DATA SOURCE: TRANSPOWER, NEW ZEALAND

DC STRAY CURRENT REDUCTIONAND BLOCKING PERFORMANCE

REVIEW OF LONG TERM GRID PERFORMANCE DATA

C O U N C I LISOCTOBER, 2014

New ZealandElectric Grid

This special report was made possibile by the

Newton D. and Rochelle F. Becker Foundation

DC STRAY CURRENT REDUCTION AND BLOCKING PERFORMANCE Review of long term grid

performance data

ForwardNew Zealand’s electricity provider, Transpower, has developed a substantial database on use of transformer “current blockers.” The company has used these transformer neutral ground-ing devices for many years to reduce Geomagnetically Induced Current (GIC) and other stray quasi-DC currents on the high voltage line linking the country’s north and south Islands. This report provides a top-level review of Transpower’s extensive history with these devices, address-ing both grid performance and risk.

EIS Council wishes to express its appreciation to Transpower, and especially to Michael Dalzell, Team Leader, HVDC & Power Electronics Engineering, for the exceptional cooperation that made this review possible.

1. IntroductionBeginning in 1992, Transpower installed resistive current-reducing devices on EHV transformer neutrals, to protect against DC currents induced by the 350 kV High Voltage Transmission line linking New Zealand’s north and south islands. More recently, in 2012 and 2013, capacitive blocking devices were also installed. This study provides an initial, top-level examination of data and grid performance with these devices, assessing performance and risk.

2. Background – Current Blockers and Dampers

• Description

Grounding (“earthing” in New Zealand usage) is an essential feature of any electric network, and is essential in a power grid for safety and for a range of system performance functions, including controlling over-voltages, improving sensitivity to system events, enabling faster breaker operating time and other capabilities.

In the Transpower System, neutral damping devices have been installed and in use since 1992, and neutral blocking devices since 2012-2013, to reduce potential damage due to stray DC currents. The Transpower current blockers and reducers are designed to withstand unwanted, abnormal current (“fault current”) of up to 30,000 amps, and up to 200 amps AC neutral imbalance current, to allow high confidence performance under a wide variety of system configurations and operating conditions.

• DC Stray Currents Reducer: A current reducer, or neutral grounding resistor, is a device that is connected between ground and the neutral point of a three phase electric system that limits currents on Neutral Grounded (WYE) systems, to prevent possible damage to affected electrical equipment. Quasi-DC GIC, among other kinds of fault currents, are reduced by such devices.

• DC Stray Currents Blocker: Blocking devices are used in a similar fashion, with the resistor replaced by a capacitive element to achieve complete blocking of the DC current.

• Using Current Blockers and Dampers

• System Performance Advantages

In the Transpower grid, stray DC currents arise typically when the 350 kV high-voltage, direct current (HVDC) link between the North and South islands is run in monopole mode, using earth as the current return path. While such effects are generally observed for conventional level geomagnetic disturbances, the far larger fault currents induced by a Severe Space Weather-induced GMD event or EMP would typically be limited or eliminated by such devices. 1

1 Note: Performance of current blockers or dampers may be substantially degraded when used on autotransformers. For more details,

see the EPRO Handbook, 1st edition.

• System Performance Risks

While installing a resistive or capacitive element can reduce or eliminate DC stray currents due to the HVDC line operation or geomagnetically Induced Current (GIC), grid implementation must be designed to prevent problems which could otherwise degrade transformer reliability and safety.

Most transformers used are solidly grounded wye systems, which typically allows these devices to be designed with lower Basic Insulation Level (BIL) ratings. If improperly configured, under stressing conditions a resistor between neutral and ground could result in voltage levels exceeding the transformers BIL rating. A capacitor, by blocking the ground path, would further increase such risks.

Current blockers and reducers are typically designed to avoid these risks by accounting for and limiting over-voltages at the neutral and other phases for the full range of operating conditions. Several different approaches are used to address these concerns.

As a primary objective of this top-level study, the impact of blocking and reducing devices on the Transpower grid was examined to provide a top-level assessment of efficacy and risk of these devices in the Transpower grid.

3. Description of Transpower Electric System – New ZealandTranspower, a State-owned company, owns and operates the national electrical grid of New Zealand. Transpower builds and maintains the transmission network and also acts as the transmission system operator (TSO). The ~12,000 km transmission network provides approximately 40,000 GWh of electricity per year from independently owned generators.

The EHV portion of the AC transmission system runs at two voltages: 220kV and 110kV. There is also a 610-kilometer HVDC link that connects the AC transmission grids on the North and South Islands, with two poles (Pole 2 and 3) that operate at +350kV. In addition, three 40 km submarine power cables cross the Cook Strait.

Power TransmissionMap of New Zealand 350 kV (DC)

North IslandConverter

kV 350

BenmoreSouth Island

Converter

Figure 1: Map of the Transpower New Zealand EHV and HVDC System

The Transpower grid has 174 substations and more than 1,000 transformers in the 10 to 800 MVA range.2 Most are three-phase bank, “core” configuration, although the HVDC converter transformers and a few AC system transformers use single-phase bank configuration. The AC and HVDC systems are connected with autotransformers using delta tertiary winding. The generator step-up transformers are generally two winding star-delta systems.

4. Transpower HVDC and GIC Protection ConfigurationTo reduce or block unwanted, DC and quasi-DC current in the New Zealand power grid, Transpower uses hardware-based protection, coordinated with emergency operating procedures and the system restoration plan. This protection system includes both capacitive and resistive devices.

• Neutral Earthing Resistors (NER) in Transpower’s System

Figure 2: Transpower North-South Island HVDC Link

The +350 kV HVDC system linking the North and South Islands can produce low levels of stray DC current, injected into the South Island AC transmission lines by entering and exiting grounded, “wye” connected power system transformers. While it was to address this problem that Transpower, in 1992, designed and installed DC current measuring devices (LEMs) and neutral earthing resistors (NERs) on vulnerable transformers, NERs also protect against the quasi-DC GIC resulting from space weather events.

2 Transpower Asset Management Plan, 2010.

NERs and LEMs were installed on 52 transformers, at all sites with grounded wye-connected transformers where analysis indicated potential stray DC current problems.3

(Figure 3 shows the locations of the key substations on South Island). In addition, one large (20 ohm) NER with a spark gap bypass configuration (Figure 4) was installed on the Pole 2 converter transformer in 1992.4

Transpower’s NERs are reportedly very low maintenance: insulation resistance measurements every four years and monthly visual inspections have proven adequate. And with over 20 years in service, after some initial minor problems with the bypass scheme were corrected, no failures or problems were reported.

• Neutral Earthing Devices (NED) in Transpower’s System

The NED is a capacitive grounding solution designed to block any DC neutral current under normal operating conditions, and to bypass the capacitor through an NER during single-phase faults on the AC system, to avoid overvoltage and improve protection system performance. This is achieved with a parallel-connected TRIAC5 supervised by the NED control system.

In 2012-2013, Transpower installed Siemens neutral earthing devices (NEDs) on the neutrals of the HVDC Pole 2 converter transformers at the Benmore substation, to protect against HVDC ground current and GIC. The NEDs have performed well, with performance data showing only event requiring maintenance – replacement of a failed NED control card.

3 The Clyde, Cromwell, Waitaki, Aviemore, Benmore, Ohau A/B/C, Tekapo B, Timaru, Ashburton, Islington, Bromley, and Kaiwharawhara substations.

4 PAPER 2.7 - DC GROUND CURRENTS AND TRANSFORMER SATURATION ON THE NEW ZEALAND HVDC LINK, by J.c. Gleadow B.J. Bisewski M.C. Stewart

5 TRIAC is a “Triode for Alternating Current”, also referred to as a “bidirectional triode thyristor”.

Figure 4: Spark gap bypass scheme on the Pole 2 converter transformer at Benmore substation

Figure 3: Key Substatitions on South Island

5. Top Level Data Review and Conclusions: Performance of Current Dampers and Blockers in the Transpower Grid

• The Data Review Process

All HVDC converter transformers are protected by NERs or NEDs, and LEM measuring devices are installed on key transformer neutrals equipped with these devices. With a usage history in the Transpower grid running from a few years to decades, a huge database is now available.

Transpower provided two forms of data: Top-level engineer assessments of the performance of these devices, and samples of the detailed performance data available.

The engineering assessments were provided as verbal reports from responsible Transpower engineering.

The data sample provided spanned the period from October 29 – 31, with data recorded at one-minute intervals at 52 locations for:

• Transformer Neutral DC Current (from the transformer DC neutral current transducers)

• HVDC ground electrode current

4,322 data points were reported for all 52 locations. Table 1 shows a small sample of this data for the current flow on the Benmore HVDC Electrode, and on transformer neutrals at Ashburton and Benmore.

Date; Time

BEN.HVDC.

.ELECTRODE

AMP

ASB.TRANS.

.T1_NER

AMP

BEN.TRANS.

.T27NER

AMP

BEN.TRANS.T29

NER.AMP

2003-10-29

00:00:00.00019.93 0.10 0.10 0.28

2003-10-29

00:01:00.00021.78 0.10 0.10 0.28

2003-10-29

00:02:00.00023.63 0.10 0.10 0.28

Table 1: Sample of the current flows on Benmore HVDC Electrode and Transformer neutrals on Ashburton and Benmore Transformers

Figure 5 shows DC current flow on the Benmore HVDC Electrode and on the neutrals of six Benmore Transformers, measured on October 29, 2003. The left scale refers to DC neutral current (Amps), and the right scale refers to DC current at the HVDC electrode (keyed to the high, rapidly varying dark blue function).

The maximum DC current at the HVDC electrode peaks at approximately 330 Amps just after 10 am. The corresponding DC current in Transformer 2, the HVDC Pole 2-converter transformer

(electrically closest to the AC-DC converter), is approximately 5.4 Amps. This transformer has a large (20 ohm) NER, with a bypass circuit (spark gap). Transpower field measurements during the design and installation of this scheme back in 1992 revealed that about 4% of electrode current would flow through the neutral of this transformer without the NER. Therefore, without the NER, the DC current in the transformer could be in the range of 13 Amps, which is damped substantially, by about 60% to 5.4 Amps. The other substation transformers have lower value NERs that are always in the neutral ground path. None of these transformers experienced DC above 1 Amp, but would also be expected to have much higher DC flows without the NERs in place.

0

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BEN.TRANS.T27NER.AMP

BEN.TRANS.T28NER.AMP

BEN.TRANS.T29NER.AMP

BEN.TRANS.T2NER.AMP

BEN.TRANS.T46NER.AMP

BEN.TRANS.T2NER.AMP

BEN.HVDC.ELECTRODE.AMP

10/29/2003

Figure 5: Measured Current Flows at the HVDC electrode and Six NER-Grounded Transformers at the Benmore Substation (October 29 – 31, 2003)

In addition, Transpower indicated that the following data is also available, if required for future research:

• Similar LEM data covering the 20+ year history of NER and 3+ year history for NED performance for transformers;

• The transformer MVAR consumption data, which can be used to assess the level of transformer saturation; and

• Network configuration data (e.g. circuit breaker status), which can be useful in determining the DC current flow.

• Top Level Conclusions

The combination of top level engineering reporting on system performance, combined with examples of the data in use by the Transpower engineering team, suggest that the NER and NED units in Transpower’s system performed nominally, meeting performance expectations and not causing system instabilities or other problems during the years they have been installed. While the scope of this study did not allow for a comprehensive analysis of all or most of the available numerical data, a more detailed analysis could be helpful to draw more detailed conclusions, and is highly recommended.

Top level engineer performance assessments

Based on the verbal reports provided by responsible Transpower Engineering, during the entire period the units were installed, there was no evidence of any grounding issues or other problems caused to the grid by these units.

Review of samples of the detailed performance data

A brief review of the sample data provided by Transpower was conducted. The preliminary review suggested that the data in use by Transpower engineers – the source of their positive assessment – provides a good basis on which to monitor performance and potential malfunction of these protective units.

The data are available in a format that can be easily used for further in-depth studies to understand the potential impact of space weather related events on transformers.

In particular, based on a top-level review of the sample data, the NER units properly reduced or blocked the occasional excessive current levels that occurred. Transpower has collected such data for all 52 NER-grounded transformers (beginning in 1992) and 2 NED-grounded transformers (beginning in 2012) since their installation.

Review of additional data (IEEE)

As part of this study a brief review was performed, examining the GIC measurement data from Transpower and supporting information available in an IEEE technical paper.6 These data indicate that the use of NER devices installed on neutrals of transformers located at strategic locations offers the benefit of controlling GICs resulting from nearby HVDC lines, reducing DC current flows into transformers by roughly 90% from anticipated currents were the transformers not protected.

The Transpower system, through proper analysis, design, testing, and installations of such devices across the system, coupled with appropriate operational procedures, has protected its key transformers from negative performance or operational impact due to stray DC and GIC currents.

6 PAPER 2.7 - DC GROUND CURRENTS AND TRANSFORMER SATURATION ON THE NEW ZEALAND HVDC LINK, by J.c. Gleadow B.J. Bisewski M.C. Stewart