Source: Ministry of Power Generation Funeng Guidian New Youth
Basic concepts
The operation of the synchronous generator is mainly manifested by the stator and rotor current, voltage, power factor, etc. The interaction between these voltage and current parameters constitutes the operating characteristics of the generator. These main parameters are summarized below:
a) Voltage: refers to the line voltage of the three-phase winding of the stator at the end of the generator when it is running.
b) Current: refers to the current flowing through each phase of the stator winding when the generator is running.
c) Power factor: the cosine value of the phase angle difference between the load voltage of the generator and the load current, that is, the ratio of power to capacity, and has the relationship of Pn= UnIncosΦn.
d) Rotor current: the value of the DC current flowing through the generator's excitation winding.
e) Rotor voltage: the DC voltage value between the positive and negative poles at the excitation winding end of the generator.
1. No-load characteristics
The no-load characteristic of a synchronous generator refers to the relationship between the no-load electromotive force and the excitation current when the stator winding is open at the rated speed of the generator (Fig. 1). It shows the saturation of the magnetic circuit of the generator, and generally speaking, the slope of the flat part of the curve is larger, indicating that the core characteristics are better. This curve can be used to check whether there is an inter-turn fault in the excitation winding. It can also reflect some problems of the magnetic conduction pathway.
Fig.1. No-load characteristic curve of synchronous generator
2. Short-circuit characteristics
The short-circuit characteristic refers to the relationship between the steady-state short-circuit current of the stator and the excitation current when the generator is short-circuited in the three-phase of the stator winding at the rated speed (Figure 2). The short-circuit current curve should be a straight line, and according to this curve, the saturation synchronous reactance to short-circuit ratio can be obtained. When there is an interturn short circuit in the excitation winding, the short-circuit characteristic curve is reduced, so it can also be used to check for such faults.
Fig.2. Short-circuit characteristic curve of synchronous generator
3. Load characteristics
The load characteristic refers to the relationship between the generator voltage and the excitation current when the speed and stator current are rated and the power factor is constant (Figure 3), i.e., U=f(IL). When the type of load is different, the power factor is different, from which a variety of load characteristic curves can be obtained. The basic parameters of the generator can be determined by using the load characteristics, no-load characteristics, and short-circuit characteristics.
Fig.3. Load characteristic curve of synchronous generator
4. External characteristics
The external characteristics of a synchronous generator refer to the relationship between the terminal voltage and the load current when the excitation current and power factors remain unchanged at the rated speed of the generator (Fig. 4). The synchronization characteristic reflects the demagnetization or magnetization of the load current to the rotor current when the load of the generator is inductive or capacitive. In the figure below, curve 1 is the inductive load curve, curve 2 is the resistive load curve, and curve 3 is the capacitive load curve.
Fig.4. External characteristic curve of synchronous generator
5. Adjust the characteristics
The regulation characteristic refers to the relationship between the excitation current and the load current when the terminal voltage and load power factors remain unchanged at the rated speed of the synchronous generator (Fig. 5). The adjustment characteristics reflect the demagnetization or magnetization of the rotor current by the load current when the load of the generator is inductive or capacitive. It is similar to the external characteristics, but the variables are different.
Fig.5. Adjustment characteristic curve of synchronous generator
6. Power angle characteristicsThe power angle characteristics refer to the relationship between the electromagnetic power and the power angle of the generator when it is connected to the power grid in a steady state (Fig. 6). The so-called power angle refers to the phase angle between the no-load potential E0 and the terminal voltage U of the generator. Simplified phasor diagram of a synchronous generator (Figure 7).
The relationship and vector diagram representing the power angle characteristics of the generator are as follows:
PG=UIcosΦ=E0U*sinδ/Xd
where: PG ------ the electromagnetic power of one phase of the generator
U------- phase voltage of the generator
I-------- phase current of the generator
E0 ------ the no-load potential of the generator
Xd ------ synchronous reactance of the generator
φ------ power factor angle
δ------ Gong angle
When the power angle is less than 90°, the electromagnetic power and the power angle change in the same direction when the voltage and excitation current at the generator end are unchanged. When the power angle is 90°, the electromagnetic power reaches the maximum value, and PGmax= E0*U/Xd(sinδ=1); When the power angle exceeds 90°, the electromagnetic power becomes smaller as the power angle continues to increase. When the power angle is greater than 180°, the electromagnetic power becomes negative, indicating that the generator becomes an electric motor and absorbs active power from the system. By increasing the excitation to reduce the power angle angle, it is an effective means to prevent the unit from losing step. Therefore, it is necessary to increase the excitation when the generator is out of sync with the system, or to limit the active power output when the generator is under-magnetic.
Fig.6. Power angle characteristic curve Fig.7. Simplified vector diagram of synchronous generator