Power Electronics have recently extended the HEMK range of solar inverters. In this article we've answered the most frequently asked questions.

1: What power ratings is the HEMK available in?
Power Electronics have recently extended the HEMK range. With 18 models now available between 1.910MVA and 4.390MVA and with MV skid connection options between 6.6kV to 33kV there is now a model for all utility scale projects. For solar projects with a rating higher than any single inverter unit – multiple inverters can simply be connected in parallel.

2: What is the form factor?
The HEMK is a modular central inverter and comes complete in an outdoor IP55 design. Each HEMK is paired with an MV station that consists of MV/LV transformer and MV switchgear.

3: How many PV inputs are there?
We have a number of configuration options depending on how the PV array is configured i.e. is it floating, grounded one pole, grounded both poles and the fuse protection selected for each pole. As standard we offer 32 PV inputs, which would suit a negative pole grounded PV array. This can be increased to 40 inputs. We have a document that assists customers/engineers select their configurations.

4: Can battery energy storage be added?
Yes, we offer our system called Bus Plus. It is possible to DC couple a BESS into the HEMK via our DC/DC converters. In its most basic form, the Bus Plus Basic uses eight DC inputs for connection of BESS. In the most advanced form, there is a cabinet called the Backpack that bolts onto the side of the HEMK solar inverter and provides 16 DC inputs and higher BESS power ratings.

5: What about corrosion resistance?
The HEMK and the MV Skid are constructed from electro-zinc plated steel and galvanized steel skid bases. The enclosures (including the MV transformer) are coated with a polymeric paint system the provides protection to C4 according to ISO 12944. This permits installation in industrial zones and inshore coastal areas with a moderate level of salinity. Given the amount of coastline NZ has and the likelihood of locating a solar farm close to the sea, it is possible to increase the protection to C5-M if required.

6: Can the Power Electronics HEMK series of solar inverters generate Reactive Power (Q)?
In practice – nearly every electricity distribution system needs to overcome reactive power issues. At very low levels of system load, transmission lines act like capacitors and increase voltages. At high levels of load transmission lines absorb reactive power and thereby lower voltages. The HEMK series of inverters can be configured to produce Kilowatts, Reactive Power, or a combination of both simultaneously. Every HEMK model is supported with a PQ curve which defines the maximum outputs of these variables so you can accurately design what size of inverter is required.

7: How does the HEMK control the level of Reactive Power (Q) being generated?
There are multiple ways that the Reactive Power levels can be controlled. This can be as simple as adding fixed levels of Q, alternatively creating a stepped Q profile based on voltage, but the most common approach is to introduce an additional Power Plant Controller (PPC) which can be configured to operate in a Voltage Regulating System mode (VRS). The PPC uses voltage measurements from the medium voltage point of interconnection to automatically exchange Q based on a voltage setpoint from within the system. This method is infinitely variable, within the rating of the inverter, and control is so close that it can often eliminate the requirement for voltage tap changing equipment on the network.

8: What maintenance is required to be performed on the HEMK?
Ongoing reliability and maximum efficiency are Power Electronics’ focus once your inverter is up and running. Power Electronics recommend that the user undertakes a simple six-monthly operational check, followed up by 12 monthly maintenance, typically undertaken by Power Electronics to inspect cooling fans, clean filters, undertake thermographic checks, and review the operational state of the inverter. With our nationwide offices and trained service team we can take care of this for you.

9: Can you get an auxiliary power supply to run some other site equipment from the HEMK?
Many sites, particularly single inverter installations, often have a requirement for a power supply to feed a small building, panel tracking equipment, site lighting etc. The HEMK comes as standard with a 5kVA user power supply which can be optioned up to 40kVA. Whilst seeming like a small thing - this feature can eliminate the need for a separate low voltage site connection and ICP reducing overall installation cost.

In cases where 40kVA is still not large enough – the HEMK can be supplied with a 75kVA connection directly to the low voltage bus within the inverter so the customer can connect their own auxiliary supply transformer of this rating.

10: What is the purpose of Night mode in the HEMK?
Unfortunately, Reactive Power isn’t only required when the sun is shining and the HEMK is capable of generation. At night-time the inverter can be configured to exchange Q with the network limiting network losses and improving the network voltage control during the night time as well. As the inverter is only exchanging Q with the network the only energy required by the unit are the internal losses of the inverter. In effect the inverter is acting as a Statcom at this point.

11: Can the HEMK be fed from an external 230V control supply or UPS?
Under normal operation the HEMK will use its own internal energy source. In applications requiring serial communication or remote MV circuit breaker reclosing, either over night time or when the MV connection is open, the HEMK comes standard with the ability to connect an external 230V supply to allow operation of critical controls. Should the user want to fit a UPS for this purpose this can also be incorporated as an option.

12: If used, is the Power Plant Controller (PPC) fitted in the HEMK?
The PPC is an external device from the HEMK. It comes in its own polycarbonate IP42 enclosure which is often fitted in a control room. The PPC, which can control up to 100 HEMK inverters, makes it control connection with the inverter via EtherNet.