Grid Impact of Neutral Blocking for GIC Protection:

Transcription

1 Report submitted to EMPRIMUS - Critical Infrastructure Protection Grid Impact of Neutral Blocking for GIC Protection: Impact of neutral grounding capacitors on network resonance Prepared By: Athula Rajapakse Date: 29 June, 2013 Electresearch

2 Third Party Disclaimer: The content of this document is not intended for the use of, nor is it intended to be relied upon by any person, firm or corporation, other than the authors of the report and EMPRIMUS LLC. Authors of this report deny any liability whatsoever to any parties for damages or injury suffered by such third party arising from the use of this document by the third party. Confidentiality: This document is restricted to the confidential use of the authors and EMPRIMUS LLC. Any retention, reproduction, distribution or disclosure to third parties is prohibited without written authorization of EMPRIMUS LLC. Electresearch Page i

3 Table of Contents 1.0 Introduction Study Methodology Results Scenario-1: System intact conditions Scenario-2: One of the 660 MVA, 22/500 kv transformers out of service Scenario-3: One of the 500 kv circuits between bus-1 and bus-2 out of service Scenario-4: 750 MVA, 500/345 kv autotransformer between bus-1 and bus-4 out of service Scenario-5: One of the 500 kv circuits between bus-1 and bus-3 out of service Scenario-6: System intact, with autotransformer neutral grounding capacitors bypassed Scenario-7: System intact, with 2 winding transformer neutral grounding capacitors bypassed Concluding Remarks References Electresearch Page ii

4 1.0 Introduction A commonly proposed solution to block the flowing of geomagnetically induced currents (GIC) in power transmission lines during solar storms is to ground the transformer neutrals through capacitors. This report details the impact of neutral blocking capacitors used for GIC blocking on network resonance at both (ii) harmonic and (ii) subsynchronous frequencies. Harmonic resonance in a power transmission network is caused by the energy exchange between capacitive elements and inductive elements in the system at one or more of the natural frequencies. Thus the harmonic resonance characteristics are mainly determined by the shunt capacitances of transmission lines, cables, and shunt capacitor banks, and the inductances of the network elements such as transmission lines, cables, transformers, generators, and reactors. Problems due to harmonic resonance are commonly manifested as significant distortion of supply voltage waveforms due to high harmonic impedance (parallel resonance) at harmonic source locations. However, series resonance conditions can also exist and lead to problems. Subsynchronous resonance is an electric power system condition where the electric network exchange energy with a turbine generator at one or more of the natural frequencies of the combined system below the nominal frequency of the system. This definition include what might be considered natural modes of oscillation that are due to inherent system characteristics as well as forced modes of oscillations driven by a particular device or control system [1]. Relevant to this study are the natural modes of oscillation, and we focus on examining the existence of natural modes of oscillation in the network that could interact with the natural torsional modes of a turbine generator shaft, setting up conditions for energy exchange at a subsynchronous frequency. The neutral blocking capacitor can introduce changes to the resonance characteristics of a transmission network, especially of the zero sequence impedances. This is because the capacitive reactance that is in series with the inductive zero sequence impedance of the transformer and other network devices can form a series or parallel resonance circuit depending on the network topology. Frequency scan analysis is the most widely used technique at present to identify the potential harmonic problems in power networks. This method is adopted in this study to identify the resonance conditions that exist in a 500/345 kv test network. All studies and analyses are conducted with the objective of understanding the impact of the neutral blocking capacitor, and thus involve comparison of frequency scans with and without the neutral grounding capacitors. In the following sections methodologies used and the results are presented. 2.0 Study Methodology The influence of the neutral grounding capacitors on the resonance characteristics of an interconnected network was investigated using frequency scan studies. Frequency scans gives the impedance seen from a specific point in the network at different frequencies. Frequency scan results can be used as screening level information to identify potential network resonance problems. In order to identify potential subsynchronous resonance issues, the calculated frequency scans should cover the frequencies below the power frequency. Harmonic resonance issues are typically due to lower order harmonics of power frequency. Thus, frequency scans presented in this report considered the frequency range from Hz (up to the 12 th harmonic of a 60 Hz system). The network used for studies is shown in Figure 1. The simulations presented in this report consider neutral grounding circuits of the two 22/500 kv two winding transformers connected between bus-7 and bus-2 (T 1, T 2 ), and the two 500/345 kv autotransformers connected between bus-1 and bus-4 (T 3 ), and bus-3 and bus-5 (T 4 ). Electresearch Page 1

5 Figure 1: Test network The main purpose of this study is to identify the influence of the neutral grounding capacitor on the overall network characteristics. Thus the frequency scan studies considered simulations with solidly grounded transformer neutrals and capacitor grounded transformer neutrals. In order to examine the effects of the value of the grounding capacitance, three different capacitor values (265 μf, 1000 μf, and 2650 μf) were considered. Moreover, different system conditions resulting from prior outages of various elements in the test circuit are also considered. Frequency scans were repeated at two locations, at bus-1 and at bus-2. In order to study the influence of the location of neutral grounding capacitors, selected cases were repeated with some of the neutral grounding capacitors bypassed. A list of simulation cases and the corresponding system conditions are given in Table 1. Simulations were carried out in PSCAD/EMTDC. The Harmonic Impedance Solution module available in PSCAD/EMTDC software, shown in Figure 2, was used to obtain the frequency scan results. This module can be used to directly obtain the frequency characteristics of the sequence impedances at the test location by selecting the proper option as shown in Figure 3. The following characteristics are presented under each of the conditions listed above. 1. Positive sequence impedance characteristics The neutral grounding capacitor is not expected to influence the positive sequence characteristics 2. Zero sequence impedance characteristics The influence of the neutral grounding capacitor is expected to appear in zero sequence impedance characteristics. Electresearch Page 2

6 Table 1 Scenarios, test locations, and test conditions Scenario Condition Location Neutral grounding Case No. Scenario-1 System intact bus-1 Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Case (1) Case (2) Case (3) Case (4) Scenario-2 One of the 660 MVA, 22/500 kv transformers out of service Scenario-3 Scenario-4 Scenario-5 One of the 500 kv circuits between bus-1 and bus-2 out of service 750 MVA, 500/345 kv autotransformer between bus-1 and bus-4 out of service One of 500 kv circuits between bus-1 and bus-3 out of service bus-2 bus-1 bus-2 bus-1 bus-2 bus-1 bus-2 bus-1 bus-2 Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Solidly grounded Capacitor grounded, C = 265 μf Capacitor grounded, C = 1000 μf Capacitor grounded, C = 2650 μf Case (5) Case (6) Case (7) Case (8) Case (9) Case (10) Case (11) Case (12) Case (13) Case (14) Case (15) Case (16) Case (17) Case (18) Case (19) Case (20) Case (21) Case (22) Case (23) Case (24) Case (25) Case (26) Case (27) Case (28) Case (29) Case (30) Case (31) Case (32) Case (33) Case (34) Case (35) Case (36) Case (37) Case (38) Case (39) Case (40) Electresearch Page 3

7 Table 1 continued Scenario Condition Location Neutral grounding Case No. Scenario-6 System intact, autotransformer bus-1 T 3, T 4 solidly grounded, T 1,T 2 capacitor grounded, C=265 μf T 3, T 4 solidly grounded, T 1,T 2 Capacitor Case (41) Case (42) neutral grounding capacitors bypassed bus-2 grounded, C=2650 μf T 3, T 4 solidly grounded, T 1,T 2 capacitor grounded, C=265 μf T 3, T 4 solidly grounded, T 1,T 2 Capacitor grounded, C=2650 μf Case (43) Case (44) Scenario-7 System intact, two winding transformer neutral grounding capacitors bypassed bus-1 bus-2 T 1,T 2 solidly grounded, T 3, T 4 capacitor grounded, C=265 μf T 1,T 2 solidly grounded, T 3, T 4 capacitor grounded, C=2650 μf T 1,T 2 solidly grounded, T 3, T 4 capacitor grounded, C=265 μf T 1,T 2 solidly grounded, T 3, T 4 capacitor grounded, C=2650 μf Case (45) Case (46) Case (47) Case (48) Electresearch Page 4

8 150 A V TL500D1 500 kv, 2 cct, 250 km A V [MVAR] [MVAR] 1.0 [ohm] 265 [uf] 'Harmonic Impedance Solution' module Z(f) A V A V BUS 1 Figure 2 Harmonic Impedance Solution module connected to calculate the frequency characteristics at BUS 1. Figure 3 Harmonic Impedance Solution module parameter options Electresearch Page 5

9 3.0 Results 3.1 Scenario-1: System intact conditions Figure 4: Frequency characteristics of the zero sequence impedance at bus-1 Figure 5: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 6

10 Figure 6: Frequency characteristics of the zero sequence impedance at bus-2 Figure 7: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 7

11 3.2 Scenario-2: One of the 660 MVA, 22/500 kv transformers out of service Figure 8: Frequency characteristics of the zero sequence impedance at bus-1 Figure 9: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 8

12 Figure 10: Frequency characteristics of the zero sequence impedance at bus-2 Figure 11: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 9

13 3.3 Scenario-3: One of the 500 kv circuits between bus-1 and bus-2 out of service Figure 12: Frequency characteristics of the zero sequence impedance at bus-1 Figure 13: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 10

14 Figure 14: Frequency characteristics of the zero sequence impedance at bus-2 Figure 15: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 11

15 3.4 Scenario-4: 750 MVA, 500/345 kv autotransformer between bus-1 and bus-4 out of service Figure 16: Frequency characteristics of the zero sequence impedance at bus-1 Figure 17: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 12

16 Figure 18: Frequency characteristics of the zero sequence impedance at bus-2 Figure 19: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 13

17 3.5 Scenario-5: One of the 500 kv circuits between bus-1 and bus-3 out of service Figure 20: Frequency characteristics of the zero sequence impedance at bus-1 Figure 21: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 14

18 Figure 22: Frequency characteristics of the zero sequence impedance at bus-2 Figure 23: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 15

19 3.6 Scenario-6: System intact, with autotransformer neutral grounding capacitors bypassed Figure 24: Frequency characteristics of the zero sequence impedance at bus-1 Figure 25: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 16

20 Figure 26: Frequency characteristics of the zero sequence impedance at bus-2 Figure 27: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 17

21 3.7 Scenario-7: System intact, with 2 winding transformer neutral grounding capacitors bypassed Figure 28: Frequency characteristics of the zero sequence impedance at bus-1 Figure 29: Frequency characteristics of the positive sequence impedance at bus-1 Electresearch Page 18

22 Figure 30: Frequency characteristics of the zero sequence impedance at bus-2 Figure 31: Frequency characteristics of the positive sequence impedance at bus-2 Electresearch Page 19

23 4.0 Concluding Remarks As expected, frequency scans show that neutral grounding capacitors has no impact on the positive sequence impedance frequency characteristics (see Figures 5, 7, 9, 11, 23). However, the impact of neutral grounding capacitors is visible on some frequency scans of zero sequence impedance, as described below. Frequency scans show that at harmonic frequencies (f > 60 Hz), the impact of neutral grounding capacitors on the zero sequence impedance is negligible when the grounding capacitance is larger than 1000 μf (see Figures 4, 6, 8, 10, 12, 22). However, at subsynchronous frequencies (f < 60 Hz), zero sequence impedance characteristics are affected by the neutral grounding capacitors. Parallel resonance conditions exist even when the capacitance is 2650 μf (see Figures 4, 8, and 12), depending on the system conditions and the location of frequency scan. However, when the value of grounding capacitance is large (i.e. in the order of 2,650 μf), peaks of the impedance plots at the resonance frequency are small and therefore, less likely to have significant impact on the system operation. Since subsynchronous resonance issues are dependent on the specific network being considered, this potential resonance issue will need to be analyzed on a case by case basis. Frequency scan results of scenarios-6 and -7 show that the effect of capacitance is local, even when the capacitance is as low as 265 μf, as described below. In scenario-6, the neutrals of the transformers T 1 and T 2 (connected at bus-2) are grounded through capacitors while the neutrals of the transformers T 3 and T 4 are solidly grounded. Frequency scans of the zero sequence impedance at bus-2 show parallel resonance conditions at subsynchronous frequencies (Figure 26). However, the zero sequence impedance measured at the remotely located bus-1 (Figure 24) is not affected at all by the neutral grounding capacitors of the transformers T 1 and T 2, for capacitances of 265 μf and larger. Similarly, in scenario-7, the neutrals of the transformers T 3 and T 4 (connected at bus-1 and bus-3 respectively) are grounded through capacitors while the neutrals of the transformers T 1 and T 2 are solidly grounded. Although the frequency scans of the zero sequence impedance at bus-1 (Figure 28) show parallel resonance conditions at subsynchronous frequencies, the zero sequence impedance measured at the remotely located bus-2 (Figure 30) is not affected by the neutral grounding capacitors of the autotransformers T 3 and T 4. The study shows that conditions for parallel resonance in the zero sequence impedance can be created at subsynchronous frequencies, when the neutrals are grounded through capacitors. Although it is less likely to have a significant impact at higher capacitor values (~2650 μf), it is recommended to conduct a through analysis to determine whether there are undue electromechanical interactions between the network and turbine generators when applying neutral grounding capacitors to a particular system. 5.0 References [1] Paul M. Anderson, B. L. Agrawal, J. E. Van Ness, Subsynchronous Resonance in Power Systems, IEEE Press, New York, NY, USA, Electresearch Page 20