The challenge of distributed generation design

The water inlet from Arthurs Lake via the mini-hydro generator to Floods Creek Dam
The water inlet from Arthurs Lake via the mini-hydro generator to Floods Creek Dam

Tasked with the design, installation and commissioning of the high voltage and associated electrical portions of the Midlands Water Scheme for Tasmanian Irrigation, Zinfra delivered a world-class distributed generation solution with a complex anti-islanding and protection component.

The Midlands Water Scheme is an engineering feat that will change the landscape of Tasmanian farming. Generating power through a mini-hydro generator to power the water scheme and sell surplus power back into the state grid, it’s a shining example of innovation, offsetting the cost of water to farmers.

The overall scheme operates when water is extracted from Arthurs Lake at an altitude of about 950m above sea level and flows under gravity through 33km of ductile iron and steel pipe, through forested rocky terrain, down to a 6MW power station at Floods Creek at 310m above sea level.

The power station can generate 50,400MWh of electricity a year. About 22 per cent of this energy is used to power three pump stations and the remaining 78 per cent is diverted to the Tasmanian electricity grid to offset the price of the water.

Fulton Hogan, one of the two main contractors on the $104 million Tasmanian Irrigation project, turned to Zinfra to design, construct and commission the electrical configuration for their contract. Zinfra’s electrical engineer Ezekiel Madzikanda started the electrical design from a one page scope document, building the system from the ground up. Here’s Mr Madzikanda’s brief of the project design:

The electrical design: a bird’s eye view

The distributed generation consists of a single 7.15MVA at 0.85pf, 3.3KV, 1000rpm synchronous generator, a step-up transformer and 15km of 22kV transmission line to the point of common coupling with the distribution network. The system includes a state-of-the-art mechanical deflector coupled with a very sophisticated electrical protection system, which uses 15km of fibre optic for fast communication.

The protection system allows engagement of the deflectors within 100 milliseconds of a grid trip to deflect a portion of water away from the runner buckets, minimising turbine overspeed and allowing continuity of power supply to the pump station and therefore reliable water delivery to the irrigators. One of the biggest challenges for Zinfra was the size and location of the distributed network, which posed a great challenge in terms of islanding. Mini-hydro islanding could go undetected if traditional islanding schemes were to be applied, and the remote location in Tasmania means there are no high-speed telecommunication networks that can be used for communication-based islanding techniques. The main components of the electrical system can be summarised as:

• 7.15MW generator
• 15km of 22kV distribution line
• 22kV substation
• Earthing
• DC supply
• Synchronisation system
• STATCOM
• Run-back system
• Anti-islanding system
• SCADA

For the 7.15MW generator, Zinfra installed and commissioned the high and low voltage cabling, the generator and step-up transformer protection panel. Power is generated at 3.3kV and transformed to 22kV through the step-up transformer to match the utility’s distribution voltage.

The 22kV transmission line from the mini-hydro to the pumping station consists of 14km of overhead line and 1.1km of underground cable.
Zinfra designed, constructed and commissioned a 22kV substation as part of the Midlands Water Scheme. The substation consists of:

• 22kV incomer panel from the mini-hydro
• 22kV transformer panel
• 22kV synchronisation panel
• 22kV STATCOM panel
• 22/0.415kV transformer
• STATCOM

In addition to the above works, Zinfra also designed and installed the earthing and protection systems for the substation.

The critical secondary systems

The protection required for this project was extensive, given the nature of the distributed generation system. Protection consists of the following:

• Transmission line protection
• Transformer protection
• 22kV bus-zone protection
• Auto synchronisation
• Run-back system
• Anti-islanding system

Both transformer and generator main protection is current differential using SEL 700G relay.

The main transmission line protection is current differential with backup distance protection. The differential protection is through a direct fibre connection between the mini-hydro and Midlands Pumping Station using SEL 311L-7 relays.

Run -back

The three constraints for exporting power to the power grid are:

• A maximum of 5MW of power can be exported into the grid
• When the STATCOM is out of service, there is a maximum of 1.0MW power that can be exported into the grid
• A maximum of 1.8MW power can be imported from the grid to Tasmanian Irrigation.

The electrical design was therefore built around these limitations, using SCADA to monitor the status of the Static Synchronous Compensator (STATCOM).

STATCOM

The STATCOM was installed as part of the system to provide voltage support and quality of supply for the distribution network and the run-back system.

An injection of active power will cause the voltage to rise at the point of connection and surrounding network, causing the installed regulators to reduce voltage by ‘bucking’. When a source of active power is suddenly removed, voltage will drop by more than 16 per cent if no static variance compensation is installed. The STATCOM was designed to be capable of 2-MVAr output for up to two seconds and 1.25-MVAr continuously so any step changes in the voltage in the network will be limited to 4 per cent.

The mini-hydro will ramp down generation to a maximum of 1MW of export on the signal the STATCOM is out of service.

Anti-islanding

For Tasmanian Irrigation to export power to the grid, an anti-islanding system is required. A credible island condition is created when the mini-hydro can be disconnected from the grid network within 500 milliseconds.

Zinfra initially designed and installed an anti-islanding system, based on a signal generator and signal detector, which it failed to successfully commission. This system was proposed by the power utility and supplied by DX3 of Canada.

During commissioning several problems, such as signal non-detection, harmonics and voltage flicker, were experienced. Due to these problems, particularly harmonics and voltage flicker, the use of a signal generator method for island detection was discounted.

Zinfra proposed two further methods for system island detection. The first method uses rate of change of frequency (ROCOF) and negative sequence protection, and the second method uses circuit breaker status (transfer tripping system).

After presenting both options to the power utility, the transfer tripping system was approved for implementation. Zinfra designed the transfer tripping system, using the power utility’s fibre optic communication platform for communication between the substations and the power station at Flood Creek. The transfer system, which was fully designed and tested by Zinfra, uses discrete programmable controllers (SEL-2440) to monitor circuit breakers at three substations and communicate the circuit breaker status to the power station. Anti-islanding is complicated at the best of times and in this case was further exacerbated by the geographical location of three substations with no direct line of sight for communication. Zinfra and the power utility installed radios to overcome this barrier. This transfer tripping antiislanding
method using a communication platform was successfully commissioned by Zinfra allowing the rest of the mini-hydro system to be synchronised to the network. Mr Madzikanda, on behalf of Zinfra, presented a paper titled A Practical Look at Anti-Islanding in Distribution Networks. Part 2: Commissioning Issues based on this anti-islanding experience, and was presented in Sydney during the Australian
Protection Symposium.

A full SCADA was designed and commissioned for the mini-hydro and Midlands Pumping Station for monitoring and control of the whole system. A FOXBORO SCD5200 device and DNP3 protocol were used.

Project success

Final commissioning of the Midlands Water Scheme was successfully completed last July, and the power station is now operating above expectations. Zinfra completed the works on budget and with no LTIs or MTIs. Ultimately, the water scheme will allow for diversified land use in a historically dry land zone, which in turn should attract longterm contracts and increase employment opportunities for this rural area.

The project has been created with a longterm vision to last 100 years.

The Midlands Water Scheme has been much lauded, acknowledged as a finalist for the Smart Infrastructure Project of the Year and Government Partnership Excellence awards, as well as winning the Tasmanian Civil Contracting Earth Award Category 5 (project value of more than $75 million) and the Tasmanian Engineering Excellence Award Project Infrastructure Category.

Tasmanian Irrigation chief executive officer Chris Oldfield described the scheme as “one of the state’s most significant civil engineering projects since the Hydro scheme”, with project manager Sven Meyer adding, “the scheme was the largest linear infrastructure project undertaken in Tasmania in 10 years”.