Protection Considerations for Interconnecting Solar Generation to the Transmission System

Authors: E. Padilla, H. Ladner-Garcia, J. Martinez-Butler, J. Deaton, M. Patel, M. A. Rios Rivera, P&C Applications - Southern Co. Services, USA

Distribution vs Transmission Connected Solar Generator Characteristics

There is a significant amount of solar generation connected to the Georgia Power Company (GPC) distribution system. Solar generation that is directly connected to the transmission network, at 46kV and below is often mistaken for distribution connected generation. GPC-Distribution has developed a philosophy for interconnecting solar generation to the system that is different from the philosophy adopted by the GPC- Transmission. Distribution connected solar generation is required to comply with IEEE 1547 standard. GPC-Distribution personnel perform a series of tests to ensure compliance for all generators greater than 250kW in capacity. The following on site tests are performed: 

  • Three Phase Disconnect Test is intended to simulate a sudden loss of utility. Upon loss of the utility connection, it is possible for the inverter based solar generator to produce a short duration transient over-voltage. Such over-voltages could damage the utility and/or customer equipment on the generator side of the open point. This test presents the worst case, as it disconnects the inverter at near maximum output and with no islanded or stranded load connected to it.  The magnitude and duration of the measured transient over-voltage has to be below pre-determined thresholds for the generator to pass this test
  • Single Phase Disconnect Test opens one of three phases of the utility source and measures the response of the solar generator. It is required that the solar generator cease operation in 2 seconds or less for it to pass this test
  • Inverter Control Mode Test ensures that the solar generator operates continuously at the pre-determined power factor. Solar generators on the distribution system are not allowed to control the voltage at the Point of Interconnection (POI)
  • Generation Start-Up Time: The solar generator is expected to curtail output for system disturbances. When the utility source returns, the generator must wait for a minimum of 5 minutes before producing power again. This test is performed to ensure just that

 

The solar generator inverters are considered utility interactive/dependent in that an external source is required for the solar generator to be operational. Hence, a utility owned anti-islanding protection scheme is typically not required. In some cases, a utility installed Direct Transfer Trip (DTT) is required to trip the interrupting device at the POI if the utility source is lost. During the sudden loss of a utility source, inverters are required to cease operation within 2 seconds. If the ratio of generation capacity to the peak load exceeds 50%, it is possible that stranded utility customers on the generation side of the disconnect switch can experience temporary over-voltage for up to 2 seconds. For these cases, a utility installed DTT is required to protect utility customers. On the contrary, transmission connected solar generators are required to:

  • Control voltage at the POI per the voltage schedule provided by the control center and/or transmission planning
  • Required to ride-through and provide support during grid disturbances    

Anti-Islanding Policy and Protection
Georgia Power Company does not allow sustainable islands for N-1 contingencies because the generator, regardless of the its type and size, may not be able to hold voltage and frequency within acceptable limits, posing a threat to utility and customer equipment. It also raises safety concerns for utility personal working in the area. For a tapped solar interconnection projects, an N-1 contingency could result in the solar generator and the utility load connected together in an island.

Is anti-islanding protection needed? If solar inverters can operate independently without the utility source anti-islanding protection must be considered. There are two types of inverters used to convert DC power into AC Power at solar farms. Line-commutated inverters need a healthy grid/utility AC voltage to be operational. They automatically shut down for a loss of utility source. Conversely, the self-commutated inverters do not need a healthy utility voltage source to convert DC power into AC power. These types of inverters may have the capability to control the grid voltage and frequency. Per company policy, all transmission connected solar generators are required to control voltage at the POI and provide support to the transmission grid during system disturbances. Hence, it was assumed that inverters used at various solar sites may have capabilities to self-commutate. Even if the line-commutated inverters are applied, there was a concern that they won’t shut down upon loss of utility power because multiple line-commutated inverters may keep supporting each other and sustain a stable island. Also, when the inverters are programmed to provide voltage control and ride-through capabilities, they do not comply with the requirements of IEEE 1547 standard. Hence, with some ambiguity in mind, it was assumed that inverters at these large utility scale solar sites would be able to operate in an island if load and the generation in the isolated region are comparable. This leads to the next question.

When should anti-islanding protection be required? If the Minimum Day Time Load (MDTL) to total generation ratio is less than 2 anti-islanding protection is required. The MDTL is determined based on actual load data from previous 12 months between the hours of 9:00 AM and 3:00 PM. If the actual load data is not available, MDTL is assumed to be equal to 15% of the peak non-industrial load. For solar panels mounted at a fixed angle, the generation usually peaks between 11:00 AM and 1:00 PM. For solar panels mounted to track the sun, the time window for which generation peaks is extended to 9:00 AM to 3:00 PM.
Figure 1 shows the 24-hour load curve for tapped load on a network transmission line for spring, summer, fall and winter. The MDTL for this line between 9:00 AM and 3:00 PM is approximately 30MWs during spring and fall season. A 20MW solar generator is proposed to connect on the low side of one of the tapped stations on this line. The load to generation ratio in this case is 1.5 and anti-islanding protection would be implemented.

IEEE standard 1547 recommends using a load-to-generation ratio of three to evaluate the need for anti-islanding protection. That recommendation was based on dynamic studies that involved synchronous machines in an island. But for solar generators, this recommendation seemed very conservative. Solar generators could provide voltage control but the performance may not be as good as with synchronous machines. Solar generators could curtail total active power generation to help high frequency grid events but are not expected to provide any support when the frequency is below the nominal value. As such, they are not expected to stay online for long when the load-to-generation ratio is high. In view of this, a load-to-generation ratio of two was selected to evaluate the need for anti-islanding protection.

How to protect for islanding? Schemes can be classified into two categories: local detection schemes and communication based detection schemes. Local detection schemes can further be classified as passive and active detection schemes.
The local passive detection scheme uses a relay or similar equipment typically installed at the POI. This relay could employ voltage, frequency, rate of change of voltage/frequency, vector shifts etc. to detect formation of an island. These types of schemes may fail to detect an island successfully, especially when load and generation in an island are comparable.
The local active detection schemes continuously try to change system parameters or inject signal into the system and observe the system response to detect an island. These schemes are expected to detect islands even when load and generation in an island are comparable.

Communication based detection schemes are simple and use fiber, Power Line Carrier (PLC) or microwave radio to send utility status to the generator site. These schemes are likely to be most expensive to implement compared to local detection schemes.
Local detection schemes were explored, but communication based DTT was chosen to protect against islanding for the following reasons:

  • Low confidence in passive detection schemes to successfully detect an island when load and generation in an island are comparable
  • Lack of experience and demonstration of successful installations of active detection schemes

Anti-Islanding Scheme Design: A simplified tapped generation interconnection is shown in Figure 2. Breaker B is the utility breaker at the POI and breaker A is the main breaker at the solar site. The anti-islanding DTT protection for the interconnection can be divided in two zones characterized as follows:

Utility Line Zone: Line C-D is typically existing construction. There may be a step-down transformer between the source line and breaker B. A unidirectional transfer trip scheme is required to trip breaker B for any condition that results in breakers C and D being open simultaneously. Power line carrier schemes are preferred for transmission voltage interconnections due to cost effectiveness. A single channel transfer trip transmitter is used at line terminals, C and D, with two receivers at terminal B. This arrangement provides security and dependability since a signal must be received from two terminals for the trip command to be initiated. Note that the logic is designed to assert trip from the receiver associated with terminal C or D if communication from that terminal is lost or the scheme is disabled. A conceptual diagram for this scheme is shown in Figure 3.

The anti-islanding scheme is considered primary and the only scheme to provide protection for islanding. Relaying at the POI breaker is equipped with undervoltage, overvoltage, underfrequency and overfrequency trip settings to provide backup to primary DTT scheme but is not considered a reliable anti-islanding protection. If communication from both terminals, C and D to B is lost or disabled, followed by a loss of line C-D, a sustained island could form. To avoid a sustained island when communication between C and D to B is out of service, a loss of communication logic is included at terminal B to bring generation offline. The logic is designed to trip the generation after a 5 minute delay for a complete loss of communication.

Tie Line Zone: These lines are always a direct dedicated feed for the solar site, usually new construction and of very short length. The interconnection agreement requires that the solar generator provide one multi-fiber fiber optic cable between the solar site and the utility substation. The fiber optic cable is required for other purposes; however spare fibers are available to facilitate protective relaying including DTT. Transfer trip to breaker A is initiated for any trip of breaker B using this fiber. 

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