DC STRAY CURRENT REDUCTION
AND BLOCKING PERFORMANCE
REVIEW OF LONG TERM GRID
PERFORMANCE DATA
OCTOBER, 2014
DATA SOURCE: TRANSPOWER, NEW ZEALAND
IS
C O U N C I L
New Zealand
Electric 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
Forward
New Zealand’s electricity provider, Transpower, has developed a substantial database on use
of transformer “current blockers.” The company has used these transformer neutral grounding 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, addressing 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. Introduction
Beginning 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.
Power Transmission
Map of New Zealand
350 kV (DC)
3. Description of Transpower
Electric System – New Zealand
Transpower, 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.
North Island
Converter
kV 350
Benmore
South 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 Configuration
To 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 wyeconnected 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.
Figure 3: Key Substatitions on South Island
Figure 4: Spark gap bypass scheme on the Pole 2 converter
transformer at Benmore substation
• 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”.
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 oneminute 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.
BEN.HVDC.
.ELECTRODE
ASB.TRANS.
.T1_NER
BEN.TRANS.
.T27NER
BEN.TRANS.T29
Date; Time
AMP
AMP
AMP
2003-10-29
00:00:00.000
NER.AMP
19.93
0.10
0.10
0.28
2003-10-29
00:01:00.000
21.78
0.10
0.10
0.28
2003-10-29
00:02:00.000
23.63
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.
400
6
350
5
10/29/2003
300
4
250
200
3
150
2
BEN.TRANS.T27NER.AMP
BEN.TRANS.T28NER.AMP
BEN.TRANS.T29NER.AMP
100
1
50
BEN.TRANS.T2NER.AMP
BEN.TRANS.T46NER.AMP
BEN.TRANS.T2NER.AMP
0
BEN.HVDC.ELECTRODE.AMP
23:23:00.000
22:22:00.000
21:21:00.000
20:20:00.000
19:19:00.000
18:18:00.000
17:17:00.000
16:16:00.000
15:15:00.000
14:14:00.000
13:13:00.000
11:11:00.000
12:12:00.000
10:10:00.000
09:09:00.000
08:08:00.000
07:07:00.000
06:06:00.000
05:05:00.000
04:04:00.000
03:03:00.000
02:02:00.000
01:01:00.000
00:00:00.000
0
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