Ring Networks for Industries

8/12/2019 Ring Networks for Industries

1/111

Consulting Electrical Engineers

Association of Maharashtra

Preamble

Urban residential and commercial electrical loads have

conventionally been fed from ring networks formed

by medium voltage underground cable networks,

ring main units and standard rated transformers (see

Figure 1).

New transformers can be joined by MV cable from

nearest existing RMU to expand the ring network.

These type of networks are particularly useful for

cities and towns where frequent digging of concrete

roads causes severe inconvenience. At times, newtransformers are also fed via a radial feeder when

existing ring networks are fully loaded or the distance

is too great to effectively complete the ring. In general,

a mix of radial and ring networks have been created

which provides a stable and sustainable power

distribution network as evidenced by the reliable

supply enjoyed by the millions of consumers in the

metro cities and townships.

MV RING FEEDING NETWORKS FOR INDUSTRIES

Logically, the MV Ring Network concept should workequally well for industrial loads.

It is commonly seen that the factory will have a single

HT yard with 1 or 2 transformers within a common

fencing complete with a long Power Control Centre

(PCC) with a large number of incoming & outgoing

ACBs with complex interlocking circuits. Masses o

large size power cables will be laid via ducts, cellars or

racks from the PCC room to departmental Sub-Panels

These distances can be long and the route can be

tricky with road crossings, floor crossing etc. It takes

the consummate skills of an experienced contractor

to design & fabricate the trays, supports and to lay the

cables in an orderly fashion. This traditional design

has an easier option when the departmental loads are

large enough to justify individual transformers. This

option is called the Internal MV Ring Network which

we shall analyze in this study and explain in detail the

various components of a Ring Network

AUTHOR

Mr. Sunil G. Vora

SunelecEmail ID : [email protected]

Fig. 1 Single Line Diagram of a MV Radial + RingNetwork


2/112



Project Size

In recent times, average Industrial project budgets

have progressed from Rs. 10 cr. to Rs. 100 cr.Correspondingly, factory site areas have gone up

from 1 or 2 acres to around 10 to 20 acres and theelectrical loads have increased from around 1000 kWto almost 5000 kW in line with larger project sizes. As

the site area increases, the plant & machinery tendsto be dispersed and the design of an economicalPower Distribution System becomes a challenge for

electrical consultants.

Problems & Solutions

Distribution of low voltage electricity or power over

long distances results in power loss (also knownas I2R losses) & voltage drop in Cables or Bus-BarTrunking. In order to compensate for voltage drop,

the cable size and numbers have to be increased.Routing & laying the masses of bulky cables becomes

a nightmare for the consultant and contractors and apermanent eye-sore for the customer ! Further, largePCCs generally have heating issues & require cooling

fans.

The cost keeps on cascading due to higher deratingfactor for parallel cables, wider trays & racks, heavier

supports, deeper trenches, extra cable boxes, higherlaying charges etc.

The termination of multiple cables in congestedpanels require highly skilled electricians, who arebecoming increasingly scarce. Over-heating at the

terminations becomes a regular complaint.

Laying, dressing and terminations of large numberof bulky cables becomes time consuming and adds

to the delay. Due to pressure for completion andcommissioning, neat dressing of cables is over-

looked resulting in a permanently shabby look to the

panel rooms.

Is there really a solution to such situations ? The answer

is Yes, and it lies in designing the power distributionsystem with smaller transformers placed at variousdepartmental levels and connected through a ring

network of MV cables.

This approach has been used by the author to design

the power distribution scheme for a number of

projects and concluded that it is possible to distribute

the load through multiple transformers instead of only

1 or 2 and yet save money, in capital costs as well asrunning costs !

This combination of lower budget with energy saving

potential makes Ring Networks a preferred option forindustries.

For industries having EHV supply i.e. 100kV / 132kV in

Maharashtra or 66kV Gujarat, the use of Ring and/or

Radial networks is the default option for the secondarydistribution level, i.e. either 6.6kV, 11kV or 22kV before

stepping down to the final voltage i.e. 415 Volts

However, when the system primary voltage is 11kV22kV or 33kV as per the norms of local DisCom, the

two options of one point step-down or Ring Network

can be considered. Both options should be carefullyevaluated and an informed decision should be taken

on the preferred option thereafter.

Steps for Evaluation

A step by step approach will make it easier to decidealthough there is no governing regulation

1. Check Factory plot size :

If the site is less than 5 acres or 20,000 sq.mts in

size, then one point step-down transformer/s may

Fig. 2 Underfloor Cabling in trays and on racks


3/113



be preferable. For larger plot sizes, both options

should be considered

2. Check Electrical Connected Load :

If the total connected load is less than 2500 kW,then one point step-down transformer/s maybe preferable. For larger connected loads, bothoptions should be considered

3. Check Department wise Loads :

If the department-wise loads are generally less than

500 kW, then one point step-down transformer maybe preferable. For higher departmental connectedloads, both options should be considered

4. Check Stand-By Power Requirement :

If all departments require full stand-by power

supply, then prefer one point step-downtransformer/s and a common PCC with DGincomers. If only certain essential load requiresDG power, both options should be considered

Comparison of Costs

Assuming that the option for Ring Network is open

then Capital & Running Cost and Comparison has to

be undertaken. This exercise is best considered using

an example of a real situation.

The client & location shall be left unnamed to maintain

confidentiality. The parameters provided by this client

were :

a) Site Layout with Factory Building Plan :

b) List of Machinery With Connected Loads :

Design with Ring Network :

Now let us look at the methodology :

1. Define Load Zones i.e. loads that can be grouped

together. Sum up the connected load, and list the

zone-wise loads


4/114




5/115



LOAD ZONES

1 UTILITIES (BOILER, THERMPACK, ETP, WTP) 400 kW

2 FABRIC PROCESSING (SOFT FLOW, STENTER, DRYERS) 600 kW

3 WEAVING (LOOMS, HUMIDIFICATION, COMPRESSOR) 750 kW

4 YARN PROCESSING & PREPARATORY (YARN DYEING, WARPING) 500 kW

5 LIGHTING, ADMIN, STITCHING, MAKING UP, PACKING, RESIDENCE 200 kW

TOTAL 2450 kW


6/116



2. Mark the Load Zones on the Site Plan

3. Identify all the locations of local sub-stations on the premises and select a convenient route for layingunderground MV cable to connect all of them in a ring


7/117



4. Conventional LT Distribution with 2 x 1250 kVA Transformers in Utility Block

5. Next, carry out costing of the installation in both cases


8/118




9/119



Costing for Option 1 : Conventional Distribution


10/1110



Costing for Option 2: 11kV Ring Network

Distribution :

As seen from above charts is that the total cost ofinstallation in this case of 11kV Ring Distribution is

0.9 Cr. as against 1.3 Cr. in case of conventional LVcable distribution showing a straight saving of Rs.

0.4 Cr which is a 30% saving. However, the runningcost of losses in transformers, cables should also be

calculated for a complete comparison

6. Running costs of both options

Due to complexity of equations, only the summary is

presented here :

Option 1: Conventional LV Distribution (for 1

month)

Transformers Losses (Iron +Copper) for 70% loading = 5700kWh each. i.e. for 2

11,400 kWh

Cable Losses in LT cables uptoSub-Panel calculated at actual loadcurrent =

4,640 kWh

Total = 16,040 kWh

Option 2: MV Ring Main Distribution: (for 1

month)

Transformer Losses for 5 transformersfor 60% loading (250 kVA 1344 kWh)+ (630kVA 2700 kWh) + (1000kVA 4583 kWh) + (630kVA 2700 kWh)

+ (630kVA 2700 kWh) =

14,046 kWh

Cable Losses in LT cables from USSto Sub-Panel =

600 kWh

Total = 14,646 kWh

Thus the savings in running cost amounts to almost

2,600 kWh (units) per month. At present rates, it

works out to almost Rs, 20,000/- per month. Hence itis preferred to install MV Ring Main network over LV

Cable Distribution.

Other sites where advantages of MV ring networks

were gained are :

a) A client in Taloja where 11 Transformers are

connected on a 6.6kV Ring Network with 2 nos. o

22kV/6.6kV, 3.15 MVA Power Transformers


11/1111



The estimated capital cost saving was in the range of

2 Cr. when compared to LV distribution

b) A client in Kutchh where 14 transformers areconnected on a 11kV Ring Network with 1 no. of

66kV/11kV Power Transformer. The plant started with6 transformers on a MV ring network initially in 2005and at present has expanded upto 14 transformersby extending the ring. When the 66kV installation was

charged, the output was simply connected to existingring with minimum changes

Components of a MV Ring Network :

a. Ring Main Unit (see separate note)

b. Distribution Transformer (can be oil-filled foroutdoor or dry type for indoor)

c. XLPE Insulated MV / HV cables

d. Remote Telecommunication Unit (if Automationand Remote operation is desired)

If required, the RMU & Transformer can be placed ina Unitized Sub-Station (USS) which is also known asCompact Sub-Station (CSS). The space occupied

by a USS typically 2.5m (width) and 3.6m (length)which is quite nominal. It also reduces safety issuesassociated with MV installations.

Note on Ring Main Units (RMUs) :

RMUs are available in variety of configurations

SF6 Insulated RMUs are compact andmaintenance-free

RMUs are available in outdoor models i.e. doesnot require a room

RMUs of standard configurations are available in ashort delivery period

RMUs have CTs & O/C +E/F relays which givebetter protection than Aerial or D.O. fuses

RMUs have Voltage Presence Indicators to identifylive supply line

Summary and Conclusions

Industries having a large and geographically diversified

load need to carefully evaluated before designingthe power distribution system. The method of MVRing Networks which is commonly used by Utility

companies for urban loads can be easily replicatedin Industries. By using Ring Networks, we bring thetransformer close to the load centre, thereby reducingLT cable lengths.

The cost advantage is calculable and can be a decidingfactor. The ease of installation is also important whenmasses of cables create execution hurdles. Theprimary reasons for the success of Ring Networks is the

development of low cost and maintenance free RMUsand economical and reliable XLPE insulated MV & HVcables. Electrical Consultants should take advantage

of these positives and offer these advantages to theirindustrial clients whenever the opportunity arises.

Documents

Ring Networks for Industries