THE NASA AMES RESEARCH CENTER IS UPGRADING ITS PROTECTION SYSTEM to meet the requirements for increased reliability under different conditions. The center is supplied with electric power at 115 kV from the local utility's transmission grid. Two 115-kV lines come to the main substation and power is then distributed to 17 115-kV/13.8-kV or 115-kV/6.9-kV transformers connected to smaller substations throughout the center.

Average demand is 10 MW to 12 MW, with a peak of more than 100 MW when the major wind-tunnel facilities are operating. The relays in the main substation as well as six other substations at NASA Ames are of great importance to the facility and are powered with 125 Vdc supplied by the independent battery rooms in the substations.

Power-system protection requires a very reliable power supply to ensure protective relays are available when a fault occurs. Thus, one must pay close attention to the design and maintenance of the power system for relays in the substations. In a very critical substation, it is common to install two battery systems and separate the primary and the backup by using two trip coils to the breakers.

However, many laboratories and office buildings are located far from the substations and lack the dc source to power the relays. It was undesirable to install a battery room for one or two relays in each building. Likewise, installation of low-voltage dc lines between the substations and buildings was not useful because of voltage drop issues. In many cases, these solutions were too costly or in other cases not possible.

LOOKING FOR A RELIABILITY UPGRADE

To have a reliable source to power the relays in some buildings and laboratories, NASA Ames considered the following:

• Depend only on 125 Vac to power the relays, since we could have voltage sag or have no voltage at all during a fault event

• Install multiple uninterrupted power sources (UPS). They need maintenance, their batteries must be changed at least every five years, and extra space is needed to accommodate them

• Evaluate self- or dual-powered protective relays.

We deemed the last option as our best choice. Many relay manufacturers are making self-powered, single-phase or three-phase relays. We decided to install a three-phase, dual-powered relay that would operate as self-powered or with auxiliary 125 Vac. We selected the AREVA (previously called the Alstom) P124 MICOM dual-powered relay.

DUAL-POWERED RELAYS

As the name indicates, dual-powered relays are normally powered from an ac or dc auxiliary supply. However, the best feature is that these power supplies need not be especially secure because the current transformer (CT) circuit can power the relay.

In a conventionally designed single battery-room substation, loss of dc affects operation of the protective relays. That loss also makes it impossible to trip any breakers. There are different options for powering protective relays and methods for tripping breakers in the absence of dc power.

CT POWER

In self-powered mode, the relay is powered from the CT circuit alone. There is a requirement for a minimum level of current flowing through at least one phase of the current transformer in order for the relay to operate. Lowering the design value of this parameter increases the burden on the current transformers and the power dissipated within the relay case. Therefore, the limits should be a compromise based on these factors:

• Minimum current to power the relay for phase faults equals 0.4 A (nominal)

• Minimum current to power the relay for ground faults equals 0.2 A (nominal).

However, a combined three-phase and ground-fault relay will operate with lower ground-fault current settings when the load current in the protected circuit is sufficient to power the relay (i.e. greater than 0.4 A). Settings less than 0.2 A are provided for ground faults but must be used with discretion.

The worst-case scenario occurs when switching onto a fault with no auxiliary power available. In this case, the relay is not powered and will be delayed in operation by the start-up time. This delay will need to be taken into account in relay coordination. The delay is the total time required for:

• The processor to initialize registers
• Perform self-checks
• Read in nonvolatile memory settings.

There will be an additional delay while the power supply builds up, but this will be less significant when using an inverse time/current characteristic because the power supply delay similarly varies with current. The start-up time is not reduced by lowering the time-multiplier setting and needs to be considered when coordinating the timing of remote backup.

Figure 1 shows the start-up time delay of a self-powered relay as a function of the current through the CTs. With pre-fault load current above the minimum level required, there will be no start-up time delay and the relays will operate within normal time settings. In cases where the start-up delay cannot be tolerated, the relay requires an auxiliary ac voltage supply. This also makes stored disturbance and event records more accurate. The recording starts only after the relay powers up and initializes, so pre-fault conditions may be missed.

That is why dual-powered relays were selected instead of self-powered relays. This ensures faster operation in the cases when:

• Auxiliary power supply is available at the time a fault occurs

• Auxiliary power supply has failed, but the load current is above the required minimum to power the relay.

A small 3-V lithium battery in the relay is used to support the existing recorded event in the case of loss of auxiliary power supply and load current below the minimum level. The start-up time delay will then only apply to the cases when there is a loss of ac power and load current below the minimum level at the time a fault occurs. But even in the worst case, the additional time delay for the fault clearing time will typically be in the range of about 50 msec.

Dual-powered relays are equipped with multiple opto-isolated inputs and relay outputs. Their operation must be considered in the analysis of the relay performance, since at the claimed minimum operating current they cannot all be energized at the same time. If they have to be simultaneously operated, the minimum operation current will have to be increased. However, in applications requiring a dual-powered relay, it is unlikely that more than two output relays will be energized at any one time.

POWER WITH AUXILIARY VOLTAGE

The addition of an auxiliary ac or dc voltage supply to power the relay will:

• Enable the settings to be changed when the protected circuit is de-energized

• Enable records to be retrieved and control functions to be carried out over the communication link

• Reduce the burden on the line CTs.

Auxiliary ac voltage can be lost during a fault. But, power will be drawn from the CT circuit to maintain the relay in a fully operational state. Thus, if the source of the auxiliary voltage is carefully chosen, it is unlikely to be lost completely during ground faults even though it may collapse up to 50% of its rated value. Provided the voltage is still above the minimum required to power the relay, very-low-ground-fault settings can be successfully applied. In the absence of the auxiliary voltage, the relay is not guaranteed to operate for ground-fault currents less than 0.2 A.

BREAKER TRIPPING

The successful clearing of a fault requires a protection relay to detect the fault condition and issue a trip signal, as well as a breaker to operate and clear the fault. Dual-powered relays may use different methods for tripping the breaker to clear the fault. The method selected for each specific application depends on the specifics of the breaker used and the auxiliary voltage available at the breaker location. These are four of the more commonly used methods for breaker tripping by self-powered or dual-powered relays:

Striker trip

A dual-powered relay can trip the circuit breaker by capacitance discharge with sufficient power (20 mJ at 12 V) output to a striker that releases the actuating mechanism of the circuit breaker.

Figure 2 shows a simplified block diagram of this breaker tripping method, and Fig. 3 gives an example of a striker.

Capacitor discharge trip

Dual-powered relays may use an additional capacitor module that is charged from the current circuit and also from the auxiliary voltage circuit (Fig. 4).

This capacitor module has storage capacity such that, in case of loss of auxiliary supply, it can supply sufficient energy to excite a standard trip coil for two consecutive tripping orders without recharge. It may be discharged directly into a suitably sensitive trip coil via one of the programmable output relays. The minimum energy fed to the trip coil is from the capacitor, but, in most cases, current from the auxiliary voltage circuit and/or the current circuit will supplement it.

When energized from current alone, the lowest current for which the relay will operate will be that necessary to start up the power supply. To use lower fault settings, an auxiliary supply is required.

The capacitance discharge circuit is not isolated from the auxiliary supply and, to prevent the relay from being damaged, no external ground connection should be made to this circuit.

Figure 4 shows a capacitor module with such storage capacity that, in the case of loss of auxiliary supply, it can supply sufficient energy to operate a standard trip coil for two consecutive breaker trips without recharge.

AC series trip

Dual-powered relays may be discharged into an auxiliary relay. This relay will be de-energized in its normal state, with its break contacts short-circuiting the trip coils of the circuit breaker (Fig. 5).

The trip coils are connected in series with the current transformer secondary circuit so that, when the auxiliary relay is operated, the full secondary current is diverted through the trip coils. To cover all fault conditions, three trip coils are required and may be necessary to limit the maximum energy that can be fed to each coil, by means of saturating shunt reactors.

Trip scheme used

The distribution breakers used on transformers throughout the NASA Ames Research Center are equipped with a built-in capacitor that has storage capacity allowing it to trip the breaker. Figure 6 shows a simplified one-line diagram of this tripping arrangement.

HUNDREDS INSTALLED

NASA Ames Research Center has installed hundreds of microprocessor relays for different applications throughout its facility. At this time, around 45 AREVA dual-powered relays are used to provide the protection of multiple remote distribution substations that do not have battery installations.

These installations, all using dual-powered relays that operate like conventional relays when auxiliary supply is available, are powered from the load or fault current in case of power loss.

John Dehkordi has worked in the NASA Ames Research Center under DMJMH+N contractor since April 1999. Prior to that, he worked for 18 years in electric utilities as a protection engineer in Virginia. Dehkordi graduated with a BSEE degree from the New York Institute of Technology in 1980. jdehkordi@mail.arc.nasa.gov