Final Task
Prepared by:
Felicity van Wyk
VWYFEL002
2013-05-15
Felicity Van Wyk- VWYFEL002
Final Task
EEE4089F-2013
Contents
Final Task............................................................................................................................................................................................................. 1
Prepared by: ..................................................................................................................................................................................................... 1
Introduction........................................................................................................................................................................................................ 3
Scope ................................................................................................................................................................................................................. 3
Protection of the Electrical System ........................................................................................................................................................... 3
Protection Criteria ...................................................................................................................................................................................... 3
Power System Protection ......................................................................................................................................................................... 4
MV/LV Substation ....................................................................................................................................................................................... 7
MV overhead line protection ....................................................................................................................................................................... 7
Calculation for Relay .................................................................................................................................................................................. 8
Future use of Generator................................................................................................................................................................................. 9
Statement of the Quality Supply................................................................................................................................................................. 9
Quality of Supply parameters ................................................................................................................................................................. 9
Implication for the Future and maintenance ..................................................................................................................................... 10
Implications ................................................................................................................................................................................................ 10
Maintenance ............................................................................................................................................................................................... 10
Conclusion ........................................................................................................................................................................................................ 10
References ........................................................................................................................................................................................................ 10
Felicity Van Wyk- VWYFEL002
Final Task
EEE4089F-2013
Introduction
This report is the last report for the electrification and design of the rural area. This report deals with the
protection of all the elements in the electricity system.
Scope
The outputs of the report are therefore:
 A proposal for the protection of all elements of the electricity system. The elements include:
 Overhead line
 Substations(MV/LV)
 LV feeders
 Recommendation for the future use of the exciting generator at the hospital.
 A statement on the quality of supply to be expected by the consumers.
 Implications for the future operations and maintenance of the system.
Protection of the Electrical System
Protection Criteria
Power systems are prone to faults, cause by external interference, aging and internally originating stress.
Power systems needs to be protected from these elements. The function of the protection systems is to limit the
severity and extent of power system disturbance and possible damage to system components.
There are different factors to consider when designing a power system:
 Redundancy
Is two protection groups, usually not identical, each independently capable of protecting the element of
the power system.
 Sensitive
Protection system needs to be sensitive to detect the smallest fault condition. The smallest the fault
current the system can detect the more sensitive the system is.
 Selective
When a fault occurs the power system must be selective and isolate the faulted elements or does
elements needed to preserve power system integrity,
 Speed
The power system should be fast enough at detecting faults and it also needs to be fast enough to limit
the system disturbances and thermal damage.
Felicity Van Wyk- VWYFEL002
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EEE4089F-2013
Power System Protection
11kV source point
MV Distribution
system
MV Feeder
A
MV/LV substation
MV/LV Transformer
B
Service distribution
system
C
C
C
C
D
D
D
D
LV Feeder
Service Connection
E
Customer Premises
Customer
Meter
F
Consumer
Distribution
Board
Figure 1: Typical Power system Layout
The above figure is a typical distribution system layout. It is divided into four areas, and all of these areas need
protection. It includes:
 MV Distribution system
 MV/LV substation
 Service Distribution System
 Customer Premises
MV Distribution System
The MV distribution system contains the MV feeder which should be protected. There is different feeder
protection but the following are commonly used:
 Underground cable system: non-directional inverse definite minimum time lag (IDMTL) overcurrent
and earth fault protection.
 MV aerial bundled conductor system: non-directional IDMTL overcurrent and earth fault protection.
Felicity Van Wyk- VWYFEL002
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EEE4089F-2013

Bare overhead conductor system: non-directional IDMTL overcurrent and earth fault protection plus
sensitive earth fault protection with single shot or multishot auto-reclose facilities.
Extremely inverse overcurrent protection should be used where the fault level is high and the conductor size is
small. It can also be used where a faster clearance time is needed. In cases where the MV fault current is limited,
the earth fault protection may be of the definite time delay type with delay times variable in the range 0 s to 10
s. With small conductors, special care should be taken to avoid excessive rises in temperature. [1]
MV/LV Substation
There are four points of protection in the MV/LV substation. The protection at the substation may consist of the
following. For the protection at point A the following can be used:
 A high rupturing capacity fuse or expulsion fuse
 Two overcurrent A.C trip coils with a time-lag fuse, or circuit-breaker, or a relay
 A link or an isolator, provided that the primary MV feeder protection will operate to clear a disruptive
fault on the distribution transformer.
For protection at point B a fuse or circuit-breaker can be used to provide overload protection for the
transformer and fault protection for the LV busbars.
For protection at point C a fuse or circuit-breaker could be used. The protection should provide overload and
fault protection for the main LV distributor and service connection distribution box. [1]
MV/LV Transformer
The transformer in the MV/LV substation needs to be protected. There are two ways to protect a transformer
and the most popular that is used in practice are:
 A self-protecting (CSP) transformer.
 A conventional transformer that complies with SANS 780.
The layout of the transformers:
Layout of a self-protecting transformer (CSP)
Figure 2: CSP Transformer
A conventional transformer that complies with SANS 780
Figure 3: Conventional Transformer with one external fuse per phase
Felicity Van Wyk- VWYFEL002
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Figure 4: Conventional Transformer with two external fuse per phase
Service Distribution System
At point D a fuse or circuit-breaker can be used for protection. The protection should provide fault protection
for the service cable. [1]
Customer Premises
At point E a fuse or a circuit-breaker can be used for protection to provide overload protection for the service
cable and the meter.
At point F a fuse or circuit-breaker can be used for protection to provide overload and fault protection for the
consumer’s distribution board. [1]
As mention above, at most points the protection contains a circuit breaker or a fuse. The fuses that are use at
this point are high rupturing capacity fuses (HRC). The LV feeder can be protected by a high rupturing capacity
fuse (HRC) or a circuit breaker. Both of the protections have their own advantages.
The advantage of using a HRC is as follows:
 They are operating fast at high currents
 Discrimination of upstream as well as downstream fuses
 They are static devices
Disadvantage of fuses:
 Single phase device
 Consumers with a three phase supply may be reduces with a supply with only one or two phases
The advantage of circuit breakers:
 Circuit breakers can be reset
 All three phases can be tripped for a single phase fault
Disadvantages of circuit breakers:
 Additional enclosure is required
 Not easily operated at ground level
 Total discrimination might not be possible for large fault current
It is not realistic to include protection at all the points mention. A more realistic and cost-effective system will
be when protection is omitted at one or more points. The following point indicates where protection can be
omitted:
 At point B, provided that the MV protection at A will operate for a fault on the LV busbar and for a
transformer overload.
 At point C, provided that the protection a B will protect the outgoing feeders over their full length. In
the case where a CSP transformer supplies a LV system.
 At point D or E, but not at both
 At point F, provided that it is fitted at E
Felicity Van Wyk- VWYFEL002
Final Task
EEE4089F-2013
The protection of the LV system will be as follows:
 A- A fuse would be use.
 For the substation the CSP protection scheme is implemented.
 B- Protection is omitted.
 C- Protection is omitted.
 D- Circuit breaker is used.
 E- Circuit breaker is used.
 LV feeders are protected using a relay.
MV/LV Substation
Choosing the protection scheme for the MV/LV substation is very crucial in the design of the protection system.
The most effective protection for a transformer would be at, near, or within each transformer. A self-protecting
(CSP) transformer would be used for the protection. The choice was based on the level of protection as well as
the cost of the equipment. In practice the protection scheme can be implemented as a CSP 100 kVA or 200 kVA
transformer with a 120 A pole-top circuit-breaker for every four service connections. The choice was based on
the following point:
 CSP is cheaper
 It enables a transformer to protect itself from faults
 Lightning protection is maximise
 It is uneconomical to operate a distribution transformer in an overloaded condition [2]
Feeder Protection
For the feeder protection current transformer (CT’s) was used to protect the electrical elements in the system.
CT’s measures the incoming and outgoing current and initiate a trip command if the level of unbalance is above
the set level. The circulating current is only driven through the relay only for the “in-zone” fault. The CT’s will
be selected in such a way that they don’t bypass the current from the relay.
Figure 5: Single phase circulating current scheme
The above figure indicates the zone of protection of an electrical component when a fault occurs in the system.
The zone of protection is the equipment that will be protected.
MV overhead line protection
For the overhead line protection relays was used for the main protection component. The following diagram
shows the MV feeder.
11 kV
S4
752.18 kVA
Felicity Van Wyk- VWYFEL002
S3
301.80 kVA
S2
4.2 MVA
Figure 5: MV feeder with relays
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S1
900 kVA
EEE4089F-2013
Symbol
= Relay
= Current Transformer
Figure 5 indicates an 11 kV radial system. When primary protection fails, like the circuit breaker and fuses,
backup protection is required. In most cases relay grading is implemented, it is the coordination of different
relays of feeder to ensure effective backup protection, while also ensuring sensitivity and selectivity.
Grading between two relay curves should be around 250-400 ms. The maximum time for a back-up operation
should not exceed 3s on MV and the tripping for the maximum fault current is should not exceed 300ms.
The plug setting of the relays was determine by using a CO-7 relay with a CT characteristics given in Figure 6
below, the same power factors for the loads was chosen, with a frequency of 50 Hz. The breaker operating time
is 5 cycles and a coordination interval of 0.3s. A 200/5 CT was use for each section, the reason for this choice of
CT ratio ensures operation in the full range of expected fault currents.
The furthers circuit breaker must have the fastest operating time
Calculation for Relay
Figure 6: Relay time-current characteristics
The fault current was calculated by using the following equation (primary current):
𝐼𝑓 =
𝐿𝑜𝑎𝑑 𝑝𝑜𝑤𝑒𝑟
√3 × 𝑉𝑝ℎ−𝑝ℎ
The secondary fault current was calculated by using the following formula (secondary current):
𝐼𝑓′ =
At
S1
S2
S3
S4
𝐼𝑓
𝐶𝑢𝑟𝑟𝑒𝑛𝑡 𝑟𝑎𝑡𝑖𝑜
If [A]
I’f [A]
47.23
1.18
267.68
6.692
283.52
7.088
322.99
8.074
Table 1: Primary, Secondary and Tap setting current
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TS [A]
4
7
8
10
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The above table shows the fault current at the primary and secondary side. The tab settings were chosen from
figure 6 above.
Future use of Generator
The generator is supplying a power of 500 kVA. In my planning, task 1; I have stated that the centre will be
supplied with electricity. According to the project outline, if the centre is being supplied with electricity the
generator can be kept for hospital standby, but can also be used for network support. The output power of the
generator is small and can therefore be used as distribution generation.
Distribution generation (DG) is where a small output power is delivered to the grid at distribution voltage
instead of transmission voltage [3]. DG is limited to generation with an output into the distribution networks of
up to 100 MW. The despatch of power is not necessarily under central control and can therefore be controlled
by the owner (rural centre).
Statement of the Quality Supply
The quality of electrical supply means a minimal number of interruptions and their duration kept at a minimal
time. The quality of supply is divided into the following:
 Commercial quality, which deals with the system operator and the consumer.
 Reliability of supply, which deals with the number and duration of disturbances to the customers.
 Voltage quality, which deals with the technical characteristic of voltage measured.
Quality of Supply parameters
Magnitude of Supply voltage
The customer supplied voltage at the LV side; the standard voltage shall be 400 V ph-ph, and 230 V ph-earth. If
the customer supplied is supplied at other voltage level, the magnitude of the declared voltage shall be as
specified in the supply agreement. The supplied voltage shall be supplied at nominal voltage.
The compatibility levels for the magnitude of supply voltage shall be as specified by the following table:
1
2
Voltage level
V
Compatibility level
%
< 𝟓𝟎𝟎
≥ 𝟓𝟎𝟎
±10
±5
Table 2: Deviation from Standard voltages
Frequency
The frequency supplied to the customers will be the standard voltage of 50Hz. The compatibility for the
frequency of supply is specified in table 2 below.
1
2
Network Type
Grid
Compatibility Level
±2% (±1 𝐻𝑧)
±2.5 % (±1.25 𝐻𝑧)
Island
Table 3: Deviation from standard frequency
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Voltage unbalance
The compatibility level for UB on LV three-ph networks shall be 2%.
Voltage flicker
The compatibility level for LV network shall be 1, this is for the long-term flicker severity.
Implication for the Future and maintenance
Implications
Power systems components are prone to faults, cause by external interference, such as overloading, lightning
strikes, storms, corrosion and vandalism. In this process components fails and disturb the system and causes
components to fail. This will have an impact on the operation of the system, and may even cause an outage.
Maintenance
Maintenance to the power system would be done on an annual basis to ensure stability, reliability as well as
increasing the life expectancy of components. Planned interruption would be enforced. The maintenance
assessment shall be carried out according to the NRS 082 standard document. When maintenance is carried out
the environmental hazards shall be removed in accordance with the organisation’s policy.
Maintenance on the following components will be done:
 Transformer
 Circuit breaker
 Fuses
 LV feeder
 CT’s
Maintenance needs to be done for optimal operation.
Conclusion
Protection plays a huge part in the electrification system; it ensures that the system is stable as well as reliable.
This is required for the system to limit the severity and extent of power system disturbances and possible
damage to the system components. The protection system has to be implemented to the highest level, cause
disturbances cost millions of Rants if a fault do occur.
References
[1] “Electricity distribution- Guidelines for the provision of electricity ditribution networks in residential
areas,” vol. 4, p. 21, 2007.
[2] T. Group, “CSP Type Transformers,” [Online]. Available: http://www.rousant.com/files/csp.pdf. [Accessed
14 May 2013].
[3] C. Gaunt, “Characteristics of Distribution Generation,” University of Cape Town, Cape Town, 2013.
[4] ESKOM, “"Electricity distribution-Guidelines for the provision of electricity distribution networks in
residential areas,” Johannesburg, First edition 2007.
Felicity Van Wyk- VWYFEL002
Final Task
EEE4089F-2013