USING SUPERIMPOSED PRINCIPLES (DELTA) IN PROTECTION TECHNIQUES IN

AN INCREASINGLY CHALLENGING POWER

NETWORK

Patricia Horton, Simon Swain– GE Grid Solutions

2017 Texas A&M Protective Relay Conference

Introduction• Superimposed techniques a lso called Delta or Incremental techniques

have been used for more than 30 years in protective relays

• Directional Comparison relays were popular in the 80s and 90s. They were called “Directional Wave relays” Some of those relays use to quote fault detection time as little as 2-5 ms

Superimposed ( DELTA ) technique

Subtracting a value of the signal, before a change from its corresponding value after the change will produce a signal that represents the change.

A Change signal produced in this way is a superimposed component.

Superimposed voltage and current

When a fault occurs on a power system, changes occur in the current and voltage signals which produce superimposed components

Sequence networks for a ph-gnd fault

Z SA1 ZL1 ZSB1

∆V1

∆I1∆I1

ZSA2 ZL2 ZSB2

∆V2

∆I2∆I2

ZSA0 ZL0ZSB0

∆V0

∆I0∆I0

IF / 3

E

Source Voltages appear short-circuited as they do not change during the fault

Voltage generator represents voltage change at fault location and is the Vph-n at fault point prior to the fault occurrence

DELTA for External faults (1)

The fault inception is represented by the closing of the switch S which connects the Superimposed voltage source ∆E, whose magnitude is the change in voltage at the fault point.

This causes a Superimposed current ∆I to flow in the Superimposed circuit

Equivalent circuit in which all signals are Superimposed Components

DELTA for External faults (2)Positive directions as defined per relay location ∆Ir

Zsa ZlEnd BEnd A

∆Ir

Zsb

∆Vr ∆Vr

∆I

Zsa * ∆Ir + ∆Vr = 0 ( Zsa + Zl )* - ∆ Ir + ∆Vr = 0

ZsaVIEndA r

r∆−

=∆:ZlZsa

VIEndB rr +

∆=∆:

+

- -

+

E

S∆I

DELTA for Directional Determination (1)

∆V

- ∆V

∆I

∠ Zs60°

RCA

For a Forward ∆I lags –∆V according to the characteristic angle of the source impedance behind the relay angle.

ZsaVIEndA r

r∆−

=∆:

DELTA for Directional Determination (2)This is ABN solid fault behind the relay location in a parallel line with heavy load flowing out from the Relay location.

Fault and prefault vectors shows how a -30° conventional memory directional line sees the fault in the forward direction

Delta directional sees the fault in the reverse direction

Using Deltas for phase selection (1)A way to use Delta principles to do phase selection is comparing the magnitudes of the three phase-to-phase superimposed currents against a threshold

This is a an example for phase A to ground fault

A single phase-to-ground fault produces the same superimposed current on two of these signals and

zero on the third

Using Deltas for phase selection (2)

This is a an example for phase AB fault

A phase-to-phase or double phase-to-ground fault produces one signal which is larger than the other two

Threshold is dynamic. For instance they are automatically increased if all current phases are less than rated current to a % of the highest phase current to prevent sporadic operation during high levels of sub-synchronous frequencies

DELTA for power swing detection (1)

PH1

PH2

Fault

PH1 : Phase 1, starts PH2 and primes PSB. Compares current value with previous 2 cycles

PH2 : Phase 2, enables distance elements. Compares current value with pre-fault value.

DELTA for power swing detection (2)Powerswing

Fault

PH1

PH2

PSB act ive& minimumthresholdincreased

PSB removedIncluding 3 ph faults

3cycles

Delta Directional Comparison protection

This has been a protect ion which has become less popular over the years as Distance protect ion have similar operat ing t imes

Delta Direct ional protect ion

Delta base Distance protection

DELTA for Z source determination

Zsa ZlEnd BEnd A

∆Ir

Zsb

∆Vr∆I

Zsa * ∆Ir + ∆Vr = 0

-

+

E

S

r

r

IVZsa

∆∆−

= With the calculation of the source impedance behind the IED, estimation of the short

circuit level is possible given a suitable event in the grid.

DELTA for SSC HVDC AC/DC Con. StationIf a HVDC converter is connected to a weak AC grid relative to the DC power, problems with power frequency stability and voltage can occur

The weaker the AC grid is, the greater the AC/DC interact ions

Short Circuit Ratio (SCR) = ratio of the AC SC to the size of DC converter power.

If SCL of the grid is 1000MVA and the size of the converter is 200MW :SCR is 1000/200 = 5.

SCR levels are categorized : high, Low and very low

A large European utility, SC Capacity Concept

• Calculate RoCof V&I due to “downstream” event (not changing source V & Z)

• Filter switching events are downstream. SCADA switching event sent to PhasorPoint to initiate SCC calculation.

ZIV

=∆∆

−

Benefit s• Ensure SCC large enough at connection

to HVDC• Relax generation dispatch constraints• Address uncertainties in model based

SCC with high renewable generation • Ensure SCC below C/B ratings• Filter switching can be initiated manually

when SCC value needed• Historical SCC for planning

Delta SCC prediction in active DN• DN with DG and loads with bi-directional power flow and plug-and-play functionality

can increase fault levels which possibly exceed the opening capacity of the related CBs

• On the other hand, when the distribution feeders loose the connexion with the grid, the fault level could be extra lower than the normal fault level.

With the changing rate of the load or an event, is possible to predict Current Fault Level and location and take respective action

• Superimposed components (delta) based principle has been used in commercially available relays for more than three decades successfully protecting the transmission lines .

• Delta directional technique have been complimented by delta based phase selection and delta current based power swing technique, to provide fast and secure operations for most faults in the power system even when they are complex.

• Most recently this principle is being used to assist smart grid in functions such as fault level prediction

Conclusions

Thank You

Questions?