Abstract: Low voltage generator sets are currently the emergency power source choice for most users, and this model usually refers to the commonly used 230V/400V diesel generator sets in the market. However, in some places, due to the distance between the diesel generator room and the electrical facilities, voltage drops may occur, resulting in the inability to use electricity normally, and in severe cases, even burning out electrical equipment. Therefore, for users who have already purchased low-pressure diesel generator sets, taking measures to upgrade from low-pressure to high-pressure becomes the best option, so as not to scrap the original low-pressure generator set and cause huge economic losses.
1、 Comparison of advantages and disadvantages of high and low pressure
1. Advantages of high-voltage generator sets:
(1) The power of the generator can be increased, and the maximum power of the high-voltage generator set can reach several thousand, or even tens of thousands of kilowatts. This is because, when outputting the same power, the current of a high-voltage generator can be much smaller than that of a low-voltage generator. So high-voltage generator windings can use smaller wire diameters. As a result, the stator copper loss of high-voltage generators will also be smaller than that of low-voltage generators. For high-power generators, when using low-voltage power, a larger stator slot is required due to the need for thicker wires, resulting in a larger diameter of the stator core and a larger volume of the entire generator;
(2) For larger capacity generators, high-voltage generators use less power and distribution equipment than low-voltage generators, and have lower line losses, which can save a certain amount of power consumption. Especially for 10KV high-voltage generators, they can directly use the grid power supply, which will reduce investment in power equipment, simplify use, and reduce the failure rate.
2. Disadvantages of high-voltage generator sets
(1) The cost of generator windings is relatively high, and the cost of related insulation materials will also increase accordingly;
(2) The requirements for the usage environment of generators are much higher than those for low-voltage generators;
2、 Boosting method for generator sets
For places that require high-voltage supply, in addition to being allocated to high-voltage generator sets, standard voltage generator sets with step-up transformers can also be used.
1. Advantages of Low Voltage to High Voltage Scheme
(1) There are two or more different voltage requirements at the construction site, or the voltage output of the generator set needs to be changed;
(2) (Isolation transformation isolation function) The high-voltage end is an angle transformer, and the three-phase three wire system has no zero line. Without a zero line, there is no zero line transfer; Isolate the harmonics generated by non line loads on the high-voltage side from the low-voltage side, making the low-voltage side clean and not affecting the operation of the automatic voltage regulator (AVR) inside the generator set, as well as solving various problems caused by zero line transfer;
(3) The Great Inertia buffering function is particularly helpful for starting large motors. Large capacity transformers contain a larger amount of copper material, and the large magnetic core plays a buffering role, reducing the impact on the generator and improving the instantaneous voltage drop.
2. Disadvantages of Parallel Connection Scheme for Low Voltage Generator Units
In a 380-415Vac generator set, if multiple generator sets are connected in parallel on the low-voltage side and then boosted by a step-up transformer; The recommended upper limit is 7500 kVA, 6000 kW. When exceeding the upper limit, the following factors should be considered:-
The capacity of the low-voltage side busbar should be close to 10kA, taking into account the busbar’s ability to withstand fault currents and the heat treatment inside the low-voltage switch (temperature rise of the low-voltage switch screen);
• The tripping capacity of low-voltage switches (up to withstand fault currents), such as up to 65kA and 100kA;
• It is necessary to calculate whether the installation of nearly 10000 ampere cables, low-voltage switches, and cost on the low-voltage side are reasonable;
3、 Renovation case
1. Equipment composition and parameters
User: A project in Macau
● Backup power supply: UPS+6000kVA generator
Total emergency capacity: 4500kVA, 3600kW
Voltage system: High voltage 11kV, 50Hz and low voltage 415 Vac50Hz
Power: 4 KTA50-GS8 models/1200kW generator sets
Generator set operation: 3 main and 1 backup, with 1 reserved for maintenance. Each generator set can be connected to the power grid for use
Generator set voltage: 415Vac/three-phase/50 cycles
● Low voltage switch screen of generator set:
5000A busbar/80kA1 seconds/three-phase four wire/50 cycles
The 5000A busbar is divided into sections A and B
Section A of the busbar is connected to two generator sets, one and two
Section B of the busbar is connected to two generator sets, 3 and 4
Installation of 5000A4 pole interconnection switch for busbar sections A and B
○ 4 × 2500A air switch → connected to 4 generator sets
3 × 3200A air switch → connected to 3 step-up transformers (low-voltage side)
● Step-up transformers: 3 sets of 2000kVA11kV/0.415kV
● High voltage switch screen of transformer: vacuum switch, 15kV600A → connected to 3 step-up transformers (high voltage side)
2. Plan analysis
(1) Four P1500 generator units are connected to the grid for use, using 3+1 generator units in parallel. Regardless of which unit needs maintenance, it does not affect the emergency power supply;
(2) In the event of a power outage, four generator sets will start simultaneously and connect four 2500A low-voltage switches and three 200A low-voltage switches on the low-voltage side, magnetizing the step-up transformer and closing three 600A high-voltage switches to supply power to various regions;
(3) Each partition does not require ATS automatic switching screens or independent generator rooms, saving a lot of costs and valuable land resources; Indirectly solve the problems of storing flammable materials, smoke exhaust, and noise caused by the generator room;
(4) In daily testing of generator sets, a starting command is issued to one or more designated generator sets by simulating a mains fault, but four 2500A low-voltage switches and three 3200A low-voltage switches do not close; And three 6000A high-voltage switches received the test program and conditionally cancelled the interlock to close. The 5000A busbar was powered on, and each generator set synchronized with the busbar. After synchronization inspection, the 2500A low-voltage switch was closed; After closing, the generator set undergoes a full load test. After the test is completed, the generator set first removes the negative pressure and trips to complete the test (first trip the 2500A low-voltage switch -3200A low-voltage switch -600A high-voltage switch);
(5) When the power supply bureau announces the need for a power outage, the generator set can be manually disconnected from the mains power supply after being loaded according to (4), so that the generator set can be powered on; Until the mains power is restored, the generator set synchronizes with the mains power under load. After grid connection, the generator set is removed and exited, and the user does not feel the temporary impact of power outage or switching throughout the entire process;
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Post time: Apr-01-2024