The results show that the stability of such systems, containing IM loads and a STATCOM, under weak grid conditions is determined by the terminal voltage controller parameters.
It can be seen from the Fig. 3 that the input electric power of IM consists of two parts: ① the power from network, which is affected by the terminal voltage vector, the transmission line and the characteristics of capacitor; ② the power from the STATCOM, which is closely related to the terminal voltage magnitude and the corresponding controller.
According to Table 1, if the reactive power remains unchanged and the active power is adjusted step-by-step, the terminal voltage mainly displays amplitude variation.
Recently, a flux magnitude and angle controller (FMAC) scheme is put forward for DFIGs, by which the terminal voltage and stator power can be controlled via the magnitude and angle of the rotor flux vector decouply [45].
Polarization curves were acquired by recording the terminal voltage V under pseudo-steady-state conditions 5 with variable external loads (Rext) and plotting the cell voltage as a function of current density (current per unit anodic area).
The power can be measured by graphically multiplying the terminal voltage and current waveforms from an oscilloscope and normalizing the energy product to 1 s (power = energy/second) or by measuring the voltage and current directly using a digital voltmeter, if it is a DC application.
The transmission line is denoted by the inductance L. The terminal voltage of the VSC is u PCC and u g is the grid voltage.
A high value of ω c will decrease the disturbance rejection capability of the PLL, and the damping factor ζ has relatively less effect on the disturbance rejection ability of the PLL. Figure 4 shows the relation between actual dq rotating reference frame and measured d′q′ rotating reference frame, which is determined by PLL technology when the terminal voltage orientation method is applied.
Here, J denotes the output current of the solar cell, J 0 the diode reverse bias saturation current, J L the current generated by the incident light, V the terminal voltage of the cell, ( frac{q}{kT} ) is ~1/25.6 mV at room temperature, and A is the diode's ideality factor.
In all these applications, the current changes are not characterized by the integer-order derivative of the terminal voltage as a consequence of the specific properties of these elements.
The terminal voltage is determined by the external grid voltage and rotor current based on (1) and (2), which can be formulated as: U_{s0} = frac{{Z_{s} }}{{Z_{0} + Z_{s} }}U_{0} + frac{{{text{j}}X_{m} Z_{0} }}{{Z_{0} + Z_{s} }}I_{r0} (3)where Z s = R s + jω e L s, X m = ω e L m, and I r0 is the steady-state rotor current.
