6.2 Inverter AC, DC and Energy-Management Functions

Quick Answer: A grid-connected PV inverter converts DC to AC. A hybrid also converts AC to DC so a battery can charge. Extra AC functions are synchronising voltage and frequency with the mains, monitoring the grid and temporarily switching off if voltage or frequency is outside limits or there is loss of mains, and keeping the waveform within harmonic and flicker limits. Extra DC functions are maximum power point tracking (MPPT) and DC-side fault monitoring. An electrical energy management system (EEMS) can watch import and export against pre-set levels, interface with a DC-coupled battery management system (BMS), interface with a smart grid to control export, and manage island-mode switching on loss of grid power. Anti-islanding must not keep a dead street live. Island-mode is a designed backup with changeover, not accidental islanding. This OpenExamPrep section is independent study material for 2922-34 handbook v1.3 LO 3.1.2.

Why functions sit next to types on the paper

Naming a hybrid on a quotation is not the same as knowing what the cabinet is allowed to do when the DNO network dies. Learning Outcome 3.1.2 after the type list is a function list. Unit 301 will happily ask you to put MPPT on the DC side, loss-of-mains on the AC side, and island-mode switching under EEMS — then to refuse the sales sentence that “backup” means the inverter may backfeed a street the linesperson believes is dead.

The Electricity Safety, Quality and Continuity Regulations 2002 (ESQCR) control parallel operation with a distributor’s network. ENA G98 and G99 are how type-tested inverters demonstrate that they synchronise, stay inside voltage and frequency windows, detect loss of mains, and cease to energise the network when they must. Those are safety and network duties, not optional app features.

Core conversion: DC to AC, and hybrid AC to DC

Every grid-connected PV inverter’s first job is to convert array DC to installation AC. Without that conversion the modules are a live DC generator that the consumer unit cannot use and the DNO cannot accept in parallel.

A hybrid adds the reverse path: AC to DC so that a battery can charge. That AC might be PV-derived AC on some AC-coupled products, or grid AC when the tariff says charge overnight. On many DC-coupled hybrids the PV MPPT also feeds the battery in DC without a second box; the handbook still wants the AC-to-DC charging capability named, because that is what lets the store take energy from the public supply.

If the machine cannot charge a battery from AC, it is not showing the hybrid function the handbook lists, even if the brochure says “battery ready” because a third-party charger could be added later. Types in the previous section named the family. This section asks what power conversions actually occur.

Additional AC functions of grid-connected inverters

Handbook extra AC functions come as a set. Learn them as one cluster.

Synchronising output voltage and frequency with the AC mains. A generator in parallel must match the supply. If the inverter’s voltage, frequency or phase disagrees with the DNO, circulating current and disconnection follow. Type-tested G98/G99 plant does this automatically. You do not “sync” a domestic string inverter with a manual synchroscope.

Monitoring the grid and temporarily switching off if voltage or frequency is outside limits, or if there is loss of mains. Under-voltage, over-voltage, under-frequency, over-frequency and loss-of-mains (anti-islanding) are why the inverter ceases to energise the network when the public supply is unhealthy or absent. “Temporarily” matters: when the mains returns inside the type-tested windows for the reconnection time, the inverter may resume. That is not the same as an installer opening the AC isolator for maintenance, and it is not a licence to disable protection because the customer wanted lights during a cut.

Ensuring the output waveform stays within specified harmonic and flicker limits. Other customers share the feeder. A distorted inverter current injects harmonics. Rapid power swings cause flicker. G98/G99 type-testing includes those emission limits. A 2922 candidate who writes “the inverter just makes 230 V” has missed a network-quality duty the handbook lists beside loss-of-mains.

Loss-of-mains is the safety sentence you must be able to say in one breath. If the DNO opens a feeder, or a fuse blows, the street can go dead. A PV inverter that kept generating would island that section: voltage on conductors that operational staff treat as isolated. Shock, backfeed into a fault, and unsynchronised reclosing are the harms. ESQCR parallel-operation duties and ENA recommendations therefore require the inverter not to keep a dead street live. That is anti-islanding, also called loss-of-mains protection. It is not optional on a grid-connected 2922 job.

Additional DC functions

Maximum power point tracking is how the inverter (or each micro, or each MPPT input) keeps the array near the knee of its current-voltage curve as irradiance and temperature move. Without MPPT the array sits at a lazy voltage and you throw away kilowatt-hours. Multi-MPPT, taught in the type section, is this function counted per tracker.

Monitoring of faults on the DC side is the inverter’s (or a separate monitor’s) watch on a generator that cannot be switched off in daylight. Insulation faults, residual current on the DC side, and other DC alarms belong here as a function name. The later design chapter on insulation-resistance and residual-current monitoring will specify location, alarm behaviour and the rule that a flashing inverter light is not a complete earth-fault alarm. On LO3 you must know that DC fault monitoring is a listed inverter function, not a nice extra from the manufacturer’s marketing deck.

Additional functions where an EEMS is incorporated

Not every 3 kW string inverter includes an electrical energy management system. Where one is incorporated — typical on hybrids and on plant with export limitation or storage — the handbook lists four extra jobs.

Monitoring imported and exported power against pre-set levels. Self-consumption logic charges the battery instead of exporting. G100 export (and import) limitation holds the boundary power to what the DNO agreed. The inverter or a paired controller measures the boundary and throttles or diverts so the pre-set is not exceeded.

Interface with a DC-coupled storage battery management system to disconnect or control power to the batteries. The BMS owns cell limits, temperature and state of charge. The inverter/EEMS must listen: stop charge, stop discharge, or disconnect, rather than pushing current the battery cannot accept.

Interface with a smart grid to control export. That may be a DNO limitation scheme, a supplier signal, or another agreed export-control path. The function is controlled export, not “dump everything whenever the sun shines.”

Management of island-mode switching on loss of grid power. This is the designed backup arrangement. It is not accidental islanding of the public main.

Anti-islanding is not island-mode

Anti-islanding / loss-of-mains: grid-connected output stops when the public supply is lost or out of limits, so the street stays dead.

Island-mode: a designed backup. A changeover (automatic or manual, depending on the product and the wiring) isolates the backup circuits from the DNO network before the inverter energises those circuits. The fridge on a backup sub-board may live. The street must not. Without that isolation, “this hybrid has backup” is a dangerous sentence: anti-islanding will trip, or worse, someone has defeated protection.

Prosumer language (PEI, EEMS goals, arbitrage) is expanded in Learning Outcome 6. Here you only need the inverter/EEMS function and the safety split.

Function versus AC, DC or EEMS

Scenario: grid fail on a hybrid advertised as “backup”

A household buys a hybrid because the brochure said “backup during power cuts.” At 21:00 the DNO feeder opens. The solar array is dark. The battery is at 60 percent.

If the hybrid is grid-tied only — no backup port, no changeover, backup loads still bonded through to the meter tails — loss-of-mains must operate. The inverter stops AC output. The house goes dark. That is correct anti-islanding. The brochure used the word backup; the functions list did not include island-mode switching on that product, or the installer never wired a backup island.

If the hybrid does incorporate EEMS island-mode management, a changeover isolates a backup sub-board (or the whole installation, depending on rating and manufacturer instructions) from the public supply, then the inverter may energise only those isolated circuits from the battery. The street remains dead. Neighbours’ sockets stay dead. DNO staff are not fed from the battery. That is island-mode, and it is the only honest reading of “backup” on a grid-connected 2922 job.

Exam trap: treating a temporary G98/G99 trip during a voltage excursion as a failed backup test. Voltage outside limits is an AC monitoring trip. Island-mode is a different function. Another trap: disabling anti-islanding “so the backup works” and thereby energising a dead street. That fights ESQCR and the type-test. The functions table is how you keep those ideas in separate rows.