System Protection Principles - 12

PAPER ON POWER SYSTEM PROTECTION PRINCIPLES

Paper by P.K GITURA, Chief Eng, System Protection.

What is Power System Protection?

A Power System Network is made up of the following components.

Generators Transformers Transmission & Distribution Lines Circuit Breakers Current Transformers Voltage Transformers Air Break Switches Auto-reclosers Voltage regulators.

Equipment in the high voltage network are from time to time subjected to stress (high voltage, overcurrent, mechanical forces, etc) of varying magnitude. This is so especially during faults in neighbouring circuits. It follows that failure of a power system component is a real possibility, depending on equipment history, and the particular forces the equipment is subjected to.

When this happens, a heavy fault current flows through the faulted equipment from the rest of the network thereby causing more damage and to some cases leading to total failure of the particular equipment.

Power system protection refers to all the combined efforts taken to isolate faulted equipments, and to reduce damage and loss from such faults.

What should be done?

System Protection Principles-11 1

Protection devices are installed to detect faulted equipment and isolate such equipment from the rest of the network to prevent further damage to equipment and to avoid causing interruption on healthy networks.

OBJECTIVES

The objective of protective devices is to detect fault occurrence and trip necessary breakers to isolate the faulty equipment.

To limit damage to the faulted equipment

Minimise supply interruption to adjacent healthy circuits, which would otherwise result in customer dissatisfaction and loss of revenue.

Reduce risk of damage to adjacent equipment.

IMPROTANT CONSIDERATIONS

Nature of Equipment to be protected [importance and cost].

Types of possible faults. Available methods of detecting these faults. Speed of operation Discrimination.


Existing protection schemes on adjacent circuits. Equipment parameters e.g. ratings, impedance, CT ratio,

VT ratio, station D.C supplies etc. Overall cost of the protection scheme.

TYPES OF FAULTS

Short circuit phase to phase [common] Short circuit phase to ground [very common] Short circuit three phase Short circuit three phase to ground Open circuits High temperature [transformers, generators] Over voltage Under voltage Over/under frequency

CAUSES OF FAULTS

Insulation Failure Overloading [leads to over heating] Oil, Leakage Ageing Lighting Wind Pollution e.g. smoke, chemicals in atmosphere. Trees Moisture Salt Rotten poles. Birds Broken Conductors

TYPES OF PROTECTION


FUSES

Used in both HV and LV systems to detect overcurrent due to earth fault or phase to phase fault. This is the most common type of protection, and is found in office and domestic appliances.

OVERCURRENT AND EARTHFAULT RELAYS

Just like the fuses, these relays detect overcurrent and earth faults. The faulted circuit is isolated when the relay issues a trip command to the associated circuit breaker.

Overcurrent relays can either be directly connected [e.g. small motors etc] or fed from current transformers. Fig 1 shows three overcurrent and earthfault connection.

SENSITIVE EARTH FAULT

This is an earth-fault relay set to pick up at very low earth fault currents. These types of faults are likely to occur on rocky ground where conductivity is poor. Fault currents are normally small and below load current. However, apart from causing voltage imbalance, a conductor on the ground is very dangerous to humans. Time delay is normally in the region of 5 seconds and above.

RESTRICTED EARTHFUALT

This is a form of differential Protection normally used on transformers. The scheme compares the currents on the star winding of a transformer and the current in the neutral. A trip command is issued if the fault is within the protected zone (fig 4).


BUCHOLZ RELAY

This relay detects transformer internal faults. The faults can be in the form gas generation or a surge, signifying a violent action within the transformer.

DIFFERENTIAL PROTECTION

Differential Protection is an example of Unit Protection, where faults are detected in specified zone only. Faults external to this zone are not detected. Differential Protection is used on transformers. Generators, Transmission Lines and Bus-bars. The scheme works by comparing currents on the incoming and outgoing side of the unit being protected. By use of current transformers, the two currents are made to be equal during normal loads. But should a fault develop within the protected zone, the currents are no longer balanced. The relay detects the imbalance and issues a trip command. See fig. [2]. The tripping circuit is illustrated in figure 3.

UNDER FREQUENCY

The condition of under-frequency occurs when a generating unit trips resulting in generation shortfall. At low frequency, the remaining generators are unstable. The under frequency relay operates and trips some loads [load shedding] so that the frequency can stabilize. The load shedding is done in stages depending on the severity of the frequency dip.

DISTANCE PROTECTION

This form of protection is used on Transmission and some distribution Lines. Using impedance relays, the scheme


works by measuring the impedance of the protected line. Under normal load conditions the impedance is high and equal to the load impedance. Under short-circuit conditions, the impedance is quite low. Since the impedance of a particular line is proportional to its length, the relay can be set to trip when the impedance measured falls below a predetermined value, which corresponds to a known length of the line km [see figure 5].

TELE - PROTECTION

Tele Protection involves communication of protection relays at two different ends of a transmission line. Communication can be through Power Line Carrier (PLC), Optic fibre, Radio Link etc. The following are examples:

1. DISTANCE PROTECTION PERMISSIVE TRIP SEND [RECEIVE]

On detecting a fault on protected line, each distance relay sends a signal to the remote relay. If the remote relay has also detected a fault, tripping is instantaneous at both ends. See fig. 6.

2. DIRECT TRANSFER TRIP

The protective relay sends a direct trip command to the circuit breaker at the remote end through the communication channel. In most cases the relay also trips the local breaker.


3. LINE DIFFEREENTIAL

Each differential relay at each end of a transmission [or distribution] line compares the local line current with the line current (magnitude and phase) data received from the remote end. The difference between the two quantities is the differential current. A differential current above a set value signifies a fault in the protected zone and the relay issues a trip command. The same will happen at the remote end.

AUTO-RECLOSERS:These are polemounted devices that can detect over-current and earth-faults in the line. In KPLC, they are found on 11KV and 33KV lines. They can trip and auto-reclose according to settings.

We have the following types:11KV:OYT, 3H, 6H, KF,KFE, OSM.

33KV:OXT, RVE, VWVE, NULEC.These figures are Manufacturers notation. Load current is from 200A to about 500Amps.

SECTIONALIZERS.Sectionalizers are pole-mounted switches( oil, vacuum or SF6) similar to reclosers, but not rated to break fault currents. They rely on a breaker behind to break the fault current before opening. They are normally mounted on spur (T-off) feeders. Type OYS exist in the KPLC 11KV system.

OTHER LINE EQUIPMENTS

(i) Voltage regulators: these are devices that assist to regulate the voltage on our lines. In KPLC they are


installed on 11 and 33KV lines. If the incoming voltage is high or low, the regulator taps up or down to bring the voltage to the nominal value of 11 or 33KV as the case maybe.

(ii) Capacitors: Capacitors are installed in 11KV substations or on 11 KV lines. Capacitors help to compensate for the lagging megavars caused by inductive load. They help to reduce the incoming line currents. By reducing current we reduce the losses since losses are a function of current in the conductors and other line equipment. Capacitors also help to boost voltage. It is therefore important to maintain these capacitors in service.

FACTORS THAT DETERMINE A GOOD PROTECTION SCHEME

Discrimination

Protection must isolate only the faulted equipment.

Speed

Fast operation is desirable to minimize damage to equipment and instability to the rest of the system.

Also, the longer the fault duration, the more the heat generated in the various elements in the fault path.

Differential and Distance relays are fastest, operating within 50 milliseconds of fault initiation. Give circuit breaker another 30 milliseconds and isolation will occur in under 100 milliseconds. Overcurrent relays are normally slower because they have to be graded with equipment downstream.


Reliability

Protection must operate when required to; as failure to operate can be damaging and expensive. Use of Back-up and duplicate protection improves reliability. Protection relays should not operate when they are not required to.

Security

Protection relays should be immune from electrical noise, vibrations and any such disturbance. They must not operate under those conditions. Such an operation is very expensive to the utility and the customers.

Requirements For Protection Schemes.

The following are some of the devices are required for a complete protection scheme:Protective relays, current transformers, voltage transformers, D.C supplies, A.C supplies, auxiliary relays, annunciators, batteries, cabling, panels, Instruments like ammeter, voltmeters etc..

MAINTENANCE

Objective

The primary objective of maintenance is to ensure that protective devices are in healthy state and will operate correctly and accurately when called upon.

Requirement

Keep the protective devices clean and in a clean environment.


Use of dust proof covers. Dust interferes with relay contacts.

Test periodically that the relay can still measure accurately [say annually for electromechanical relays).

Confirm AC and DC station supplies healthy. Test that the relay trips the required circuit breaker(s). Test that all the alarms and indications operate. If auto-reclose is provided, this should be tested. Timing tests where critical. Use of test sheets.

PROTECTION TERMINOLOGY

The following are some of the terminology commonly encountered.

DiscriminationThe ability of a Protective System to distinguish between Power System conditions for which it is intended to operate and those for which it is not intended to operate.

Inverse time delay relayA time delay relay having an operating time which with inverse characteristic.

Inverse time relay with definite minimum time (I.D.M.T.)An inverse time relay having an operating time that tends towards a minimum value with increasing values of the electrical quantity.

Knee-point e.m.f.That sinusoidal e.m.f. applied to the secondary terminals of a current transformer, which, when increased by 10% causes the exciting current to increase by 50%.


Main ProtectionThe Protective system which is normally expected to operate first in response to a fault in the protected zone.

Back Up Protection.A protection system intended to supplement the main Protection in case the latter should be ineffective, or to deal with faults in those parts of the Power System that are not readily included in the operating zones of the main Protection.

Pick UpA relay is said to ‘pick up’ when it changes from the unenergised position to the energised position.

Protected Zone.The portion of a power system protected by a given protective system or a part of that protective system. Through Fault CurrentThe current flowing through a protected zone to a fault beyond that zone.

Unit ProtectionA protection system which is designed to operate only for abnormal conditions within a clearly defined zone of the power system

Unrestiricted Protection.A Protection system which has no clearly defined zone of operation and which achieves selective operation only by time grading.

PROTECTION REFERENCE NUMBERS


(ANSI, IEC 617 and IEEE C37.2-1991)

Protection relays are assigned numbers according to their

functions. The following are some of the more common ones.

2 Time Delay

21 Distance Protection

25 Synchronising Check

27 Undervoltage

30 Annunciator

32 Directional Power

37 Undercurrent or Under Power

40 Field Failure

46 Negative Sequence

49 Thermal

50 Instantaneous overcurrent

51 Time Delayed Overcurrent

51N Time Delayed Earthfault

52 Circuit Breaker

52a CB auxiliary Switch – Normally Open

52b CB auxiliary Switch – Normally Closed


59 Overvoltage

60 Voltage or current Balance

64 Instantaneous Earth Fault [High Impedance]

67 Directional Overcurrent

67N Directional Earthfault

74 Alarm

79 Auto-Reclose

81 Frequency

85 Signal Receive

86 Lock-Out

87 Differential

94 Tripping

Fig 1. OVERCURRENT AND EARTH FAULT RELAY CONNECTION.

Current TransformersR

Y

B


OvercurrentRelays

Earthfault

Fig 2. DIFFERENTIAL PROTECTION PRINCIPLE

Protected Unit CT CT


Relay

Fig.3. TRIP CIRCUIT

+ve

Protective RelayContact

Circuit Breaker Trip coil

-ve


Fig.4 RESTRICTED EARTH-FAULT SCHEME

CT

X Y

CT REFRelay

Fig.5. DISTANCE RELAY PRINCIPLE

CT

VTDistance

Relay


Fig.6 TELE PROTECTION PRINCIPLE

STATION A STATION

B

RelayRelay

A AProtected Line

Send ReceiveCommunication

CommunicationEquipment Equipment

Receive Send


Communication Equipment can be:-

- Power line carrier (PLC)- Optic Fibre- Radio Link- Pilot Wires


Documents

System Protection Principles - 12