What are the common problems in parallel debugging of diesel generator sets?

Two parallel diesel generator sets are more and more common, often because of the increase of power demand, the service of parallel diesel generator sets is required. With the development of modernization, generator sets are more and more used in national defense engineering and weapon systems. , field work and other projects. In order to meet the requirements of heavy load or uninterrupted power supply, it is often necessary to connect two or more units in parallel. In parallel operation of units, the phenomenon of uneven power distribution often occurs. Excessive uneven power distribution will seriously affect the safety and reliability of the power station system, and will cause serious harm to the generator set. This kind of harmfulness is rooted in the circulation problem of the system, and it is also the most common and difficult to deal with in the debugging of parallel power plants. Based on some experience in debugging parallel units, the technicians of our company put forward the reasons for the occurrence of circulating current, some factors affecting the average power distribution and the treatment methods.

Static Analysis of Circulation Occurrence

Taking the modular parallel control system as an example, the parallel commissioning of generator sets generally balances the circulating current of the parallel generators in parallel at no-load to meet the requirements of small and stable operation, and then adjusts and balances the active power through the load divider. The circulating current problem when the load is connected in parallel. Taking two units in parallel as an example, the problems often presented in no-load parallel connection are as follows:

(1) The circulating current is too large, far exceeding 10% of the rated current of the parallel unit;

(2) After parallel connection, the circulating current gradually increases with the working time until the reverse power alarm;

(3) The circulation is unstable, and the randomness fluctuates.

How to deal with these problems? Let's take the parallel connection of two equal-power generators as an example, and first analyze the reasons for the occurrence of circulating currents.

Circulating current U1: 1# unit terminal voltage, U2: 2# unit terminal voltage, R3: load of two units working in parallel, I0: circulating current, I1: 1# unit output current, I2: 2# unit output current. Haifeng diesel generator sets provide technical support. If two units work in parallel, and the circulating current I0 is 0 under any load, it is necessary that U1=U2, that is, the terminal voltages of the two units are equal under any load. No-load parallel connection is equivalent to infinite load, and the no-load terminal voltages U01 and U02 should also be equal. That is, U01=U02(1-2) We know that the average distribution of active power depends on the characteristics of the diesel engine and its speed control system, while the distribution of reactive power depends on the characteristics of the generator and its excitation system, that is, the generator set its own voltage regulation characteristics. The voltage regulation characteristic is a U=f(I) curve, U is the terminal voltage of the generator set, and I is the current. In order to facilitate the analysis of the problem, the curve is usually replaced by a straight line approximation. Assuming that there are two parallel generator sets, respectively with the voltage regulation characteristics shown in Figure 2, set: δ1=tgβ1, δ2=tgβ2, δ1: the voltage regulation characteristics of the 1# unit, δ2: the voltage regulation characteristics of the 2# unit .

From the above analysis it can be seen that:

(1) To connect two units in parallel, firstly, adjust the no-load voltage and voltage regulation characteristics of the two units to be exactly the same. This is the precondition to ensure the complete and even distribution of reactive power between the two units, and it is also the subsequent adjustment of the two units. Basis for equal distribution of power. When the above two items are adjusted and balanced, it is possible to ensure that the output voltages of the two units working in parallel are equal under any load, and at the same time to ensure the equal distribution of power, and to ensure that the circulating current is 0 (ideal case). It is indicated that the root cause of circulating current is that the no-load voltages of the two units are not completely equal or the voltage regulation characteristics are different, resulting in unequal output voltages and circulating currents.

(2) The no-load voltage and voltage regulation characteristics of the two units are equal, but the output currents of the two units are not equal, that is, the power distribution of the two units is uneven, which will also cause U1 and U2 to be unequal, and a circulating current will occur. .

(3) There are many factors that affect the distribution of reactive power, such as the characteristics of the automatic voltage regulator, the stabilizing effect of the voltage equalizing line, etc., which will not be analyzed here.

The dynamic balance analysis of power distribution takes the parallel connection of two equal-power generator sets as an example. GB/T12786-2006 General technical conditions and rules for automated internal combustion engine power stations: the difference in active power distribution between 20% and 80% of the calibration rating is ≤±10%, and the average distribution of active power between two parallel units is mainly by the two units. This is done by balancing the chain between load dividers. Schematic diagram of the balance chain operation. Be

After adjusting the "load distribution" potentiometer of the load distributor, the power distribution parameters of the two load distributors can be adjusted to be equal, that is, η1=η2. Assuming that the loads of the two units are definitely equal, that is, P1=P2, then u1=u2, I0=0. This is the ideal situation of perfect balance. In fact, this is a dynamic balance process: it is assumed that the two units are working in parallel with the loads of P1 and P2 respectively, during which P1=P2.