Emergency power system

Himatangi Radio emergency power generators

Himatangi Radio emergency power generators. Each of the two large Mirrlees diesel engines at the left powered an alternator producing 240KW at 400V. Photo courtesy Chris Underwood

By Rex Johnson
with additional information from Jon Asmus

The use of mains power was a critical factor at Himatangi Radio. The station was charged for each month’s usage of electricity by the peak amount used during the month. Therefore, if a transmitter was wound up on test to its full power (some transmitters having outputs in the region of 50 kilowatts) for too long, then the power authority started rubbing its hands together. By contrast, a Chief Technician might be seen trotting out from his office into the main hall and advising a lax young technician to get his meter readings and close the transmitter down.

Where high power transmitter service was required it might be arranged that the station would change over to its emergency power supply for a time, so that the energy bill could be paid in gallons of diesel rather than in an exorbitant power bill.

There were two large 300kVA emergency alternators, each powered by an 8-cylinder, 396 horsepower Mirrlees 2TL8 diesel engine made in England. From the picture above it can be seen that two smaller generators were also originally on site as backups to the Mirrlees units. By 1969 those smaller generators had been replaced by an even smaller Lister quickstart generator. This engine provided the automatic-startup emergency power for lighting and essential services.

Mirrlees No 2 and the small Lister quick start generator. Date unknown.

Mirrlees No 2 and the small Lister quick start generator in the early 1980s. Photo: Alan Turner. Click to enlarge.

The Mirrlees engines were constructed for use in submarines. This use was highlighted at their installation when the mechanics chose to strip them down for a detailed check before starting them up for the first time. They found iron filings in the piston bearings, which would have effectively sabotaged any submarine and would have put Himatangi’s standby power capability out of service for some time. Who sabotaged the engines, and why, is not known.

To ensure that the station equipment was free of any vibrations from these large engines when operating at 500 revolutions per minute, considerable precautions were taken in their initial installation. Each engine alternator unit was mounted on a large concrete block which in turn was mounted on a series of massive leaf springs installed below floor level. The resultant vibration-free operation of the two Mirrlees engines was no doubt further assisted by the fact that the flywheel fitted to each engine was of substantial proportions.
George King, Manager of Himatangi Radio 1964-1980

The big Mirrlees diesel engines and their alternators in 1974

The big Mirrlees diesel engines and their alternators in 1974. Note the holes in the flywheels for turning the engine by hand. Photo: Rex Johnson

The engines were started by compressed air. This involved gradually feeding compressed air into a turbine, causing the engine to turn over and slowly gain speed. When it had reached a good rotation rate the fuel lever would be rocked over and the engine would immediately commence operation on its diesel fuel supply.

Bent connecting rod from Mirrlees engine at Himatangi Radio

The unfortunate conrod. Photo: Jon Asmus

Unfortunately, this relatively simple startup operation came to grief one day in the mid 1960s after cooling water had leaked into one of the cylinders. Air and fuel compress, but water does not. When the engine was started a large steel connecting rod was bent into the shape of a banana and a solid steel cam had a large section of its lobe smoothed off as if it had been warm butter. Murphy’s Law ensured that the water leak was in the last cylinder to go through the compression cycle, meaning the engine had a bit of momentum at that stage and could suffer maximum damage.

The ruined connecting rod and camshaft were kept on site as proof to young technicians that it was worth using the alternative startup method.

Sigh, the alternative method! For the weekly test run of the engines this involved an element of hard labour for the lucky technician rostered for this duty. First, a series of air cocks were to be opened at the head of each cylinder. Second, a massive iron pole had to be lifted (grunt) and inserted into a hole in the rim of the engine flywheel (gasp). The technician then swung on the pole (feet flapping in the air) until his weight caused the flywheel to turn, the pistons to rise and fall and any water in the cylinders to be squirted out. Two full revolutions were good (wheeze, puff) before the cylinder cocks could be closed and the air-start method used to fire up the engine.

The perspiring technician could then turn his attention to the second engine and repeat the procedure.

The electrical control board for the diesel alternators in 1974

The electrical control board for the diesel alternators in 1974. Photo: Rex Johnson

Having got two engines running nicely, the technician would go to the main control panel and attempt to synchronise the engines and lock their outputs together. The synchronising process was quite delicate considering the size and power of the Mirrlees engines. The main aim was to lock the engines in phase rather than to get them running exactly with 50Hz output.

Failure to synchronise could cause one engine to try and drive the other while its opposite number fought back and tried to take control. This would be accompanied by loud noises from the engines and rapidly flickering lights on the control panel, plus the sound of feet pounding across the transmitter hall floor from the direction of the Chief Technician’s office.

Alternatively, there would be a satisfying ‘clunk’, the engines would purr and the technician could take the time to wipe his sweaty hands and mop his brow.

Bill Murray (Mechanic) at the Himatangi Radio emergency power controls

Bill Murray (Mechanic) at the Himatangi Radio emergency power controls. Photo courtesy Chris Underwood

In the photo above, Bill Murray is standing in front of two separate engine control panels and has his hand on the speed adjustment control for one of them. He is looking across to a smaller ‘swing-out’ panel that displayed the phase difference between the engines and he is trying to minimise that difference. The top meter had lamps either side of it which flickered with the difference in phase and which could gradually be brought to a very slow dimming and brightening. These, along with the needle on that meter were the indicators used for synchronising. When the speeds were matched the large contactor switches on each engine panel would be operated and the engines locked together.

The next step was relatively simple, using one speed control to adjust both engines so the output frequency was about 52Hz. The operator would then go farther down the row of bays to his right and drop the station off mains power and on to emergency power. The engines would grunt a bit and the station load would cause the output frequency to sag to say 48Hz. The operator would then simply wind the combined engine speed up for a 50Hz output.

Another view of the two Mirrlees engines in 1974

Another view of the two Mirrlees engines in 1974. Photo: Rex Johnson

After Himatangi Radio closed, one of the engines (Number 2) and the control panels were taken to the Tokomaru Steam Museum for preservation. Their picture of the engine shows the flywheel with one of the holes used in poling it over.

Admiring one of the two Mirrlees 2TL8 diesel engines, which produced 396bhp at 500rpm to drive a 300kVA alternator

Admiring one of the two Mirrlees 2TL8 diesel engines, which produced 396bhp at 500rpm to drive a 300kVA alternator. Photo courtesy Jon Asmus

Two Mirrlees 8-cylinder engines (896bhp at 500rpm) driving 300kVA alternators, plus Lister Quick Start plant (green) which provided power to the North and South VHF link terminal stations and the transmission equipment within the station

Two Mirrlees 8-cylinder engines (896bhp at 500rpm) driving 300kVA alternators, plus Lister Quick Start plant (green) which provided power to the North and South VHF link terminal stations and the transmission equipment within the station. Courtesy Jon Asmus