What’s really important to achieve in transmission line protection relaying?

Transmission line protection

As you already know, the real purpose of transmission line protection is to detect faults or abnormal operating conditions and to initiate corrective action. Protective relays must be able to evaluate a lot of parameters to choose and establish right corrective action.

What's really important to achieve in transmission line protection relaying?

Obviously, a relay cannot prevent the fault. Its primary purpose is to detect the fault and take the necessary action to minimize the damage to the equipment or to the system.

The most common parameters which reflect the presence of a fault are the voltages and currents at the terminals of the protected apparatus or at the appropriate zone boundaries.

The fundamental problem in power system protection is to define the quantities that can differentiate between normal and abnormal conditions.

This problem is compounded by the fact that ‘‘normal’’ in the present sense means outside the zone of protection. This aspect, which is of the greatest significance in designing a secure relaying system, dominates the design of all protection systems.

The Nature of Relaying

1. Reliability
2. Zones of Protection
3. Relay Speed
4. Primary and Backup Protection
   1. Remote Backup Protection
   2. Local Backup Protection
      1. Substation Local Backup
      2. Circuit Local Backup
5. (Automatic) Reclosing
   1. How it works | AR-Sequence
6. System Configuration

1. Reliability

Reliability, in system protection parlance, has special definitions which differ from the usual planning or operating usage. A relay can misoperate in two ways: it can fail to operate when it is required to do so, or it can operate when it is not required or desirable for it to do so.

To cover both situations, there are two components in defining reliability:

1. Dependability – which refers to the certainty that a relay will respond correctly for all faults for which it is designed and applied to operate.
2. Security – which is the measure that a relay will not operate incorrectly for any fault.

2. Zones of Protection

The property of security is defined in terms of regions of a power system – called zones of protection for which a given relay or protective system is responsible. The relay will be considered secure if it responds only to faults within its zone of protection.

Figure 1 shows typical zones of protection with transmission lines, buses, and transformers, each residing in its own zone. Also shown are ‘‘closed zones’’ in which all power apparatus entering the zone is monitored, and ‘‘open’’ zones, the limit of which varies with the fault current.

3. Relay Speed

It is, of course, desirable to remove a fault from the power system as quickly as possible. However, the relay must make its decision based upon voltage and current waveforms, which are severely distorted due to transient phenomena that follow the occurrence of a fault.

The relay must separate the meaningful and significant information contained in these waveforms upon which a secure relaying decision must be based. These considerations demand that the relay take a certain amount of time to arrive at a decision with the necessary degree of certainty.

The relationship between the relay response time and its degree of certainty is an inverse one and is one of the most basic properties of all protection systems.

Although the operating time of relays often varies between wide limits, relays are generally classified by their speed of operation as follows:

1. Instantaneous — These relays operate as soon as a secure decision is made. No intentional time delay is introduced to slow down the relay response.
2. Time-delay — An intentional time delay is inserted between the relay decision time and the initiation of the trip action.
3. High-speed — A relay that operates in less than a specified time. The specified time in present practice is 50 milliseconds (3 cycles on a 60 Hz system).
4. Ultra high-speed — This term is not included in the Relay Standards but is commonly considered to be operation in 4 milliseconds or less.

4. Primary and Backup Protection

The main protection system for a given zone of protection is called the primary protection system. It operates in the fastest time possible and removes the least amount of equipment from service.

On Extra High Voltage (EHV) systems, i.e., 345kV and above, it is common to use duplicate primary protection systems in case a component in one primary protection chain fails to operate. This duplication is therefore intended to cover the failure of the relays themselves. One may use relays from a different manufacturer, or relays based on a different principle of operation to avoid common-mode failures.

The operating time and the tripping logic of both the primary and its duplicate system are the same.

It is not always practical to duplicate every element of the protection chain.On High Voltage (HV) and EHV systems, the costs of transducers and circuit breakers are very expensive and the cost of duplicate equipment may not be justified.

On lower voltage systems, even the relays themselves may not be duplicated. In such situations, a backup set of relays will be used. Backup relays are slower than the primary relays and may remove more of the system elements than is necessary to clear the fault.

Selection of CT

1)    Indoors or Out Door:

• Determine where CT needs to be used. Indoor transformers are usually less costly than outdoor transformers. Obviously, if the current transformer is going to be enclosed in an outdoor enclosure, it need not be rated for outdoor use. This is a common costly error in judgment when selecting current transformers.

2)    What do We need:

• The first thing we need to know that what degree of accuracy is required. Example, if you simply want to know if a motor is lightly or overloaded, a panel meter with 2 to 3% accuracy will likely suit for needs. In that case the current transformer needs to be only 0.6 to 1.2% accurate. On the other hand, if we are going to drive a switchboard type instrument with 1% accuracy, we will want a current transformer with 0.3 to 0.6 accuracy. We must keep in mind that the accuracy ratings are based on rated primary current flowing and per ANSI standards may be doubled (0.3 becomes 0.6%) when 10% primary current flows. As mentioned earlier, the rated accuracies are at stated burdens. We must take into consideration not only the burden of the load (instrument) but you must consider the total burden. The total burden includes the burden of the current transformers secondary winding, the burden of the leads connecting the secondary to the load, and the burden of the load itself. The current transformer must be able to support the total burden and to provide the accuracy required at that burden. If we are going to drive a relay you must know what relay accuracy the relay will require.

3)    Voltage Class:

• You must know what the voltage is in the circuit to be monitored. This will determine what the voltage class of the current transformer must be as explained earlier.

4)    Primary Conductor:

• If you have selected a current transformer with a window you must know the number, type and size of the primary conductor(s) in order to select a window size which will accommodate the primary conductors.

5)    Application:

• The variety of applications of current transformers seems to be limited only by ones imagination. As new electronic equipment evolves and plays a greater role in the generation, control and application of electrical energy, new demands will be placed upon current transformer manufacturers and designers to provide new products to meet these needs

6)    Safety:

• For personnel and equipment safety and measurement accuracy, current measurements on conductors at high voltage should be made only with a conducting shield cylinder placed inside the CT aperture. There should be a low electrical impedance connection from one end only to a reliable local ground. An inner insulating cylinder of adequate voltage isolation should be between the shield cylinder and the conductor at high voltage. Any leakage, induced or breakdown current between the high voltage conductor and the ground shield will substantially pass to local ground rather than through the signal cable to signal ground. Do not create a “current loop” by connecting the shield cylinder to ground from both ends. Current flowing in this loop will also be measured by the CT.

7)     CT output signal termination:

• The CT output coaxial cable should preferably be terminated in 50 ohms. CT characteristics are guaranteed only when CT is terminated in 50 ohms.  The termination should present sufficient power dissipation capability.  When CT output is terminated in 50 ohms, its sensitivity is half that when terminated in a high-impedance load.

Current transformers

Principle of operation of CT

• A current transformer is defined as “as an instrument transformer in which the secondary current is substantially proportional to the primary current (under normal conditions of operation) and differs in phase from it by an angle which is approximately zero for an appropriate direction of the connections.”
• Current transformers are usually either “measuring” or “protective” types.

Some Definitions used for CT:

1)    Rated primary current:

• The value of primary current which appears in the designation of the transformer and on which the performance of the current transformer is based.

2)    Rated secondary current:

• The value of secondary current which appears in the designation of the transformer and on which the performance of the current transformer is based.
• Typical values of secondary current are 1 A or 5 A. In the case of transformer differential protection, secondary currents of 1/ root 3 A and 5/ root 3 A are also specified.

3)    Rated burden:

• The apparent power of the secondary circuit in Volt-amperes expressed at the rated secondary current and at a specific power factor (0.8 for almost all standards)

4)    Rated output:

• The value of the apparent power (in volt-amperes at a specified power (factor) which the current transformer is intended to supply to the secondary circuit at the rated secondary current and with rated burden connected to it.

5)    Accuracy class:

• In the case of metering CT s, accuracy class is typically, 0.2, 0.5, 1 or 3.
• This means that the errors have to be within the limits specified in the standards for that particular accuracy class.
• The metering CT has to be accurate from 5% to 120% of the rated primary current, at 25% and 100% of the rated burden at the specified power factor.
• In the case of protection CT s, the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P, as well as composite error at the accuracy limit factor of the CT.

6)    Current Ratio Error:

• The error with a transformer introduces into the measurement of a current and which arises from the fact that actual transformation ratio is not equal to the rated transformer ratio. The current error expressed in percentage is given by the formula:
• Current error in % = (Ka(Is-Ip)) x 100 / Ip
• Where Ka= rated transformation ratio ,Ip= actual primary current, Is= actual secondary current when Ip is flowing under the conditions of measurement

7)    Accuracy limit factor:

• The value of primary current up to which the CT complies with composite error requirements. This is typically 5, 10 or 15, which means that the composite error of the CT has to be within specified limits at 5, 10 or 15 times the rated primary current.

8)    Short time rating:

• The value of primary current (in kA) that the CT should be able to withstand both thermally and dynamically without damage to the windings, with the secondary circuit being short-circuited. The time specified is usually 1 or 3 seconds.

9)    Instrument security factor (factor of security):

• This typically takes a value of less than 5 or less than 10 though it could be much higher if the ratio is very low. If the factor of security of the CT is 5, it means that the composite error of the metering CT at 5 times the rated primary current is equal to or greater than 10%. This means that heavy currents on the primary are not passed on to the secondary circuit and instruments are therefore protected. In the case of double ratio CT’s, FS is applicable for the lowest ratio only.

10) Class PS X CT:

• In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS (X) CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.

11) Knee point voltage:

• That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).
• The ‘Knee Point Voltage’ (Vkp) is defined as the secondary voltage at which an increase of 10% produces an increase in magnetizing current of 50%. It is the secondary voltage above which the CT is near magnetic saturation.

12) Core balance CT (CBCT):

• The CBCT, also known as a zero sequence CT, is used for earth leakage and earth fault protection. The concept is similar to the RVT. In the CBCT, the three core cable or three single cores of a three phase system pass through the inner diameter of the CT. When the system is fault free, no current flows in the secondary of the CBCT. When there is an earth fault, the residual current (zero phase sequence current) of the system flows through the secondary of the CBCT and this operates the relay. In order to design the CBCT, the inner diameter of the CT, the relay type, the relay setting and the primary operating current need to be furnished.

13) Phase displacement:

• The difference in phase between the primary and secondary current vectors, the direction of the vectors being so chosen that the angle is zero for the perfect transformer. The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually express in minutes

14) Highest system voltage:

• The highest rms line to line voltage which can be sustained under normal operating conditions at any time and at any point on the system. It excludes temporary voltage variations due to fault condition and the sudden disconnection of large loads.

15) Rated insulation level:

• That combination of voltage values (power frequency and lightning impulse, or where applicable, lightning and switching impulse) which characterizes the insulation of a transformer with regard to its capability to withstand by dielectric stresses. For low voltage transformer the test voltage 4kV, at power-frequency, applied during 1 minute.

16) Rated short-time thermal current (Ith):

• The rms value of the primary current which the current transformer will withstand for a rated time, with their secondary winding short circuited without suffering harmful effects.

17) Rated dynamic current (Idyn):

• The peak value of the primary current which a current transformer will withstand, without being damaged electrically for mechanically by the resulting electromagnetic forces, the secondary winding being short-circuited.

18) Rated continuous thermal current (Un)

• The value of current which can be permitted to flow continuously in the primary winding, the secondary windings being connected to the rated burdens, without the temperature rise exceeding the specified values.

19) Instrument security factor (ISF or Fs):

• The ratio of rated instrument limits primary current to the rated primary current. The times that the primary current must be higher than the rated value, for the composite error of a measuring current transformer to be equal to or greater than 10%, the secondary burden being equal to the rated burden. The lower this number is, the more protected the connected instrument are against. 

20) Sensitivity

• Sensitivity is defined as the lowest value of primary fault current, within the protected zone, which will cause the relay to operate. To provide fast operation on an in zone fault, the current transformer should have a ‘Knee Point Voltage’ at least twice the setting voltage of the relay.

21) Field Adjustment of Current Transformer Ratio:

• The ratio of current transformers can be field adjusted to fulfil the needs of the application.  Passing

more secondary turns or more primary turns through the window will increase or decrease the turns ratio.  

Actual Turns Ratio = (Name Plate Ration- Secondary Turns Added) / Primary Turns.