Basic Concept
The operation status of the synchronous generator is mainly manifested by the stator and rotor current and voltage, power factor, and so on. The mutual influence relationship between these voltage and current parameters constitutes the operating characteristics of the generator. The main parameters are briefly described as follows:
a) Voltage: refers to the line voltage of the three-phase winding of the stator at the machine end when the generator 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 generator load voltage and the load current, that is, the ratio of power to capacity, and has a relationship of Pn=UnIncosΦn.
d) Rotor current: the value of DC current flowing through the excitation winding of the generator.
e) Rotor voltage: DC voltage value between positive and negative terminals of generator excitation winding.
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 (Figure 1). It proves the saturation of the magnetic circuit of the generator. 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 a turn-to-turn fault in the field winding. It can also reflect some problems of the magnetic conduction path.
Fig.1 No-load characteristic curve of synchronous generator
2. Short circuit characteristics
The short-circuit characteristic refers to the relationship between the stator steady-state short-circuit current and the excitation current when the three-phase short-circuit of the stator winding is under the rated speed of the generator (Figure 2). The short-circuit current curve should be a straight line. According to the curve, the saturated synchronous reactance and short-circuit ratio can be obtained. The short-circuit characteristic curve is reduced in the event of a turn-to-turn short circuit in the field winding and can therefore also be used to check for such faults.
Figure 2 Synchronous generator short-circuit characteristic curve
3. Load characteristics
The load characteristic refers to the relationship curve between the generator voltage and the excitation current when the speed and the stator current are the rated values, and the power factor is constant (Figure 3), that is, U=f(IL). When the types of loads are different, the power factors are different, and various load characteristic curves can be obtained from this. With load characteristics, no-load characteristics, and short-circuit characteristics, the basic parameters of the generator can be determined.
Figure 3 Load characteristic curve of synchronous generator
4. External characteristics
The external characteristics of a synchronous generator refer to the relationship curve between the terminal voltage and the load current when the generator is at the rated speed and the excitation current and power factor are kept constant (Figure 4). The synchronous characteristic reflects the demagnetization or magnetization of the rotor current by the load current when the load of the generator is inductive or capacitive. Curve 1 in the figure below is the inductive load curve, curve 2 is the resistive load curve, and curve 3 is the capacitive load curve.
Figure 4 External characteristic curve of synchronous generator
5. Adjustment features
The regulation characteristic refers to the relationship curve between the excitation current and the load current when the terminal voltage and load power factor of the synchronous generator are constant at the rated speed (Figure 5). The adjustment characteristics reflect the demagnetization or magnetization of the rotor current by the load current when the generator load is inductive or capacitive. It is similar to the external characteristics, but the variables are different.
Figure 5 Synchronous generator adjustment characteristic curve
6. Power angle characteristics
Power angle characteristics refer to the relationship between the electromagnetic power of the generator and the power angle when the generator is connected to the power grid for steady-state operation (Figure 6). The so-called power angle refers to the phase angle between the no-load potential E0 of the generator and the terminal voltage U. Simplified phasor diagram of a synchronous generator (Fig. 7).
The relational expression and vector diagram representing the power angle characteristics of the generator are as follows:
PG=UIcosΦ=E0U*sinδ/Xd
In the formula: PG——electromagnetic power of one phase of the generator
U——-phase voltage of the generator
I——–phase current of the generator
E0——No-load potential of the generator
The xd —— synchronous reactance of the generator
φ——power factor angle
δ——power angle
When the power angle is less than 90°, when the generator terminal voltage and excitation current are constant, the electromagnetic power and the power angle change in the same direction; when the power angle is 90°, the electromagnetic power reaches the maximum value, at this time PGmax= E0*U/ Xd (sinδ=1); after the power angle exceeds 90°, as the power angle continues to increase, the electromagnetic power becomes smaller; when the power angle is greater than 180°, the electromagnetic power becomes negative, indicating that the generator becomes a motor, from The system absorbs active power. By increasing the excitation to reduce the power angle, it is an effective means to prevent the unit from out of step. Therefore, the excitation needs to be increased when the generator loses synchronization with the system, or the active power output needs to be limited when the generator is under-excited.
Figure 6 Power Angle Characteristic Curve
Fig.7 Simplified vector diagram of synchronous generator