Busbars Are A Vital, Yet Often Overlooked, Part Of The Power System
By Paul Marot, P.Eng.
Busbars are a vital, often overlooked, part of the power system. Busbar faults are rare. However, when one occurs damage is widespread and plant downtime is substantial. This soon reminds users of busbars' importance and, in particular, the importance of good protection.
Busbar Protection And Requirements
The high fault levels associated with busbars require that protection be fast. Typical fault clearing time should be less than 100ms; with fast breakers this means measuring time should be about 20 to 30 ms.
In order to minimize the interruption to the plant the protection system must correctly identify the area of the fault and open only the necessary, and minimum number, of breakers. To achieve this, it must discriminate properly -- but because of speed requirements, discrimination based on time delays is not acceptable. It is therefore preferable to have a clearly defined zone of protection or unit scheme.
Busbars, the connection nodes of multiple power circuits, must have very secure protection since tripping of a busbar usually has widespread power interruptions. The risk of an unnecessary trip must be kept to a minimum. This immediately brings stability into consideration as it is usually a fault just beyond the zone of busbar protection -- commonly known as through faults -- which has similar fault levels to the bus that causes a mistrip of the busbar protection. The protection must be stable for these though faults.
It should be pointed out that the above requirements can, depending on the application and relay principles involved, be competing -- an improvement of one means a deterioration of the other. For example, an increase in security would probably be achieved at the expense of tripping time.
Relaying Principles Impact
On CT Requirements
To understand the current transformer (CT) requirements of different busbar protection systems, it is worth looking at the various relaying principles or measuring devices available for use with busbar protection systems. The most common types are basic circulating current differential, biased differential, and overcurrent and direction overcurrent relaying.
The relaying principle of basic circulating current differential uses the well known Mertz Price circulating current principle. It compares current signals and requires them to be matched, thus requiring very accurate current signals from the CT's as well as matched CT ratios. Errors in the current matching or transient CT errors can cause relay operation, so that some time will be spent examining CT's and sources of errors.
Biased differential uses the Mertz Price principle but compares the difference between current signals and the average of the current signals and requires the result to be above a certain percentage before operating.
It basically has an operate winding which looks at the difference current and restrain windings which looks at the average current.
Obviously this relaying principal can accommodate greater sources of error in CT's. It is a more secure principle but not as fast as the circulating current principle.
Most modern overcurrent relays use a principle of peak or RMS measurement of the current. The measured signal is obtained by passing the CT secondary current through an input CT on the relay. The relay measurement requires very little energy from the measured signal, so by design, these relay input CT's impose very little burden on the main CT's. Consequently CT's for use with overcurrent relays have no stringent requirements besides those of standard relaying accuracy.
Directional overcurrent relaying would be used in a direction comparison type scheme described later but the measuring element is usually an overcurrent relay that is directionalised. Such relays do not pose any more limitations on the CT's than overcurrent relays.
CT Performance And Requirements
The simplified CT equivalent circuit given in Figure 1 is first examined to establish a device's limitations when applied to relaying.
The CT takes magnetizing current from the primary circuit and since this current does not flow into the relay branch, there is an error between the primary and relay currents. This is a major source of ratio error.
Winding resistance is part of the CT burden and needs to be taken into account when establishing the CT kneepoint voltage (Vk) requirement for a particular application. This should be known and the smaller it is the better.
The kneepoint of the CT gives an indication of the burden the CT can accommodate. Recall that we are trying to get accurate signals up to current values in excess of 20 times rated and this is especially likely with busbar CT's where fault levels are high. The kneepoint is the point on the magnetization curve where a large increase in magnetizing current produces a minimal increase in the output voltage needed to drive current through the secondary burden. This value is given directly in the CT specification: 10L200 where the 200 figure means the CT can produce 200 volts on the output with the current being within the stated accuracy. It must be remembered that the ratio of a CT greatly influences the maximum kneepoint that can be obtained by a CT. The kneepoint is basically the maximum secondary output voltage the CT can develop. The physical size of the CT and how much core iron is used determines the volt per turn on the secondary winding. This typically varies from 1.5 to 2.5V per turn, so given the number of turns which is the CT ratio, one can estimate the maximum obtainable kneepoint. Don't try and expect a 400V kneepoint out of a 400/5 ratio CT!
If the CT becomes saturated because of high flux in the core, it will not produce any secondary output current. In fact, the secondary will appear as load to the other parallel connected CT¹s. This is illustrated by the short across the magnetizing branch in Figure 2 and the load is simply the CT winding resistance.
Busbar Protection Schemes
In order to examine different relay principles, and some basic CT performance characteristics, various forms of busbar protection will be reviewed. The busbar protection scheme, namely the blocking scheme, will also be reviewed in light of all the features found in modern overcurrent relays.
For the purposes of simplicity only single busbar systems will be considered. These are common in HV sub-transmission level power systems. The variety of busbar configurations such as breaker and a half, double busbar and ring bus arrangements found on EHV and HV transmission systems utilize the same principles but only in more complicated systems.
High Impedance Differential
This is a simple yet fast scheme with a low relaying cost, but it requires good CT inputs with well matched ratios with high CT kneepoints. This is most easily obtained by utilizing CT's from one manufacturer only which is usually possible with a new installation.
All incoming busbar CT's are paralleled and connected in opposite polarity to the paralleled outgoing busbar CT's. The differential relay is then connected across the paralleled CT's.
The relays used most often in this scheme are simple attracted armature relays which have fast operating times. This scheme can thus easily provide 20ms operating times including the trip relay time.
The main application consideration with this scheme is stability for heavy through faults. The stability is determined by the CT's and relay settings which in turn affect the sensitivity of the scheme.
Consider a simple two CT differential circuit shown in Figure 3. If the right hand side CT saturates, which means it appears as a load as previously described, then it will cause a voltage to develop across the relay which will be If(2Rl + Rct). If this voltage is greater than the setting voltage, the relay will operate. This is undesirable. Therefore, this becomes the setting criterion; the relay should be set to a voltage > If(2Rl + Rct).
Once the setting is determined, the scheme sensitivity can be calculated by considering the relay operating current reflected to the primary plus the magnetizing current of the CT¹s connected in parallel. Recall that the CT primary provides the magnetizing current not reflected in the secondary. The primary operating current for the scheme can be found from the formula:
Ip= N (Ir + nIu) where N= CT ratio
Ir= relay operating current at setting
Iu= magnetizing current at setting
voltage (obtained off CT magnetizing curve)
n= number of CT set connected
If the relay setting voltage is high, the CT magnetizing current at the setting voltage totaled up for all the CT's can be significant.
For an internal fault, the CT's will attempt to force all the secondary current through the relay. With a high impedance differential relay, this would require very high secondary voltages, beyond the CT's capabilities, again causing saturation. However, reliable operation of the relay does take place if there is a significant time before saturation occurs. This is because, if the kneepoint is well above the setting voltage, the relay will receive sufficient voltage to operate before saturation occurs. Manufacturers thus recommend a voltage of at >