Why is both RTD temperature and thermal overload protection needed?

Although both methods give protection against overload and thermal rise, both have own pros and cons. Especially with the important objects, both methods are used because they complement each other.

RTD basics

A fundamental physical property of a metal is that its resistivity changes with temperature. For many metals, this relationship is quite linear over wide temperature ranges making them ideal for measuring temperatures. A Resistance Temperature Detector (RTD) is a resistor designed to measure temperature using the known resistance vs. temperature relationship of metals. An RTD element is the actual temperature-sensing unit.

RTD’s are used to measure temperatures of protected device windings or shaft bearings. A rise in temperature may indicate overloading of the protected device, or the beginning of a fault.

Thermal overload protection basics

Thermal overload protection is based on a mathematical model of the thermal behavior of the protected device (motor etc.) The protection relay continuously measures the phase current amplitudes and calculates the thermal image. The thermal model might consist of one or more time-constant, separate protection for stator and rotor etc.

Why both are needed?

Although both methods give protection against overload and thermal rise, both have own pros and cons. Especially with the important objects, both methods are used because they complement each other.

RTDThermal overload protection
Resistors must be located in the protected object (isolation, wiring, etc)Requires only the phase current measurement (current transformers)
Temperature measured in certain spots, typically only in the statorMore overall (average) thermal image, can have separate images for stator and rotor
Measures actual (true) temperatureDoes not recognize thermal rise caused by reduced cooling (dirt or cooling system failure)
Slow response for rapid changes in loadBetter (faster) operation in start-ups and heavy overload (might include hot spot behavior modeling)
Can be used for temperature measuring of bearings etc.Does not see bearing etc. faults unless fault causes rise in the phase current amplitude

Protection relay codes

Introduction:

Numerical codes are sometimes used when defining the protection relay, whereas in other cases symbols are used. The numerical codes refer to the IEEE C37-2 Standard, whereas the symbols refer to the IEC Standards. In the definition of the symbols the IEC Standards have not detailed all the symbols to be used and so in practice, one still uses the codes mentioned in C37-2.

ANSI Protetion Relay Codes:

An extract of the numerical codes is given below, as given in the C37-2 Standard relative to protection systems. The description is a summary of what is given in the Standard:

– 2 starting timer;
– 21 distance relay (impedance);
– 24 overfluxing relay (volts per hertz);
– 25 synchronizer or synchronism verifier;
– 26 apparatus for temperature control;
– 27 undervoltage relay;
– 32 directional power relay;
– 37 undercurrent or under-power relay;
– 40 loss of field relay;
– 46 negative sequence relay or for current balance by
means of current measurement;
– 47 cyclic sequence relay by means of voltage measure-
ment;
– 48 incomplete sequence relay;
– 49 thermal relay for transformers or machines;
– 50 instantaneous overcurrent relay;
– 51 overcurrent relay with inverse time;
– 55 power factor control relay;
– 59 overvoltage relay;
– 60 voltage balance relay;
– 62 stop timer;
– 63 pressure sensor;
– 64 relay to identify ground faults (not used for networks
with grounded neutral);
– 66 apparatus which detects a certain number of opera-
tions;
– 67 directional overcurrent relay for alternating current;
– 68 locking relay (for example to prevent reclosing after
loss of step);
– 74 alarm relay;
– 76 overcurrent relay for direct current;
– 78 loss of step relay or for measurement of phase angle;
– 79 reclosing relay for alternating current;
– 81 frequency relay;
– 82 reclosing relay for direct current;
– 83 automatic changeover relay of for selective control;

– 85 pilot wire relay;
– 86 look-out relay;
– 87 differential relay;
– 90 regulator device;
– 91 directional voltage relay;
– 92 directional power voltage relay;
– 94 trip relay.

A Little Explanation:

The meaning of the codes used most frequently is given in
more detail since they are often the cause of misinterpretations
and misunderstandings.

code 48: is a little known code which is, however, now commonly used to indicate the protection against prolonged motor starts. Sometimes it is confused with the protection called 51LR (‘locked rotor’ overcurrent). There are two codes to be used to indicate the protections which serve to control motor starting and locked rotor: 48 for the starting phase (prolonged starting) and 51LR for the locked rotor (when the motor is already running);

– code 50: for the Standard this is an overcurrent protection of instantaneous type. The definition of instantaneous relay was valid for the electromechanical, now the various thresholds of the overcurrent relays always have the possibility of introducing a delay. In common practice it is considered to be the overcurrent protection which identifies strong currents typical of short-circuit;

– code 51: for the Standard, this is an overcurrent protection of the dependent (inverse) time type. The definition of a relay with inverse time is typical of American tradition. In common practice code, 51 is used both for overcurrent relay with dependent (inverse) time characteristic and with independent (definite) time characteristic. In general, it is considered the overcurrent protection which identifies weak currents typical of an overload or of short circuits with high fault impedance.

Further clarifications are necessary in defining the numerical codes to be used for the protection against ground faults. The C37-2 Standard only specifies a code to be used for ground faults: 64, but specifies that this code cannot be used for the protections connected to the CT secondary in grounded networks where code 51 must be used with suffixes N or G. In defining the N and G suffixes, the C37-2 Standard is very clear and they are used as follows:

– N when the protection is connected by means of transducers which measure the phase parameters and the vectorial sum of the parameter to be measured (current or voltage) is sent to the relay. This connection is generally called residual connection (Holmgreen);

– G when the protection is connected directly to the secondary of a transducer (CT or VT) which measures the homopolar parameter directly (current or voltage);

Therefore it is correct to use the following definitions for protection against ground fault:

– 51G for the overcurrent protection connected to the secondary of a ring CT which measures the ground current;

– 51G for the homopolar overcurrent protection connected to the secondary of a CT positioned on the grounding of the machine (star point generator or transformer);

– 51N for the homopolar overcurrent protection connected with residual connection to three phase CTs;

– 59N for the homopolar overvoltage protection con- nected on the vectorial sum of the three phase VTs (open delta – residual voltage);

– 59G homopolar overvoltage protection connected to the VT secondary positioned on the machine grounding (star point generator or transformer);

– 64 only applicable in networks with isolated neutral both for overcurrent and overvoltage protection.

Apart from the N and G suffixes, sometimes other suffixes are added to indicate the application of the protection in detail.
For example:

– G generator (for example 87G differential protection for generator);
– T transformer (for example 87T differential protection for transformer);
– M motor (for example 87M differential protection for motor);
– P pilot (for example 87P differential protection with pilot wire);
– S stator (for example 51S overcurrent stator);
– LR motor protection against locked running rotor (51LR);
– BF failed opening circuit-breaker 50 BF (BF = breaker failure);
– R used for different applications:
– reactance (for example 87R differential protection);
– undervoltage to indicate residual voltage (27R);
– rotor of a synchronous machine (64R ground rotor);
– V associated with the overcurrent protection (51) it indicates that there is voltage control or voltage restraint (51V);
– t indicates that the protection is timed (for example 50t protection against overcurrent short-circuit with delay added).

All About Induction Machines

Introduction

Induction machines, also called asynchronous machines, can be used as generators or motors. Induction machines can be either of one or three-phase construction. The following discussion focuses on the three-phase machines and their properties. The maximum of the applications for three-phase induction machines is with power ranges varying from a few kilowatts to a few hun-dred kilowatts with rated voltages below 1 kV. The range can be extended roughly up to 20 MW with rated voltages up to 15 kV. The simple, robust and low loss construction of an induction machine has contributed to its wide-spread success in different applications.

squirrel cage motor
squirrel cage motor
wound winding rotor motor with slip rings
wound winding rotor motor with slip rings

The stator has basically the same construction as with synchronous machines. It is fed by three-phase alternating current providing rotating flux. This flux rotates at synchronous speed.

The rotor is a three-phase short-circuited winding. This winding can be a normal wound wind-ing or it can be done by casting aluminum “cage” windings into the slots in a laminated iron rotor construction. In the earlier case, it is referred to as “wound rotor machine,” and in the latter case, it is referred to as “squirrel cage rotor machine.”

The rotating stator field induces a flux in the rotor windings. Since the rotor windings are short-circuited, the induced flux will create a current in the rotor. This current will produce a flux of its own, opposing the flux that created it. As a result, there will develop a torque on the machine shaft. When the developed torque is higher than the resisting load torque, the machine starts to rotate as a motor.

During operation, under no-load conditions, the speed of the rotor is very close to the syn-chronous speed, thus the currents induced in the rotor will have a low frequency. The rotor currents will have a frequency corresponding to the rotating speed difference between stator field and rotor (shaft). This difference is referred to as slip. Normally, the slip is stated in relation to the synchronous speed i.e. relative slip (s).

When the slip is positive, the machine is working as a motor, and when it is negative, the machine is working as a generator. The slip is often given as percentage value s%.

The name “induction machine” comes from the fact that the induced voltage in the rotor wind-ings is due to the rotating stator field, whereas the term “asynchronous machine” refers to the fact that the rotor is always rotating at a different speed than the stator field.

The performance of an induction machine can be studied based on a one-phase equivalent circuit.

Equivalent circuit of an induction machine
Equivalent circuit of an induction machine

The reduced rotor current flowing through the slip-related rotor resistance component de-scribes actually the mechanical power developed at the rotor shaft at each operation point. Also the rotor leakage reactance value is depending on the slip since the rotor current frequency is depending on the slip. Under light load conditions, the significance of the rotor leakage reactance is negligible, but the situation will change as the load, and slip increases.

Torque-versus-slip characteristic of a three-phase induction motor

From the below figure, it can be seen that when the load torque requirement rises, the slip increases until a point of equilibrium is found. The torque can be increased until the breakdown torque point is reached, and sliding on the left-hand side of this point with a constant-load torque would mean stopping of the motor. It can be also noted that the starting torque is much less than the maximum, breakdown, torque. Also, the rated torque of the motor is below the maximum, at least by a relation of 1.6

Torque as a function of slip with a three-phase induction motor
Torque as a function of slip with a three-phase induction motor

Increasing the rotor resistance increases the starting torque, and this lowers the torque curve gradient between no-load and breakdown torque points. On the other hand, the rotor re-sistance does not affect the maximum, breakdown, torque value.

This phenomenon is utilized with induction motors having wound winding-type rotor with slip rings. External variable resistance is connected to the non-short-circuited rotor windings using the slip rings. By adjusting the external resistance, it is possible to increase the starting torque, lower the starting current and within certain limits to control the motor speed. The drawbacks are the losses in the external resistor, limited speed adjustment range, speed varia-tions with variable torque (load) and more complex structure of the motor.

Electrical characteristics

(Peak) making current:

The peak value of the first major loop of the current in one pole of a
switching device during the transient period following the initiation of current during a
making operation.


Peak current:

The peak value of the first major loop of current during the transient period
following initiation.


Breaking current:

Current in one pole of a switching device at the instant of initiation of
an arc during a breaking process.


Breaking capacity:

Value of the prospective breaking current that a circuit-breaker or load switch can break at a given voltage under prescribed conditions for application
and performance; e.g. overhead line (charging current) breaking capacity.


Short-line fault:

Short circuit on an overhead line at a short but not negligible distance
from the terminals of the circuit-breaker.


Out of phase (making or breaking) capacity:

Making or breaking capacity for which the specified conditions for use and behavior include the loss or the lack of synchronism between the parts of an electrical system on either side of the circuit breaker.


Applied voltage:

The voltage between the terminals of a circuit-breaker pole immediately
before making the current.


Recovery voltage:

Voltage occurring between the terminals of a circuit-breaker pole
after interruption of the current.


Opening time:

The interval of time between application of the auxiliary power to the opening release of a switching device and the separation of the contacts in all three
poles.

Closing time:

The interval of time between application of the auxiliary power to the closing circuit of a switching device and the contact touch in all poles.

Break time:

interval of time between the beginning of the opening time of a switching
device and the end of the arcing time.

Make time:

the interval of time between application of the auxiliary power to the closing circuit of a switching device and the instant in which the current begins to flow in the
main circuit.

Rated value:

value of a characteristic quantity used to define the operating conditions
for which a switching device is designed and built and which must be verified by the
manufacturer.

Rated normal current:

the current that the main circuit of a switching device can
continuously carry under specified conditions for use and behaviour. See below for
standardized values.

Rated short-time withstand current:

current that a switching device in a closed position can carry during a specified short time under prescribed conditions for use and behaviour. See below for standardized values.

Rated voltage:

The upper limit of the highest voltage of the network for which a switching device is rated. See below for standardized values.