Small (kW range) portable AC generating units are used to drive isolated AC loads and normally consist of a diesel or petrol prime mover and a wound-field synchronous generator.
The ability to operate these units in parallel is convenient as it allows operational and efficiency advantages. Replacing the wound-field generator with a permanent magnet generator improves the system efficiency and reliability. This paper experimentally investigates the parallel operation of the proposed permanent magnet generator and demonstrates good parallel operating performance.
Portable AC generating units designed to drive small (kW range) isolated AC loads and normally consist of a diesel or petrol prime mover and a wound-field synchronous alternator. A control unit is used to regulate both the AC output frequency by varying the speed of the prime mover, and the output voltage by varying the field current of the synchronous generator.
It is often preferable to have two or more smaller generating sets (gen-sets) operating in parallel rather than a single larger unit as smaller units are easier to transport and it allows one of the units to be stopped at light loads which can significantly improve the fuel efficiency. Parallel operation also enables easier maintenance, future expansion and improved reliability based on the ability to have a level of spinning reserve or redundancy.
When alternators are paralleled, they operate at the same voltage and frequency and it is important that they share the real (kW) and reactive (kVAr) power load in proportion to their rating. The real power sharing is related to the prime mover speed (throttle) control and the reactive power sharing is related to the field current excitation control .
A common passive means to parallel alternators is using droop control, where the gen-sets are set to have the same no-load speed and voltage, and the speed and voltage of both units is designed to fall (droop) by the same amount (typically 3 to 5%) at full-load. Alternatively a more complex active parallel control system can be used which monitors the real and reactive output of each generator and controls them to be in proportion to their rating without requiring the need for a change in the output voltage or frequency with load .
Recently a novel interior permanent magnet (PM) alternator has been developed as a replacement for the wound-field alternator for small (kW range) gen-sets . Despite its lack of field control, by careful design of the alternator it has an acceptable voltage regulation. Compared to the conventional wound-field alternator it offers higher efficiency and smaller size and weight.
This paper examines the parallel operation performance of the interior PM alternator.
Tests were performed using two small single-cylinder engines, one diesel and one petrol driven, both with identical two-pole three-phase interior permanent magnet (PM) alternators attached. The diesel engine was rated at 12 hp (9 kW) while the petrol engine was rated at 13 hp (9.7 kW). The engines were made by different manufacturers. The different engine types were chosen to test the ability of paralleling systems with different load characteristics.
A paralleling switch was set up with a three-phase contactor and a simple electronic circuit that detected the point when the phase waveforms were in synchronism. The neutrals of the three-phase circuits were connected together, leaving the three phases open to be connected when the contactor closed for paralleling.
This paper describes the series of tests done to verify the parallel operating performance of the alternators. Section II describes the steady-state load sharing testing, section III discusses the step load transient waveforms and section IV investigates the capability of the paralleled alternators to start induction machines.
II. Steady-State Parallel Testing
This section of the testing investigated the ability of the paralleled alternators to share load under steady-state conditions with a three-phase resistive load bank.
The first step was to conduct load tests on each of the two generating sets (gen-sets) separately and generate a load voltage droop curve for each. These are shown in Fig. 3 (a). Although the alternators are the same, the engine throttle responses are quite different. This results in different voltage versus load curves for the two engines. This would generally make it difficult to parallel the two generating sets (gen-sets) with simple controls.
The two gen-sets were then paralleled using the simple switching unit described earlier. The paralleling method used in this test was droop paralleling. This method requires the voltage of the alternator to drop as the load current increases. This allows the alternators to automatically balance the load between the two paralleled alternators. Conventional wound-rotor alternators need to add a droop current transformer sensor with feedback to the automatic voltage regulator (AVR) to do droop paralleling
The first tests were taken without any attempt to adjust the engine throttle to balance the voltages at no load.
Fig. 3 (a) also shows the load voltage curve when the two alternators were running in parallel. The paralleled curve follows the petrol engine voltage load curve until the load gets to 3,500 W, and then the diesel engine begins to share the load. From 4,500 W to the maximum load of 12,500 W the diesel engine takes an increasing share of the load (based on its output current) until it is in proportion to the engine ratings at maximum load. The line THD stayed in the range 1.4-2% while the phase THD increased from 2 to about 6%.
Fig. 3 (b) shows the output currents of the two paralleled alternators as well as the circulating current betweenhttps://rflalternators.com/wp-content/uploads/2016/07/featured-image.jpg 300 550 Jason Clegg https://rflalternators.com/wp-content/uploads/2016/06/rfl_alternators_logo.png Jason Clegg2016-07-11 12:07:382017-10-10 07:04:40Parallel Operation of Small Interior Permanent Magnet Alternators