Wind Farm Protection Using an IEC 61850 Process Bus

Architecture
1. Cardenas
1
, V. Muthukrishnan
2
, D. McGinn
2
, R. Hunt
3

General Electric
1
(SP), General Electric
2
(CA),General Electric
3
(USA)



Keywords: Wind turbine generator, Merging units,
IEC61850, Protection & Control, Wind Iarms..
Abstract
Wind generation and wind Iarms are becoming an important
part oI the generating capacity oI the modern utility grid.
Protection oI wind turbines, and wind Iarm collector systems,
has been the subject oI numerous technical papers over recent
years, as wind generation grows more prevalent. These papers
have Iocused on the basics oI protecting the wind turbines,
protecting the wind Iarm, and modelling oI wind Iarms Ior
stability studies. However, Iew have Iocused on practical
aspects oI protection and control systems when wind turbines
and collectors systems are distributed over many square
kilometres oI surIace area. There are design, construction, and
maintenance decisions to make Ior a wind Iarm, such as
where to physically mount protective relays, laying out
copper wiring between measurement sources and protective
relays, and realistic and aIIordable maintenance programs Ior
protective relays. Maintenance can quickly become an issue iI
relays are located at each individual wind turbine. These
decisions become much more complicated when the wind
Iarms are located oIIshore, as the cost oI installation, and the
cost oI maintenance, increase dramatically.

This paper proposes a protection and control (P&C) approach
based on IEC61850 process bus architecture. Ruggedized
Merging Units (MUs) can be installed at individual wind
turbines to acquire AC measurements (as raw sampled values)
and status inIormation, as well as provide control points. Data
is transmitted via IEC 61850 messages over a Iibre optic
communications network to a Central Relaying Unit (CRU)
which executes P&C Iunctions Ior a whole wind Iarm.
1 Introduction
Wind power has become cost-competitive with other
conventional means oI power generation. It can provide a
signiIicant amount oI energy Irom a renewable resource with
minimum adverse impacts on the environment and is the
Iocus oI 'green power marketing programs throughout the
world. Wind generation and wind Iarms are rapidly becoming
an important part oI the generating capacity oI the modern
utility grid. Emerging trends such as government incentives,
carbon limits, and decreasing costs oI wind turbine
technology will all lead to increased number oI wind Iarms in
the coming years.
There are two kinds oI wind Iarms: (1) large wind Iarms
located onshore or oIIshore consisting oI numerous wind
turbines connected together and distributed over several
square kilometres, with a single interIace to the transmission
system; and (2) a single wind turbine directly connected to the
distribution utility`s system. The Iocus oI this paper shall be
the large wind Iarms. Typically modern wind Iarms consist oI
20-150 individual wind turbines clustered into many groups
depending on the total number oI turbines. The capacity oI
each turbine is in the range oI 0.5 3MW, with some turbines
as large as 5MW.

A typical wind turbine generator unit consists oI the wind
turbine itselI, an induction generator, turbine/generator
control, generator breaker, and step-up transIormer. Recently
power converters have been employed to permit variable
speed operation in order to maximize the output power and
provide reactive power. Generation voltage is typically 690V
and this is stepped up to 34.5kV. Numerous wind turbine
outputs are connected together and tied to the collector bus
through a circuit breaker. Multiple collector Ieeders are
combined and Ied to a utility transIormer, which steps up the
voltage to transmission level and transIers the power. OIten,
reactive power compensation units such as capacitor banks
are also provided at the collector bus. There are other modern
ways such as the use oI FACTS devices or advanced control
oI induction generator Ior providing reactive power support.

When it comes to protection and control requirements oI a
wind Iarm, the wind power industry has been using
conventional and simple approaches. Even though rapid
advancements are being introduced in various Iields related to
wind power such as wind turbine and induction generator
design, wind turbine/induction generator control, ride-through
ability, and reactive power control, approaches adopted Ior
implementing protection have not seen signiIicant
advancement. It is not that protection and control industry has
not made technology breakthroughs; in Iact there have been
remarkable developments. However, there might have been a
gap in tuning these to the needs oI wind Iarm application.

The objective oI this paper is to present a new approach Ior
wind Iarm protection and control implementation.
Advancements in the protection and control industry have to
be brought into the domain oI wind Iarm application to
address the speciIic needs and challenges oI this application.
This paper begins by presenting an over view oI protection
and control requirements Ior a wind Iarm highlighting the key
challenges. A key proposal in the presented concept is the
idea oI using a Centralized Relaying Unit (CRU) to meet the
P&C requirements oI the whole wind Iarm consisting oI
numerous wind turbine generators, their step-up transIormers,
and the collector Ieeder.

It is important to note that the capacity oI individual wind
turbine generators and wind Iarms as a whole continue to
increase. The simple and basic protection approaches such as
Iuses will no longer be suIIicient to protect these systems.
More elaborate protection Iunctions and schemes will be
required in order to enhance the availability and reliability oI
wind Iarms. The time is ripe Ior the wind power industry to
look at innovative technology that not just meets their
protection and control needs, but goes beyond to solve their
various other economic, operation and maintenance
challenges, while remaining simple and using proven
methods.
2 Protection & Control Requirements of a
Wind Farm
A wind Iarm has many protection zones Irom the traditional
P&C perspective. Figure 1 illustrates the various protection
zones in a typical wind Iarm. This structure remains similar
whether it is an onshore or oIIshore wind Iarm. Each
protection zone has one or more protection relays and at times
other means oI protection such as Iuses depending on the size
and arrangement oI a wind Iarm. The disconnecting switches
are not shown in the diagram. The wind turbine generator,
which is normally an induction generator with its associated
power electronics converter and control, is protected by a
separate wind generator protection relay. The step-up
transIormer has normally a Iuse at the MV side. However Ior
larger transIormers (~1MW), a separate transIormer
protection relay is used. The collector Ieeder is protected by a
separate Ieeder protection relay, while the collector bus uses a
bus bar protection relay. The high voltage transIormer is
always protected by a transIormer protection relay while the
transmission line has a distance or current diIIerential relay.
There are speciIic challenges in a wind Iarm protection that
are worth mentioning. There exist diIIerent topologies Ior
connecting wind turbine generators to the collector Ieeder.
Radial, BiIurcated radial, Feeder-SubIeeder and Looped
Ieeder are the diIIerent types oI topologies used. Each
topology has its advantages and disadvantages. Some provide
the opportunity to isolate the Iaulted area and continue
running the rest, while other topologies have lower cost.
However whatever may be the conIiguration, the MV side oI
wind turbine generator step-up transIormer has normally a
delta winding. So the entire collector Ieeder depends on the
ground oI the High Voltage transIormer. This may lead to a
situation where a collector Ieeder with all its wind generators
runs as an island without a ground. This may result in
transient or sustained over-voltages and wind turbine
generator relays won`t be able to see MV ground Iaults. The
behavior oI the power converter and associated controls can
act to produce voltage transients due to neutral shiIt. Thus,
sometimes a grounding transIormer is installed on each Ieeder
circuit. Even then, it is not desirable to run the collector
Ieeder as an island due to saIety reasons. Various
communications based transIer trip solutions are used to trip
the wind turbine generator breakers iI the collector Ieeder
breaker trips |2|. In addition new approaches to identiIy Iaults
in diIIerent areas oI a collector Ieeder will be a blessing since
speciIic areas can then be easily isolated while rest oI the
system could continue to run.
Control
I G
1
Control
I G
2
Control
I G
3
Control
I G
n
Capacitor Bank Relay Zone
Collector Feeder 1
Collector Feeder 2
Collector Feeder 3
Collector Feeder n
Wind Generator Relay Zone
Collector Feeder Rela y Zone
Collector Bus
Relay Zone
690 V / 34. 5kV
34. 5 kV / 220kV
HV Transformer Relay Zone
Transmission Line Relay Zone
Wind Generator Relay Zone
Wind Generator Relay Zone
Wind Generator Relay Zone
Figure 1: Protection Zones in Wind Farm
The Iact that the protection relays Ior wind generators are
located at the base oI the tower brings in several challenges
associated with commissioning, operation, and maintenance.
Monitoring oI a wind Iarm protection system Irom a central
location means interconnecting all the individual protection
relays and conIiguring each relay to access required
inIormation. This could be an elaborate process. For example,
a wind Iarm with 36 wind turbines on a single collector Ieeder
would have 36 relays minimum. To install, conIigure, test,
commission and maintain these individual relays distributed
over a wide area and that too located at the base oI the wind
turbine tower could be quite expensive in terms oI labor, time,
and resources. Any new technology Ior wind Iarm protection
should address these challenges associated with installation,
commissioning, operation and maintenance.
4. Protection and Control System Based on IEC
61850 Process Bus
At a time when the protection industry is looking at what
should be the next step in the evolution oI protective relays, it
should be recognized that merely enhancing the capabilities
oI numerical relays will not be suIIicient to meet the real
challenges Iacing the utilities such as dealing with huge
copper cabling installation, scarcity oI workIorce, reduction
in available time Ior new or retroIit project executions, labour
cost and cyber security threats etc. Thus a holistic approach
taking into account the business expectations oI the utilities is
essential. This section presents one such protection and
control solution based on the IEC 61850-9-2 process bus.
This implementation is based on the concept oI distributed
data acquisition units interIacing with various primary
apparatus and exchanging inIormation with numerical relays
over Iiber communication.
This IEC 61850 process bus architecture uses merging units
to collect CT/VT signals and circuit breaker/process control
and status signals. The IEC 61850-9-2 output oI each merging
unit is connected via pre-terminated Iiber cable to a patch
panel that directs the appropriate signals to each relay.
In reIerence to Figure 2, the system includes merging units
mounted at the primary apparatus, relay, pre-terminated
cables, and Iiber patch panels Ior cross-connecting the
merging units and relays.


Figure 2: Protection & Control System using IEC 61850
Process Bus
The merging units are designed to interIace with all signals
typically used Ior substation automation and protection as
close to their respective origins as practical, including AC
currents and voltages Irom instrument transIormers, breaker
status and alarms, breaker control, disconnect switch status
and control, temperature and pressure readings, etc. The
merging units are designed Ior harsh environments including
temperature extremes, shock and vibration, electromagnetic
compatibility, sun load eIIect, pressure washing and exposure
to salt and other harsh chemicals.
Each merging unit contains multiple independent digital cores
each composed oI a microcontroller with individual bi-
directional (bi-di) Iiber links providing dedicated point-to-
point communications with a single relay. Sampled value
communications used conIorm to IEC 61850-9-2, and
GOOSE communications to IEC 61850-8-1.
All cables are connectorized and pre-terminated Ior ease oI
deployment and replacement using standard military/avionic
grade components. Rugged outdoor Iiber cables connect the
merging units in the Iield to the patch panels in the control
room.
The process bus architecture presented above brings in
numerous beneIits to the end user in terms oI reduction in
total installed cost, ease oI engineering, installation,
commissioning, operation and maintenance. The rugged
merging units with pre-Iabricated/connectorized cabling and
highly reliable point-point Iiber communication makes this
approach suitable Ior wind Iarm applications as explained in a
later section.
Patch panels are used to land and organize the outdoor cables.
Standard patch cords are used to accomplish 'hard-Iibering,
making all the necessary IEC 61850 connections between the
relays and the merging units as dictated by the station
conIiguration on a one-to-one basis, without the use oI
switched network communications as detailed.
Each relay provides protection Ior one basic zone,
conIorming to established protection philosophies. It receives
the signals to perIorm its Iunction over a secure and dedicated
network consisting oI direct hard-Iibered links to each oI the
associated IEC 61850 merging units. Due to the completely
deterministic data traIIic on these dedicated links, a simple
and robust method is used Ior synchronization whereby each
relay controls the sample timing oI the connected merging
unit cores over the link without relying on an external clock
Ior process bus data synchronization. Figure 3 depicts how
the classical numerical relay hardware platIorm presented in
the previous section has been modiIied to adapt to the process
bus based system. The process bus relay has the process bus
interIace card but does not have the conventional DSP and
Contact I/O modules as these Iunctionalities are moved to the
merging units.
Power Supplg
Centrcl Processor 0nit Process 8us interfcce
mptg spcce |cvcilcblel
Inter- relcg communicction
Module

Figure 3: Hardware Architecture oI a Process Bus Based
Relay
3 Integrated Protection and Control System
Knowing that numerical relays are built on a common
hardware platIorm and in Iact have the soItware programming
required Ior various protection Iunctions hosted within the
same unit, it is practical to think about integrating all the
protection and control requirements oI a substation into a
single numerical relay. There could be two challenges to this
approach to begin with: (1) the capability oI the processors
used in numerical relays, (2) the need to bring all the copper
wire based signals Irom the whole station into a single relay.
The Iormer challenge is easily addressed by ever increasing
capabilities oI the processors and perhaps by various other
architectural considerations. The distributed processing and
the all-Iiber numerical relay architecture presented in the
previous section resolve the second challenge. BeIore delving
into this idea, it is worth recapturing some oI the eIIorts in the
past that were invested in similar lines.
6. Wind Farm Protection & Control Using IEC
61850 Process Bus

The present practice using conventional numerical relays or
process bus based relays is to use an individual protective
relay dedicated to each zone oI protection. II the same
approach is adopted, one relay will be needed Ior each wind
turbine, and collector Ieeder protection. However, use oI the
integrated protection and control system allows Ior one
protective relay to acquire signals Irom multiple zones oI
protection, and provide protection Ior multiple zones, Iurther
reducing maintenance and installation costs. Protection and
Control requirements oI the whole wind Iarm can be met by a
single protection relay with distributed merging units.
ReIerring to Figure 4, this system has merging units placed at
the wind turbines and Ieeder breaker. They are brought to the
Central Relaying Unit (CRU) using point-to-point Iiber
communication. The CRU is essentially an all-Iiber numerical
relay and has multiple process bus interIace cards to connect
to the numerous merging units out in the Iield. Each process
bus interIace card connects to a pre-deIined number oI
merging units and each CRU has a Iixed maximum number oI
process bus interIace cards it can support.

The CRU implements the parallel processing approach. The
CRU uses multiple processors running in parallel to execute
the protection and control Iunctions oI the whole wind Iarm.
Each process bus interIace card in turn consists oI multiple
processors that have the power and ability oI a single
numerical relay. So it can be said that the CRU essentially
contains multiple relays. The protection Iunctions oI a whole
wind Iarm are distributed among the diIIerent process bus
interIace cards and the main processor card.

Figure 4 also shows the application oI this integrated
protection and control system in the typical wind Iarm
example explained in Section 2. Each wind turbine generator
has a merging unit as part oI its generator breaker, which can
easily be installed, wired and tested by the breaker
manuIacturer beIore even shipping to site. This merging unit
acquires current, voltage and breaker signals. Similarly each
Ieeder breaker merging unit is installed as part oI it to acquire
CT and breaker interIaces.
Control
I G
1
Control
I G
2
Control
I G
3
Control
I G
n
Collector Feeder1
Collector Feeder2
Collector Feeder3
Collector Feedern
Integrated Wind
GeneratorProtection
690 V / 34.5 kV
34.5 kV / 220 kV
CollectorBus
Bar Relay
MergingUnit1
MergingUnit2
MergingUnit3
Merging Unit ' n '
HVTransformer
Relay
Capacitor Bank
Relay
Feeder 1
MergingUnit
P
S
I
n
t
e
r
-
R
e
la
y
C
o
m
m
s
C
P
U
CRU
Transmission Line
Relay

Figure 4: Wind Farm Protection using IEC 61850 Process Bus and Integrated P&C System
The CRU executes all protection and control requirements Ior
a speciIic collector Ieeder and all the wind generators
connected to it. This means the CRU instantiates a copy oI all
the protection Iunctions mentioned as part oI Wind Generator
Zone in Section 2 Ior each oI the wind generator in that
collector, and also instantiates the collector Ieeder zone
protection Iunctions. ThereIore, a single relay has the
visibility into the whole oI that collector section in the wind
Iarm. The success oI such a system is as dependent on the
user interIace Ior the system as the hardware and Iirmware.
The soItware Ior such a system must consider various aspects
such as intuitive appearance, simplicity oI conIigurations,
ease oI navigation, transparent monitoring screens and one-
shot view oI the connected system.
One CRU shall be used Ior each oI the collector Ieeders. The
collector bus shall be protected by a single bus bar protection
relay that has access to the merging units Irom all collector
Ieeders and the HV transIormer breaker merging unit. The
capacitor bank protection relay, HV transIormer protection
relay, and transmission line protection relay all shall access
appropriate merging units to execute the respective protection
elements. It is possible to integrate all these into the collector
bus bar protection relay, or a diIIerent CRU, as well. Such an
arrangement will result in one CRU Ior each collector Ieeder
and a CRU Ior collector bus bar, capacitor bank, HV
transIormer and transmission line protections.
This integrated protection approach Ior the whole wind Iarm
makes the installation and commissioning easy as the merging
units come installed and tested with the breaker. Further the
need to conIigure and test individual relays have been
eliminated and only a single relay - CRU needs to be
conIigured, tested and maintained. Further, procedures such
as transIer trips will no longed be needed as the CRU can
issue trip signals to the Ieeder breaker and all the wind
generator breakers simultaneously.
The advantages oI the integrated wind Iarm protection and
control system over the conventional ways oI protecting the
wind Iarms are summarized below:
Rugged merging units, suitable Ior outdoor
installation and Iully connectorized Ior ease oI
installation and maintenance.
SigniIicant part oI site tests could be done in Iactory.
Merging units have no settings or Iirmware identity,
essentially dumb units and so ease oI maintenance.
Eliminates the need Ior separate relays Ior each wind
generator and the need to set, conIigure, test, and
maintain the individual relays.
One setting Iile Ior the whole collector section and
so ease oI engineering.
Eliminate the need Ior transIer trips as simultaneous
trips to Ieeder and generator breakers possible as the
CRU has both Ieeder and generator breakers access
instantly.
Possibility oI new operational schemes within the
collector section Ior Iault location and to
automatically isolate Iaulty sections and restore
operation oI rest oI the system.
Monitoring the whole collector section is
conIiguration Iree as CRU has all the inIormation
already.
Disturbance recording at each wind turbine and
single sequence oI event recording.
Easy integration and exchange oI data with other
collector Ieeders or interconnection systems.
Last but not the least, use oI a widely accepted open
standard protocol IEC 61850.
The integrated protection and control system explained here
Iind its application not only in wind Iarms; it can easily be
applied Ior large bus bars with numerous Ieeders, MV
switchgears with multiple Ieeders and incomers. In Iact, it is
also possible to have the protection and control requirements
oI a whole HV substation met by this integrated protection
system.
7. CONCLUSIONS
Wind energy is a rapidly growing environmentally Iriendly
generating means. The technology around wind energy is also
growing very Iast. However the protection and control
requirements oI a wind Iarm are still met using conventional
means oI protective relaying only. The protection industry is
evolving at its own rate with breakthrough next generation
P&C systems that addresses key challenges Iacing the utility
industry today. This paper brieIly presented the protection
and control requirements oI a wind Iarm and discussed the
evolution oI technology in protective relays. An Integrated
Protection and Control System Ior Wind Farms is presented
that uses the concepts oI IEC 61850 process bus and a
centralized relaying unit with parallel processing ability. The
application oI this system in a wind Iarm brings in signiIicant
improvements such as reduction in P&C system total
installation cost; ease oI installation, commissioning and
maintenance; ease oI conIiguration; Iaster and reliable
transIer tripping schemes; and new approaches to identiIy and
isolate Iaulty sections and restore operations.

References
|1| 'Integrated Protection System Ior Rural Substations
SIPSUR. GE Protection & Control and Union Electrica
Fenosa (Spain) under a PIE research project, 1990.A. B.
|2| | 'Wind Farm System Protection and Stability Using Peer
to Peer Communications, M.L.Reichard, D. Finney,
J.T. Garrity, CIGRE Canada, ConIerence on Power
Systems, 2006.