QP Civil
QP Civil
QP Civil
of
Distributed Control Systems and Power Electronic Systems
Dr. V. R. Kanetkar
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Index
Chapter 1 Introduction
References
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 1
Introduction
Bad quality of power supply is usually associated with parameters such as large
voltage / frequency variation, transients, sustained sags or swells, large voltage
distortions, brown outs or blackouts. Similarly, bad quality of grounding /
earthing is associated with large ohmic resistance of earth, improper
connections from earth to the system / its cubicles, improper ratings of earth bus
bars and cables, and finally the improper philosophy adopted for grounding and
earthing.
While, the Power Electronic Systems (PES) handle low to high powers based on
different applications, the Distributed Control Systems (DCS) handle relatively
low powers or rather require low power for their functioning. However, the
proper power quality, grounding and earthing become the necessity for their
smooth functioning. These factors define their required availability, reducing
the downtime substantially.
Apart form the Power Quality; the other important factor to be noted in this
connection is the Electro Magnetic Interference (EMI) or Noise. The EMI or
noise is more predominant in case of Power Electronic Systems as compared to
the Distributed Control Systems. The Power Electronic Systems generate EMI
or noise and can affect other electronic equipment in vicinity. On the other side,
the Distributed Control Systems are more vulnerable to EMI or noise effects
and can easily mal-operate due to its presence.
It is hence imperative to discuss the power quality, EMI, actual grounding and
earthing, and philosophy of grounding and earthing in that order. This document
hence is arranged in the same order and should form a proper guide for
grounding and earthing of “Distributed Control Systems (DCS)” and “Power
Electronic Systems (PES)”.
References [1] – [7] used for preparing this document are given at the end of
this document.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 2
There is always a ground plane, which passes through the neutral point, dc bus
mid point, the motor neutral point, and neutral point of both ac side filters (if
they are used). It balances both ac sides correctly like a two pan weighing
balance. If neutral is not earthed, the balance and the symmetry are lost.
The power supply can have voltage transients (dips or sudden voltage rises /
spikes), voltage variation (sag or swell), frequency variation, voltage distortion,
and interruptions (brownouts or blackouts). It is hence necessary to protect the
DCS and PES from such poor-quality parameters.
As far as PES are concerned, normally the design will take care of most of these
parameters and also islanding in safe operating region, especially during
brownouts or blackouts. However, it is essential to have adequate protection
against transients or especially surges passed on from the incoming supply to
the equipment. An appropriate surge suppressor network (Shunt R-C
combination across the supply in case low kVA ratings and a diode rectifier dc
capacitor based surge suppressor for higher ratings) hence needs to be employed
in the PES. In case of DCS, an appropriately designed shunt R-C network across
the incoming single-phase power supply is adequate to protect the system.
It is always a good practice to have the power supply to the “Distributed Control
System (DCS)” or for the “Power Electronic Systems (PES)” obtained from a
separate isolation transformer. The isolation transformer helps in reducing the
impact of transients. The transformer should be installed as near as possible to
the equipment.
The PES does operate with higher incoming voltage distortion (as, many times,
these are designed for handling higher distortions such as 6 to 8%). However,
such is not the case with DCS. For DCS, the recommended voltage distortion on
load should not be more than 2.5%. To achieve this the DCS should make use of
UPS of adequate capacity and installed as near as possible to the DCS.
The primary source must be free from non-repeating power interruptions greater
than 20 milliseconds. Otherwise, it can cause loss of data, control, or erase the
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
control software and cause mal-operation of the system. Normally, an UPS will
not cause such interruptions.
Line conditioners or EMI filters, as described later, can be used while supplying
power to the DCS or Control Electronics of the PES.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 3
EMI mitigation
Sudden change in cable currents (as observed when large rating motors
start, especially on direct on-line)
Switching frequencies other than the normal power frequency and
associated sidebands
Transformer inrush currents
On / off control of current carrying power contactors, breakers, and
solenoids
Sudden loads being thrown off from the supplies
The sudden changing fields or even strong fields associated with the large
power currents cause electromagnetic fields, which can cause circulating
currents in other relatively small current carrying conductors in the vicinity and
alter the electrical information. The EMI effect can hence cause mal-operation
of near by small kVA equipment.
There are two important aspects of the EMI. First aspect is related to reliable
functioning of electronic equipment in a given EMI environment developed by
by equipment in vicinity. This is called as susceptibility. The other aspect is the
acceptability level of generated EMI. This is called as acceptable emission level.
These are governed by the IEC standards in series 61000.
Without getting into much detail, it is at least necessary to understand that PES
should function with acceptable emission levels (as the power levels are high)
and DCS should function with acceptable susceptibility levels.
In case of PES, normally the controls and cubicles are designed in such way that
emission is kept at minimum. The cubicles function with internally taken care of
EMI to see that the PES function properly. In case of DCS, major care needs to
be taken in respect of the supply connections. The power supply cables should
be routed away from motor or transformer or for that matter high current
carrying power cables. The same is also valid or applicable for the field signals
received or transmitted by the DCS. The UPS supplying power should be kept
as near as possible to the DCS. Shielding of the signal wires with shield
connected to the grounding bus bar (and hence to earth) at one end, gives
reliable operation. The supply cable distance to DCS should be kept as
minimum as possible. The grounding inside the cubicles and subsequent proper
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
grounding and earthing of the cubicles helps in achieving proper performance
against the EMI. It is also better to use field signals as current signals (0 to 20
mA or 4 to 20 mA) instead of low-level voltage signals (such as 5 V signals).
This improves the immunity against the EMI. Particularly in DCS, which is low
kVA sensitive equipment, it is better to use isolation transformers with three
screen windings for internal power distribution (as discussed subsequently) and
also for the incoming power supply. Such an isolation transformer is shown in
fig. 1.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 4
The basic difference between grounding and earthing is that when the system is
grounded, it is still not connected to earth. The system has a ground bus bar
inside or outside located at an appropriate place to which all internal grounding
connections are returned. Once the final ground bus bar is connected to an
actual earth pit or earth grid that the system gets finally earthed.
As an example, in a typical PES there could be many cubicles. Each cubicle can
have an internal grounding bus bar to which internal components such as power
module, shields of various input / outputs, fans, controller power supply
transformer screens, cores of electrical components like power inductors etc. are
connected. These bus bars then are returned to a final ground bus bar from
where the connection is then taken to earth pit or earth grid.
Similarly, in case of DCS, each cubicle can have a ground bus bar to which the
controller chassis, shields, transformer screens etc. can be connected. These bus
bars then are returned to a final ground bus bar from where the connection is
then taken to earth pit or earth grid.
The earth pit must have a very small earth resistance (much less than an one
Ohm). Usually, the earth resistance can be measured by using a three-probe
method as shown in fig. 2. The voltage here is applied between probes E and P,
and the current is measured in the loop E, C, and in between earth path. The
resistance is then calculated by using Ohm’s law. The current should be
accurately measured using milliampere meter.
Table -1 gives distances between probes E and P and probes E and C against the
depth of ground probe at point E. For more details, please see the reference [7].
Table -1
Depth of ground rod at E Distance between E and P Distance between E and C
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 5
Use a separate power distribution system for each location containing the
control systems. This needs to be strictly adhered to.
Use good regulated power supply with distortion less than 2%. Tap the
highest available power system voltage for feeding the isolation
transformers used for electronics. If this is not possible, use UPS for
supplying the power supply loads (control electronics in Power Electronic
controllers and power supplies in Distributed Control Systems). However,
keep the distance between UPS and the load to minimum or locate it
locally.
The signal and other control cables should run separately away from ac
power lines, transformers, rotating electrical machines, solenoids, and
other high power equipment. The recommended separation distance
between signal and power cables is given in Table-2 below.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Table-2 (Recommended separation distance)
0 to 125 V 0 to 10 A 30 cms
125 to 250 V up to 50 A 38 cms
250 to 440 V up to 200 A 46 cms
Note: For higher currents, the distance can be extrapolated with safety factor of
1.3.
The signal wires should be shielded and the shield should be connected to
earth only on one side.
Grounding and earthing: There are four layers required for proper and
effective grounding and earthing. First is the “Dedicated Plant Earth Grid
(G4)”, second is “Control System Ground (G3)”, the third is “Isolated
Common Ground Reference (G2)” for local area, and the fourth is
“ Isolated Local Ground (G1)”.
The “Control System Ground (G3)” is where the incoming power supply
isolation transformer secondary is grounded along with the ground
connection obtained from the “Isolated Common Ground Reference
(G2)”. This “Control System Ground (G3)” is considered as the final
earth pit for that location.
The “Control System Ground (G3)” or the final earth pit is then finally
terminated on the “Dedicated Plant Earth Grid (G4)”.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
The “Dedicated Plant Earth Grid (G4)” may or may not exist. If exists, it
is supposed to have the lowest earth impedance and is considered as the
most stable ground or earth. It consists of many earth pits connected in a
grid fashion.
The size is also based on the incoming power supply conductors or cable
size. Thus, between the two (power supply conductor or cable size and
the size as per above Table –3), the size of earth connection cable should
be selected based on whichever is of higher size.
Similarly, the ground bus bars to be used should be copper bus bars, with
approximately 10 mm as the thickness and 50 mm as the width.
The “Control System Ground (G3)” or the final earth pit connected to the
local control systems should be a separate earth pit, which then can be
connected to the “Dedicated Plant Earth” system. This “Control System
Ground (G3)” should not be shared with other plant systems.
The field wiring cable shields should be terminated on shield bars, which
are to be used along with the I/O carriers. These shield bars then should
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
be connected to the “Isolated Ground References (G2)” and / or then to
separate earth pit.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 6
UPS: The UPS supplies power to the system at one location or sometimes
two systems at different locations. This needs to be avoided. It should
supply power to only one location. The UPS should be as close to the
system as possible. The system load determines the kVA of the UPS and
usually at one location it does not exceed 50 kVA. The UPS voltage
distortion on no load (without system connected) should be less than 1%
and on load it should not exceed 2.5%. Usually, the UPS has an inbuilt
step up isolation transformer whose secondary side is 110 V ac. This is
the supply taken for the control system. Cables from the UPS should be
traceable, should run separately with proper separation distance in case
required with respect to near by other power cables, and the neutral can
be grounded provided isolation transformers are used after the Power
Distribution Boards, as discussed later. The UPS body needs to be
separately earthed, as shown in fig. 4.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
for the connections. One isolation transformer can supply power to one or
two regulated dc power supplies.
DC power supplies of SMPS type take isolated inputs as they have inbuilt
EMI filters. The isolation transformers supply this power.
The grounding and earthing scheme required is now shown in fig. 4. The
individual components of the control system cubicles / panels are
grounded to an “Isolated Local Ground (G1a)”. These components are
chassis of power supplies, DIN rails of the controller, secondary side
screen of the isolation transformer, etc. Since the power ratings are small,
the cubicle / panel frames also can be connected to this ground. The
operator station is connected to a separate “Isolated Local Ground
(G1b)”. The field input shields are also connected to a separate and
another “Isolated Local Ground (G1c)”. These grounds G1a, G1b, and
G1c are basically isolated ground bus bars.
Grounds G1a and G1b are now connected to another ground (ground bus
bar) called as “Isolated Common Ground Reference (G2)”. The two
grounds G2 and G1c are now connected to separate earth pits, which
could be called as “Control System Grounds (G3a and G3b)”. The
“Isolated Common Ground Reference (G2)” is defined here for the sake
of convenience. However, it is not absolutely necessary. The two grounds
G1a and G1b can be connected directly to G3a and the ground G3c is
then connected to G3b.
In case proper Earth Grid is available in the vicinity, the grounds G3a and
G3b should be connected to this Earth Grid, called as “Dedicated Plant
Earth Grid (G4)”.
The connections from grounds G3a and G3b to Earth pits 1 and 2 and are
also to the “Dedicated Plant Earth Grid (G4)”, as shown in fig. 4, should
be with aluminum bus bars. The minimum size should be 100 mm * 12
mm. If split bus bar is used (two numbers of bus bars instead of a single
bus bar), each will have half the cross section (100 mm * 6 mm). This is
preferred.
From fig. 4 the most important aspect to be noted is that all connections
terminating on G1 grounds and finally terminating in “Dedicated Plant
Earth Grid (G4)” are all radial connections.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
transformer is available with ungrounded two wire cables. When new
UPS is to be ordered, it is better to order it with mid point connection also
available from this secondary. The mid point then can then be directly
terminated on “Control System Ground (G3a)” in fig. 4. The neutral then
gets earthed further through this ground G3a. The mid point grounding of
the neutral gives the necessary ground plane symmetry as discussed in
Chapter 2.
Note:
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 7
Depending upon the system or the drive rating, there may be a dedicated
transformer or there may not be a one. There is also a possibility that many
systems may exist in one location simultaneously. These could be from one
manufacturer or from different manufacturers.
Accounting the scenario above, the earthing recommendations are given below.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
separate earth pit). The neutral should be sized based on minimum of half
the rated line current rating.
In case proper Earth Grid is available in the vicinity, this ground G3a
should be connected to this earth grid, called as “Dedicated Plant Earth
Grid (G4)”. If the motor is placed near the drive, the motor body / frame
should then be connected to this ground G3a. if not, then it should be
connected to a separate “Control System Ground (G3b)” (as a direct and
radial earth connection). It should then be returned to “Dedicated Plant
Earth Grid (G4)”, if available in the vicinity and as discussed above.
If there are more than systems present in a location, one earth pit can be
used to connect few of the systems based on the collective ratings. Large
rating drives and associated motors should have separate “Control System
Grounds (G3’s)”. All of them can then be returned to “Dedicated Plant
Earth Grid (G4)”, if available in the vicinity.
The cable connecting the inverter and ac motor (in case of VFD’s) should
be an armored cable. It should be a three-core cable with symmetrically
place current carrying conductors and three ground conductors,
symmetrically embedded in it. The armor should be of corrugated
aluminum. The armor and the ground conductors should be earthed at
both ends (drive as well motor end). These end earth connections from
the armor and ground conductors, and the motor frame / enclosure
earthing connection should be returned separately / radially to the
“Control System ground (G3a)”. If the motor is not near to the drive, then
the armor earth connection and the motor frame / enclosure earthing
connection should be returned separately / radially to “another” “Control
System ground (G3b)” as shown in fig. 5.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
From fig. 5 the most important aspect to be noted is that all connections
terminating on G1 grounds and finally terminating in “Dedicated Plant
Earth Grid (G4)” are all radial connections.
1 kHz 60 meters
3kHz 50 meters
12 kHz 30 meters
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 8
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Chapter 9
Check that the incoming power to the PES is coming from an isolation
transformer and that its neutral is earthed as shown in fig. 5.
Check that transformer capacity is adequate for the PES and that
voltage distortion at the Point of Coupling is within acceptable limits
(less than 2.5%).
Check that cables from transformer are routed separately and are not
in vicinity of any other signal cables. The safe distance for the signal
cables from the transformer cables should be as per Table –2.
Check that all the signal wires received from field are shielded and are
grounded as in fig. 5. Further check that these wires are not in vicinity
of any “other power cables”. If so, the separation distance should be as
per Table –2.
Check that the grounding and earthing scheme is as per fig. 5 and all
connections to grounds and to earth are radial as explained in Chapter
7.
Check that the earth connecting bus bars from grounds G3’s to earth
pits and the earth grid G4 are sized as given Chapter 7.
Check that the earthing resistance is much less than one ohm for all
the earth pits as discussed in Chapter 4.
Check that there is an earth inspection schedule agreed and drawn for
future use.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
References
[1] David Brown, David Harrold, and Roger Hope,” Control Engineering: Control
system power and grounding better practice,” published by Elsevier Inc., 2004.
[2] IEEE standard 1100-2005: Recommended practice for power and grounding
sensitive electronic equipment.
[5] IEEE standard 81-1983 and 81.2-1991: Guide for measuring earth resistivity,
ground impedance, and earth surface potentials of a ground system.
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Annexure -
Figures for Earthing and Grounding of DCS and PE systems
Primary Secondary
Primary Secondary
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
Ohm meter
Earth Ground tester
E P C
Earth
Probes
used for
testing
62% D
D
L L
Other G1's in
Enclosure frame
and
ground
other enclosure
frame grounds in
same area
Control System
Ground (G3)
Earth pit Isolated Common
Ground Reference
(G2)
Dedicated Plant
Earth Grid (G4)
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems
UPS
DCS System
Single-phase AC
supply Number of small kVA Shields of field I / O's
power supply
L L transformers
Operator
N Station Grounding of individual
Isolation components
transformer Grounding of
(normally a part of individual
UPS ) components
(G1b)
Earth pit 3
Earth pit 2
Grounding of
Individual
Components PE System
(G1b) (Converter or VFD)
Armored cable
R R
Three -phase AC
Y Y
supply
Induction
Motor
B B
N
Frame
Grounding of individual
components
Three-phase
isolation
transformer Isolated Local Ground
(G1a)
Enclosure frame
ground (G1b) Isolated Common
Ground
Reference (G2)
Control System
Ground (G3c)
Control System
Control
Ground (G3a)
System
Earth pit 1 Ground (G3b)
Earth pit 3
Earth pit 2
AR/01-Grounding and Earthing of Distributed Control Systems and Power Electronic Systems