Meeting the protection challenges of DER interconnections to a distribution system
With distributed energy resources (DER) growing in prevalence, power suppliers are experiencing new challenges regarding their distribution system planning and operation. The impact of DER installations on system short circuit behavior and existing protective devices is one such challenge. DER interconnections can have varying and opposing effects on an electrical system’s short circuit currents depending upon the DER interconnecting transformer and grounding configuration. As a result, the entirety of the electrical system should be modeled to examine the impact of DER installations on every connected consumer and generator. This article addresses the impact of DER installations on generating plant single phase to ground fault protection.
Medium voltage synchronous generators have a low single phase to ground fault withstand rating. These generators typically have high impedance grounding to limit single phase to ground faults. As a result of the low single phase to ground faults, the ground overcurrent element for these generators must be very sensitive.
The transformer connection and grounding method for a DER installation can have varying effects on the system short circuit levels – depending on the method – affecting the existing generator ground protection and possibly causing damage despite the generator grounding scheme. The following two examples demonstrate DER transformer connections and their impact on the generators.
In our first example, a 10 MVA, 11 kV synchronous generator is connected to a power grid as shown in Figure 1. The generator has a grounding resistance of 160 ohms that limits single phase to ground fault current to 40 amps at the generator bus. A 10 MVA inverter-based DER is also connected to the power grid with a 10 MVA, 11kV:0.48kV delta-grounded wye transformer as shown in Figure 1. The DER is modeled to have 1.2 per unit three-phase fault current contribution. The zero-sequence impedance for the DER unit is set to infinite since inverters will not provide zero sequence fault current contribution.
Figure 1 - Example 1
The total single phase to ground fault current will be limited to 40 amps, as dictated by the generator’s 160-ohm grounding resistor. When the DER site is not connected to the grid, the entire 40 amps of single phase to ground fault current will flow through the generator. However, if the DER is connected, the delta primary transformer will provide a path for positive and negative sequence current flow during the single phase to ground fault as shown in Figure 2. In this case, the 40 amps of ground fault current will split between the generator and the delta winding of the DER transformer. This results in a single phase to ground current lower than 40 amps at the generator. While this may seem beneficial to the generator, it could cause misoperation of the generator’s ground overcurrent protection. If the ground overcurrent protection at the generator is set above 34 amps in this example, it will not operate.
Example 1 is also applicable to DER sites with wye-grounded to wye-grounded interconnecting transformers.
Figure 2 - Example 1: Single Phase Fault Current Flow
In our second example, the DER site transformer is changed to a wye-grounded delta as shown in Figure 3. A wye-grounded delta transformer will provide a zero-sequence source for the system during a single phase to ground fault.
Figure 3 - Example 2: Oneline
When the DER site is not connected to the grid, 40 amps of single phase to ground fault current will flow through the generator. However, if the DER is connected, the wye grounded-delta transformer will provide a zero-sequence source to the fault, increasing the overall single phase to ground fault current as shown in Figure 4. This fault current may exceed the generator rating and cause damage.
Figure 4 - Example 2: Single Phase Fault Current Flow
Both examples show the problems that can occur when DER sites are combined with existing medium voltage generators. If the DER connection causes reduced fault current at the generators, it could prevent fault current detection and require setting adjustments. If the DER connection causes excessive fault currents at the generators, it could cause damage to the generators and will require modifications to the DER grounding or transformer(s). When generators and DER interconnections are present on a system, a detailed and thorough short circuit analysis should be conducted to ensure coordination and equipment protection are maintained and mitigations can be implemented.
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