	918	111	653
2019	2652	290	1866	180	316	876	51	611
2020	2652	290	1929	180	253	832	-12	566
2021	2652	290	1992	180	190	787	-75	522
2022	2652	290	2055	180	127	743	-139	478
2023	2652	290	2118	180	63	699	-202	434
2024	2652	290	2182	180	0	655	-265	390
2025	2652	290	2245	180	-63	610	-328	345
* An export of 250 MW to ESKOM has been considered for this supply-demand analysis. This export may be considerably reduced during peak load to cater to ZESCO's own power requirements.

The 330-kV developments within Tanzania were also studied, since transmitting power to the northern load centers and onward to Kenya requires reinforcement of the Tanzanian network. At 330 kV, lower losses are incurred compared to the 220-kV system. Therefore, staged-voltage upgrading of the 220-kV network in Tanzania was also proposed to realistically meet the power-transfer requirements. Up to the year 2017, transfer trip of the Zambia — Tanzania interconnector was recommended for any outage of 330-kV line within Tanzania. This would result in a power deficiency in the Tanzania and Kenya system, which would undergo underfrequency load shedding to arrest the system-frequency decline. In the case of tripping of the interconnector while carrying 400 MW from Zambia to Tanzania, the system frequency in the combined Tanzania-Kenya systems drops down to 49.3 Hz before it is arrested by the underfrequency load-shedding scheme.

Static var compensators (SVCs) would be necessary at the Mbeya and Singida substations in Tanzania to provide voltage support for proper operation of the system. These SVCs would have to be designed to damp the system oscillations during disturbances as well.

Transient stability studies revealed that the rotor angle and voltage oscillations appear after three-phase or single-phase disturbances on the Zambian or Tanzanian systems. In some cases, the oscillations continue for the duration of the simulation period and beyond. This phenomenon is not uncommon where weak systems are connected with long tie lines. One of the most effective and economic methods to address these oscillations is to install power-system stabilizers on all fast exciters in the system. These stabilizers need to be properly tuned to effectively dampen the interarea oscillations.

330-KV TRANSMISSION LINE CHARACTERISTICS

The basic design of the line was considered in determining the tower type for double-circuit 330 kV, its outline and main dimensions, the conductor type and bundle arrangement and the basic tower span.

Conductors: The conductor costs, especially for a double-circuit bundle arrangement, represent a significant proportion of the total transmission line's capital investment. In addition, the conductors' mechanical characteristics have a considerable influence on the tower design and spacing, thereby further affecting the project's total capital cost. Therefore, particular care must be given to the conductor selection.

The first criterion is the conductors' current-carrying capacity or circuit ampacity. However, for long lines like the Zambia-Tanzania interconnector, this is not normally a major issue. In this case, the focus was on the following issues:

• Optimization of losses on the line

• Minimization of corona losses

• Optimization of mechanical characteristics

• Stability issues and power transfer capacity.

The following classical conductors were selected because they satisfy the ampacity criterion:

• ACSR BISON to BSS 215 part 2

• ACSR TERN to ASTM B232

• ACSR RAIL to ASTM B232

• ACSR BLUEJAY to ASTM B232

Also, the following innovative AAAC conductors were also considered:

• AAAC AERO-Z 455 to NBN C 34-100

• AAAC AERO-Z 504 to NBN C 34-100

The AERO-Z conductors are recent additions to the range of overhead-line conductors, and they offer a combination of good electrical and mechanical characteristics provided by the aluminium-alloy material and an innovative design of the conductor strands, which are interweaved. Improved thermal heat dissipation, high mechanical breakdown values and good corona withstand characteristics are therefore offered by these conductors.

Calculations made using the characteristics and manufacturers' cost estimates for all of the conductors under consideration showed that either the BISON or AERO-Z 455 offer the best cost and characteristics compromise for the interconnector.

The BISON is a standard conductor for both ZESCO and TANESCO and could be a good choice. However, the all-aluminum-alloy AERO-Z conductor offers some excellent electrical (lower resistance) and mechanical (higher breaking strength) properties, which could make it the best conductor for this project by providing the lowest overall cost for the line. More detailed studies will be required to select the final conductors, based on the cost of aluminum on the world market at the time of project implementation.

Insulator strings: The transmission line will be operating in a part of Africa that has a high isokeraunic level. Yet this line must have the highest level of reliability that can be economically achieved. These objectives can be realized by adequate tower design, low tower footing resistance and the selection of high-quality insulator strings. Therefore, the process of insulator selection and string-length determination is also a high-priority exercise.

This interconnector should be designed to minimize circuit tripping or outages, especially during the first years of operation when only one circuit will be in operation. This is because of the effect that tripping would have on the system stability of the Zambian and Tanzanian transmission systems and the problems that would occur when the system operator has to resynchronize both systems. This issue is so important that a double-circuit line is the best option for this project, and the lightning-withstand voltage of the line is so important that utmost care will have to be given to reduce the probability of the line's outage rate to the lowest level that can be economically achieved.

Because high levels of pollution and humidity will prevail along the length of the circuit, it is necessary to use insulators that are fairly immune to puncture risks and are designed to avoid the formation of any conductive paths on the insulator surface to minimize the risk of flashover, whatever the pollution level. Therefore, it was decided to specify toughened-glass insulators for this circuit in spite of the slightly higher costs.

Tower outline: The tower outline selected for this double-circuit 330-kV transmission interconnector is shown in Fig. 2.

The tower outline will use an OPGW conductor as one of the guard wires to provide communications facilities between Zambia and Tanzania, in addition to the necessary communications channels required for line and systems operation.

THE TANZANIAN NETWORK REINFORCEMENT

To accommodate the incoming power on the Zambia — Tanzania interconnector and with the flow of part of this power to Kenya, some significant reinforcement will be required on the Tanzanian HV network.

The major characteristics of these reinforcements, including increasing line voltages, interconnecting nodes and substation requirements, have been selected, optimized and designed in this study. It is also probable that the type of conductors used could be either the BISON or AERO-Z, as for the interconnector. The tower outlines selected for Tanzania's HV network are shown in Fig. 3.

REINFORCEMENT BENEFITS

The major benefit of this project is economical because this interconnector will minimize overall investments in power generation and transmission. In the case of Zambia, it will provide a significant income from the surplus power that is available. Other impacts of the interconnection on Zambia include:

• The Zambia — Tanzania interconnector will facilitate the transfer of power available in Zambia and also from other SAPP countries, to Tanzania and Kenya, thus reducing the overall cost of system operation in these countries. This will benefit both Zambia and Tanzania. Zambia will derive a significant income for some years to come, and Tanzania and Kenya will cope with their increasing power demands in an economical way.

• The plan is to route the interconnector through the Kasama Substation located in the northern part of Zambia. This 330-kV/66-kV connection at Kasama will provide voltage support in the region, thus providing an additional welcome benefit to Zambia.

The impact of the interconnection on Tanzania includes:

• The development of a 330-kV network within Tanzania as part of the interconnection not only helps in facilitating the transmission of power through Tanzania, but also provides a low loss path that relieves the existing 220-kV network in Tanzania.

• With a strong 330-kV transmission network in Tanzania, voltage control within Tanzania will be greatly improved.

• Before the interconnector is in operation, the largest single contingency in the Tanzania system is the dropping of the 60-MW Keneryzi generator, which results in a frequency drop of 0.28 Hz during the peak load condition. With the Zambia — Tanzania interconnector in operation, outage of the same unit would not significantly affect the system frequency at all because the system would be connected to the rock-solid SAPP system.

• The spinning reserve for Tanzania and Kenya systems can be maintained outside the respective systems where it is more economical to do so.

• The addition of the interconnector has a beneficial impact on the reactive power margins in Tanzania and Kenya.

RESULTS OF ECONOMIC AND FINANCIAL ANALYSIS

The proof of the viability of a project such as this comes from economic analysis of the project compared to other available alternatives.

The justification for this interconnector is to supply cheaper power to Tanzania to meet its increasing load demand in the most economical way. Therefore, it was necessary to consider the Tanzanian power master plan for coping with its future load demand and to cost the alternatives available for meeting this load increase. In fact, the number of alternatives was limited:

• No interconnection alternative.

• 330-kV interconnection alternative.

There is also a remote possibility that Tanzania will use its 220-kV network to interconnect to Zambia.

The economic studies analyzed the costs of the three options over the time period to 2025, omitting all components common to each scheme, as they cancel out in the analysis. A discount rate of 12% was used to determine the respective net present value, and the results confirmed that a 330-kV interconnector would not only technically but also economically be the best solution.

Daniel Fayolle has an M.Sc. in electrical power engineering from both The Conservatoire National des Arts et Métiers, Paris, and Bath University, United Kingdom. Fayolle is a Chartered Engineer and a member of both the IEE and IMechE in the United Kingdom. He is a private consultant in the field of power and was with Scott Wilson Piésold when working as project manager on this interconnector project. Fayolle has worked all his professional life in the power sector and is an expert in high-voltage transmission and power generation, especially in hydropower. Scott Wilson Piésold has been involved in a large number of major power projects throughout the world over the past 25 years, and particularly in Africa and Asia. fayolled@free.fr

Ata Rehman earned his bachelor's degree in electrical engineering from the University of Engineering and Technology, Lahore, Pakistan, in 1987 and his master's degree from the University of Texas, Arlington, in 1993. Rehman, who is currently working for Alberta Electric System Operator in Canada, was with Acres International Ltd. in Toronto when he worked on the Zambia-Tanzania interconnector project. Ata.rehman@aeso.ca

John Wright obtained his BE degree from the University of Zambia, and he also holds a certificate in hydropower development. Wright joined ZESCO after graduation and became the director of generation and transmission in 1988. In 1999, he joined the Office for Promoting Private Power Investment (OPPPI), Ministry of Energy and Water Development, Zambia, as manager, and he is responsible for the Zambia-Tanzania interconnector project on behalf of OPPPI. oppi@zamnet.zm

Clement Sasa was awarded a B.Eng. degree in electrical engineering by the University of Zambia, Lusaka, in 1994. He joined ZESCO as a planning engineer in the Generation and Transmission Development Division. In 2000, Sasa joined OPPPI, Lusaka. csasa@zamnet.zm