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ABB Protection Relay School Webinar
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© ABB Group October 27, 2015 | Slide 1
Relion. Thinking beyond the box.Designed to seamlessly consolidate functions, Relion relays are smarter, more flexible and more adaptable. Easy to integrate and with an extensive function library, the Relion family of protection and control delivers advanced functionality and improved performance.
This webinar brought to you by the Relion® product family Advanced protection and control IEDs from ABB
© ABB Group October 27, 2015 | Slide 2
ABB is pleased to provide you with technical information regarding
protective relays. The material included is not intended to be a complete
presentation of all potential problems and solutions related to this topic.
The content is generic and may not be applicable for circumstances or
equipment at any specific facility. By participating in ABB's web-based
Protective Relay School, you agree that ABB is providing this information
to you on an informational basis only and makes no warranties,
representations or guarantees as to the efficacy or commercial utility of
the information for any specific application or purpose, and ABB is not
responsible for any action taken in reliance on the information contained
herein. ABB consultants and service representatives are available to
study specific operations and make recommendations on improving
safety, efficiency, and profitability. Contact an ABB sales representative
for further information.
ABB protective relay school webinar seriesDisclaimer
Transformer protection fundamentalsOctober 27, 2015
ABB protective relay school webinar series
© ABB Group October 27, 2015 | Slide 3
© ABB Group October 27, 2015 | Slide 4
Presenter
Bob Wilson
Bob graduated from Purdue University and joined Westinghouse Electric Corp. After receiving a Masters degree in Electrical Engineering from Carnegie Mellon University, Bob was a Systems Analysis Engineer responsible for software designed to automate system-wide coordination. He then transferred to Kansas City where he assumed the role of District Engineer and eventually moved to the Houston area where he currently resides.
In his current role as Regional Technical Manager, he supports ABB’s Substation Automation and Protection Division, providing technical support to customers throughout the South Central United States. Bob is a senior member of IEEE and has authored and presented several papers in power system protection at a variety of technical conferences throughout the United States. He is a Registered Professional Engineer in the states of Pennsylvania and Texas.
© ABB Group October 27, 2015 | Slide 5
Learning objectives
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 6
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 7
Power transformer:
1. HV side bushings2. LV side bushings3. Load tap changer4. Load tap changer
operating device5. Control panel6. Oil thermometer7. Gas relay8. Radiators9. Oil conservatorN. Neutral bushings
Transformer basicsConstruction
© ABB Group October 27, 2015 | Slide 8
Power transformer
Iron core
HV voltage winding
LV voltage windingMV voltage winding
Transformer basicsConstruction
© ABB Group October 27, 2015 | Slide 9
Transformer basicsDifferent winding arrangements
© ABB Group October 27, 2015 | Slide 10
Winding failures
Turn-to-turn insulation failure
Moisture
Deterioration
Internal and external faults Mechanical and insulation integrity
Tap changer failures
Mechanical
Electrical
Short circuit
Oil leak
Overheating
Transformer basicsTypes of failures
© ABB Group October 27, 2015 | Slide 11
Bushing failures
Aging, contamination, and cracking
Flashover due to animals
Moisture
Low oil
Core failures
Core insulation failure
Ground strap burned away
Loose clamps, bolts, wedges
Transformer basicsTypes of failures
© ABB Group October 27, 2015 | Slide 12
Miscellaneous failures
Bushing current transformer failure
Metal particles in oil
Damage in shipment
External faults
Poor tank weld
Overvoltages
Overloads
Transformer basicsTypes of failures
Cause of Transformer Failures* %
Winding failure 55
Tap changer failures 21
Bushing failures 10
Terminal board failures 6
Core failures 2
Miscellaneous failures 6
All causes 100
*IEEE Guide
Transformer basicsCause of failures
© ABB Group October 27, 2015 | Slide 14
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 15
( ) ssmhppp InIIn-In =+
This quantity cannot be directly measured
Single-phase transformer
© ABB Group October 27, 2015 | Slide 16
Considerations for three-phase transformers
Winding connections
Number of windings
Core construction and operating characteristics
Three-phase transformer
© ABB Group October 27, 2015 | Slide 17
No connections High voltage bushings
H1, H2, H3 => system A, B, C H0 if neutral provided
Low voltage bushings X1, X2, X3 => system A, B, C X0 if neutral provided
Tertiary Third winding Y1, Y2, Y3 => system A, B, C
Turns ratio - N High-to-low
Basic Three-phase transformer
© ABB Group October 27, 2015 | Slide 18
ANSI standard
High voltage reference leads the low voltage reference by 30O
High voltage reference is in phase with low voltage reference
© ABB Group October 27, 2015 | Slide 19
No phase shift Effective turns ratio = N Same applies for delta - delta
connection Auto-transformers
Wye-wye connected transformer
© ABB Group October 27, 2015 | Slide 20
ANSI Standard ConnectionsHigh Voltage Low Voltage
High voltage reference phase voltage leads the low voltage reference phase voltage by 30O
Wye-delta Delta-wye
© ABB Group October 27, 2015 | Slide 21
Phase shift H1 leads X1 by 30O
Effective turns ratio
3Nn =
Wye-delta connected transformer
© ABB Group October 27, 2015 | Slide 22
Phase shift H1 leads X1 by 30O
Effective turns ratio
3Nn =
Delta-wye connected transformer
© ABB Group October 27, 2015 | Slide 23
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 24
IA-1IB-1IC-1 IA-3
IB-3IC-3
IA-2IB-2IC-2
Winding-3 Inputs(3-Winding units only)
Winding-1 Inputs
Winding-2 Inputs
Typical transformer phase differential configuration
X/1
N:1 (Phase shift δ)
M:1 (Phase shift φ)
Y/1
Z/1
Y or ∆
Y or ∆
Y or ∆
Transformer differential protection
© ABB Group October 27, 2015 | Slide 25
IA-1IB-1IC-1 IA-3
IB-3IC-3
IA-2IB-2IC-2
Winding-3 Inputs(3-Winding units only)
Winding-1 Inputs
Winding-2 Inputs
Zone of protection defined by current transformers (CT’s)
X/1
N:1 (Phase shift δ)
M:1 (Phase shift φ)
Y/1
Z/1
Y or ∆
Y or ∆
Y or ∆
Transformer differential protection
© ABB Group October 27, 2015 | Slide 26
IA-1IB-1IC-1 IA-3
IB-3IC-3
IA-2IB-2IC-2
Winding-3 Inputs(3-Winding units only)
Winding-1 Inputs
Winding-2 Inputs
Non-trip zone for phase differential protection
X/1
N:1 (Phase shift δ)
M:1 (Phase shift φ)
Y/1
Z/1
Y or ∆
Y or ∆
Y or ∆
Transformer differential protection
© ABB Group October 27, 2015 | Slide 27
IA-1IB-1IC-1 IA-3
IB-3IC-3
IA-2IB-2IC-2
Winding-3 Inputs(3-Winding units only)
Winding-1 Inputs
Winding-2 Inputs
Ideally what comes in equals what goes out:IIN = IOUT
X/1
N:1 (Phase shift δ)
M:1 (Phase shift φ)
Y/1
Z/1Y or ∆
IIN
IOUT
+
Transformer differential protection
© ABB Group October 27, 2015 | Slide 28
IA-1IB-1IC-1 IA-3
IB-3IC-3
IA-2IB-2IC-2
Winding-3 Inputs(3-Winding units only)
Winding-1 Inputs
Winding-2 Inputs
Transformer differential protection is generally quite simple, but requires the correct application and connection of current transformers and an understanding of the power transformer winding connections, characteristics and operation.
X/1
N:1 (Phase shift δ)
M:1 (Phase shift φ)
Y/1
Z/1
Y or ∆
Y or ∆
Y or ∆
IIN IOUT
Transformer differential protection
© ABB Group October 27, 2015 | Slide 29
R R
RR
RR
O
O
O
Ia
Ib
Ic
nIa
nIb
nIc
n(Ia-Ic)
n(Ib-Ia)
n(Ic-Ib)
n(Ia-Ic)/CT2
n(Ib-Ia)/CT2
n(Ic-Ib)/CT2
Ia/CT1
Ib/CT1
Ic/CT1
25 MVA69kV 13.8kVCT1=250/5 CT2=1200/5
A
B
C
Transformer differential protection
© ABB Group October 27, 2015 | Slide 30
( )303
3
92.10458.13*3
10*25
2.20969*3
10*25
jL
H
eAI
AI
−⋅==
==
( )30.136.4240/1046
.118.450/2.209j
LSec
HSec
eupAIupAI
−⋅===
===
upeIII jLSecHSecDIFF .52.011 30
=−=+= − ==> TRIP!
Tap settingsCT secondary currents
Rated primary currents
Differential current
Transformer differential protection
© ABB Group October 27, 2015 | Slide 31
R R
RR
RR
O
O
O
Ia
Ib
Ic
nIa
nIb
nIc
n(Ia-Ic)
n(Ib-Ia)
n(Ic-Ib)
n(Ia-Ic)/CT2
n(Ib-Ia)/CT2
n(Ic-Ib)/CT2
Ia/CT1
Ib/CT1
Ic/CT1
25 MVA69kV 13.8kVCT1=250/5 CT2=1200/5
A
B
C
Transformer differential protection
IA’= I1 + I2 + I0
IA’ nIA’
nIA’= nI1 + nI2 + nI0
nI0= n(Ia-Ic) + n(Ib-Ia) + n(Ic-Ib) = 0
IA’≠ nIA’
© ABB Group October 27, 2015 | Slide 32
R R
RR
RR
O
O
O
Ia
Ib
Ic
nIa
nIb
nIc
n(Ia-Ic)
n(Ib-Ia)
n(Ic-Ib)
n(Ia-Ic)/CT2
n(Ib-Ia)/CT2
n(Ic-Ib)/CT2
(Ia-Ic)/CT1
(Ib-Ia)/CT1
(Ic-Ib)/CT1
A
B
C
CT1=250/5 69kV 25 MVA 13.8kV CT2=1200/5
Ia/CT1
Ic/CT1
Transformer differential protection
IA= I1 + I2
IA
nIA
nIA= nI1 + nI2
Change CT connections from wye to delta
© ABB Group October 27, 2015 | Slide 33
Issues
Tap setting
Phase Shift
( ) ( )( )30
30
.136.4240/1046
.124.750/2.209j
LSec
jHSec
eupAIeupAI
−
−−
⋅===
⋅==⋅= j30e3
011 3030=−=+= −− jj
LSecHSecDIFF eeIII ==> NO TRIP!
Tap settingsCT secondary currents
Differential current
Transformer differential protection
© ABB Group October 27, 2015 | Slide 34
Transformers with delta and wye windings Phase shift and magnitude (√3) compensation must
be applied Zero sequence currents for external ground faults
must be blocked Solution
Conventional differential protection CT on the wye side connected in delta CT on delta side connected in wye
Numerical protection Connect all winding CTs in wye Apply compensating factors and I0 filtering
Vendor Specific
Transformer differential protection
© ABB Group October 27, 2015 | Slide 35
Compensation factors for numerical relays
HIGH LOWANGLE
(H leads L)
30 0
0 0
30 0
0 0
330 0
COMPENSATINGFACTOR
H L
1 1
1 1
1
1
1
1
3
3
3 REFERENCE
3 0 0
60 090 0
120 0
150 0
180 0
210 0
240 0
270 0
300 0
330 0
ANGLE HV LEADS LV
Transformer connections
330 0 3
*ANSIAll CT'S assumed wye connected
Transformer differential protection
*
*
© ABB Group October 27, 2015 | Slide 36
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 37
Example:Transformer data:
25MVA (Max); 69 kV to 13.8/7.967 Grd Y; Xt=7%CT1=250/5CT2=1200/5
1. Phase shift is 30 degrees, low side CF = √32. High side current 209.2(A) and low side current 1046(A)
)(924.104510*8.13*3
10*25
)(184.20910*69*3
10*25
3
6
3
6
AI
AI
L
H
==
==
Differential protection settings
© ABB Group October 27, 2015 | Slide 38
3. Through fault current:
4. CT ratio: CT1=250/5 = 50 and CT2 = 1200/5 = 2405. Secondary current
IH (sec) = 209.2/50=4.18(A) IL (sec) = 1046/240=4.36(A)
6. Relay current at maximum load are: high side = 4.18 (A) low side = 4.36*√3 = 7.55 (A)
7. Tap settings are 4.2 (A) and 7.5 (A)
)(942,1407.0*10*8.13*3
10*25
)(298807.0*10*69*3
10*25
3
6
3
6
AI
AI
L
H
==
==
Differential protection settings
© ABB Group October 27, 2015 | Slide 39
8. Set harmonic restraint method9. Select a linear percentage slope of 30% for
transformer with +/- 10% load tap changer 10. Select minimum operating current of 0.3pu with +/-
10% load tap changer 11. Set 87H (unrestrained) to 6.0 [times Tap]
Differential protection settings
© ABB Group October 27, 2015 | Slide 40
0 1 2 3 4 5 6 7 8
1
2
3
4
5
6
IRES in pu
I DIF
Fin
pu
Operating Region
Unrestrained Operating Region
Restraining Region
IOP - MIN
Differential protection settings
© ABB Group October 27, 2015 | Slide 41
0 1 2 3 4 5 6 7 8
1
2
3
4
5
6
IRES in pu
I DIF
Fin
pu
Unrestrained Operating Region
IOP - MIN0
1
1
• Margin• LTC Range • Max difference in HV & LV
CT error for min. thru-fault• Excitation Current
5-10%10%
20%
IOP - MIN
Operating Region
2%
Differential protection settings
© ABB Group October 27, 2015 | Slide 42
0 1.25 2 3 4 5 6 7 8
1
2
3
4
5
6
IRES in pu
I DIF
Fin
pu
Operating Region
Unrestrained Operating Region
LTC Range - 10%
Max difference in HV & LV CT error for max. thru-fault - 20%
Margin - 5-10%
Differential protection settings
© ABB Group October 27, 2015 | Slide 43
Example:
CT error = +/- 10%LTC variation = 10%Excitation current at rated voltage = 1.4%Margin = 5%
Relay minimum operating current setting:(CT error)@HV side+(CT error)@LV side +LTC + Ie +Margin = 0.1+0.1 +0.1+0.02 + 0.05=0.37 pu
Relay slope setting:
(CT error)@HV side+(CT error)@LV side +LTC + Margin = 0.1+0.1 +0.1+0.05 = 35%
Differential protection settings
© ABB Group October 27, 2015 | Slide 44
0.1
1
10
100
0.1 1 10 100
I - Larger Restraint Current in Per Unit of Tap
I -O
pera
te C
urre
nt in
Per
Uni
t of T
ap
HU 35%
HU 30%
IopTH
IopMIN
Westinghouse HU electro-mechanical relay characteristic
Differential protection settings
© ABB Group October 27, 2015 | Slide 45
Differential protection settings
1 32 4 5
1
2
3
4
5
6
6IRES in pu
I DIF
F in
pu
m2
Region 3Region 2Region 1
m3
Unrestrained Operating Region
IUNRES
Operating Region
Restraining Region
IOP-MIN%100⋅
∆∆
=RES
DIFF
IIm
% Slope
© ABB Group October 27, 2015 | Slide 46
Differential protection settings
If LTC position is monitored improved sensitivity can be provided
In this case the minimum operate current setting and slope can be set in the range of 0.15-0.25 pu.
© ABB Group October 27, 2015 | Slide 47
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 48
Zero sequence current
Transformer inrush
CT saturation
Overvoltage (over-excitation)
Differential measurement issues
© ABB Group October 27, 2015 | Slide 49
R R
RR
RR
O
O
O
Ia
Ib
Ic
nIa
nIb
nIc
n(Ia-Ic)
n(Ib-Ia)
n(Ic-Ib)
n(Ia-Ic)/CT2
n(Ib-Ia)/CT2
n(Ic-Ib)/CT2
(Ia-Ic)/CT1
(Ib-Ia)/CT1
(Ic-Ib)/CT1
A
B
C
CT1=250/5 69kV 25 MVA 13.8kV CT2=1200/5
Ia/CT1
Ic/CT1
Zero sequence elimination
IA= I1 + I2
IA
nIA
nIA= nI1 + nI2
Change CT connections from wye to delta
© ABB Group October 27, 2015 | Slide 50
The peak value of the magnetizing current is generally higher for smaller transformers
Duration of the inrush current is longer for the larger transformers
Switching instant
The maximum inrush current will happen when the transformer is switched at voltage zero transition
Statistical data indicates every 5th or 6th transformer energization will result in high values of the inrush
Inrush currentInfluencing factors
© ABB Group October 27, 2015 | Slide 51
Energizing side Inductance is function of geometry. Low voltage winding that
is wound closer to the magnetic core has less impedance than the outer winding. Consequently, energizing the transformer from the LV winding will cause more inrush than energizing from the HV winding.
Typical values: LV side: magnitude of inrush current is 10-20 times the rated
current
HV side: magnitude of inrush current is 5-10 times the rated current
Remanence (residual flux) in the core Higher remanence results in the higher inrush
Impedance of the system Energizing from lower source impedance results in the higher
inrush Highest inrush generally at generating plants
Inrush currentInfluencing factors
Harmonic analysis-typical
Transformer Inrush Current
Internal Fault Current
Differential current due to:
Component Internal Fault CT Saturation Magnetic Inrush
Peak 145% 126% 244%
DC 38 0 58
Fund. 100 100 100
2nd 9 4 63
3rd 4 32 22
4th 7 9 5
5th 4 2 32
6th 6 1 4
7th 2 3 3
Internal Fault Current with CT Saturation
Values in % of fundamental© ABB Group October 27, 2015 | 52
© ABB Group October 27, 2015 | Slide 53
Protection commonly uses 2nd harmonic value in % of fundamental to distinguish between inrush current and short circuit current
Short circuit current has only a small 2nd harmonic
Inrush current has significant 2nd harmonic
Minimum % 2nd harmonic and maximum peak on inrush occur when the transformer is energized at voltage-angle = 0º
Protection engineers should set relaying based on data provided by the transformer manufacturer
Inrush current
© ABB Group October 27, 2015 | Slide 54
CT saturation
Waveform restraint – designed to detect the 2nd harmonic wave form, differentiate it from saturated CT secondary current, and block tripping
Adaptive 2nd harmonic operation
Setting
2nd harmonic always in
Conditional 2nd harmonic
Logic to detect when 2nd harmonic restraint is applied
Transformer energization
CT saturation on external faults
How to avoid delayed trips for an internal fault that saturates a CT?
Overvoltage/overexcitation current
Ie
0 90 180 360 540 720
Total Flux
Transformer Magnetizing
Characteristics
S
0 90 180 360 540 720
Over E
xcitation Current
ZS (saturated) Overexcitation waveform produces predominately high odd harmonics 3rd, 5th, 7th, etc.
Typical % of fundamental
2nd = 0
3rd = 70 (*never use)
4th = 0
5th = 30
6th = 0
7th = 2
* 3rd harmonics are a prevalent quantity on the power system produced from many sources
© ABB Group October 27, 2015 | Slide 56
Over-excitation exists if the per unit V/Hz exceeds the design limit of transformer
Voltage exceeds 105% at full load or 110% no load at rated frequency
The frequency goes below 95% (fl) to 90% (nl) of rated at rated voltage
Use manufacturer’s V/Hz – Time curve
How to detect over-excitation?
V/Hz relay using the VT input from the primary
Differential relay with fifth harmonic current restraint can be used if VT unavailable …Apply blocking or tripping with caution !
Over-excitation detection
© ABB Group October 27, 2015 | Slide 57
Transformer protection
Transformer basics
Transformer winding connections
Basic differential protection
Basic differential protection settings
Differential measurement issues
Complementary protection functions
© ABB Group October 27, 2015 | Slide 58
Ground differential protection (87G) - REF
Restricted Earth Fault (REF) Protects only the wye-grounded
transformer winding for all internal ground faults Idiff = IN + 3I0 Ires = Max(Ia, Ib, Ic, IN), I1
A directional criteria is also applied in order to increase stability against heavy external faults with CT saturation
Circuit Breaker
Optional Z
I0
I0
I0
3I0
X
Circuit Breaker
Optional Z
I0
I0
I0
3I0
X
3I0 3I0
3I0 3I0
6I0
0
Internal Fault
External Fault
Turn-to-turn fault detection Turn-to-turn fault
Usually involves a small number of adjacent turns
A small unbalance in primary to secondary turns ratio,
(Np-Nt)/Ns
Undetectable with normal differential protection
High current in shorted turns (Np-Nt)Ip + NtIt + NsIs = 0 Not measurable (no access)
Sudden Pressure Relay (SPR) Slow Tendency to mis-operate Block on thru-fault current
Negative sequence differential
Np Np
Np - Nt
NsNs Ns
Nt
Negative sequence differential protection
Negative sequence current for external fault Relative phase between them is 180 degrees
Relay
E2f
Z2S1 Z2S2
I2S1 I2S1 I2S2
I2S1 I2S1Negative Sequence Zero
Potential
I2diff = 0
Negative sequence differential protection
Negative sequence current for internal fault Relative phase between them is 0 degrees
Relay
E2f
Z2S1 Z2S2
I2S1 I2S2
I2S1 I2S2Negative Sequence Zero
Potential
I2diff = I2S1 + I2S2
© ABB Group October 27, 2015 | Slide 62
Overcurrent protection coordination
Through fault protection
C57.109-1985 considers both thermal and mechanical effects for through fault (external faults)
Category I transformer ==> thermal effect is considered
Category II and III transformer ==> thermal effect is considered; mechanical effect depending on the frequency of external fault
Category IV transformer ==> thermal and mechanical effect is considered
Category Single phase (Minimum nameplate kVA)
Three phase (Minimum nameplate kVA)
I 5-500 15-500 II 501-1667 501-5000 III 1668-10,000 5001-30,000 IV Above 10,000 Above 30,000
© ABB Group October 27, 2015 | Slide 63
Transformer capability curve
Category I
Category II and III with low fault frequency
Overcurrent protection
Fuse
Overcurrent relays
© ABB Group October 27, 2015 | Slide 64
Transformer capability curve
Category II and III with high fault frequency
Category IV
High speed differential protection
Backup overcurrent protection
© ABB Group October 27, 2015 | Slide 65
Overcurrent protection coordination
R
TransformerProtection
FeederProtection
Fuse
Recloser
XX
X
Infrequent Fault Zone Frequent Fault Zone
Fault cleared by feeder protection
Fault cleared by transformer differential or primary side device
Fault cleared by transformer primary or optional secondary main breaker device
© ABB Group October 27, 2015 | Slide 66
Overcurrent protection coordination
© ABB Group October 27, 2015 | Slide 67
Overcurrent protection coordination
Time-overcurrent protection
Inverse time characteristic relay provides the best coordination
Pickup settings of 200 to 300% of the transformer’s self-cooled ratings
Fast operation is not possible (coordination with other relays)
Instantaneous protection
Fast operation on heavy internal faults
Settings 125% of the maximum through fault (low side 3Φ fault)
Settings should be above the inrush current
© ABB Group October 27, 2015 | Slide 68
This webinar brought to you by the Relion® product family Advanced protection and control IEDs from ABB
Relion. Thinking beyond the box.Designed to seamlessly consolidate functions, Relion relays are smarter, more flexible and more adaptable. Easy to integrate and with an extensive function library, the Relion family of protection and control delivers advanced functionality and improved performance.
© ABB Group October 27, 2015 | Slide 70
Thank you for your participation
Shortly, you will receive a link to an archive of this presentation.To view a schedule of remaining webinars in this series, or for more
information on ABB’s protection and control solutions, visit:
www.abb.com/relion