Читать книгу Smart Solar PV Inverters with Advanced Grid Support Functionalities - Rajiv K. Varma - Страница 49

Low‐frequency electromechanical power oscillations (typically 0.1–2 Hz) are recognized as one of the major limiting factors in power transfer over long transmission lines [85]. Conventionally, these oscillations are damped by Power System Stabilizers (PSSs) integrated with synchronous generators. Displacement of fossil fuel‐based steam turbine generators by inertialess IBRs such as solar PV systems results in overall reduction in damping of electromechanical and inter‐area oscillations, as conventional generators [86–89]. This especially becomes a concern during system transients such as faults or large equipment/line outages.

Figure 1.17 Typical behavior of power systems with different levels of stored kinetic energy during an overfrequency event.

Source: Reprinted with permission from EPRI [11].

An example study of reduced damping due to high penetration of solar PV systems is presented in [87]. Eigenvalue analysis and transient stability studies are performed on a test system representing the entire Western Electricity Coordinating Council (WECC) network ranging from 34.5 to 500 kV. The synchronous generators are modeled with excitation systems, PSS, and governors. Solar PV systems comprising both rooftop systems and utility‐scale plants are added in a region with a high potential of their growth. The utility‐scale PV systems are fixed at 600 MW while the amount of rooftop PV systems are varied to achieve different PV penetration scenarios.

The percent PV penetration is considered to be the ratio of total PV generation to total system generation. The solar PV penetration is increased by displacing conventional generators while still keeping critical generators providing reactive power support in service. To maintain the generation–load balance, the outputs of the critical generators are reduced to accommodate the increased penetration of solar PV systems. The rooftop PV systems are modeled with unity power factor while the utility‐scale solar PV systems are represented by full converter model and having reactive power based voltage control capability. A scenario of high solar PV output and low load is considered. The DSA Tools software package is used for performing both small‐signal studies and transient stability studies.

Critical electromechanical modes having a damping less than 10% are obtained from Eigen analysis. The variation of damping ratio of this system mode with increasing solar PV penetration is illustrated in Table 1.1 [87]. It is seen that increasing PV penetration has a detrimental impact on the damping of the critical electromechanical mode.

The generators having highest participation in this critical mode are identified through participation factor analysis. A single‐line diagram of the region close to these generators which are connected at buses 2102 and 2103 is depicted in Figure 1.18 [87].

A three‐phase fault at 345 kV bus numbered 2104 is initiated for four cycles. The speed of generator located at bus 2103 is portrayed in Figure 1.19 [87].

Clearly, the speed of this generator becomes more oscillatory with increasing solar PV penetration, i.e. with increasing displacement of conventional synchronous generators. It is, therefore, recommended that when solar PV penetration levels are likely to increase in a system, critical generators must be identified and still kept in service to maintain adequate damping levels of the critical electromechanical modes.

Table 1.1 Impact of PV penetration on damping of critical mode.

Source: Eftekharnejad et al. [87].

PV penetration (%)	Real part of critical eigenvalue	Imaginary part of critical eigenvalue	Frequency (Hz)	Damping ratio (%)
0	−1.0926	11.2221	1.786	9.69
20	−0.9135	11.1828	1.7798	8.14
30	−0.9423	11.1346	1.7721	8.43
40	−0.5173	11.0029	1.7512	4.7
50	−0.4876	10.9947	1.7499	4.43

Figure 1.18 Single‐line diagram of the study system near the participating generators.

Source: Eftekharnejad et al. [87].

Figure 1.19 Speed of generator 2103 after a three‐phase fault at bus 2104.

Source: Eftekharnejad et al. [87].