How the Harrow System works.


The system has four major parts.

  1. The collection array
  2. The energy Store
  3. Main and reserve Inverter
  4. Protection and Changeover system.

1. The Array

The array is in five sections.  Each section delivers energy at 24 volts DC to the energy store.  The sections are distributed and the orientations are chosen to tame the midday peak in exchange for an extension to the operating time at each end of the day.  

The site suffers from shadows from nearby trees and buildings, the distribution of the array sections helps minimise the effects as the shadows move during the day.


2.  The Energy Store.

The energy store currently consists of 6 110 AH caravan batteries in three strings of two.  Energy is stored at 24 volts under control of a Stecca PR3030 Charge controller.   Each section has short circuit and overload protection by a 32A mcb.  The controller is protected from the effects of a short internally or on the array by a 32A mcb between it and the energy store.


3.  Inverters.

The main inverter is a Studer AJ2400 unit, fed from the energy store by a 100A BS88 fuse for short circuit and DC overload protection.   The reserve inverter, for winter use, is a Studer AJ400, fed from the load output of the charge controller via a 20A mcb.   In winter, the risk of discharging the energy store is greater, owing to poorer sun conditions, and as a protective measure, the charge controller is used to give more cautious deep discharge shutdown.   The input current at full power of the main inverter exceeds the current rating of the load output of the charge controller, and so this inverter is fed from the energy store directly.   It is in use between spring and mid autumn, when the chances of sustained poor sun are lower, and the consequent risk of deep discharge is less. 


4.  protective measures and Changeover arrangements.

In addition to their internal overload and short circuit protection, each inverter is protected by the combination of an RCD and mcb, 10A for the main inverter and 6 for the reserve.   In both cases the RCD has a tripping current of 30 mA to give supplemental protection from direct contact.  The neutral terminal of each inverter is connected to the system’s earth electrode, a standard rod type earth electrode mounted in the garden.  This establishes the neutral as a true neutral and allows the standard electric shock protective measures to be used.  To maintain the equipotential zone in the flat, the generator earth is bonded to the grid supply earth terminal.

Each mcb/RCD combination feeds its own BS4343 socket on the case of the generator cabinet.A BS4343 plug and flexible cord is taken to a flex outlet unit, and thence in fixed wiring and a second meter to the generator consumer unit to the house.This unit houses three MCBs, which feed the second inputs of three changeover switches, one for the lighting circuit and one each for the socket circuits in the flat.To prevent the solar generator accidentally operating in parallel with the mains supply, the changeover switches incorporate a centre off position..