Northeast Utilities Service Company Application Docket N

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Northeast Utilities Service Company Application Docket No. 272

to the Connecticut Siting Council for a

Certificate of Environmental Compatibility and

Public Need (“Certificate”) For The Construction

of a New 345-Kv Electric Transmission Line

Facility and Associated Facilities Between Scovill

Rock Switching Station in Middletown and

Norwalk Substation In Norwalk, Including the

Reconstruction of Portions of Existing 115-kV and

345-kV Electric Transmission Lines, the Construction

of Beseck Switching Station in Wallingford, East

Devon Substation in Milford, and Singer Substation

in Bridgeport, Modifications at Scovill Rock

Switching Station and Norwalk Substation, and the

Reconfiguration of Certain Interconnections November 22, 2004


  1. How large, physically, would the C-type filter installations be compared to the size of the basic shunt capacitor installation they would be replacing, and if the C-filters would be larger than the shunt capacitors, has KEMA investigated whether there is adequate existing space in the system or whether further property interests would have to be acquired for the C-type filters?

A: The final size of the C-Type filter will depend on the detailed design of the filter and selected manufacturer. From other C-Type filter installations it can be indicated that the C-Type filter design may require 50 – 100% larger footprint when compared with and standard mechanically switched capacitor bank of the same rating. Some of the C-Type filter components (resistor and reactor) are rated at lower voltages and can thus be stacked vertically if footprint size is at a premium.

KEMA made no attempt to determine the optimal locations and space allocations for each C-Type filter.

  1. KEMA states that it investigated the maximum length of the proposed Phase II 345 kV line that could be installed underground, based solely on technical feasibility, rather than optimizing the system based on economics. Does this statement suggest that optimizing the system based on economics might result in a different 345 kV proposal for Phase II than a 345 kV proposal based solely on maximum technically feasible underground installation?

A: If a complete techno-economical optimization were done on a desired configuration, it is likely that it would differ from a 345 kV system proposed based solely on maximum technically feasible underground installation.

  1. Can KEMA give an estimate as to how much each C-type filter installation would cost?

A: The price per bank can only be determined in cooperation with manufacturers, after a specification is finalized. It is however important to note that the components in the existing capacitor banks may be used in the converted C-Type design. Budget prices for new installations, based on previous installations may be in the range of $12 - 18 per kVAr for a 150 MVAr, 115 kV design.

  1. Please give KEMA’s best estimate of how many filter installations of the type, size, and voltage class recommended by KEMA for installation in Southwestern Connecticut (“SWCT”) there are in the United States and in the world? What is the next closest size filter installation that is currently in operation or under construction?

A: These filters have been in operation since the early 1980’s on HV systems in the 115 kV to 400 kV voltage levels. The exact number of banks installed in the USA and the world cannot be determined on such a short notice. From previous work in this area KEMA has knowledge of several 150 – 225 MVAr banks on the 132 kV, 275 kV and 400 kV system of National Grid (UK). These filters are already more than 5 years in operation. In some cases existing mechanically controlled banks were converted to C-Type filter designs on the 132 kV system. The relocatable SVCs on NGC’s grid are also equipped with C-type designs. TenneT, the Dutch 150 – 380 kV Transmission System Operator, has installed 2 years ago, ten 150 MVAr C-Type capacitor banks on most of the 380 kV and 220 kV substations for reactive power control. A more complete list and references can be compiled in cooperation with several US and European manufacturers US.

  1. Please discuss the implications of a C-type filter or filter component failure and describe the potential modes of failure.

A: Most of the C-Type filter components are similar to standard capacitor bank components and installations. Similar failure modes can be expected. The shunt connected damping resistor may be the exception. The voltage across the resistor is under normal operating conditions at a low level. With the resistor failing open circuit, the damping provided by the filter would be lost, but the unit would still operate as a filter. If the resistor failed short-circuit the filter would operate as a regular capacitor bank. The switching transients across the damping resistor should be investigated during capacitor switching and may require an over-voltage arrester for added protection.

  1. While KEMA’s report makes reference to harmonic victims, it does not appear to indicate the extent of system disruption which may inflict damage on harmonic victims or the damage such victims and victim equipment may suffer. Please describe in detail the extent of system disruption which may occur as a result of harmonic-related distortion and the extent of damage which may be caused to harmonic victims and victim equipment.

A: Good textbook and other references exist, describing the effects of harmonic distortion to systems and components. Heating effects to network and load components may be the result of steady-state harmonic current injections from non-linear loads. Over-voltages may also occur on systems due to harmonic voltage amplifications near parallel resonances, triggered by transient switching conditions, e.g. the in-rush current due to transformer energization. If these over-voltages cannot be mitigated, component failures may occur resulting in system disruptions.

  1. What happens to harmonic resonant impedance as load level decreases and the capacitor/filter installations are switched off for voltage control? What are the implications for transient voltage performance?

A: This is one of the main advantages of using the C-Type filter design. The C-Type filter does not alter the harmonic frequency characteristics of the system. It just provides added damping at the different resonance frequencies, without amplification of the harmonic voltages at the point of connection. This implies that the frequencies, associated with the different harmonic resonance points, will not change when the filter bank is switched off. The damping will diminish when the filters are switched out. Therefore, the system will benefit by keeping the filters in operation as much of the time as possible.

  1. Section 2.2.2 of the study indicates that the harmonics generated "are normally well damped and no high overvoltage transients will result". The studies undertaken by GE have not indicated harmonics to be well damped and have indicated temporary overvoltages to be high. How do you explain these differences between your studies and the GE studies?

A: KEMA has not had an opportunity to examine the referenced GE studies. Therefore, we cannot explain the referenced differences.

  1. Section 3.5 of the KEMA study models the Devon-Beseck circuits as 1750kcmil. Is it KEMA’s opinion that these are adequate under steady state conditions? If so, is it based on actual testing and what were the results?

A: KEMA used 1750 kcmil XLPE cable in its study, based on data supplied by the Applicant. KEMA is currently reviewing this assumption and will report the results of its review upon completion.

  1. Section 5 of the study indicates that energizing the 345/115kV transformers will result in distortion events of 0.1-0.3 seconds. Please state your basis for this relatively short 0.1 to 0.3 second duration and state whether such short durations are necessarily the case in low system strength applications such as SWCT, where the transformer energization can be an order of magnitude higher, such as 5 seconds.

A: The specific magnitude and duration of inrush magnetizing current during transformer energization, depend on the system configuration, system strength, specific transformer transient saturation parameters and point on wave switching timing. This ultra high level of magnetizing current may persist typically between 0.1 to 0.3 seconds, but depends on the specific system configurations. Section 5 did not present specific results or characteristics from the SWCT system, but provided background information on some of the key issues. In weak EHV systems with highly efficient transformers, the high levels of inrush currents may persist for longer than 0.3 seconds. KEMA cannot comment on whether the specific inrush currents during transformer energization can exist for up to 5 seconds. No specific measurements of the SWCT system’s transformer inrush currents were available at the time.

  1. Section 5 of the study indicates that inrush phenomina can be mitigated through "operational procedures, protection and mitigation techniques". Please clarify:

(a) how operational procedures and protection measures will help limit post

fault recovery scenario given that these are not "planned" events; and (b) what the protection and mitigating techniques are.

A: Fault recovery and contingency analysis for unplanned events were not studied by KEMA. For any new design these detailed fault recovery analyses should be performed during the design phase. It was also not in the scope of KEMA’s study to develop detailed operational procedures for transformer energization. These are important issues relevant for any system. In KEMA’s study the different alternatives were compared from a harmonic performance point of view. Due to the improved damping of Phase II with extended undergrounding, fewer problems can be expected when compared with Phase I alone.
KEMA’s report emphasized that, with proper operational procedures, system overvoltages can be minimized as a result of transformer energization. It is important to configure the system such to move the low resonance points to higher levels before a large transformer is energized. Mitigation techniques may include correctly dimensioned surge arresters, well-dimensioned damped C-Type or other filters in the system and over- and undervoltage protection schemes on equipment and capacitor banks.

  1. Section 5.2, Table 6 of the study indicates parameters for the C-type filter. Please: (a) clarify the practicalities of retrofitting the filters to the existing capacitor banks recognizing the size of the additional capacitors and inductances required; and (b) quantify the costs associated with filter losses.


(a) The existing capacitor bank components may be re-used in a retrofitted design. The reactors, damping resistors and improved protection protocols have to be added to the original design. This approach was also followed by NGC (UK) on a number of capacitor banks for their 132 kV system. The details of such a retrofitting design have to be evaluated in cooperation with manufacturers.
(b) System losses are not uniformly appreciated by all utilities. In the C-Type design the series reactor contributes to most of the losses. The loss appreciation should be weighed against the quality of the reactor (series resistance of reactor). In a final design these two parameters should be optimized over the lifetime of the filter. KEMA has not evaluated the costs associated with filter losses.

  1. Section 5.1 of the study indicates that STATCOMs can be designed to "provide some damping at key low order harmonics". Please clarify what level of damping the STATCOM(s) can provide, given their ratings, for the post fault recovery scenario during which inrush to multiple transformers occurs.

A: A STATCOM can provide dynamic reactive power support during contingencies by regulating the voltage within acceptable margins. Furthermore, the STATCOM can provide active filtering to low order harmonic currents. Using these features together with the C-Type filter characteristics, the overvoltages associated with the inrush currents can be mitigated. The specific ratings of the STATCOM and C-Type filters should be investigated in a detailed design study.

  1. Section 5.1 also indicates that STATCOMs will improve "dynamic voltage support and transient performance...". Even if this is true in theory, is it true in practice, given the status of the technology? Please comment.

A: High power STATCOMs and SVCs have been utilized for a number of years to provide dynamic voltage support and improved transient performance to utilities world-wide. Some of the references in the KEMA report provide more information on the characteristics of STATCOMs and SVCs

  1. Section 7.3.3 notes that the C-type filter design provides the lowest impedance except for the 2nd. How effective or ineffective are C-type filters for 2nd harmonic mitigation?

A: The proposed design was tuned to the 3rd harmonic for comparing the different alternatives. Looking at Norwalk 345 kV, it was found that the filters would effectively filter harmonic currents in the 2.5 – 3rd harmonic range. In a detailed design, the optimal tuned frequency and locations should be investigated for each filter. By tuning some or all of the filters closer to the 2nd harmonic, the system characteristics can be altered to provide filtering to 2nd harmonic inrush currents generated by transformer energization.

  1. Section 7.3.3, Figure 11 indicates that the C-Type filter has effectively moved the first resonant frequency above the 3rd harmonic. This marginal move from a first resonance point of ~2.8 to ~3.2 has been achieved effectively by adding 400MVAr of filter capacitance. Please confirm or deny that further filtering will be required if further reinforcements are introduced to the SWCT system beyond Phase II.

A: This would depend on the specific planned reinforcements, cable v/s overhead lines. These proposed filters are also not new, but are usually retrofits to existing capacitor banks needed for controlling voltage and reactive power levels on the system. It should be noted however that even without any mitigation the harmonic impedance at most of the characteristic frequencies, especially 2nd , 3rd , 5th ,etc. is much lower with Phase II compared to Phase I for most alternatives, even though the first resonance peak moves to lower values. This should indicate a reduction in harmonic related problems .

  1. Figure 12 indicates that the filters appear to mitigate a 5.5 harmonic; however, they appear to produce an even lower 3.7 harmonic. Please indicate whether or not this suggests that there could be other situations where even lower order harmonics could be generated and explain why or why not.

A: The harmonic impedance at 5.5 is damped and the original undamped first resonance at 2.6 is shifted to 3.7 in the comparison graph, shown as Figure 12.

  1. Figure 15 indicates a second lowest resonance approximately between the 8th and 11th harmonic. Given that converter equipment generates harmonics at the 11th, will this now not create a problem around the 11th harmonic? Further, does this not suggest the need for more complex filter design than the KEMA report proposes, again, at additional cost and requring further space?

A: The C-Type filter configuration does actually provide damping around the 11th harmonic present in the system. Without the C-Type filter this harmonic will even be less damped. For the 10-mile undergrounding, this resonant does move closer to the 11th harmonic. This needs to be investigated further, but in terms of total system performance this harmonic is reasonably damped at around 250 ohm.

  1. How will the C-type filters be affected by changes in system conditions including:

(a) capacitor switching, (b) load level changes, (c) line maintenance outages, and (d) generator status changes.

  1. The filters will not be affected by other capacitor switching. If these other capacitor banks are not C-Type designs, they will influence the system’s harmonic impedance when they switch. If they are of the C-Type design, they will not affect the harmonic impedance frequency. They will just provide additional damping to the system when they are in operation.

  2. Changes in load level in general provide more or less damping to the system, but will not affect the C-Type filters directly. If the load consists of a large percentage harmonic current, the damping resistor of the filter will receive higher loading.

  3. Line maintenance will not affect the C-Type filters. The line outage will affect the system harmonic impedance, but the filter will still be tuned to the specific harmonic (3rd in this harmonic study) .

  4. The generator allocation does not affect the C-Type filter. It will only affect the system harmonic impedance.

  1. Testing was performed at 70% to 100% load level. How effective are the C-type filters at lower load levels with lower loads, capacitors and generation?

A: A number of cases at half (50%) loading, with capacitors off, with minimum and light generator dispatch were included in KEMA’s report. The effectiveness of the filters as a mitigation option does not change under these variable conditions. When the C-Type filters are in operation they provide more system damping, but do not affect the resonance frequency points at the sub-stations where they are connected.

  1. If C-type filters are tuned to the 3rd harmonic, is it possible that they can shift harmonic resonances at the 3rd harmonic to levels below the 3rd harmonic?

A: A C-Type filter tuned to the 3rd harmonic does not amplify harmonic voltages above the 3rd harmonic, but filter harmonic currents at the third harmonic. The system impedance between the busbar on which the filter is connected (e.g. 115 kV substation), and another busbar (e.g. 345 kV substation) will affect the filtering characteristics seen from the distant busbar (345 kV). As seen from the results on page 65 of the report, the combined filtering characteristics of all the filters are shifted from 3 to around 2.6 at Norwalk. This does not mean that the resonance is shifted below 3rd harmonic, it is the filtering characteristics that operates at around 2.6 for this case.

  1. The study results suggest that up to 20 miles of additional undergrounding could be enabled due to installation of C-type filters, based on 70% to 100% load level testing. If lower loads were tested, wouldn’t the studies potentially indicate lower order harmonic resonance at less than 20 additional miles?

A: When the system is at lower than 70% load levels, some of the capacitor banks will be switched off, resulting in the first resonance point staying at the same frequency or move higher due to less capacitance in the system. It is assumed that all capacitors banks will still be in operation at levels of around 70% of load. Loads below this level will result in capacitor banks being switched off since no voltage support will be required.

  1. KEMA has observed that ISO New England is concerned about the complexity of multiple STATCOMs in SWCT from an operational perspective, and KEMA has stated that C-type filters are not as complex as STATCOMs and will not negatively affect system operations.

  1. Does KEMA have any operational experience with respect to C-type filters, and if so, what additional operating considerations does KEMA believe they impose on system operators, and has KEMA encountered any operational difficulties?

A: KEMA has aided utilities in the HV harmonic measurements, design, specification, tender evaluation, commissioning and performance evaluation of C-Type filters. From an operational point of view, no major changes to regular capacitor bank operations are encountered. See also comments on existing installations in the answer to Question 4.

  1. If KEMA does not have any direct operational experience, does KEMA nevertheless know what additional operating considerations C-type filters impose on system operators, and what operational difficulties have been encountered in connection with C-type filters?

A: See response to (a)

  1. Does KEMA believe that the use of multiple STATCOMs in close proximity, as originally proposed for SWCT in Case 5, would present undesirable operational complexity?

A: We have not studied the operational aspects of multiple STATCOMs in close proximity.

24. KEMA recommends C-Type filters, either alone or in the combination with one or

two STATCOMs. Would the C-type filters be less effective as a mitigation tool if operated without one or two STATCOMs? If not, please describe how much less effective they would be.

A: From a harmonic performance perspective alone, the C-Type filters will be as effective as a mitigation tool with or without the STATCOMs.

  1. When KEMA recommends one or two STATCOMs, would the Glenbrook STATCOM be included in, or in addition to, the recommended STATCOMS?

A: It may be possible that Glenbrook can be part of this group, but detailed voltage and transient stability analyses, beyond the current harmonic study, should be performed to confirm this statement. From a harmonic performance perspective alone, the effects of the Glenbrook STATCOM, were already included in the study.

25. What purpose does KEMA have in mind in recommending the possibility of STATCOMs in combination with C-filters, and if no STATCOMs are employed, would that purpose be effectively served by some other means or would there be added reliability risk or operational complexity in the system?

A: From a harmonic performance alone, point of view, the C-Type filters will be as effective as a mitigation tool with or without the STATCOMs.The STATCOMs would mainly be used for regulating dynamic voltage profiles and would contribute to post-contingency performance of the system, including the concerns over transformer energization after contingencies. The improved harmonic performance that may be obtained from the STATCOMs would be a secondary benefit. The best locations, ratings and control algorithms have to be studied in detail based on voltage and transient stability analyses.

26. KEMA has concluded that when C-Type filters and/or STATCOM mitigation schemes are employed, Phase II with a 20-mile underground extension is a workable solution from a system resonance point of view, and KEMA has recommended that transient analyses should be performed with a detailed system model of the selected options with the mitigation solutions in place and that an IEEE 519 analysis be performed. If the these analyses yield results that are not satisfactory, would KEMA conclude that a 20-mile underground extension is not workable?
A: If the analyses yield results that are not satisfactory, and if no acceptable mitigation is possible, then KEMA would conclude that such an underground extension was not workable.
28. If not already in the record in this proceeding, please provide a copy of the curriculum vitae or professional biography of each of the authors of the KEMA report and of any other principal contributor to the KEMA report.

A: Curriculum vitae are attached for each author of the KEMA report. There were no other principal contributors to the report.

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