Smart Solar PV Inverters with Advanced Grid Support Functionalities. Rajiv K. Varma

distortions can potentially cause overheating and failure of equipment such as transformers, motors, and other voltage sensitive equipment connected in the vicinity. Cases have been reported where harmonic amplification due to network resonances has led to shutdown of wind turbines.

      In a practical scenario of high voltage distortion due to network resonance, it may be difficult to identify if the harmonic resonance is caused by series or parallel resonance. Background harmonics need to be measured by harmonic monitoring equipment over a prolonged period of time, especially before the installation of the wind plant (in this case). The harmonic injections are typically provided by PV inverter manufacturers and wind plant inverter under laboratory conditions, where ambient harmonics are not present. The harmonic injections may be different when connected to the grid having ambient harmonics.

      Still, harmonic measurements at the PV plant and wind plant during no active power output conditions and operation at different power levels under different grid conditions can help in identifying the cause of harmonic distortion and developing mitigation strategies.

      In a utility study [62], it is shown that bus capacitors interact with system short circuit impedance (inductance) and cause network impedance resonances. For realistic short circuit levels, the network impedance resonant frequencies get aligned with the harmonics injected by the PV inverters and result in harmonic amplification and voltage THD in excess of the stipulated limits. For the study system, even though 20 MW of PV generation does not cause any violation of limits related to steady‐state voltage and TOV, only 6.9 MW of PV generation can cause the utility THD limit to be violated. Thus the limit of PV connectivity was dictated by total harmonic distortion considerations than from overvoltage criterion.

      Another example of harmonic resonance in a 54 MW solar PV plant caused by capacitor switching is described in [63].

The various harmonic mitigation measures adopted in utilities include: (i) changing the size and sequence of energization of capacitor banks within a plant, (ii) detuning the network resonances by network reconfiguration, (iii) reduction of harmonic generation, (iv) converting existing power factor correction capacitors into passive filters, (v) utilizing separate passive filters, and (vi) employing active filters, etc. [60].

      1.2.14 Low Short Circuit Levels

      As solar PV systems increasingly displace conventional synchronous generators, the amount of short circuit current capability in the power system decreases resulting in weak grids [16]. The stability of inner current control loops and the phase‐locked loop declines when inverters are connected to networks which are weak, i.e. where the network voltage is susceptible to substantial variability. Low short circuit levels may also cause challenges in disconnecting solar PV systems during unintentional islanding scenarios.

      1.2.15 Protection and Control Issues

      Protection systems of power distribution systems comprise overcurrent relays, fuses, and reclosers that are designed to operate for only one direction of power flow, i.e. from the grid to the loads or to the possible fault location [20, 64]. With the high proliferation of solar PV systems, the following challenges are experienced [16, 20,64–69, 70]:

      1 The fault current may become bidirectional as it will be fed both from the grid and the different solar PV systems in the distribution system.

      2 It is not necessary that the fault current will always increase with the introduction of solar PV systems: it can both increase and decrease depending upon the location of the concerned PV systems with respect to the fault location [71]. Solar PV systems installed upstream of the fault location would increase the fault current while locating them downstream could reduce the fault current level. Hence the protection coordination done earlier without the solar PV systems, i.e. with only unidirectional flow of power will not apply when multiple solar PV systems are connected.

      3 The power output from solar PV systems is variable. It can be either zero as in the night or maximum with full solar irradiance. Furthermore, the power output is variable due to varying cloud coverage. These operating conditions lead to changing fault current levels. Hence, variability in solar and wind DER power production can adversely impact the operation of overvoltage and overcurrent protection systems.

      4 The conventional power systems were not designed for power flow in reverse direction. The relays and protection systems have to be modified to adapt to the backflow of power. Presence of high amount of solar PV systems results in current flows from multiple sources, which makes protection coordination even more complex and difficult. Protection device settings need to be modified or relays may need to be replaced in distribution systems where reverse power flow is expected [26].

      1.2.16 Short Circuit Current Issues

      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