Strength through networking
Like many developing countries, Kenya is faced with a strong electricity demand growth rate that could soon reduce the reliability of the country`s transmission network. But KPLC, the national utility, has acted fast strengthening the national grid for the pressures to come.
In Kenya, the annual electricity load growth has been in the order of six per cent over recent years, and is anticipated to continue at this rate for the foreseeable future. As a consequence, national electricity utility Kenya Power and Lighting Company (KPLC) recently began to reinforce its transmission system, enlisting the help of UK consultants Mott MacDonald.
Mott MacDonald is assisting KPLC by providing engineering services throughout the development of two projects: a new 220 kV interconnection from the Tana River hydropower complex to Nairobi; and an additional 132 kV link between Kipevu power station and Rabai substation.
Feasibility studies, conceptual designs and environmental impact assessments of these projects have been undertaken. Construction contracts for the overhead lines and substations will be placed in early 1999.
A new interconnection
The transmission system in Kenya is generally very reliable, but the current growth in electricity demand could reduce this. KPLC has therefore decided to invest in the network to help guarantee future supplies.
Kenya has a total installed generation capacity of 882 MW, 713 MW of which is in the northern region with the balance in the coastal area. Nairobi is the major load centre with a 1997 peak demand of 730 MW. A substantial portion of this load is supplied from the Tana River hydropower stations of Masinga, Kamburu, Gitaru and Kiambere via a double circuit 220 kV transmission line from Kamburu to the Nairobi substations of Dandora and Embakasi, and also over a 132 kV line from Kindaruma to Juja Road.
The reliability of the KPLC transmission system over recent years has been very high. All 220 kV and 132 kV lines are equipped with single-pole autoreclosing and although there have been several line faults most have been of a transient nature with successful automatic line restoration.
Although loss or reduction of power to Nairobi is a rare occurrence, the consequences of power failure to the country`s capital could be significant. KPLC therefore asked Mott MacDonald to examine ways of reinforcing the transmission links to the Tana River hydro stations.
An economically justifiable solution was a direct interconnection between Kiambere and Embakasi substations using one of the existing 220 kV circuits between Dandora and Embakasi, connected to a new section single circuit line to Kiambere, as illustrated in Figure 3.
The economic evaluation considered both the costs of reduced losses and the load shed. It concluded that the value of reduced losses would be so significant due to the new section of line, that only a very small reduction in load shed (150 MWh) per year, for each year of the circuit`s life, would be necessary to achieve a project rate of return equal to KPLC`s discount rate. The increased reliability afforded by the interconnection would more than achieve this level of reduction.
Mott MacDonald`s environmental specialists undertook an Environmental Impact Assessment of the project, examining land use, ecological, tourism and socioeconomic impacts.
Over much of its 140 km length, the line will run parallel to the existing Kamburu-Dandora lines, thereby keeping impacts to a minimum. One section of the line passes through the Ol Donyu Sabuk National Park, which has appreciable concentrations of important wildlife.
The transmission line route, however, crosses a section of the National Park which is only likely to be inhabited by small antelope and rodents, all fairly ubiquitous in Kenya.
It is not expected that the towers will impede the movement or migration of local wildlife. KPLC will record bird strike incidents as part of its environmental monitoring and management programme and bird deflector devices will be installed if necessary.
The proposed route will not have a significant impact on socio-economic aspects such as education, transportation, sociology, employment and health, nor will it adversely affect the tourist industry.
The new section of 220 kV transmission line from Kiambere will be based on KPLC`s standard design. The towers will be lattice type, flat formation with single `Canary` conductors, the size used on the existing Kamburu-Dandora line. Although both porcelain and glass insulators have an equivalent performance, porcelain insulators have been selected as experience in Kenya has shown that glass is more susceptible to vandalism, to the extent that it has a marked influence on the reliability statistics.
The lines will be fitted with two earthwires to provide enhanced shielding of conductors from direct lightning strikes. In the past, both earthwires have been made from galvanised steel but, in this case, KPLC is considering the introduction of one optical fibre ground wire (OPGW) to increase the capacity of its communication system.
The Kiambere switching station is presently laid out for a breaker-and-a-half configuration but is operated as a four-switch mesh. To accommodate the new Embakasi line, one additional circuit breaker along with associated disconnectors will be added and the substation will continue to form a mesh.
No additional primary plant is required at Embakasi, nor are any changes necessary to the primary equipment at Dandora. There is, however, a need to co-ordinate the new line protection at Kiambere with the existing relays at Embakasi. Because of its importance the line will be protected by two sets of multi-zone distance protection, one operating in a permissive scheme, the other set up to operate in blocking mode.
The distance protection will be supplemented with duplicated directional earth fault protection to detect high resistance faults beyond the scope of the distance relays.
Power line carrier equipment will be used for telecontrol from the National Control Centre in Nairobi and for protection signalling and telephony. Duplicate power line carrier systems and signalling channels are also being installed. As well as new equipment for the Kiambere-Embakasi circuit, additional power line carrier and signalling is being fitted on the remaining Dandora-Embakasi circuit to achieve the duplicate signalling.
KPLC`s Kipevu power station currently delivers power to Rabai substation over two 132 kV circuits. Kipevu I, a 75 MW diesel station, engineered by Mott MacDonald and due for completion in 1999, forms the first phase of a new development aimed at increasing Kenya`s thermal capacity.
The development will also comprise Kipevu II, a similar sized diesel station which will be financed, constructed, owned and operated by Tsavo Power Company. Mott MacDonald is assisting KPLC with this project which includes a new 132 kV switching station.
Based upon KPLC`s planning criterion of a single circuit outage contingency (n-1) for export of power from major power installations such as Kipevu, the export capability of the existing Kipevu link is adequate. However, with generation expected to increase to 312 MW by the year 2000, and an ultimate capacity of 355 MW envisaged, the transmission interconnection to Rabai must be strengthened.
Several reinforcement options were considered for the Kipevu-Rabai project, including replacement, uprating and removal of the existing circuits. Some sections along the route are double circuit steel towers while others are single circuit on steel towers and single circuit on wood poles. The steel tower sections were found to be in good condition with a reasonably long life expectancy, despite being exposed to saline and pollution.
The wood poles were not in such good condition, but since they form a relatively small part of the overall installation, it was concluded that the existing circuits could be retained.
The lowest cost reinforcement option, when considering switchgear developments at Rabai, as well as the eventual replacement of the wood pole section, was to develop a new double circuit line running parallel and close to the existing line over most of its route. One circuit of the new 132 kV line will be connected to the existing switching station and the other will be terminated in the new station to provide a direct export route for the new plant.
Two types of overhead line supports were considered, lattice steel towers and tubular steel poles. Although the latter would require a smaller extension to the existing wayleave, the total installed cost of the steel pole construction would be at least double the cost of the steel towers.
Another drawback of the steel poles concerns the maintenance techniques, and the associated equipment, which are considerably different to those used on the conventional lattice steel towers or wood poles normally used in Kenya. The decision was therefore taken to use towers familiar to KPLC and similar to those used on the double circuit section of line as it enters Kipevu.
Rabai substation is laid out for a breaker-and-a-third configuration, but operated as a seven switch mesh. Under this project a complete new diameter will be constructed and an existing partially equipped diameter completed by additional switchgear so that the substation will become a classic three diameter, breaker-and-a-third arrangement. This will provide the high level of security needed for such a key substation.
Construction work on both projects is expected to begin in March of this year with completion of the Kipevu-Rabai 132 kV reinforcement scheduled for May 2000. The Kiambere-Embakasi 220 kV interconnection will be completed towards the end of 2000.
Figure 1. The KPLC transmission system
Figure 2. Kipevu-Rabai 132 kV interconnector
Figure 3. Tana River 220 kV interconnector and Kipevu-Rabai reinforcement
Figure 4. Schematic line diagram of the KPLC transmission system