Electrical Power Systems

Introduction

Almost all systems in an aircraft use electrical power in some capacity. Because of the complexity inherent in aircraft electrical design, electrical power considerations must be addressed early in the design process. More often than not, the testing of power system specifications is done only at the LRU/component level and not at the complete system level. Because of this, there have been different problems in later system-test stages arising from this lack of system-level approach in specification requirements for electrical power.

A basic understanding of electrical power systems and their operation under different conditions would improve control in vendor testing and, later, system testing. This understanding could also be applied to the benefit of other design considerations, such as the effect of high intensity radiated field (HIRF) and other transients on power quality and tolerances throughout systems.

In this section the following areas are addressed:

• • Power sources and distribution structure
• • Power quality and operating modes
• • Design considerations

Power Sources and Distribution Structure

The normal source for aircraft power begins with generators in each engine that convert mechanical power to electrical power. Additionally, when the aircraft is grounded the auxiliary power unit (APU) supplies the electrical power for the aircraft, as well as starting air for the engines.

Depending on aircraft power requirements, these generators can produce AC or DC power.

Generally, the commercial aircraft standard for a primary power-generating system is 3-phase, 115 VAC, 400 Hz. The main reason for this high frequency is to reduce weight and size of the equipment; 400 Hz does not need the huge transformers seen in 50 or 60 Hz equipment. Filtering is much simpler. Some military aircraft use 270 VDC. The space station uses a 20,000 Hz system; smaller aircraft do not need more than a 28 VDC system.

Types of Power Generation

Basically, there are two types of generating systems used for primary power.

1. 1. Constant speed drive (CSD)/generator
   The first of these generator types uses a mechanical means of converting—through a hydromechanical means—the variable speed of the engine into a constant speed. A 3-phase generator is coupled to the CSD and produces a constant 400 Hz electrical power. Until recently, this was the only feasible method to obtain 115 VAC, 3-phase at a steady 400 Hz.
   An evolution of this CSD/generator is the integrated drive generator (IDG) that combines these two functions into one component.
2. 2. Variable speed—constant frequency (VSCF)
   The advances in solid-state high-power electronics are making the usage of another type of primary electrical generation possible—variable speed–constant frequency (VSCF). Instead of converting the variable engine speed to a constant number of revolutions to drive the generator, the VSCF system uses a generator producing power at a variable frequency that is transformed to the required 400 Hz by electronic conversion. The number of generators associated with an aircraft is a function of the number of engines the aircraft has. The electrical power distribution system for each generator is configured to allow each generator to power its own electrical distribution system, which is referred to as a generating channel. Multiple generating channels allow us to build redundant power systems and to distribute the loads appropriately.

Emergency Power Generation

Backup power sources take over automatically, or by manual actuation, when the primary power sources fail.

Many different forms of backup power sources are available, including:

1. 1. Static sources such as batteries, capacitors, solar cells, atomic piles, or
2. 2. Dynamic sources such as generators, windmills, hydraulic generators, etc.

Power Bus Structure

AC power bus

Each generating channel has a main power distribution point called the AC Generator Bus. Power on the AC Generator Bus is distributed to individual AC buses. The rectifier units (used to generate DC power), are connected directly to the AC Generator Bus. Otherwise, LRUs are connected to the secondary AC buses. (A bus is a circuit over which data or power is transmitted or received.)

To ensure correct sizing of power feeder wires, accurate information must be given to the Wiring Systems organization.

Inaccurate power requirement information can lead to:

1. 1. Inadequate wiring installations providing lower than required voltage values at the LRU during periods of peak power usage (if actual power requirements are greater than those specified, equipment malfunction may occur if voltage dips are severe enough);
2. 2. Excess weight in the wiring installation if actual power requirements are less than those specified.

DC power bus

The supply for the aircraft’s 28 VDC power requirements is obtained by transforming the AC power into a 28 VDC current. This is accomplished through the use of transformer rectifiers (TRs) connected to the AC Generator Bus. Generally, one or more of these units, per primary generator, is installed to provide for the required power demands and redundancy. The resultant DC is called Primary DC Power.

Paralleled Systems

Multiple generators can be configured to operate in parallel, meaning that the power output of the generators is tied together. With this approach, the loss of one generator will not be noticed and the loads will continue to be powered by the remaining generator(s).

For reliability purposes, a “split-parallel” configuration can often be the best approach. For example, a four-engine aircraft could have two independent generator systems, each with two parallel generators. A tie bus relay could then be employed to isolate each of the four generating channels under special system conditions. During most normal flight operations, all three generating channels are connected in parallel. Two exceptions are: (a) during ground operations where only those systems and buses necessary are powered up; and, (b) during autoland when the generator buses are isolated for maximum safety and reliability.

Redundant Loads and Power Supplies

The power supply separation philosophy needs to be maintained throughout the design process for all systems. Keep the following in mind when assigning loads to power buses:

• Redundant loads need to be connected to redundant power supplies. Criticality of redundant loads must be taken into consideration when assigning these loads to buses. Critical or essential redundant loads are usually connected to different generating channels. For redundant loads that are not critical or essential, two different AC buses within the same generating channel might be used.

Important facts

Understanding the AC and DC power quality specifications for normal, abnormal, and emergency operating modes are important for the systems/design engineers.

ARINC 413A defines these electrical power systems’ operating modes as follows:

1. 1. Normal: Normal operation of the primary electric system is all the functional electric-system operations required for aircraft operation, aircraft mission, and electric-system controlled continuity. These operations occur at any given instant and any number of times during flight preparation, takeoff, airborne conditions, landing, and anchoring. Examples of such operations are switching of utilization equipment loads, engine speed changes, bus switching and synchronization, and paralleling of electric power sources. Switching of utilization equipment loads is a type of system operation that occurs the greatest number of times.
2. 2. Abnormal: Abnormal operation of the electric system is the unexpected but momentary loss of control of the electric system. The initiating action of the abnormal operation is uncontrolled and the exact moment of its occurrence is not anticipated. However, recovery from this operation is a controlled action. This operation occurs, perhaps, once during a flight, or because of equipment malfunction, or it may never occur during the life of an aircraft. An example of an abnormal operation is the faulting of electric power to the structure of an aircraft and its subsequent clearing by fault protective devices. Abnormal limits accommodate the trip band of protective equipment in the primary power generating system. Besides momentary loss of control, abnormal operation also can include steady-state operation outside the normal control limits, but within the overvoltage/undervoltage protection trip limits of the system.
3. 3. Emergency: Emergency operation is defined as that condition of the electric system during flight when the primary electric system becomes unable to supply sufficient or proper electric power, thus requiring the use of a limited independent source of emergency power.

Ground operation power transfers, takeoff, cruise, descent and landing, autoland, are all examples of normal operation where the power supply qualities are stable and all power switching operations perform as expected.

Abnormal operation is generally associated with faults in the power distribution system, loss of a power source, and equipment failures.

In emergency operations, when the primary sources of power have failed, we rely on the emergency power supplies to maintain only critical and essential equipment.

In an all-engine-out, the following functions are required to be operational:

• • Cockpit and cabin lighting
• • Altitude indication
• • Attitude indication
• • Indicated airspeed
• • Direction
• • Engine indication
• • Engine ignition
• • Engine fuel controls
• • Passenger address
• • VHF 1
• • Fire detection
• • Fire agent discharge
• • Other functions required for continued safe flight and landing

In an all-generator-out, extended-over-water emergency, all of the preceding functions must be operational, as well as:

• • Navigation and landing guidance
• • High frequency radio
• • Air traffic control (ATC)—transponders
• • Fuel pump

Emergency power sources are switched on either manually or automatically after a loss of normal electrical power has occurred. Equipment intended to operate on emergency power must be designed to handle long-term power interrupts, power up safely, and begin performing its function immediately.

AC/DC Power Quality

For electrical power systems, the operating-mode classification centers around the varying states of power quality, specifically to the steady-state and transient characteristics found (or not found) in the power supply under varying conditions.

Steady state

1. 1. A condition in which circuit values remain essentially constant, occurring after all initial transients or fluctuating conditions have settled down.
2. 2. A term used to specify the current through a load or electric circuit after the inrush current is complete. It is a stable run condition.

7.6.9.2 Transient

1. 1. A momentary surge on a signal or power line. It may produce false signals or triggering impulses and cause insulation or component breakdowns and failures.
2. 2. Signals that exist for a brief period prior to the attainment of steady-state conditions.

Normal Operation Power Qualities

1. 1. Steady-state characteristics: Steady-state characteristics are based on the electrical power system’s ability to regulate to a nominal frequency and voltage value.
2. 2. Transient characteristics: Transients are usually induced by the switching of loads but may also be induced by lightning strikes and other external environmental factors. For normal operation, transient characteristics are based upon the electrical power system’s ability to return to the nominal frequency and voltage value. Transients should not cause loss of memory or system switching to occur, which would lead to flight crew action to reperform preflight, or to take corrective action to place the system back into a normal mode of operation. In looking at the transient characteristics for normal operation, it is important to understand that there are two different types of switching logic commonly found on aircraft. These two switching methods produce different transient characteristics. The first switching logic is break power transfer (BPT) which, as its name implies, causes a momentary break in the supply of power. The second is no break power transfer logic (NBPT). As its name implies, power is maintained throughout the switching process by momentarily placing both power sources on the line at the same time.

Abnormal Operation Power Qualities

1. 1. Steady-state characteristics
   Abnormal steady-state characteristics are based upon the electrical power system’s protective trip limits for voltage and frequency.
   In abnormal operation, voltage regulation components may fail, causing voltage or frequency to exceed normal steady-state limits, but not exceed protective trip limits.
2. 2. Transient characteristics
   For abnormal operation, transient limits are based upon failures that trip the transient protective circuitry. This includes failures in electrical power system components, the distribution system, or utilization equipment.

Emergency Operation Power Qualities

Emergency operation power qualities are defined by steady-state characteristics only. Transient characteristics remain the same as for abnormal operation. Steady-state characteristics for emergency operation are based upon the electrical power system’s ability to regulate to a nominal frequency and voltage. Except for steady-state AC voltage limits, the limits associated with emergency vary around a larger tolerance than normal operation due to the greater regulation range associated with the emergency power supplies.