Integrating Solar PV Power with Existing Distribution Circuits; Part 2

(This Blog is a continuation of an ongoing series on integrating inverter-based solar photovoltaic generation with existing electric distribution circuits. Link to Part 1)

Solar PV (shorthand for photovoltaic) generation is growing in support and implementation in part because of a supportive regulatory environment. Among the more common types of interconnection terms are NEM and FIT.

NEM — Net Energy Metering. Residential and commercial utility customers who own and operate an eligible renewable energy generation system up to a generating capacity of 100 kW and intend to connect to utility grid. The NEM customer is allowed to connect their renewable generator to the utility grid, allowing it to export surplus electricity into the grid, and to receive credits at full retail value which may be used to offset electricity purchases over a 12-month period.

FIT — Feed-in Tariff. Programs designed to encourage addition of more renewable energy projects with pre-established rates and standardized contract terms for individuals, small businesses, governmental entities, or other developers. Typically applies to PV facilities greater than 20 kW and up to 2,000 kW.

In Part 1 of this series, we identified the various technical areas that may potentially be impacted by a new solar PV installation. In this Part 2, we go into further details on the following aspects:

  • Power Flow Assessment: The power flow study addresses voltage profile issues, thermal loading of electrical equipment, changes in system losses and power factor requirements under a number of pre-defined operating conditions such as minimum and maximum daytime load.
  • Stability Assessment: This addresses the performance of the system with integrated PV inverters during faulted conditions and what effect these may have on the electric power system dynamic response.
  • System Operation and Limitations Assessment: This assessment determines the variability of PV inverter output and its impact on the normal operation of the system.

To illustrate the issues associated with each of the aspects listed above, a sample system representing the island of Lanai, Hawaii is used. The load on the island is to be served by several conventional diesel generators, inverter-based PV and a chemical battery installation. Customer demand ranges from about 2 to 4 MW.

POWER FLOW ASSESSMENT

Power flow assessment addresses two aspects: thermal loading of equipment and steady-state voltage along the distribution feeder. Typically, these are analyzed under various operating conditions such as at peak and day minimum load, various equipment outages and emergency operating states. (Day minimum is used because the solar PV is only active during the daytime hours.) Steady-state voltage is applicable in the timeframe from one to several minutes. In addition, the operating duty of transformer tap-changers, voltage regulators, capacitor banks and other power conditioning equipment is assessed for possible degradation due to the new PV. PV, in particular, are susceptible to transitory outages when cloud cover shuts down then unveils, at full output, irradiation of the PV arrays.

For the sample system, several power flow base cases, each corresponding to a typical daily operating state were used in the assessment. Specifically, these represented operation at 3am, 7am, 1pm and 7pm. To each base case, the power flow assessment applied the following criteria:

  • Thermal loading of transmission facilities, lines and transformers are within 100% of the Normal Rating for these equipment.
  • Voltage criteria – steady-state voltages are within 95 and 106.5 percent of nominal voltage.

Power flow analysis of the sample system showed that thermal loading and voltages are within acceptable ranges for all normal and contingency conditions. In situations where thermal loading is observed, upgrades of existing transmission or new transmission may be required. This may be too high a cost to be supported by a single PV installation of less than a MW capacity.

Distribution utilities may minimize impacts on thermal loading and steady-state voltages by limiting the amount of generation connecting to particular feeders; i.e., setting a maximum of 15% penetration on each feeder before more detailed studies, and potentially significant upgrades, are conducted.

STABILITY ANALYSIS

As a non-rotating generation resource, solar PV installations introduce a unique impact to the synchronism of the interconnected system and to the ability of power control systems to maintain voltage and frequency within acceptable ranges. For the sample system, two types of disturbances were simulated for stability analysis, as follows:

  • Three-phase faults which are subsequently cleared by tripping a transmission segment.
  • Loss of one of the diesel generators

These two types are typically the most severe with respect to voltage and frequency stability.

Several operating scenarios were developed with various combinations of the diesel generator and PV units in service, and with and without the battery. The battery is set to Low Frequency Control (“LFC”) mode. Some of the PVs are modeled with frequency ride-through capability, but not all. The frequency settings are summarized in Table 1.

Table 1

The frequency response comparisons for the different faults and generation dispatches at the day peak condition are shown in Figure 1. Although, there are some frequency fluctuations, these are within acceptable ranges. The system voltages (not shown) were likewise within acceptable ranges. Hence, the system can operate stably with the PV in service.

Figure 1

SYSTEM OPERATIONS AND LIMITATION ASSESSMENT

The primary system operations issue is typically the amount of spinning reserve. In the case of the sample system, the question is: How many diesel units need to be online in order to have sufficient spinning reserve to withstand the loss of one diesel unit?

After simulating various scenarios, it was determined that at least three diesel units need to be in service. (The battery is in excursion control mode). With only two units, the voltage collapses on loss of one of the units. The voltage response is shown in Figure 2.

Figure 2

Additionally, the potential impact on system operation caused by solar radiation shifting from maximum to minimum was assessed. This phenomena is associated with cloud cover going across the field of solar arrays, first covering then uncovering the arrays. A ramp rate of 60kW/sec for each PV plant was used. The resulting frequency excursion during day peak conditions is shown in Figure 3.

Figure 3

The range of frequencies are within acceptable limits.

CONCLUSIONS

In this Blog, we discussed the methods for assessing impact of new PV on a distribution system with respect to three aspects: power flow, stability and operating limits. A sample system was developed to illustrate the method representing the island of Lanai, Hawaii.