Locally made inverters for new solar farms

July 16th, 2019, Published in Articles: Energize

RWW Engineering, a South African manufacturer of electrical equipment for industry and utilities, has been appointed the South African manufacturer of inverters for Spanish company Green Power Technologies. The inverters will be used at three 75 MW solar PV farms which are being built as part of Round 4 of South Africa’s Renewable Energy Independent Power Producers Procurement Programme (REIPPPP).

The three farms, all of which are situated south-west of Upington, will use solar panel trackers to improve the farms’ daily electrical output. These trackers will also be powered by the inverters. RWW Engineering builds the inverters, known as WD3 inverters, using components supplied by the Spanish company. The units are high capacity in power rating and modular in construction.

Fig. 1: A complete inverter with its three-phase units on the left and the head module on the right.

The inverters, which produce 3-phase AC power, are controlled by a head unit which operates as a central unit providing communication and control functionality. The head unit also houses the AC output bridge, the AC busbar connections and provides additional protections and functions. The local company is responsible for the steel work, final assembly and testing. The company will also install the inverters at the solar farms and commission them in preparation for connection to the local distribution grid.

Fig. 2: A view of the inverters under construction.

Trevor Wood, the project manager and engineer in charge of the project at RWW Engineering, says that these inverters, which are rated at 4 MW, receives 1500 V DC from 15 000 PV panels, converts that to 660 V AC and feeds it into a 4,7 MVA step-up transformer which produces a distribution-level voltage of 33 kV. To date, two of these inverters have been shipped and more are under construction. Wood says the build schedule will run at three units per week. The entire project calls for 60 units.

Fig. 3: Inside view of one of the three power modules.

The inverter panels are highly sophisticated and use high-power insulated gate bipolar transistors (IGBTs) switching at 1950 kHz. Although these panels, and some other parts, are imported, the project has a high local content of between 55 and 65%.

Fig. 4: The inverters in a typical installation with step-up power transformer connected.

Cooling

The cover of the unit is designed to draw cooling air into the cabinets to cool the equipment contained within. Warm air exits from the base of the cabinet. This is achieved by means of fans positioned in strategic positions within the cabinet. Power for the fans is de-coupled from grid voltage and is therefore independent of grid instabilities. This system prevents dust from the surrounding area from being drawn into the cabinets, as it would be should the cool air be draw in from the base and exit from the top. This helps to keep the equipment operating for longer between routine maintenance checks.

MPPT options

The maximum power point tracker (MPPT) is an electronic DC to DC converter and forms part of the inverter design. This device ensures that the maximum output from the solar PV panels is supplied to the high frequency switcher in the inverter. The MPPT in these units is designed to operate in either single- or multi-input mode. In single mode, the output from the PV panels is shared equally between all three modules. In multi-input mode, three independent streams of power are supplied from three banks of PV panels. In both cases, however, the output from these inverters is a pure sinusoidal three-phase waveform which feeds the step-up transformer. The modular construction of these units makes them easy to install and maintain. Furthermore, the way the busbars exit the module makes for a quick and easy installation of the step-up transformer.

The company, which was established in 1986, also manufactures a line of motor-starting equipment, capacitors and reactive power and harmonic filtering equipment, as well as switchgear under licence to Schneider Electric and reclosers under licence to Noja Power.

Contact Jeremy Wood, RWW Engineering, Tel 011 433-8003, jwood@rww.co.za

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