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Protection Scheme Problem

Standard overcurrent protection schemes for passive radial systems assume single direction current flow. The addition of distributed generation (DG) presents issues for the protection scheme, as current can flow from multiple directions. This study investigates the impact of DGs on overcurrent protection designed for a radial system, and proposes solutions to address the issues.

Protecting Distribution Feeders from the Effects of Distributed Generation
Protecting Distribution Feeders from the Effects of Distributed Generation

A realistic feeder system and its protection scheme are developed in PSCAD/EMTDC. A point-of-common-coupling (PCC) is identified, indicating the portion of feeder that can potentially operate as an island. One DG, with output adjusted to maintain a specified power flow at the PCC, is added to the feeder system. The performance of the overcurrent protection in the presence of line, ground, and three-phase faults is analyzed.

A second machine, outputting full capacity at unity power factor, is added to the feeder system.

The strategies used to develop the single-DG modified protection scheme are applied to the two-DG system. The functionality of the modified protection scheme is verified.


Two DG Feeder Protection

In this chapter, the strategies which were used in Chapter 4 to develop the modified protection scheme are applied to a feeder system with two DG. Three operating conditions are selected and it is verified that they are within the machine’s operating limits in both grid connected mode, and after transitioning to islanded mode.

It is confirmed that in all three operating conditions, the first coordination time interval – CTI (0.3s) is not beyond the maximum time for which the generator can tolerate a fault without becoming unstable.

Based on the results from the single-DG case, the radial feeder overcurrent protection scheme would exhibit similar issues in the two-DG feeder system. The modified protection scheme from the single-DG case would also require changes, as the settings for the breaker protecting the generator’s zone were unmodified from the original radial feeder protection.

The generator overcurrent protection delay of both machines is set to the first coordination time interval, to allow the required coordination to be achieved. Directional and overcurrent relays are used for CB2, CB3, and CB4. The downstream coordination time interval is maintained at 0.3s and the upstream coordination time interval is 0.15s.

For the similar current levels for faults at two different locations on the lateral protected by CBT3, the system is designed to island, allowing proper coordination to be maintained downstream.

Simulations are conducted to evaluate the performance of the modified protection scheme.

Generator protection disconnects the machine in a limited number of cases due to overfrequency or undervoltage. In the remaining cases, the modified scheme succeeds in isolating faults without unnecessarily isolating loads.


1. Two-DG Feeder Layout

A synchronous generator S1, rated at 1.3MVA and 4.16kV, is connected through a step-up transformer to the distribution feeder presented in Chapter 4 (see Figure 1) on subfeeder 2. The machine parameters are typical for a 1.3MVA unit and are presented in Appendix D.

The layout of the study system with two DGs is shown in Figure 1. The portion of system downstream of the PCC can be islanded.

Figure 1 – Study system with two DGs layout (click to zoom)

Study system with two DGs layout
Figure 1 – Study system with two DGs layout

2. Generator Overcurrent Protection

The overcurrent protection of generator S1 is based on the decrement curve presented in Section 3.3. Based on the conclusions from the single-DG case in Chapter 4, the tripping time is increased to the first CTI for both generators. The decrement curve, trip characteristic, and ten second current rating for the second machine (rated at 1.3MVA) are shown in Figure 2.

The ten second current rating of the machine is higher than the fault current for a 3! fault at the terminals for 0.3s. This verifies that the machine can survive the fault duration.

Figure 2 – Generator S1 decrement curve and overcurrent trip characteristic (click to zoom)

Generator S1 decrement curve and overcurrent trip characteristic
Figure 2 – Generator S1 decrement curve and overcurrent trip characteristic

3. Generator Operation and Stability

3.1 Machine Operating Conditions

When operating in grid connected mode, the output of generator S4 is controlled so that the required real and reactive power at the PCC is maintained (see Figure 1). The output of generator S1 is the full machine MVA capacity at unity power factor.

Three operating conditions are utilized in this study:

  1. Case #1: zero power exchange across the PCC (CB2)
  2. Case #2: net import downstream of 0.20pu of P and Q
  3. Case #3: net export upstream of 0.20pu of P only

The export scenario (Case 3) is achieved without the removal of any loads. For each operating condition, the steady-state real and reactive power outputs of generator S4 in both grid connected and islanded modes are confirmed to be contained inside the machine operating limits.

Similar to in Chapter 4, the three operating points are shown in Figure 3 and the P and Q values are shown in Table 1.

Figure 3 – Operating limits of synchronous generator S4 with three operating points

Operating limits of synchronous generator S4 with three operating points
Figure 3 – Operating limits of synchronous generator S4 with three operating points

Table 1 – Output values of generator S4 for grid connected and islanded modes

Machine Output (Grid)Machine Output (Islanded)
Operating ConditionP (pu)Q (pu)P (pu)Q (pu)
Case #10.580.230.910.28
Case #20.340.020.910.28
Case #30.810.250.910.28

3.2 Transient Stability

In Chapter 4, the modified protection scheme increased the generator overcurrent protection delay of S4 to the first CTI (0.3s). As a result, the generator disconnected at the first CTI, and the reduced machine operating range associated with increasing the protection delay is deemed acceptable.

To meet the above requirements, it is verified that for all three operating conditions, the generator S4 can sustain a fault for at least the first CTI without becoming unstable.

It is assumed that generator S1 operates at full power at unity power factor. If S1 trips due to overfrequency before one CTI has elapsed, it can resynchronize back to the system.

Title:Protecting Distribution Feeders from the Effects of Distributed Generation by Tim Chang
Format:PDF
Size:14.40 MB
Pages:133
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Further Study – Proper selection and overcurrent coordination of LV/MV protective devices

Proper selection and overcurrent coordination of LV/MV protective devices

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