Читать книгу Smart Solar PV Inverters with Advanced Grid Support Functionalities - Rajiv K. Varma - Страница 44

Solar PV systems contribute fault currents in a different manner than synchronous generators and hence impact the operation of protection systems differently [27].

The fault currents from PV inverters are much smaller than those from synchronous generators and are for much shorter duration (typically less than a cycle to few cycles) [72]. The magnitude and duration of fault currents are determined by inverter control and not so much by the impedance between the PV system and the fault location. This unique behavior of DER inverters during system faults may cause different issues such as enhanced short circuit currents, relay desensitization, sympathetic tripping, breaker reduction of reach, protection coordination issues, etc. [16].

The short circuit current contribution from solar PV systems is dependent on the controls employed in inverter systems. This is quite different from the behavior of synchronous generators during faults, which depends upon its different effective reactances during the fault, i.e. subtransient, transient, and steady‐state reactances. Moreover, the inverter fault current does not include zero sequence component and the negative sequence current is typically partially or fully suppressed depending on the inverter control [73].

The short circuit current impact of solar PV systems is a function of the following factors:

1 size of solar PV system

2 location of solar PV system

3 nature of controlled short circuit current from the solar PV system, which is dependent upon the inverter control employed

4 trip time of solar PV systems with decreased terminal voltages subsequent to the fault occurrence

5 constitution of the short circuit current, i.e. the relative magnitudes of active and reactive currents therein. It is noted that solar PV systems operating on unity power factor control will inject primarily real short circuit current, whereas solar PV systems operating with LVRT characteristics will emanate a higher component of reactive currents

6 configuration of interconnection transformer

7 grounding techniques employed, and the resulting flow of zero sequence currents.

All the above factors impact conventional protection and relaying schemes. Studies have been reported [74] that even if the flow of power is unidirectional, increasing penetration levels of solar PV systems can cause miscoordination between the different protection devices.

Proliferation of solar PV systems in the network increases the short circuit level due to their short circuit current contribution during faults [21,75–77]. The short circuit current contribution from a PV system inverter is typically in the range of 1.2 times rated current for the large size inverter (1 MW), 1.5 times (500 kW) for medium size inverter and between 2 and 3 times for smaller inverters [72, 78]. While the short circuit current contribution from an individual solar PV system may be small, the total amount of short circuit current contribution may become appreciably large for high penetration of PV DERs [79].

It is a reasonable expectation that short circuit current contributions from a large number of solar systems in the distribution networks may add up to levels that could damage circuit breakers. Hence, circuit breakers will need to be upgraded and substations will need to be modified at a significant cost to the concerned utility. This apprehension actually resulted in the denial of about 45% applications for solar farm connections in Ontario, Canada, during 2011–2013. Consequently, a major effort was launched by CanSIA, the national trade association representing the solar energy industry in Canada, to investigate the actual impact of fault currents from PV inverters, and their remedial measures [72], so that more PV systems can be connected in Ontario.

The problem of short circuit currents can be solved by substation upgrades, installing series reactors [80], or by employing fault current limiters [81].