Maraetai Power Station
Encyclopedia
Maraetai Power Station is a hydroelectric power station on the Waikato River
, in the North Island
of New Zealand
. It is the fifth of the eight hydroelectric power stations on the Waikato River, and at 360 MW, is the largest hydroelectric station on the Waikato.
The station has two powerhouses - Maraetai I powerhouse is directly in front of the Maraetai Dam, while Maraetai II powerhouse is located 550 metres downstream of Maraetai I. Although both are completely separate, both powerhouses are operated as one power station.
Maraetai is owned and operated by state-owned electricity generator Mighty River Power.
, announced a full development of the Waikato River, with the first three stations planned at Okakuri, Whakamaru
, and Maraetai. Although Ohakuri was chosen first, the choice soon shifted to Whakamaru as it would be the best location for transmission needs. However, more investigation had been done for the Maraetai site, and starting Whakamaru first would make it more complex to divert the river for Maraetai later, so it was decided to build Maraetai first.
Investigations into a hydroelectric station at Maraetai had been completed between 1940 and 1945. The site was in a deep narrow gorge, west of Tokoroa
and 20 miles (32.2 km) upstream of Arapuni
. Because of the narrowness of the gorge, the powerhouse took up the entire length of the base of the dam, meaning no orthodox spillway
could be constructed. This meant that the spillway had to be designed as part of the diversion tunnel. The site was composed of volcanic ignimbrite
, which had cracked extensively on cooling meaning extensive grouting of the rock was required.
Construction of Maraetai I began in 1946, with the construction of an access road from Tokoroa, and establishing services and accommodation for the workers of the dam. The service and accommodation camp formed the town of Mangakino
, on the south shore of what would become Lake Maraetai
. Problems were encountered right from the beginning - the Second World War had just ended, which meant there was a great deal of shortages of labour and materials. The Public Works department's work-force was less than two-thirds of its pre-war size, with thousands of vacancies not filled. Concrete
and steel
were also in short supply, as steel imports were less than half their pre-war levels, and local contractors could only produce two-thirds of the required amount of concrete. Combined with industrial unrest
and the nature of the site, the target completion date of 1951 became less achievable.
Work on the diversion tunnel - 25 feet (7.6 m) in diameter and 1685 feet (513.6 m) in length - began in late 1946 on the southern side of the site. However, work was slowed by large amounts of water that poured into the tunnel until it was extensively grouted with cement and bitumen and lined with concrete. Because of the constant water seepage, it was not uncommon for the workers to work in knee-high or even waist-high water. By 1948, some 800 workers were working on the site. The tunnel was completed in March 1949, when the northern portal was opened and the first water flowed through the tunnel. Work then started by building cofferdam
s to block the natural channel of the river and allow dam construction to begin. The Taupo
control gates were shut to lower the water level in the Waikato, and 11 bulldozers frantically pushed spoil into the river to stop the natural flow. The process was hampered by unexpected heavy rain which caused the river to rapidly rise, with it taking 27 hours to finally dam the river's natural course.The spillway tunnel was then constructed, which dropped at a 45-degree angle to intersect with the diversion tunnel.
In late 1949, work finally started on the dam foundations. Extensive and careful grouting of the rock took place, reaching 200 feet (61 m) below the river bed, and along the sides for a length of 1000 feet (304.8 m). The grouting involved drilling thousands of holes in the rock face, and 11000 tonnes (10,826.2 LT) of cement was used in the process.
Construction of the dam began in September 1950, and soon the work force had reached its peak of 1200. A large mechanised batching plant had been bought on site to produce in excess of 153000 cubic metres (200,116.4 cu yd) of concrete that was required for the dam, which was reinforced with 5000 tonnes (4,921 LT) of steel. For the first time in New Zealand, a system of cooling mass concrete was used. This involved embedded pipes through which refrigerated water was passed.
The powerhouse was constructed rapidly to have the machines operating as soon as possible. The foundations were started in early 1950, and the powerhouse was completed within a year. It consisted of a steel structural frame wit a pre-cast concrete sectional roof. The transformer
station was placed at the back and on top of the powerhouse itself due to the confines of the gorge.
The Public Works Department wanted to source major equipment from outside the usual United Kingdom
because of tight delivery dates, and British firms' lack of experience in constructing large turbines, generators and 220 kV equipment. Eventually, contracts were let to Canada
for the generators and the United States
for the turbines, both being more experienced and the price being 40% less than the British equivalent. For the spillway gates and frames, a tender from West Germany
only five years after the Second World War was quite surprisingly accepted by Cabinet, being one-third lower and one year faster than the lowest British offer.
By mid-1951, the penstock
s had been installed and the first three machines had been installed. On 31 October 1952, Maraetai generated its first electricity, producing 30 MW on a reduced load and half head. Temporary flumes transported water through the powerhouse from the two remaining penstocks. The reduced head was maintained while the diversion tunnel was stopped with a huge 30 metres (98.4 ft) long concrete plug
. The head was raised to 175 feet (53.3 m) in April 1953, and the station was officially opened on 5 September 1953 - two years after the target completion date.
The fourth machine was installed in January 1954, followed by the fifth and final machine in April. The lake was then raised to its full height of 200 foot head.
, led by Sir Keith Holyoake
, shifted priority to building the HVDC Inter-Island
("Cook Strait cable") between the North and South Islands, following a significant increase in electricity demand.
With the inter-island link completed in 1965, work recommenced on Maraetai II, with the first two turbines and generators were finally commissioned in July 1970. The last turbine and generator, and the last on the Waikato River, being commissioned in February 1971 - almost 25 years after construction of Maraetai began.
which impounds the Waikato River behind the Maraetai I powerhouse, and creates Lake Maraetai. The dam is 133 metres (436.4 ft) long, 87 metres (285.4 ft) high, and is 15.2 metres (49.9 ft) wide at the base and 11 metres (36.1 ft) at the crest. At one time, the dam was the largest hydroelectric dam in the Southern Hemisphere
.
The dam's spillway
is located south of the dam. Because of the dam's location in a narrow gorge, there was insufficient space to construct an orthodox spillway, so the diversion tunnel used in construction was reconfigured to become the dam's spillway. The spillway tunnel is 521 metres (1,709.3 ft) long, and 7.6 metres (24.9 ft) in diameter, and is blocked by three spillway gates, each 7.1 metres (23.3 ft) high and 7.16 metres (23.5 ft) wide.
s, each 70 metres (229.7 ft) long and 4.8 metres (15.7 ft) in diameter. The water is used to turn five Vertical Francis turbine
, each revolving at 167 rpm. Water is then deposited back into the Waikato River.
Each of the five turbines turns a generator, each generating 36 MW of electricity at 11,000 volts. For one megawatt of electricity to be generated, 2000 litres (4,226.8 US pt) of water must pass through the turbines every second. Electricity from each generator is then conveyed to a 40,000 MVA transformer
, where the electricity is stepped-up to 220 kV for transmission.
A sixth penstock is fitted to Maraetai I, which takes water to two auxiliary turbines and generators. These generators are used to generate electricity for the requirements in both Maraetai I and Maraetai II.
Water is taken from the canal to the powerhouse via five steel penstocks, slightly different to Maraetai I's. The penstocks are 102 metres (334.6 ft) long and 4.6 metres (15.1 ft) in diameter. The water is used to turn five Vertical Francis turbines, revolving at a higher speed of 187 rpm.
The generators at Maraetai II are the same as at Maraetai II, producing 36 MW each at 11,000 volts. This is stepped-up by 40,000 MVA transformers to 220 kV for transmission.
From Whakamaru, electricity generated at Maraetai is distributed via Transpower
's 220 kV grid to Hamilton
, Taupo
, Palmerston North
, and Auckland
.
Waikato River
The Waikato River is the longest river in New Zealand. In the North Island, it runs for 425 kilometres from the eastern slopes of Mount Ruapehu, joining the Tongariro River system and emptying into Lake Taupo, New Zealand's largest lake. It drains Taupo at the lake's northeastern edge, creates the...
, in the North Island
North Island
The North Island is one of the two main islands of New Zealand, separated from the much less populous South Island by Cook Strait. The island is in area, making it the world's 14th-largest island...
of New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
. It is the fifth of the eight hydroelectric power stations on the Waikato River, and at 360 MW, is the largest hydroelectric station on the Waikato.
The station has two powerhouses - Maraetai I powerhouse is directly in front of the Maraetai Dam, while Maraetai II powerhouse is located 550 metres downstream of Maraetai I. Although both are completely separate, both powerhouses are operated as one power station.
Maraetai is owned and operated by state-owned electricity generator Mighty River Power.
Maraetai I
In early 1946, the Minister of Public Works Bob SempleBob Semple
Robert "Bob" Semple was a union leader and later Minister of Public Works for the first Labour Government of New Zealand....
, announced a full development of the Waikato River, with the first three stations planned at Okakuri, Whakamaru
Whakamaru Power Station
Whakamaru Power Station is a hydroelectric power station on the Waikato River, in the North Island of New Zealand. It was the fourth hydroelectric power station on the Waikato River to be installed....
, and Maraetai. Although Ohakuri was chosen first, the choice soon shifted to Whakamaru as it would be the best location for transmission needs. However, more investigation had been done for the Maraetai site, and starting Whakamaru first would make it more complex to divert the river for Maraetai later, so it was decided to build Maraetai first.
Investigations into a hydroelectric station at Maraetai had been completed between 1940 and 1945. The site was in a deep narrow gorge, west of Tokoroa
Tokoroa
Tokoroa is the third-largest town in the Waikato region of the North Island of New Zealand and largest settlement in the South Waikato district. Located 30 km southwest of Rotorua, close to the foot of the Mamaku Ranges, it is mid-way between Taupo and Hamilton on State Highway One...
and 20 miles (32.2 km) upstream of Arapuni
Arapuni Power Station
Arapuni Power Station is a hydroelectric power station on the Waikato River, in the North Island of New Zealand. It is the seventh and penultimate hydroelectric power station on the Waikato River...
. Because of the narrowness of the gorge, the powerhouse took up the entire length of the base of the dam, meaning no orthodox spillway
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
could be constructed. This meant that the spillway had to be designed as part of the diversion tunnel. The site was composed of volcanic ignimbrite
Ignimbrite
An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere....
, which had cracked extensively on cooling meaning extensive grouting of the rock was required.
Construction of Maraetai I began in 1946, with the construction of an access road from Tokoroa, and establishing services and accommodation for the workers of the dam. The service and accommodation camp formed the town of Mangakino
Mangakino
Mangakino is a small town on the banks of the Waikato River in the North Island of New Zealand. It is located close to the hydroelectric power station at Lake Maraetai, southeast of Hamilton. Its population in 2001 was 1257...
, on the south shore of what would become Lake Maraetai
Lake Maraetai
Lake Maraetai is one of several artificial lakes formed as part of a hydroelectricity scheme on the Waikato River in the North Island of New Zealand...
. Problems were encountered right from the beginning - the Second World War had just ended, which meant there was a great deal of shortages of labour and materials. The Public Works department's work-force was less than two-thirds of its pre-war size, with thousands of vacancies not filled. Concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
and steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
were also in short supply, as steel imports were less than half their pre-war levels, and local contractors could only produce two-thirds of the required amount of concrete. Combined with industrial unrest
Industrial unrest
Industrial unrest is the term used to describe activities undertaken by the workforce when they protest against pay or conditions of their employment....
and the nature of the site, the target completion date of 1951 became less achievable.
Work on the diversion tunnel - 25 feet (7.6 m) in diameter and 1685 feet (513.6 m) in length - began in late 1946 on the southern side of the site. However, work was slowed by large amounts of water that poured into the tunnel until it was extensively grouted with cement and bitumen and lined with concrete. Because of the constant water seepage, it was not uncommon for the workers to work in knee-high or even waist-high water. By 1948, some 800 workers were working on the site. The tunnel was completed in March 1949, when the northern portal was opened and the first water flowed through the tunnel. Work then started by building cofferdam
Cofferdam
A cofferdam is a temporary enclosure built within, or in pairs across, a body of water and constructed to allow the enclosed area to be pumped out, creating a dry work environment for the major work to proceed...
s to block the natural channel of the river and allow dam construction to begin. The Taupo
Taupo
Taupo is a town on the shore of Lake Taupo in the centre of the North Island of New Zealand. It is the seat of the Taupo District Council and lies in the southern Waikato Region....
control gates were shut to lower the water level in the Waikato, and 11 bulldozers frantically pushed spoil into the river to stop the natural flow. The process was hampered by unexpected heavy rain which caused the river to rapidly rise, with it taking 27 hours to finally dam the river's natural course.The spillway tunnel was then constructed, which dropped at a 45-degree angle to intersect with the diversion tunnel.
In late 1949, work finally started on the dam foundations. Extensive and careful grouting of the rock took place, reaching 200 feet (61 m) below the river bed, and along the sides for a length of 1000 feet (304.8 m). The grouting involved drilling thousands of holes in the rock face, and 11000 tonnes (10,826.2 LT) of cement was used in the process.
Construction of the dam began in September 1950, and soon the work force had reached its peak of 1200. A large mechanised batching plant had been bought on site to produce in excess of 153000 cubic metres (200,116.4 cu yd) of concrete that was required for the dam, which was reinforced with 5000 tonnes (4,921 LT) of steel. For the first time in New Zealand, a system of cooling mass concrete was used. This involved embedded pipes through which refrigerated water was passed.
The powerhouse was constructed rapidly to have the machines operating as soon as possible. The foundations were started in early 1950, and the powerhouse was completed within a year. It consisted of a steel structural frame wit a pre-cast concrete sectional roof. The transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
station was placed at the back and on top of the powerhouse itself due to the confines of the gorge.
The Public Works Department wanted to source major equipment from outside the usual United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
because of tight delivery dates, and British firms' lack of experience in constructing large turbines, generators and 220 kV equipment. Eventually, contracts were let to Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
for the generators and the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
for the turbines, both being more experienced and the price being 40% less than the British equivalent. For the spillway gates and frames, a tender from West Germany
West Germany
West Germany is the common English, but not official, name for the Federal Republic of Germany or FRG in the period between its creation in May 1949 to German reunification on 3 October 1990....
only five years after the Second World War was quite surprisingly accepted by Cabinet, being one-third lower and one year faster than the lowest British offer.
By mid-1951, the penstock
Penstock
A penstock is a sluice or gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydraulic turbines and sewerage systems. It is a term that has been inherited from the technology of wooden watermills....
s had been installed and the first three machines had been installed. On 31 October 1952, Maraetai generated its first electricity, producing 30 MW on a reduced load and half head. Temporary flumes transported water through the powerhouse from the two remaining penstocks. The reduced head was maintained while the diversion tunnel was stopped with a huge 30 metres (98.4 ft) long concrete plug
Plug
Plug may refer to:* Plug for the plughole in the bathtub, washbasin and sink* Plug , a short lived British comic that ran from 1977 until 1979, when it merged with The Beezer, which featured a character from The Bash Street Kids called Plug as it's main star.* Plug , a family of fishing lures*...
. The head was raised to 175 feet (53.3 m) in April 1953, and the station was officially opened on 5 September 1953 - two years after the target completion date.
The fourth machine was installed in January 1954, followed by the fifth and final machine in April. The lake was then raised to its full height of 200 foot head.
Maraetai II
The construction of the Maraetai II powerhouse, the last powerhouse to be built on the Waikato River began in 1959, with a 500-metre long canal taking water down the south side of the gorge to the powerhouse. In 1961, work on Maraetai II was suspended when the newly-elected National governmentSecond National Government of New Zealand
The Second National Government of New Zealand was the government of New Zealand from 1960 to 1972. It was a conservative government which sought mainly to preserve the economic prosperity and general stability of the early 1960s...
, led by Sir Keith Holyoake
Keith Holyoake
Sir Keith Jacka Holyoake, KG, GCMG, CH, QSO, KStJ was a New Zealand politician. The only person to have been both Prime Minister and Governor-General of New Zealand, Holyoake was National Party Prime Minister from 20 September 1957 to 12 December 1957, then again from 12 December 1960 to 7...
, shifted priority to building the HVDC Inter-Island
HVDC Inter-Island
The HVDC Inter-Island link is a high-capacity, bipolar high-voltage direct current transmission system connecting the electricity transmission networks of New Zealand's two main islands to form the National Grid. The HVDC link is owned and operated by state-owned transmission company Transpower...
("Cook Strait cable") between the North and South Islands, following a significant increase in electricity demand.
With the inter-island link completed in 1965, work recommenced on Maraetai II, with the first two turbines and generators were finally commissioned in July 1970. The last turbine and generator, and the last on the Waikato River, being commissioned in February 1971 - almost 25 years after construction of Maraetai began.
Dam
Maraetai's dam is a concrete arch damArch dam
An arch dam is a type of dam that is curved and commonly built with concrete. The arch dam is a structure that is designed to curve upstream so that the force of the water against it, known as hydrostatic pressure, presses against the arch, compressing and strengthening the structure as it pushes...
which impounds the Waikato River behind the Maraetai I powerhouse, and creates Lake Maraetai. The dam is 133 metres (436.4 ft) long, 87 metres (285.4 ft) high, and is 15.2 metres (49.9 ft) wide at the base and 11 metres (36.1 ft) at the crest. At one time, the dam was the largest hydroelectric dam in the Southern Hemisphere
Southern Hemisphere
The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...
.
The dam's spillway
Spillway
A spillway is a structure used to provide the controlled release of flows from a dam or levee into a downstream area, typically being the river that was dammed. In the UK they may be known as overflow channels. Spillways release floods so that the water does not overtop and damage or even destroy...
is located south of the dam. Because of the dam's location in a narrow gorge, there was insufficient space to construct an orthodox spillway, so the diversion tunnel used in construction was reconfigured to become the dam's spillway. The spillway tunnel is 521 metres (1,709.3 ft) long, and 7.6 metres (24.9 ft) in diameter, and is blocked by three spillway gates, each 7.1 metres (23.3 ft) high and 7.16 metres (23.5 ft) wide.
Maraetai I
Water for Maraetai I is taken from Lake Maraetai at the dam, and is conveyed to the powerhouse by five steel penstockPenstock
A penstock is a sluice or gate or intake structure that controls water flow, or an enclosed pipe that delivers water to hydraulic turbines and sewerage systems. It is a term that has been inherited from the technology of wooden watermills....
s, each 70 metres (229.7 ft) long and 4.8 metres (15.7 ft) in diameter. The water is used to turn five Vertical Francis turbine
Francis turbine
The Francis turbine is a type of water turbine that was developed by James B. Francis in Lowell, Massachusetts. It is an inward-flow reaction turbine that combines radial and axial flow concepts....
, each revolving at 167 rpm. Water is then deposited back into the Waikato River.
Each of the five turbines turns a generator, each generating 36 MW of electricity at 11,000 volts. For one megawatt of electricity to be generated, 2000 litres (4,226.8 US pt) of water must pass through the turbines every second. Electricity from each generator is then conveyed to a 40,000 MVA transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
, where the electricity is stepped-up to 220 kV for transmission.
A sixth penstock is fitted to Maraetai I, which takes water to two auxiliary turbines and generators. These generators are used to generate electricity for the requirements in both Maraetai I and Maraetai II.
Maraetai II
Water for Maraetai II is diverted down a canal south of the dam to the powerhouse. The canal to Maraetai II is 550 metres (1,804.5 ft) long, 12 metres (39.4 ft) wide, and 12 metres (39.4 ft) deep.Water is taken from the canal to the powerhouse via five steel penstocks, slightly different to Maraetai I's. The penstocks are 102 metres (334.6 ft) long and 4.6 metres (15.1 ft) in diameter. The water is used to turn five Vertical Francis turbines, revolving at a higher speed of 187 rpm.
The generators at Maraetai II are the same as at Maraetai II, producing 36 MW each at 11,000 volts. This is stepped-up by 40,000 MVA transformers to 220 kV for transmission.
Transmission
Electricity from both stations is sent to the Maraetai switchyard, between the two powerhouses. Electricity from the downstream Waipapa Power Station links into Maraetai via a single-circuit 220 kV line (MRI-WPA-A), and electricity from Maraetai and Waipapa is then transmitted to Whakamaru via two single-circuit 220 kV lines (MRI-WKM-A and MRI-WKM-B).From Whakamaru, electricity generated at Maraetai is distributed via Transpower
Transpower New Zealand Limited
Transpower New Zealand Limited is the state-owned enterprise responsible for electric power transmission in New Zealand. Transpower performs two major functions in the New Zealand Electricity Market...
's 220 kV grid to Hamilton
Hamilton, New Zealand
Hamilton is the centre of New Zealand's fourth largest urban area, and Hamilton City is the country's fourth largest territorial authority. Hamilton is in the Waikato Region of the North Island, approximately south of Auckland...
, Taupo
Taupo
Taupo is a town on the shore of Lake Taupo in the centre of the North Island of New Zealand. It is the seat of the Taupo District Council and lies in the southern Waikato Region....
, Palmerston North
Palmerston North
Palmerston North is the main city of the Manawatu-Wanganui region of the North Island of New Zealand. It is an inland city with a population of and is the country's seventh largest city and eighth largest urban area. Palmerston North is located in the eastern Manawatu Plains near the north bank...
, and Auckland
Auckland
The Auckland metropolitan area , in the North Island of New Zealand, is the largest and most populous urban area in the country with residents, percent of the country's population. Auckland also has the largest Polynesian population of any city in the world...
.