DC Electrics - Aircraft Electrical Power Systems — Page 137, Lesson 138

I want to walk you through the concept of load shedding and what happens when a generator or alternator fails in a DC electrical system. Let's start with load shedding. Load shedding is the overall reduction of the electrical loads on the power supply system. It happens when the generators cannot supply all of the load that's being demanded. In some aircraft, this is achieved automatically. In other aircraft, the pilot must monitor the electrical load by using the ammeters or load meters and keep the total load within the rated value of the generator or alternator. So, after a generator failure, some non-essential loads would be switched off to prevent overloading the remaining generator or the battery. This reduces the current demand from the bus bar and allows the essential loads to keep being supplied. Now, let's look at the indications of a generator or alternator failure. You would see a generator or alternator warning light illuminate. At the same time, the ammeter or load meter would show either zero, or if it's the centre-reading type, it would show a discharge. The typical actions you carry out in the event of a generator or alternator failure follow a clear sequence. First, switch off all unnecessary electrical loads. The specific items to shed are detailed in the aircraft handling notes. Second, isolate the generator or alternator electrically by turning the master switch or alternator switch to "off". This breaks the field circuit, and the output voltage falls to zero or a residual value, making the failed system safe. Third, in most cases, a failure of the generator causes the reverse current relay to operate, which isolates the generator output from the bus bar. Let me also clarify a point about the generator warning light. The warning light will be illuminated when the battery voltage exceeds that of the generator and the cut-out has opened. That's the correct condition — not at night only, not when the generator is supplying current to a fully charged battery with no loads on, and not when the battery charge current is lower than required to maintain its fully charged state. Now, let's tie this to the block diagram you can see on screen. This diagram shows a multi-DC generator system. You have two generators, each producing 28 volts — No.1 GEN and No.2 GEN. There's also an external power connection, labelled EXT. PWR. The batteries are 24-volt units. The bus bars are arranged as No.1 BUS, No.2 BUS, and a CENTRE BUS BAR. There's a BATTERY BUS, also called the HOT BUS, which supplies VITAL CONSUMERS. The centre bus bar feeds ESSENTIAL DC CONSUMERS and, through inverters INV1, INV2, and INV3, supplies ESSENTIAL AC CONSUMERS and NON ESS AC CONSUMERS. The non-essential DC consumers are fed from the No.1 and No.2 buses. Each generator has its own generator circuit breaker, and there's a battery switch controlling the batteries. So, in a failure, load shedding would remove the non-essential DC consumers and non-essential AC consumers first, protecting the essential and vital loads. The generator circuit breakers and the reverse current relay handle the isolation.