Andy Green has worked for Emerson Control Techniques for 25 years, holding various positions including those of Systems Sales Director and General Manager. For the past three years, in the position of Industry Development Manager – Solar Inverters, he has channelled his knowledge of grid tie inverter applications in support of Emerson’s goal of becoming a leading player in the central inverter market.
PES: Welcome back to PES. When the company last appeared in the magazine you told us about your transformerless central inverter system for utility-scale photovoltaic power plants. Can you tell us a little more about how this and about how sales are progressing?
AG: Our SPV inverter systems offer a range of transformerless central inverters which been extremely well received in the market. Each inverter is constructed from multiple standard 175kW power modules which are mass produced and derived from modules used in industrial motor control applications This approach has given us a very robust and reliable inverter solution.
For more information on Emerson’s SPV inverter system, see panel
PES: You also say that the inverter solutions ‘enable investors to exceed their return on investment expectations through high-yield inverter systems’. Can you tell us a little about how they can achieve this?
AG: A PV inverter – be it central or string – can appear as a bottleneck to a PV plant if the system is poorly engineered. A Central Grid Tie inverter, such as the Emerson SPV product, has three principle effects on the total PV plant yield:
1. System losses:
In other words, how efficiently does the inverter convert the available DC power from the PV modules to grid quality AC power? Losses are attributable to switching and conduction losses inside the inverter and associated components such as filters, cabling and fuses. The Intelligent Inverter Control system matches the optimum inverter capacity to the solar load presented by the PV modules, therefore only the losses that are absolutely necessary are incurred.
2. Maximum Power Point tracking (MPPT):
The instantaneous power presented to the inverter is not only a function of light intensity hitting the active material in the PV module, less the DC cable losses, but also of the ability of the inverter controls to detect and operate the PV modules at their maximum power point or MPP. There are many types of MPP trackers available ranging from simple hardware implementations to sophisticated software based systems.
3. Inverter System Availability:
This is easily overlooked. However it has the most important effect on the overall plant yield. Take a moment to place a value on one day of total failure and relate that as a percentage of inverter cost. Traditionally those who consider such scenarios tend to opt for smaller inverters in larger quantities thus the loss of any single inverter is seen as less catastrophic. This is balanced by an increased likelihood of failure due to the number of inverters deployed and by increased cost per kWp of the inverters. In reality this means that there is actually no increase in availability but a large increase in cost from the additional inverters, shelters, medium voltage switchgear, transformers MV cabling to each inverter, access roadways and additional SCADA connections.
