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What Is Generator Droop Control

Aug. 19, 2019

The so-called droop control is to select the frequency droop characteristic curve (Droop Character) similar to the traditional generator as the micro-source control method, that is, to obtain stable frequency and voltage through P/f droop control and Q/V droop control respectively. The control method separately controls the active power and reactive power of the micro-source output in the micro-grid, without the communication coordination between the gensets, realizes the goal of micro-source plug-and-play and peer-to-peer control, and ensures the unity of power balance and frequency in the micro-grid under the island, which has the characteristics of simplicity and reliability.

Using the frequency active droop characteristic (Droop Character), the unbalanced power of the system is dynamically allocated to each unit to ensure the uniformity of the frequency and voltage in the micro-grid system, which is simple and reliable. The droop control adopts voltage and current double loop control, and the current inner loop dynamic response speed is fast, which is used to improve the power quality of the inverter output. The voltage outer loop controller has a slow dynamic response speed, which can control the output voltage of the system and generate the reference signal  of the inner loop.

The voltage-current double closed-loop control used in droop control includes voltage outer loop control and current inner loop control. Firstly, the voltage and current of the load point are collected by the measurement module, and the output instantaneous active and reactive power of the micro power supply are calculated, and then the corresponding average power is obtained through the low-pass filter LPF.

The principle is that the inverter power source detects the output power of each power, and performs independent decoupling control on the active and reactive parts. The droop characteristic is used to obtain the reference value of the output frequency and the voltage amplitude, so as to reasonably distribute the active power and reactive power of the system.

generator set

Generator Droop/Isochronous mode analysis

The ISOCH mode is the no-difference mode, but this mode is used when the generator is running, that is, set the frequency (Speed), and then the positive feedback value is adjusted by the load change to achieve a stable frequency. When two or more generators are running in parallel, if the ISOCH mode is adopted, the speed regulation ring of each generator will act when the load changes. Because each speed regulating ring is carried out separately, the speed regulation will be uneven and the adjustment will be too larger or too small, causing the generator to run off. However, if the speed regulation ring of multiple generators are connected together in the ISOCH mode, then the adjustment parameters can be corrected to achieve parallel operation of multiple generators and stable frequency operation.

Droop is a differential mode. Droop itself means drooping, that is, its speed and load curve is a drooping straight line (ISOCH mode is a parallel straight line of X-axis), and the speed control system will modify the frequency adjusted by the speed-regulating ring according to the load. The adjusted frequency, which is not the frequency we want to stabilize, but requires a secondary speed (such as manual) in order to achieve a stable frequency. The purpose of this is to prevent the generator from being overloaded or being reversed to cause a trip when the load rate of the generator set is suddenly high and low.

Two operating modes of the generator set: 1. Droop mode, the speed is sacrificed with load. 2. ISO mode, using PMS to achieve constant speed operation mode of the generator. Both Droop and ISO refer to the speed regulation characteristic mode of the governor, which is differential speed regulation and non-differential speed regulation.

Active power output regulation

1.1 Control mode

It is divided into Droop, Isochronous and BaseLoad modes. The generator only works in Droop and Isochronous mode when the main circuit breaker is broken. The BaseLoad mode may be effective when the main circuit breaker closes and the generator runs with load.

1.2 Droop mode

Droop refers to changing the speed and frequency of the generator to proportionally change the load. That is, when the load of the generator increases, the speed or frequency is reduced by simple load feedback.

When the generator is running at full load, the Droop mode can proportionally reduce the speed of the generator. Given a suitable DRP value, the generator will always produce the same power at a fixed frequency. Therefore, the DRP mode is sometimes called the percent speed regulation mode.

If all generating sets are set to the same DRP mode, they will distribute the load correspondingly to each other. The load size depends on their speed setting. If the system load changes, the frequency of the system will also change. At this point, the speed given value will need to be changed to offset the change in feedback and return the system to its original frequency. In order for all the generators in the system to maintain the original power distribution state, each generator set needs to make the same change at the same time.

In DRP mode, when a generator set is connected to the system, the load control will not load the generator at the moment when the main circuit breaker is closed, that is to say, the generator load is set at zero. At this point, the input control point of LoadRaise must be used to make the controller load the generator. As long as the generator is operating in DRP mode, the user can increase/decrease the load of the generator at any time through the load adjustment switch.

1.3 Isochronous mode

Synchronous mode means that the generator is always running at the rated synchronous speed regardless of its power. The ISC mode has no power feedback link, so in most cases it can only be applied to the case where only one generator is running in the grid. When more than two generators are running in parallel, there must be a load feedback method to manage the load of each generator.

1.4 Load distribution in ISC mode

The EGCP-3LD can distribute the load between generators operating in multiple ISC modes. The feedback signal is provided by the LON network and the speed can be kept constant as the load changes.

The LON information is based on the rated power of the generator. If the rated capacity of each generator is different, LON will maintain their same load percentage.

The EGCP-3LS automatically adjusts the load distribution between generators operating in multiple ISC modes to maintain the output frequency at the rated value.

1.5 BaseLoad mode

The BSL mode is similar to the DRP mode and is used when running in parallel with an infinite grid or facility. The advantage is that the frequency does not change when it is separated from the facility. Simply set the trimming signal to zero to separate from the facility and return to ISC mode operation.

In BSL mode, the load reference can be changed either internally or remotely. Remote refers to the method of inputting through an external analog signal.

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