MV system coordination doesn't have to be complicated, as long as you know the types of protective relays available, their function, and how they work.
Recently, solid-state electronic relays, just like the solid-state trip unit shown at the right, have become more popular. These relays perform all the same functions as electromechanical relays. But, because of their electronic circuitry and microprocessors, they provide even more functions.
Protective relays are a mystery to many These devices are arguably the least understood components of medium-voltage (MV) circuit protection. In fact, some believe MV circuit breakers operate by themselves, without direct initiation by protective relays. Others believe the operation and coordination of protective relays is much too complicated to understand. Both of these beliefs are wrong. MV breakers do need initiation from protective relays, usually through appropriate sensors. (See sidebar "MV Breakers Are Not True Circuit Breakers" on page 37.) And, coordination doesn't need to be complicated-knowing some basic relay and sensor information will help considerably. Let's try to clear up the mystery.
Electromechanical relays are not a new concept They've been in use for many years. Built like fine watches, these relays offer great precision and sometimes come with jeweled bearings. They have earned a well-deserved reputation for accuracy, dependability, and reliability. There are two basic types of operating mechanisms: the electromagnetic-attraction relay and electromagnetic-induction relay.
Magnetic-attraction relays, as shown in Fig. 1, have either a solenoid that pulls in a plunger, or one or more electromagnets that attract a hinged armature. When the magnetic force is great enough to overcome the restraining spring's force, the movable element begins to travel, and continues until the contact(s) close or the spring's force, the movable element begins to travel, and continues until the contact(s) close or the magnetic force is removed. The pickup point is the current or voltage at which the plunger or armature begins to move. In a switchgear relay, you van set the pickup value very precisely.
These relays are usually instantaneous in action, with no intentional time delay, closing as soon after pickup as the mechanical motion permits. You can add time delay by means of a bellows, dashpot, or a clockwork escapement mechanism. However, the timing accuracy is considerably less precise than that of induction-type relays. As such, users seldom choose these relays with time delay in switchgear applications.
Attraction-type relays can operate with either AC or DC on the coils. Therefore, the DC component of an asymmetrical fault definitely affects these relays using thisprinciple. They must be set to allow for this.
Induction relays, as shown in Fig. 2 (on page 39), are available in many variations to provide accurate pickup and time-current responses for a wide range of simple or complex system conditions. They're actually like induction motors. On the relay, the moving element (rotor) is usually a metal disk, although sometimes it's a metal cylinder or cup. The stationary part (stator) is one or more integral electromagnets, with current or potential coils inducing currents in the disk, causing it to rotate. Until the rotational forces are great enough to turn the disk and bring its moving contact against the stationary contact, a spring restrains the disk motion. This closes the circuit the relay is controlling. The greater the sensed fault, the greater the current in the coils, and the faster the disk rotates.
A calibrated adjustment called the time dial sets the spacing between the moving and stationary contacts; this varies the operating time of the relay from fast (contacts only slightly open) to slow (contacts nearly a full disk revolution apart). Reset action begins upon removing the rotational force, either by closing the relay contact that trips a breaker or by otherwise removing the malfunction the relay is sensing. The restraining spring resets the disk to its original position. The time required to reset depends on the type of relay and the time-dial setting (contact spacing).
With multiple magnetic coils, these relays can sense several conditions of voltage and current simultaneously. The coils' signals can be additive or subtractive in actuating the disk. For example, a current-differential relay has two current coils with opposing action. If the two currents are equal, regardless of magnitude, the disk does not move. If the difference between the two currents exceeds the pickup setting, the disk rotates slowly for a small difference and faster for a greater difference. The relay contacts close when the difference continues for the length of time determined by the relay characteristics and settings. By using multiple coils, directional relays can sense the direction of current or power flow, as well as magnitude. Since induced magnetic fields from AC magnets create the disk's movement, induction relays are almost completely unresponsive to the DC component of an asymmetrical fault.
Most switchgear-type relays come enclosed in a semiflush-mounting drawout case. Installers typically install relays usually on the door of the switchgear cubicle. They bring sensor and control wiring to connections on the case. The relay inserts into the case and connects by means of small switches or a bridging plug, depending on the manufacturer. As such, you can disconnect and withdraw it from the case without disturbing the wiring. When the relay is disconnected, the current transformer (CT) connections in the case are automatically shorted to short circuit the CT secondary winding and protect the CT from overvoltages and damage.
Many relays are equipped with a connection for a test cable, which allows you to check the relay calibration with a test set. The front cover of the relay is transparent and removable for access to the mechanism. It has provisions for wire and lead seals to prevent tampering by unauthorized personnel.
Recently, solid-state electronic relays have become more popular. These relays perform all the same functions as electromechanical relays. But, because of their electronic circuitry and microprocessors, solid-state relays provide many functions not previously available.
In general, solid-state relays are smaller and more compact than their mechanical counterparts. For example, you can use a 3-phase solid-state overcurrent relay in place of three single-phase mechanical overcurrent relays. And, the solid-state unit is smaller than any one mechanical overcurrent relay.
The precision of electronic relays is greater than that of mechanical relays, which allows you to achieve a closer system coordination. And because there is no mechanical motion and their electronic circuitry is very stable, electronic relays keep their calibration accuracy longer. Reset times can be extremely short if desired because there's no mechanical motion involved.
Electronic relays require less power to operate than their mechanical counterparts, producing a smaller load burden on the CTs and PTs supplying them. Because solid-state relays have a minimum of moving parts, they are very resistant to seismic forces and are especially well suited for areas susceptible to earthquake activity.
In their early versions, some solid-state relays were sensitive to the severe electrical environment of industrial applications. They were prone to failure, especially from high transient voltages caused by lightning or utility and on-site switching. Today's relays, however, withstand these transients and other rugged application conditions. As such, this essentially eliminates this type of failure.
Solid-state relays have gained a strong and rapidly growing position in the marketplace as experience proves their accuracy, dependability, versatility, and reliability.
There are literally hundreds of different types of relays. The catalog of one manufacturer of electromechanical relays lists 264 relays for switchgear and system protection control functions. For complex systems (many voltage levels and interconnections over great distances) such as utility transmission and distribution, relaying is an art to which some engineers devote their entire careers. For more simple industrial and commercial distribution systems, relay protection is less elaborate, although proper selection and application are still very important.
The most commonly used relays and devices are listed in the original article's Table (page 42) by their American National Standards Institute (ANSI) device-function number and description. These standard numbers are used in one-line and connection diagrams to designate the relays or other devices, saving space and text.
Where a relay combines two functions, both the function numbers show. The most frequently used relay is the overcurrent relay, combining both instantaneous and inverse-time tripping functions (designated as device 50/51). As another example, device 27/59 would be a combined undervoltage and overvoltage relay. The complete ANSI standard lists 99 device numbers, reserving a few for future use.
You can classify relays by their operating-time characteristics. Instantaneous relays are those with no intentional time delay. Some can operate in one-half cycle or less; others may take as long as six cycles. High-speed relays operate in three cycles or less.
Time-delay relays can be definite-time or inverse-time types. Definite-time relays have a preset time delay not dependent on the magnitude of the actuating signal (current, voltage, or whatever else is being sensed) once the pickup value is exceeded. The actual preset time delay is usually adjustable.
Inverse-time relays, such as overcurrent or differential relays, have operating times that do depend on the value of actuating signal. The time delay is long for small signals and becomes progressively shorter as the value of the signal increases. The operating time is inversely proportional to the magnitude of the monitored event.
You usually use overcurrent relays on each phase of each circuit breaker in switchgear applications. Often, you may need an additional overcurrent relay for ground-fault protection. Conventional practice is to use one instantaneous short-circuit element and one inverse-time overcurrent element (ANSI 50/51) for each phase.
In the standard electromechanical relay, both elements for one phase are combined in one relay case. The instantaneous element is a clapper or solenoid type, and the inverse-time element is an induction-disk type.
In some solid-state relays, you can combine three instantaneous and three inverse-time elements in a single relay case smaller than that of one induction-disk relay.
Overcurrent relays respond only to current magnitude, not to direction of current flow or to voltage. Most relays operate from the output of a standard ratio-type CT, with 5A secondary current at rated primary current. A solid-state relay needs no additional power supply, obtaining the power for its electronic circuitry from the output of the CT supplying the relay.
On the instantaneous element, only the pickup point can be set, which is the value of current at which the instantaneous element will act (with no intentional time delay) to close the trip circuit of the circuit breaker. The actual time required will decrease slightly as the magnitude of the current increases, from about 0.02 sec maximum to about 0.006 sec minimum, as seen from the instantaneous (bottom left) curve in Fig. 3 (in the original article's page 41). This time will vary with relays of different ratings or manufacturers and also between electromechanical and solid-state relays.
Note that this curve is based on multiples of the pickup setting for the instantaneous element, which is usually considerably higher than the pickup setting of the inverse-time element.
You can select time delays over a wide range for almost any conceivable requirement. Time-delay selection starts with the choice of relay. There are three time classifications: standard, medium, and long time delay. Within each classification, there are three classes of inverse-time curve slopes: inverse (least steep), very inverse (steeper), and extremely inverse (steepest). The time classification and curve slopes are characteristic of the relay selected, although for some solid-state relays these may be adjustable to some degree. For each set of curves determined by the relay selection, the actual response is adjustable by means of the time dial.
On the inverse-time element, there are two settings. First the pickup point is set. This is the value of current at which the timing process begins as the disk rotates on an electromechanical relay, or the electronic circuit begins to time out on a solid-state relay.
Selection of time-dial setting is next. This adjusts the time-delay curve between minimum and maximum curves for the particular relay. See typical inverse, very inverse, and extremely inverse curves in Fig. 3. A given relay will have only one set of curves, either inverse, very inverse, or extremely inverse, adjustable through the full time-dial range. Note that the current appears in multiples of pickup setting.
Each element, instantaneous or time delay, has a flag that indicates when that element has operated. You must reset this flag manually after operation.
How do you go about setting the pickup point? The first step in setting the relay is selecting the CT so that the pickup can be set for the desired primary current value. The primary current rating should be such that a primary current of 110% to 125% of the expected maximum load will produce the rated 5A secondary current. The maximum available primary fault current should not produce more than 100A secondary current to avoid saturation and excess heating. You may not be able to fulfill these requirements exactly, but they are useful guidelines. As a result, you may have to make some compromises.
On the 50/51 overcurrent relay, the time-overcurrent-element (device 51) setting is made by means of a plug or screw inserted into the proper hole in a receptacle with a number of holes marked in CT secondary amperes, by an adjustable calibrated lever or by some similar method. This selects one secondary current tap (the total number of taps depends on the relay) on the pickup coil. Determine the primary current range of the settings by the ratio of the CT selected.
For example, assume the CT has a ratio of 50/5A. Typical taps will be 4A, 5A, 6A, 7A, 8A, 10A, 12A, and 16A. The pickup settings would range from a primary current of 40A (the 4A tap) to 160A (the 16A tap). If you want a 60A pickup, you select the 6A tap. If you want a pickup of more than 160A or less than 40A, you'll have to select a CT with a different ratio or, in some cases, a different relay with higher or lower tap settings.
Various types of relays are available with pickup coils rated as low as 1.5A and as high as 40A. Common coil ranges are 0.5A-to-2A (for low-current pickup such as ground-fault sensing); 1.5A-to-6A (medium range); and 4A-to-16A (the range usually chosen for overcurrent protection). CTs are available having a wide range of primary ratings, with standard 5A secondaries or with other secondary ratings, tapped secondaries, or multiple secondaries.
You can find a usable combination of CT ratio and pickup coil for almost any desired primary pickup current and relay setting.
The instantaneous trip (device 50) setting is also adjustable. The setting is in pickup amperes, completely independent of the pickup setting of the inverse-time element or, on some solid-state relays, in multiples of the inverse-time pickup point. For example, one electromechanical relay is adjustable from 2A to 48A pickup; a solid-state relay is adjustable from two to 12 times the setting of the inverse-time pickup tap. On most electromechanical relays, the adjusting means is a tap plug similar to that for the inverse-time element. With the tap plug, it's possible to select a gross current range. An uncalibrated screw adjustment provides final pickup setting. You'll need to use a test set to inject calibration current into the coil if the setting is to be precise. On solid-state relays, the adjustment may be a calibrated switch that you can set with a screwdriver.
How do you set the time dial? For any given tap or pickup setting, the relay has a whole family of time-current curves. You select the desired curve by rotating a dial or moving a lever. The time dial or lever is calibrated in arbitrary numbers, between minimum and maximum values, as shown on curves published by the relay manufacturer. A typical set of time-dial curves for an inverse-time relay is shown in Fig. 4 (on page 43). At a time-dial setting of zero, the relay contacts are closed. As the time dial setting is increased, the contact opening becomes greater, increasing relay operating time. You can make settings between calibration points, if you want, with the applicable curve interpolated between the printed curves.
You select the pickup points and time-dial settings so the relay can perform its desired protective function. For an overcurrent relay, the goal is that when a fault occurs on the system, the relay nearest the fault operates. The time settings on upstream relays should delay their operation until the proper overcurrent device has cleared the fault. You'll need a selectivity study, plotting the time-current characteristics of every device in that part of the system examined. With the wide selection of relays available and the flexibility of settings for each relay, you'll be able to achieve selective coordination for most systems.
You can select and set relays other than overcurrent relays in similar fashion. Details will vary, depending on the type of relay, its function, and the manufacturer.
How does the relay operate? An electromechanical relay will pick up and start to close its contacts when the current reaches the pickup value. At the inverse-time pickup current, the operating forces are very low, and timing accuracy is poor. The relay timing is accurate at about 1.5 times pickup or more, and this is where the time-current curves start (Fig. 4). Make sure you consider this fact when selecting and setting the relay.
When the relay contacts close, they can bounce, opening slightly and creating an arc that will burn and erode the contact surfaces. To prevent this, overcurrent relays have an integral auxiliary relay (with a seal-in contact in parallel with the timing relay contacts) that closes immediately when the relay contacts touch. This prevents arcing if the relay contacts bounce. This auxiliary relay also activates the mechanical flag, which indicates the relay has operated.
When the circuit breaker controlled by the relay opens, an auxiliary contact on the breaker deenergizes the relay coil. This protects the relay contacts, which are rated to make currents up to 30A, but should not break the inductive current of the breaker tripping circuit, to prevent arcing wear. The disk then returns to its initial position by the spring to reset the relay. Reset time is the time required to return the contacts fully to their original position. Contacts part about 0.1 sec (six cycles) after the coil deenergizes. The total reset time varies with relay type and time-dial setting. For a maximum time-dial setting (contacts fully open), typical reset times might be 6 sec for an inverse-time relay and up to 60 sec for a very inverse or extremely inverse relay. At lower time-dial settings, contact opening distance is less; therefore, reset time is lower.
A solid-state relay doesn't depend on mechanical forces or moving contacts for its operation but performs electronically. Thus, timing is very accurate even for currents as low as the pickup value. There is no mechanical contact bounce or arcing, and reset times are extremely short.