SIZING GROUNDING TRANSFORMERS AND RESISTORS

TABLE OF CONTENTS
1.0 PURPOSE & SCOPE..........................................................................................................................3
2.0 DEFINITIONS....................................................................................................................................4
3.0 APPLICABILITY...............................................................................................................................4
4.0 RESPONSIBILITIES..........................................................................................................................4
5.0 PROCEDURE / REQUIREMENTS...................................................................................................5
6.0 REFERENCES..................................................................................................................................14
7.0 ATTACHMENTS..............................................................................................................................14
1.0 PURPOSE & SCOPE
This procedure outlines the method to be followed for preparation of the Generator Neutral
Grounding and Transformer Neutral Grounding and Resistor Sizing and is intended to assure
uniformity of all such systems.
This procedure depicts grounding schemes as well as sizing of grounding equipment for the
following electrical systems and equipment:
 Main Generator,
 Main Transformer,
 Unit Auxiliary Transformer, Startup Transformer,
 Low-Voltage Load Center Transformer,
 Diesel Generator (if provided),
 AC and DC UPS systems
2.0 DEFINITIONS
Generator Neutral GroundingThe purposes of generator neutral grounding through an
impedance are to limit the damage at the point of fault, limit transient over-voltages and provide
a sensitive means of detection for insulation deterioration. There are two acceptable methods of
grounding unit connected generators; resistor grounding and resonant (reactor)
grounding.Resistance GroundingA resistance grounded system uses a resistor in the neutral-to-
ground circuit to limit the maximum value of short circuit current during a line to ground fault
limiting the current and reducing the extent of damage to equipment caused by the heating and
mechanical stresses generated by the short circuit currentsLow Resistance GroundingIn a low
resistance grounded system the value of resistance is usually selected so that the ground fault
current is large enough to trip protective devices selectively, yet low enough to limit damage to
equipmentHigh Resistance GroundingIn a high resistance grounded system the value of
resistance is selected so that it is of sufficient value to limit fault currents to very low values,
usually equal to the total charging current of the systemResonant (Reactor) GroundingThis
grounding scheme is similar to resistance grounding except the resistor is replaced with a reactor
equivalent to the system charging current to form a tuned (resonant) circuit.
3.0 APPLICABILITY
This procedure is applicable to all projects.

4.0 RESPONSIBILITIES
4.1 Lead Electrical Engineer
The Lead Electrical Engineer is responsible for sizing the grounding transformers and resistors.
5.0 PROCEDURE / REQUIREMENTS
5.1 Equipment Grounding Requirements
5.1.1 Main Generator
Main generator shall always be high resistance grounded.
5.1.2 Main Transformer
Main transformer for the generating plant shall always be solidly grounded at the high side.
Low side connected to the generator is always delta connected. No special equipment is
required for high side grounding.
5.1.3 Unit Auxiliary Transformer
Unit auxiliary transformer connected to the generator leads shall always have the high side delta
connected. Low side of this transformer is generally wye connected and shall be low resistance
grounded allowing a maximum of 1000A ground fault current. Grounding resistor directly
connected to the neutral shall have a minimum of 10-sec rating. Low resistance grounding shall
always be used unless specifically required by the client.
Cases, where low side of this transformer is also delta connected, due to phasing, grounding
bank of T-type or zig-zag type with a resistor allowing a 1000A maximum ground fault shall be
used. Rating of the grounding equipment shall of a minimum of 10 sec.
When the contract requires a different method, a calculation similar to Attachment 7.2 shall be
performed.
5.1.4 Startup Transformer
Startup transformer, when used, may have either delta or wye connection for high or low side
winding. For high side, wye connection, neutral shall be solidly grounded. For low side wye
connection, neutral shall be low resistance grounded. For low side delta, grounding bank of T-
type or zig-zag transformers with a resistor allowing a 1000A maximum ground fault current.
Rating of this equipment shall of a minimum of 10 sec.
5.1.5 Load Center Transformer
Transformer feeding low voltage load center shall be delta/wye-solidly grounded, neutral, unless
specifically required by the client to be different.
5.1.6 Diesel Generators
Diesel generators, when provided shall be low resistance grounded at the medium voltage and
solidly grounded at low voltage distribution. Low resistance grounding shall allow a maximum
of 1000A ground fault current. Grounding resistor directly connected to the neutral shall have a
minimum of 10 sec rating.

5.2 Generator Neutral Grounding Calculation Method

Attachment 7.1 provides a recent project calculation for sizing the generator and transformer
grounding transformer.
The following consideration must be analyzed when recommending and selecting a method of
Generator Grounding.
1) The damage at the point of fault is greatly reduced when the unit is resonant grounded as
compared to resistor grounded.
2) Surges on the high voltage system will not appear between generator neutral and ground
because of the wye-delta transformation of the main transformer.
3) Transient overvoltages on the unfaulted phases are limited by resonant grounding. Arcing
or current surges are reduced by resonant grounding. This precludes the possibility of
transient overvoltages due to arcing grounds.
4) The appearance of zero sequence voltage on the transmission system for a line to ground
fault impresses a transient voltage on the generator system. This voltage is a function of the
capacitive coupling reactance in the unit step-up transformer and the zero sequence
impedance of the generator system. The transient voltage on the generator system (phase to
ground EO) will be higher in magnitude with resonant grounding than with resistor
grounding. This voltage must be controlled within acceptable limits by the proper selection
of the coil constant of the reactor.
5) Resonant grounding provides increased sensitivity for detection of insulation deterioration
at any location in the generator system and neutral connected apparatus.
6) When resistance grounding is used, there is a dead zone where faults cannot be detected.
This dead zone is a circle at the system neutral concentric with all zero sequence fault
voltages. By using the resonant grounding, this dead zone can be offset by introducing a
voltage to displace the system neutral.
This procedure summarizes the required data and the calculations that must be performed to
specify the distribution transformer, reactor or resistor ratings.
5.3 Required Data
5.3.1 GENERATOR: MVA, three phase

Eg = Phase to phase voltage = V

Cg = Single phase to ground capacitance = mmf
E30g = Zero sequence normal third harmonic voltage = V
Eog = Zero sequence normal fundamental frequency voltage = V

5.3.2 MAIN TRANSFORMERS: MVA, three phase

Nm = Number of transformers =
Cm = Single phase to ground
capacitance (gen. side) = mmf
Ct = High to low side winding per phase capacitance = mmf
EHO = High side zero sequence voltage for a fault on the high side
of the transformer = V

5.3.3 UNIT STATION SERVICE TRANSFORMER: MVA, three phase

Nu = Number of unit station service transformers =

Cu = Single phase to ground capacitance (gen. side) = mmf

5.3.4 ISOLATED PHASE BUS:

Cb = Total single phase to ground capacitance = mmf

5.3.5 SURGE CAPACITORS:

Cs = Single phase to ground capacitance (gen. side) = mmf

Em = Maximum acceptable voltage the system equipment can take without damage = V
5.4 Calculations For Resonant Grounding

Eg
Ege = Generator phase to neutral voltage = 3 = V

C = Total system capacitance = 3. (Cg+NmCm+NuCu+Cb+Cs) = mmf

1 1012
Xc = Total system capacitive reactance =   C or 2  f  C = ohms

1
Ege 
Ic = System capacitive fault current = Xc = amps

XL = Effective inductive reactance for resonant ground = Xc = ohms

ELP = Distribution transformer primary voltage  Ege = V available

ELS = Distribution transformer secondary voltage = 120/240 V

Continuous distribution transformer rating = Ic  Ege  10

-3
TR = = KVA

If no delay is intended for the unit trip, this rating can be reduced by the factors
from the following table:

TIME FACTOR

1 Minute 0.212
5 Minutes 0.357
30 Minutes 0.55
1 Hour 0.625
2 Hours 0.714

Xct = Capacitive coupling reactance between high and low side of

1012
the main transformer =
2  f  C t = ohms

LS = Assumed total losses for the reactor = 5%

R = Effective zero sequence resistance in the generator

100
3 XL 
neutral for this loss = LS = ohms
K.Eog = Neutral voltage displacement due to generator's
100
 Eog
fundamental zero sequence voltage = LS = V

ER = Neutral displacement due to fault on the high side

EHO  R
of the main transformer =
R  j  X ct = V

E = Phase to ground voltage for a ground fault on the high

of the main transformer = Ege + KEog + ER = V

If E calculated above is more than the maximum acceptable voltage (E M)

increase the reactor losses (LS) to a higher value and recalculate R, KEog and ER

XLS = Reactance in the secondary of the distribution transformer

 E LS  2
XL 
 E LP  2 = ohms

RLS = Resistance in the secondary of the distribution transformer

LS
X LS 
100 = ohms

KL = Coil constant = 100

LS

ELP
Ic 
Is = Secondary current =
ELS = amps

E LS
E ge 
Vs = Secondary voltage =
E LP = V

XLSKVA = Reactor’s KVA = Is . Vs .10-3 =

KVA

rF = Fault resistance = ohms

E ge
IFO = Zero sequence fault current =
R  3  rF = amps

IF = Fault current = 3 . IFO = amps

Eao = Percentage neutral voltage displacement (Relay Sensitivity)

I FO  R  100
E ge
= = amps
 wF = Energy into the fault = IF2 . rF = watts

Changing the value of fault resistance from zero upward in reasonable steps,
calculate and plot wF and Eao against resistance.

5.4.1 180 Hz, No Fault Or Normal Conditions

Using the above distribution transformer with percentage reactance (X) equal to impedance (Z) =%

3  Z  10 2   ELP 
2

Xe = Primary reactance = TR  103 = ohms

XL3 = Third harmonic reactance in the generator neutral 3 XL = ohms

(Including distribution transformer reactance)

E30 g
I30p = Third harmonic zero sequence current =
j  3  X L 3  3  RL  j  X c = amps

ELP
I 30 p 
I30s = Reactor current =
ELS = amps

V30s = Third harmonic normal voltage across the reactor

= I30s . 3 (RL + jXL3 = jXe) = V

5.5 Calculations For Resistor Grounding

Eg
Ege = Generator phase to neutral voltage = 3 = V

C = Total system capacitance = 3(Cg + NmCm + NuCu + Cb + Cs) = mmf

1 1012

Xc = Total system capacitive resistance =
wc 2    f c = ohms

1
E ge 
Ic = System capacitive fault current =
Xc = amps

Ireq = Assuming neutral current (Minimum 5 amps) = Ic = amps

E ge
I req
Req = Equivalent resistance in the generator neutral = = ohms

Ep = Distribution transformer primary voltage  Ege =

Vavailable

TR = Continuous distribution transformer rating

= Ireq . Ege . 10-3 = KVA

If no delay is intended for the unit trip, this rating can be reduced by the factors
given under resonant grounding.

Es = Distribution transformer secondary voltage = 120/240 V

Req 
 Es  2
Rs = Grounding resistor in the secondary =
E  p
2
= ohms

Es
 E ge
Ep
Vs = Maximum secondary voltage = = V

Ep
I req 
Is = Secondary current =
Es = amps

RsKVA= Grounding resistors KVA = Is . Vs . 10-3 = KV

rF Fault resistance = ohms

 E ge
R  j  X x 
3  rF  3  eq
IFO = Zero sequence fault current =
 Req  j  X c  = amps

IF = Fault current = 3 . IFO = amps

WF = Energy into the fault = IF2 . rF = watts

I FO  3
 R  j  X 
eq c

 R  j  X  100
eq c

Ege
Eao = Percentage neutral voltage displacement = = %

Changing the value of fault resistance from zero upward in reasonable steps,
calculate and plot WF and Eao against fault resistance. = %

5.5.1 180 Hz, No Fault, or Normal Condition:

Using the above distribution transformer with percentage reactance (X) equal to impedance (Z)

3  Z  10 2   E p 
2

Xe = Primary reactance = TR  103 = ohms

E30 g
3   Req  j  X e   j  X c
I30p = Third harmonic zero sequence current = = amps

Ep
I 30 p 
I30s = Ground resistor currents =
Es = amps

I 30 s  3  Rs
V30s = Ground resistor normal voltage = = V
5.6 Required Calculation Inputs
The following is a summary of the required inputs to the calculation to satisfy this procedure:
1) Contract grounding requirements of medium-voltage and low-voltage auxiliary system.
2) Unit auxiliary, startup and load center transformer connections.
3) Capacitance to ground for:
 Generator
 Surge arrester (if applied between generator and transformer)
 Generator step-up and unit auxiliary transformers
 Isolated Phase Bus Duct

5.7 Required Calculation Outputs

The following is a summary of the required outputs to the calculation provided by this
procedure:
1) Generator Neutral Grounding Calculation and Equipment Rating.
2) Medium-Voltage Neutral Grounding Equipment Rating.
6.0 REFERENCES
6.1 EPRI Power Plant Electrical Reference Series, Volume 8, “Station Protection.”

7.0 ATTACHMENTS
7.1 Sample Project Calculation- Generator and Transformer Grounding Equipment Ratings (pdf)
7.2 Sample Project Calculation- Neutral Grounding Transformer and Resistor Sizing Calculation-
4 kV Auxiliary distribution System (zip)