Mjolka (Milk) River Power Station

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Click to enlarge

Introduction

In the year 1956 the State Electric Power Works started building the power station at Mjolka River, using the fall from Borgarhvilft to Borgarfjordur, around 210 metres. This power station began generating electricity in 1958.  In the years from 1958 to 1960 the connection of the Mjolka power station to the towns from Patreksfjordur to Bolungarvik was completed, as well as a connection to the Reidhjallar power station. This is quite an extensive system of power lines, spanning the high mountain passes of the Westfjords and sea lines across the deep Arnarfjordur and Dyrafjordur. This power system was called Mjolka Power Grid.    In 1972 the State Electric Power Works started planning another power plant at Mjolka, this time using the fall from Lake Langavatn down to Borgarfjordur, a fall of around 490 metres.  In the years 1971 to 1972 a dam was built at Lake Langavatn to regulate the water to the power station at Mjolka. Construction of Mjolka II started in 1973, including the Lake Tangavatn regulatory dam. This power station was operational in 1975. The present Mjolka Power Station is, therefore, two power stations using the same station house and facilities at the head of Borgarfjordur. The older station (Mjolka I) is smaller, 2.4 MW, and Mjolka II is 5.7MW.

On the 1st of January 1978 the Orkubu Vestfjarda (the Wesfjords Power Service) was established and it took over the running of Mjolka Power Station as well as other power stations in the Westfjords. The Construction of the West Line was finished in 1980 and this connected Mjolka Power Station to the national grid. Until that time the power station at Mjolka was the basic power supply for the area, but today it is run as a control station for the electricity the Orkubu buys from Landsvirkjun. An estimate has been made toward increasing the size of the Mjolka Power Station (Mjolka III). This increase includes building a reservoir at Lake Stora-Eyjavatn and directing the flow of water off an area of 13.5m2 via canals and pipes to the intake reservoir of Mjolka II (Langavatn) and then to direct water from the upper Holsa river basin to the intake reservoir.

Houses
The power stations of Mjolka I and II are housed together in a common building with one generator room and a basement. During construction of Mjolka II the building was lengthened by 9 metres to the south. The control room of both stations is housed in the control room for the older station, in one end of the generator room.

Mjolka I
The main dams and intake are situated by lake Borgarhvilftsvatn. The intake dam is an 80 metre rock dam with a concrete centre. The overflow dam is also a rock dam with a concrete centre, 390 metres long and 9.5 metres high. This dam contains a 30 metre concrete overflow regulator, situated just north of the Borgarhvilft creek. The top of the overflow regulator is at an altitude of 217.5 metres above sea level. The volume of this reservoir is just under 0.4 GL.
The intake structure is situated in the reservoir and is connected to land by a bridge. Two intake openings that can be closed by iron shutters are situated low down on the wall.

A shutter structure is placed just below the intake dam. This houses two shutters: a manual sliding shutter and a hinged shutter with a counterweight.

The preasure pipeline is 987 metres in length. It has a diameter of 800 mm at the top with a wall thickness of 7 mm, changing gradually to a diameter of 700 mm and wall thickness of 12 mm at the bottom.

The Hofsa duct was built in the summer of 1976, the river Hofsa being dammed at an altitude of 452 metres above sea level. A 2,320 metre asbestos pipe was laid underground over to the basin of Borgarhvilft creek. This pipe has an inside diameter of 600 mm.

Generators and electrical equipment
All generators and equipment are made by Skoda of Czechoslovakia except for the voltage regulators.
The turbine is of a Pelton type with two jet intakes on a horizontal shaft, 3,600 hp, 500 rpm. The turbine wheel and its shovels are cast in one block. The net fall is 200 metres, water use 1.5 m3/sec under maximum load.
The generator is 2.400 kW, cos j 0.8, voltage 6.3 kV, 50 Hz.

Construction
Construction plans and engineering calculations: Almenna byggingarfelagid hf.
Architect: Gudmundur Kr. Kristinsson
Main contractor: Phil & Sön
Pressure pipeline: Landssmidjan
Installation of electrical and mechanical equipment: Electricity dept of the energy commission.

Mjolka II
Lake Tangavatn is situated at a height of 55 metres above sea level It is a reservoir of about 1 GL. The lake is dammed with a 95 metre log earthen dam with a concrete watertight wall and a 60 metre long concrete overflow. The highest point of the dam is 10 metres. The dam has a shutter house and the water is piped through a 600 diameter pipe with a counterweighted shutter.

Lakes Langavatn and Holmavatn are found at an elevation of 490 metres. They form a reservoir of 3.2 GL. Lake Langavatn is dammed with an earthen dam waterproofed with a plastic sheet. Combined length of the dams is 280 metres and the overflow is of concrete, 70 metres in length. The dam reaches a height of 9 metres.
The intake for the power station is in the southernmost dam. A concrete intake structure placed in the lake, contains grids and emergency shutters, and is connected to the dam via a concrete bridge. On the back wall of the dam is a shutter house with an intake shutter backed by emergency equipment. The total volume of the Lakes Langavatn and Tangavatn reservoir is 4.2 GL.

The preasure pipe is 3,980 metres in length. The top of it has a diameter of 900 mm and a wall thickness of 8 mm, narrowing to 700 mm diameter and 14.2 mm wall thickness at the bottom. The last 455 metres of the pipe are covered with earth and fastened down with concrete anchors. The top half of it rests on concrete foundations and is isolated with urethane.

Generators and electrical equipment
The turbine is of a Pelton type with two jet intakes on a horizontal shaft, 8,400 hp, 750 rpm made by Litostroj in Yugoslavia. The speed regulator is from Litostroj and Asea. The net fall is 478 metres, water use 1.7 m3/sec under maximum load.
The generator is from Rade Koncar in Yugoslavia, 5.700 kW, cos j 0.8, voltage 6.3 kV, 50 Hz.

Construction
Construction plans and engineering calculations: Almenna verkfraedistofan hf.
Architect: Gudmundur Kr. Kristinsson
Main contractor: Istak, Islenskt verktak hf.
Pressure pipeline: Stahlform, Berlin, Germany
Design of mechanical and electrical equipment: Rafteikning hf.
Installation of electrical and mechanical equipment: Construction Dept of RARIK
Design of Lake Langavatn reservoir: Planning Dept of RARIK
Contractor for Lake Langavatn reservoir: Vesturverk, Bolungarvik

Operation of the power station
Three men work at the power station, handling the day to day running, maintenance, and inspection of the station. In addition they supervise the buying of electricity by the Orkubu from Landsvirkjun through the Vesturlina power line.

Local conditions
The Mjolka river basin is situated on the western edge of the Glama plateau, at the head of Borgarfjordur in Arnarfjordur. The main rivers of the area are the Mjolka and Hofsa rivers. Between them runs the creek Borgarhvilftarlaekur into the intake reservoir of Mjolka I, Lake Borgarhvilftarvatn. These rivers are fresh water rivers that are mostly fed by springs, but derive some water from snow caps that do not melt completely during the summer. These snow caps have been diminishing over the last decades, and Glama is no longer counted a real glacier.

The average flow of Mjolka at the station is around 2.3m3/sec. During the driest quarter (from mid January to mid April) around 12% of the average yearly water runs to sea, whereas during the wettest quarter, 40% of the water runs to sea. The spring river part of Mjolka can therefore be seen to be substantially more than that of Hofsa and other rivers measured in the Westfjords, except for the river Dynjandi.

The water basin of Mjolka at the power station is around 29km2. Of this, 20.5km2 are above Lake Langavatn and the water basin of Lake Borgarhvilftarvatn, and 8.6km2 below Lake Langavatn. The Hofsa reservoir water basin is 14km2 and thus the combined water basin of Mjolka I is 22.6km2.

When the dam is full and water spills over, Mjolka falls down the steep hill above the station house in many white, spluttering waterfalls.

Power production
The production of electricity in the station varies with the seasons, but has, during the past years, averaged around 54 GWh per year. Of this the production of Mjolka I has been 11-16 GWH per year while Mjolka II has produced 38-45 GWh per year.