São Carlos, 19 de Setembro de 2013
AC grid protection for grids
with
VSC-HVDC converters
Helder Leite
INESC TEC/University of Porto
Outline
 Transmission Line Protection Principle
 Short Lines
 Medium Lines
 Long Lines
 Distance Relays: Step coordination
 Pilot-Aided Distance-Based Schemes
 Overreaching Distance Functions
 The influence of a VSC-based HVDC link on the AC SCR
Transmission Line Protection Principle
 VSC-HVDC converters are going to connect to AC Grid
 Transmission lines are going to take this bulk of energy and provide the path to
transfer power to the load;
 Transmission lines are : A Vital Part of the Power System
• Operate at voltage levels from 69kV to 765kV;
•
Deregulated markets, economic, environmental requirements have pushed
utilities to operate transmission lines close to their limits.
Transmission Line Protection Principle
Classification of line length depends on:
 Source-to-line Impedance Ratio (SIR), and
 Nominal voltage
Length considerations:
 Short Lines: SIR > 4
 Medium Lines: 0.5 < SIR < 4
 Long Lines: SIR < 0.5
ZS
SIR 
ZL
 Zs- Source Impedance
 ZL - Line Impedance
(1)
Transmission Line Protection Principle
• Typical Protection Schemes (Short Lines)
 Current differential
 Phase comparison
 Permissive Overreach Transfer Trip (POTT)
 Directional Comparison Blocking (DCB)
Transmission Line Protection Principle
• Typical Protection Schemes (Medium Lines)
 Phase comparison
 Directional Comparison Blocking (DCB)
 Permissive Underreach Transfer Trip (PUTT)
 Permissive Overreach Transfer Trip (POTT)
 Unblocking
 Step Distance
 Step or coordinated overcurrent
 Inverse time overcurrent
 Current Differential
Transmission Line Protection Principle
• Typical Protection Schemes (Long Lines)
 Phase comparison
 Directional Comparison Blocking (DCB)
 Permissive Underreach Transfer Trip (PUTT)
 Permissive Overreach Transfer Trip (POTT)
 Unblocking
 Step Distance
 Step or coordinated overcurrent
 Current Differential
Transmission Line Protection Principle
Z3 (remote backup)
Z2 (time delayed)
Z1
Transmission Line Protection Principle
• Distance relay Coordination
Over Lap
Local Relay – Z2
Remote Relay – Z4
Transmission Line Protection Principle
• The Impedance Relay Characteristic
R 2  X 2  Z r21
X
Plain Impedance Relay
Operation Zone
Z  Z r1
Zr1
Radius Zr1
R
Transmission Line Protection Principle
• Step Distance relays
• Zone 1:
Trips with no intentional time delay
Underreaches to avoid unnecessary operation for faults beyond
remote terminal
Typical reach setting range 80-90% of ZL
•
Zone 2:
Set to protect remainder of line
Overreaches into adjacent line/equipment
Minimum reach setting 120% of ZL
Typically time delayed by 15-30 cycles
•
Zone 3:
Remote backup for relay/station failures at remote terminal
Reaches beyond Z2, load encroachment a consideration
Transmission Line Protection Principle
• Need directionality
F2
1
F1
2
3
4
RELAY 3
Operation Zone
5
6
X
F1
F2
Nonselective
Relay Operation
R
Transmission Line Protection Principle
• Directionality improvement F1
F2
1
2
3
RELAY 3
Operation Zone
4
6
X
F1
F2
The Relay Will
Not Operate for
This Fault
5
Directional Impedance
Relay Characteristic
R
Transmission Line Protection Principle
Mho Element Characteristic (Directional Impedance Relay)
Operates when:
V  I Z M cos    MT 
Z  Z M cos    MT 
X
ZM
Z
 MT

R
Transmission Line Protection Principle
Distance Relay Characteristics)
X
PLAIN
IMPEDANCE
X
OFFSET
MHO (2)
R
R
X
X
OHM
QUADRILATERAL
R
R
Transmission Line Protection Principle
Inverse-Time Relay Coordination
I
Distance
t

T
 T
 T
Distance
Transmission Line Protection Principle
Inverse-Time Relay Coordination
t
Relay
Operation
Time
I
Radial Line
Fault
Load
Transmission Line Protection Principle
Directional Comparison Pilot Protection Systems
L
IR
IL
T
Relays
R
Communications
Channel
Exchange of logic information
on relay status
R
Relays
T
R
Transmission Line Protection Principle
Pilot-Aided Distance-Based Schemes
 DUTT – Direct Under-reaching Transfer Trip
 PUTT – Permissive Under-reaching Transfer Trip
 POTT – Permissive Over-reaching Transfer Trip
 Hybrid POTT – Hybrid Permissive Over-reaching Transfer Trip
 DCB – Directional Comparison Blocking Scheme
 DCUB – Directional Comparison Unblocking Scheme
Transmission Line Protection Principle
Pilot-Aided Distance-Based Schemes –DUTT Scheme
Zone 1
Bus
Bus
Line
Zone 1
Transmission Line Protection Principle
Pilot-Aided Distance-Based Schemes –DUTT Scheme
Zone 2
Zone 1
To protect end of
line
Bus
Bus
Line
Zone 1
Zone 2
Transmission Line Protection Principle
Pilot-Aided Distance-Based Schemes –POTT Scheme
Zone 2
Zone 1
Bus
Bus
Line
Zone 1
Zone 2
Key XMTR
Zone 2 Elements
RCVR
AND
Trip
Transmission Line Protection Principle
Directional Comparison Blocking Scheme
Bus A
1
RVS
2
Bus B
3
4
5
6
FWD
FWD
RVS
Overreaching Distance Functions
Impedance Seen by Distance Relays
Power swings can be seen as multi-phase faults by distance relays;
Power swing protection monitor the rate of change of the positive-sequence
impedance;
During a major disturbance, a distance relay may see a power swing as a multiphase fault if the positive sequence impedance trajectory enters the operating
characteristic relay;
As an example:
- Simple two-source system
A
ES
ZS
Relay
Irelay
B
ZL
ZR
ER0
Overreaching Distance Functions
Impedance Seen by Distance Relays
- Simple two-source system
A
ES
Relay
ZS
Irelay
- The current in Bus A
I relay
ZT  Z S  Z L  Z R
B
ZL
ZR
E S   E R

ZS  ZL  ZR
ER0
(2)
- The impedance measure by the relay at bus A
Z seen ( relay ) 
Vrelay
I relay

E S   I relay Z S
I relay
 E S 
  Z S  
 E S   E R

 Z T (3)

Overreaching Distance Functions
Impedance Seen by Distance Relays
- Let us define
- K=1
ES
k
ER

1
Z seen ( relay )   Z S  Z T 
 1  cos   j sin 
Z
Z

 ZS  T 
j T cot
2
2 
2


a cons tan t offset
- K≠1
Z seen

 

perpendicu lar line segment
k ( k  cos  )  j sin  
  ZS 
Z T (5)
2
2
( k  cos  )  sin 
(4)
Overreaching Distance Functions
Impedance Seen by Distance Relays
- K=1
K≠1
Overreaching Distance Functions
Impedance Seen by Distance Relays
- The distance relay located at bus 1 will misoperate on load if the magnitude and
phase angle result in a measure impedance that falls within the relays
characteristic;
2
Z a  R  jX 
-
Za
V
V
( P  jQ )
P Q
2
2
is the impedance measure by a distance relay
is the measure voltage
- P and Q is the active and reactive power
(6)
Overreaching Distance Functions
Impedance Seen by Distance Relays
- The distance relay located at bus 1 will misoperate on load if the magnitude and
phase angle result in a measure impedance that falls within the relays
characteristic;
2
Z a  R  jX 
-
Za
V
V
( P  jQ )
P Q
2
2
is the impedance measure by a distance relay
is the measure voltage
- P and Q is the active and reactive power
(6)
Overreaching Distance Functions
Impedance Seen by Distance Relays
Overreaching Distance Functions
Impedance Seen by Distance Relays
- The real and imaginary parts of the measured impedance
2
R
V
2
P
P Q
2
X 
2
V
Q
P Q
2
(7)
2
4
- R and X can be written as
R X 
2
- Substituting (7)and (8) results in:
2
R X 
2
2
RV
P
2
R X 
2
2
XV
Q
2
V
P Q
2
2
(8)
2
4


V
V
2


 R
X 

2 P 
4P2


2 2
4


V
V
2


R  X



2Q 
4Q 2


2
(9)
(10)
Overreaching Distance Functions
Impedance Seen by Distance Relays
- The real and imaginary parts of the measured impedance
Overreaching Distance Functions
Effect of Fault Resistance in Loop System
Vm
Zm 
 sZ mn  R f
I mf
 If

 I mf
Vn
Zn 
 (1  s ) Z mn  R f
I nf



 If

 I nf
(11)



(12)
- Vm and Vn are the voltages at the buses; Imf and Inf are the currents flowing in the line;
If is the fault current flowing into the fault F
Overreaching Distance Functions
Effect of Fault Resistance in Loop System
- The currents Imf and Inf are usually shifted,
- Current in the fault resistance is phase displaced from currents at the line
terminals;
- Because of these phase displacements, errors can be obtained on fault location
estimation;
- The fault resistance appears to the fault locator as an impedance that both
resistive and reactive components;
Overreaching Distance Functions
Effect of Fault Resistance in Loop System
Impact of VSC-HVDC Links on AC Power System
- The VSC-HVDC can practically instantaneous change the active and reactive
power within the capability curve of the converter
- Can be used to support the grid with the best mixture of active and reactive power
during stressed conditions (even if power reversal is necessary).
- Why to know the Impact of VSC-HVDC Links on AC Power System?
- Voltage and power stability on the AC Network
- Rating of circuit breaker
- Power system Protection
Impact of VSC-HVDC Links on AC Power System
- The basics control patterns for VSC-HVDC
a) Frequency control;
b) AC voltage control;
c) Active power
d) Reactive power control
e) DC voltage control
f) AC current control
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
 Contribution of the VSC-HVDC Links to short circuit currents may have some
significant impact on the ratings for the circuit breakers in the existing AC
 Active power control (Control of active power and DC voltage reference)
 Reactive power control (control of reative power and AC voltage control)
 VSC can be seen as voltage souce control under normal condition and a current
source under abnormal condiction
 The higher the SCR of the AC network the less contribution in current form the
VSC- HVDC
 Different control modes and different operation points may change the short circuit
current contribution from the VSC.
 Limited contribution
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
- E.g. AC Current mode
- When the AC voltage lowers the current limit is reached, the active power
reduces
- the current limitation is implemented by limiting current components (dand q- axis).
- The converter closer to the AC side fault is operating under DC voltage
control and reactive power
- In the sending end of the VSC-HVDC, active and reactive orders are
adjusted proportionally, according with the pre-specified power factor, in
order to avoid DC overvoltages, assuming also the sources behind the
converter can reduce their production.
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
- Working Example
-
Fault @ bus 3 | VSC –power control
Zf (p.u.)
Fault @ bus 3
j0.005
7.41%
j0.01
7.68%
0.005
11.32%
0.01
15.45%
0.05
15.49%
Icc(p.u.A)
10.24<74.2º
9.79<74.7º
11.00<81.5º
10.91<78.4º
9.49<57.2º
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
- Working Example 2
 Active power control (Control of active power and DC voltage reference)
 FRT PROVISION BY HVDC GRIDS
HVDC-VSC technology is able to remain connected to the AC grid during low
voltage sags.
For the converters connected to onshore AC grids, a voltage sag in the AC side
will lead to a current increase, since the converters will try to remain
transmitting the same amount of power.
Nevertheless, power electronic converters have a maximum current limit
associated with the characteristics of the power electric switches.
Such characteristic results on a significant reduction of power being injected to
the faulted AC mainland grid, which leads to a DC power unbalance that
results on DC overvoltages
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
- Working Example 2
FRT
Ability of remaining connected to
the AC network during moments
were voltage profile is extremely
low (normally due to fault
occurrence)
Inject reactive current in order to
support AC voltage
Impact of VSC-HVDC Links on AC Power System’s Protections
VSC-HVDC CONTRIBUTION TO SHORT CIRCUIT CURRENTS
•
•
Fault occurrence at onshore AC grid #1
Severe 500 ms 3 - phase fault:
• Assessment of the impact in DC voltage.
• The non-injected power in the AC grid #1 is partially re-direct to AC grid #2 –
influenced by P/V droop and DC grid dynamics
• Overvoltage occurrence in the DC grid